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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Look east In September 2013 – and take a journey from mythology to science

As we travel September skies we’ll move from the age of mythology to the age of science.

First, the age of mythology. Had you been born a few hundred – or even a few thousand – years ago, the eastern sky in September shortly after sunset would look something like this to your imaginative eye.

For most of recorded human history different cultures turned the stars into familiar patterns that  illustrated familiar mythological stories. In our September eastern skies shortly after sunset we have a wodnerful collection of five related mythological figures - Cepeheus (king), Cassiopeia (queen), Andromeda (princess), Perseus (hero), and Pegasus, the flying horse. (Developed froma screen shot of  Starry Night Pro. Click for larger version.)

For most of recorded human history different cultures turned the stars into familiar patterns that illustrated familiar mythological stories. In our September eastern skies shortly after sunset we have a wonderful collection of five related mythological figures – Cepheus  (king), Cassiopeia  (queen), Andromeda  (princess), Perseus  (hero), and Pegasus, the flying horse. (Slightly modified screen shot of Starry Night Pro. Click for larger version.)

The tale is easy to remember. The king (Cepheus) and queen (Cassiopeia) felt their kingdom was threatened by a sea monster, so as a sacrifice to the monster they tied their daughter, Andromeda, to a coastal rock. m But don’t worry, our hero Perseus, fresh from slaying Medusa, appears to rescue Andromeda, and they ride off across the starry heavens on his faithful steed, Pegasus, the flying horse.  Really – today the king and queen  would be tried for child abuse!

Of course as usual with the ancient constellations, the figures bear only the crudest relationship to the pattern of bright stars, so a lot of imagination is required to see them.  But that said, I do find this myth an easy way to remember these five constellations. In modern times we’ve drawn complex boundaries around each constellation and used these imaginary celestial boundaries to name and locate stars.  But more importantly, we’ve developed a celestial coordinates system much along the lines of Earthly longitude and latitude.

If you imagine the Earth’s latitude lines projected onto the dome of the sky, they become circles indicating declination – how far in degrees a point is from the celestial equator. The celestial equator itself is a projection onto the sky dome of Earth’s equator.  Longitude is projected and marked in 24 hours of “right ascension” so the whole celestial clock appears to pass overhead in the course of a day. I found it difficult remembering where these hours begin until I learned about the “Three Guides.” These are three bright stars –  indicated by arrows in the chart below – that fall very close to the zero hour line of right ascension. Above them (think of these as preceding them, for they rise first)  the hours count backward from 24. Below them – think of these  as following them, since they rise afterwards – are the hours counting up from 0.

Prepared from a Starry Nights Pro screen shot - click for a larger version.

I prefer to remember my sky in terms of bright stars and asterisms, so Cassiopeia becomes the “W.” Andromeda  becomes “Andromeda’s Couch,”  and the flying horse becomes the “Great Square.”   But the Three Guides – the three bright stars indicated by arrows – allow you to see the sky in more scientific terms, for these are the starting points for laying a grid on the sky to create a precise address for each star in terms of its places on the grid. This grid is indicated above by the red lines. (Prepared from a Starry Nights Pro screen shot – click for a larger version.)

For a printer-friendly version of this chart, download this.

First, let’s look at the “Great Square” – or perhaps we should say “Great Diamond,” since that’s what it looks like when rising. Once overhead, it is certainly a square, and it forms the heart of Pegasus – the flying horse. The stars are all second and third magnitude – about the brightness of the stars in the Big Dipper – so wait until about an hour after sunset, then look east and you should be able to pick this out. Its stars mark out a huge chunk of sky that is nearly empty of naked-eye stars, which is why I sometimes call it the “Great Empty Square.”

Andromeda’s Couch, ties to the northern corner of the square. In fact, it shares a star with this corner. “Andromeda’s Couch” is just my memory device – others would simply call this “Andromeda” because that’s the name of the constellation it dominates. I have difficulty seeing the lovely maiden chained to a rock by looking at these stars.  But knowing that in myth Andromeda was a lovely woman who was rescued by Perseus, I like to think of this graceful arc of stars as her couch with her a misty fantasy figure lying there in alluring fashion. That said, notice three things about it:

1. The bright star at the right – southern – end is also a corner of the Great Square, as we mentioned. In fact, it is the brightest star in the Great Square.

2. The three brightest stars in the “couch” – I’m ignoring the second star which is fainter – the three brightest are about as close to being identical in brightness as you can get – magnitude 2.06, 2.06, and 2.09. They also are pretty equally spaced. Hold your fist at arm’s length and it should easily fit in the gaps between these stars, which means there are 10-15 degrees between each star. That’s similar to the spacing between the four stars in the “Great Square” as well.

3. The second star, as mentioned, is dimmer by more than a full magnitude (3.25), but it’s what gives this asterism a couch feeling to me – or maybe a lounge chair – marking a sharp, upward bend.

And where’s the hero Perseus? he should be nearby, right? Well he’s on his way, rising in the northeast after Cassiopeia, but we’ll leave him for next month when he’s more easily seen.

Now for the pièce de résistance!

This is a group of stars that are new to me, at least in this role, and I love them! They’re called “The Three Guides,” but I think of it as four guides They can all be tied together by a long, graceful arc that represents the great circle of zero hour right ascension – which is the “celetsial meridian” as defined in the equatorial coordinate system.

As mentioned, the equatorial coordinate system is essentially a projection of the Earth’s latitude and longitude system onto the sky to enable us to give a very precise address for any star or other celestial object, as seen from our planet. On Earth we require an arbitrary circle be chosen as the zero longitude line, and this is the circle that passes through the poles and Greenwich, England.

In the heavens we also need such a circle, and the one chosen is the one that passes through the point where the Sun crosses the celestial equator at the vernal equinox. But that point is not represented by any bright star, so how do we know where this “zero hour” circle is? We need it to put numbers to the entire system. Enter “The Three Guides.”

They start with the star Beta Cassiopeia. This is the western most star in the familiar “W”  – the one which rises first and leads the rest. (Remember – all stars appear to move westward as the earth turns.)  From there draw an arc to Alpha Andromedae. This is the star mentioned before where Andromeda and the Great Square are joined – they both share this star.

The third star of this trio is Gamma Pegasi – the star that appears to be at the bottom of the Great Square when we see it as a diamond when rising. (If this is not clear, one glance at the accompanying chart should make it so.)

When I look at this great arc, however, I always start to trace it right from the North Star, Polaris. All the great circles representing meridians of right ascension pass through the north and south celestial poles.

As you move upward from this zero line in the general direction of the Summer triangle, the hours count backwards counting the Zero Hour as 24. Move downward, towards the horizon and the hours count forward from zero. This sequence is marked on our chart around Polaris.

Taking a wide view of the “Three Guides” to incorporate the North Star and Summer Triangle as well. Here’s what we should see about an hour after sunset. Click image for larger version. (Derived from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, download this.

What’s important is to be able to visualize this one circle in the sky and connect it with the another circle crossing it at a right angle – the celestial equator. If you can do that, you will have identified the two zero points on the equatorial coordinate system and moved your knowledge of finding things in the sky from the mythological arena to the scientific one. That’s why these three “guides” excite me so. When you can look up at the night sky and see not only a dome, but a curved grid projected on it, and on this grid be able to attach meaningful numbers, then you have graduated to sky explorer, first class!

. . . and the rest of the guideposts?

If you’ve located the new September asterisms and identified The Three Guides, then it’s time to check for the more familiar stars and asterisms you might already know, assuming you have been studying the sky month by month. (If this is your first month, you can skip this section.) So here are the guidepost stars and asterisms still visible in our September skies.

  • The Summer Triangle is now high overhead, though still favoring the east. Vega, its brightest member, reaches its highest point about an hour after sunset and moves into the western sky. Altair and Deneb are still a bit east, but will cross the meridian within about three hours of sunset.
  • The “Teapot,” marking the area of the Milky Way approaching the center of our galaxy, is due south about an hour after sunset. Well into the southwest you’ll find the red star Antares that marks the heart of the Scorpion.
  • Arcturus (remember, follow the arc of the Big Dipper’s handle to Arcturus) is due west and about 25 degrees above the horizon as twilight ends.
  • The Keystone of Hercules and the circlet that marks the Northern Crown can both be found high in the western sky by tracing a line between Vega and Arcturus.

Look north in September 2013 – the king’s on the rise!

Yes, that’s Cepheus, the King – remember that Cassiopeia (the “W” ) is the Queen. Though Cepheus makes a familiar “home plate” asterism, it’s not nearly so memorable as the “W” of Cassiopeia, primarily because its stars are dimmer than those of the “W.” In fact, you might have difficulty picking it out at first, but here’s a tip: Follow the familiar “Pointers” of the Big Dipper to the North Star – then keep going. The first bright star you meet will mark the tip of the Cepheus home plate. (It’s about one fist away from Polaris – the Pointer stars are nearly three times that far in the other direction.)

Also coming up below the “W” is the “Bow” asterism that marks Perseus, who is carrying the head of Medusa, which contains the “Demon Star,” Algol. We’ll take that up next month when they’re higher in the sky and easier for all to see. Here’s a chart.

Click image for a larger version. (Developed from Starry Nights Pro screenshot.)

For a printer-friendly version of this chart, download this.

To review the connecting mythology, which helps me remember the related constellations, here’s the story in brief.

Cepheus and Cassiopeia have a daughter Andromeda whose beauty makes the sea nymphs jealous. They enlist Poseidon to send a sea monster to ravage the coastline of Ethiopia, the kingdom of Cepheus and Cassiopeia. To appease the monster, the good king and queen chain Andromeda to a rock along the coast, but Perseus rescues her and together they escape on Pegasus, his flying horse.

You meet Andromeda and Pegasus – the flying horse is much easier to identify as the “Great Square” – in the “look east” post this month. Also in the “Look East”  post we detail the “Three Guides,” three stars that mark the zero hour in the equatorial coordinate system used to give a permanent address to all stars. The first of those Three Guides is Beta Cassiopeia, visible in our northeastern sky, and so on the chart with this post.

Moving from mythology to science, Cepheus is probably best known today for a special type of star called a Cepheid variable. This is a star that changes in brightness according to a very precise time table. What’s more, it was discovered that the length of a Cepheid’s cycle – that is the amount of time it takes to grow dim and then brighten again – is directly related to its absolute magnitude. The absolute magnitude of a star is a measure of how bright it really is as opposed to how bright it appears to us. (How bright it appears is, of course, related to how far away it is.) That makes Cepheid variables a sort of Rosetta Stone of the skies.

It is relatively easy to time the cycle of a variable, even if the star is quite faint from our viewpoint. These cycles usually cover a few days. If you can identify the length of this cycle, you then can know the absolute magnitude of a star. And if you know its absolute magnitude, it’s a simple matter to compare that to how bright it appears to us and thus determine its approximate distance from us.

This is a huge breakthrough. Without Cepheid variables astronomers were at a loss for determining the distance of anything that was more than a few hundred light years away. The distance to such “close” stars could be determined using a very common method known as parallax – that is, determining how the star appeared to change position slightly from opposite sides of the Earth’s orbit. But that change in position is extremely tiny and difficult to measure even with very close stars. With the Hipparcos satellite and computer analysis, it has been possible to use this parallax system for stars as far as 3,000 light years. But that still is close by astronomy standards. (Keep in mind our galaxy is about 100,000 light years across.) But Cepheid variables can even be found in other galaxies. In fact, they played a huge role in proving that “spiral nebulae” were really other “island universes” – that is, other galaxies. The Hubble Space Telescope has found Cepheids out to a distance of about 100 million light years – a huge leap from the 3,000 light years we can reach with the parallax method.

There are other ways of making an educated guess at an object’s distance, and they frequently are quite complex and indirect. But the Cepheid variable has been one of the most important tools in the astronomer’s tool kit for the past century. It was in 1908 that Henrietta Swan Leavitt, a $10.50 a week “calculator” at Harvard Observatory noticed a pattern while doing tedious work cataloging stars and saw it’s importance. Though she published a paper about it, she never really received the credit she deserved during her lifetime for this breakthrough discovery.

So when you look at this “home plate” in the sky, see if you can find the fourth magnitude star, Delta Cephei – it’s not hard to spot under good conditions. (See the chart above.) When you find it, pay homage to it for the key role it has played in unlocking the secrets of the universe – for once astronomers know the distance of an object they can make all sorts of deductions about its composition, mass, and movement.

Mars will NOT be close to Earth, nor “as large as the Moon” in August – this year or ever!

Forget what you might have read in an email, or on Facebook, or whatever – this is the truth about how we see Mars on those every-other-year occasions when we see it at its best!   This graphic was created  by Tom Harradine and highlights the  simple reality – Mars will never be as big or as bright as the Full Moon, or even a crescent Moon.

mars_hoax

Click the graphic to get a larger version.

Sorry I have to write this, but every August since 2003 I have gotten questions about a spectacular showing of Mars in our sky because an anonymous email   or social media post makes the rounds of the Internet causing people to get excited. THIS EMAIL IS NOT TRUE. I wrote about it other years pn this site. Here’s my updated version.

The most outrageous claim in this email goes something like this:

On the night of Aug. 27, the planet Mars will come closer to Earth than it has in the past 60,000 years, thereby offering spectacular views of the Red Planet.  Mars will appear to the naked eye as bright as and as large as the full moon. No one living today will ever see this again!

No one living today will ever see this. Period. OK. here’s the simple truth.  An hour before dawn  August 27, 2013 Mars will be rising in the east and be about two fists – 20 degrees – above the horizon. It will shine a little dimmer than  two nearby stars, Castor and Pollux. As always it will have a reddish tinge to it, but to the naked eye look just like any bright star. 

In a very good telescope, Mars will look like a very tiny planet, about one-fifth the size it is when it actually does make a relatively close approach to Earth.  Mars and the Earth are relatively  close to one another  – 35 to 60 million miles apart – every two years.  In AUgust 2013 Mars is about 214 million miles away, or bout 6 times as far away as it it is on closest approach and this appearing – telescopes – about one sixth the size it appears on closest approach.   (Mar’s at it’s absolute closest is still – in telescopes – only about half the size of Jupiter.

Is there any truth in the email? Yes, there are some grains of truth here, but even they are usually distorted. This all began because in August 2003 Mars really did make an unusually close approach to Earth – but by unusual we mean just a bit closer than it gets routinely every two years.  During a close approach such as the one in 2003 Mars can be 24 seconds of arc across – and sometimes it’s “close” approach means its half that size.

In short, this Mars email is just another Internet urban legend telling us things that would be fun if they were true, but really are far, far from the truth. For details on this, please visit the Snopes.com site – in fact, anytime you receive an email that sounds too good to be true – on almost any subject – check snopes.com before you forward the email to friends. It’s a great clearing house for getting out the truth on these legends.

August 2013 – last good look at Saturn, and a Moon-free Perseids shower

The Big Dipper's handle can guide you first to bright Arcturus, then to yellowish Saturn and blue Spica - both will be about the same brightness. Venus is much birghter, but best seen about half an hour after sunset when it is about 10 degrees above the western horizon. By an hour after sunset it ishalf that or less and even if you have an unobstructed horizon, may be lost in mist and twilight.

The Big Dipper’s handle can guide you first to bright Arcturus, then to yellowish Saturn and blue Spica – both will be about the same brightness. Venus is much brighter, but best seen about half an hour after sunset when it is about 10 degrees above the western horizon. By an hour after sunset it is half that or less and even if you have an unobstructed horizon, may be lost in mist and twilight. CLick for larger image. (Prepared from Starry Nights Pro screen shot.)

For a printer friendly version of the above chart, click here.

If you have a small telescope, August 2013 will give you your last good look at Saturn for the year and if you live on the right side of the globe – not where I live – the Perseids  meteor shower should be spectacular this year with no interference from a waning Moon.  Venus, meanwhile, continues to reign low in the western sky just after sunset.

The sky north of east early on the morning of August 12, prime time to watch for Perseids meteors. (Created froma Starry Nights Pro screenshot.

The sky north of east early on the morning of August 12, prime time to watch for Perseids meteors. (Created from a Starry Nights Pro screenshot.)

For a  printer friendly version of the above chart click here.

The Perseids should reach their peak on August 12 at about 19:00 UTC. To find what time that is for your region, go here.  For about half the world that’s good news, for the other half it’s bad because you really want to see this shower in the early morning hours and you will get the best show if the shower’s peak falls during those hours for your time zone.

Locally, on the East Coast of the United States, I’m going to watch the weather and if either the morning of August 11 or the morning of August 12 is forecast to be clear, I plan to start observing about 2 am. But I am not expecting a big Perseids show – just a nice summer night with a much better chance than usual of seeing a bright meteor.

Meanwhile, I’m bracing myself to hear a lot of promotional blather about the Perseids locally from TV weather folks and others who should know better, but the truth is in North America the timing of this year’s shower could hardly be worse.  The shower is best for a couple hours either side of its peak and its peak is forecast to come at 19 hours GMT on August 12 – for Eastern Daylight Time that translates to 3 pm – broad daylight.  What’s worse, even if the peak was in the early evening hours, the Perseid’s radiant point doesn’t get high in the sky until the early morning. That’s why the best time to see Perseid meteors – regardless of the peak time – is still  between midnight and  a couple hours before dawn.

So can we in America hope to see any Perseids at all? Yes, of course we can.  Just don’t expect a “shower.” In fact, I have to say that i always wince a little at the times and rates of meteors frequently given in news reports. Hey, just the word “shower” implies a lot more than most people usually see, especially from their typically light-polluted back yards.  When someone reports that the Perseids will peak at better than 100 meteors an hour, they usually fail to mention that three conditions have to be met for you to see that peak.

1. You need the Perseids radiant point to be nearly directly overhead – for EDT that occurs in a twilight sky, but is reasonably high from midnight on. The meteors may appear in any part of the sky, but they will appear to radiate from that point, so the higher it is, the better chance we have of seeing a meteor.

2. You need very dark skies – skies that will allow you to see magnitude 6.5 stars, if you are going to experience those real high rates. I have never experienced such dark skies, but they certainly exist. However,  with my reasonably dark skies I am very happy when I can detect a star of magnitude 5.

3. And, of course, you need the shower’s peak to coincide with the radiant point being very high in your sky.

One more caution – anything can happen. This is a forecast and usually reliable. But there could be a burst of meteors at a different time. You may get lucky.

And if all these  condition aren’t met for your location? Well, it’s reasonable to expect to see a Perseid meteor about every 10-15 minutes – of course you  may get two or three in a row hardly separated at all, then not see another one for  an hour. But be patient and you will get results – just not the meteor spectacular that some reports will imply. Last year they were coming in at a rate of 15-20 an hour four hours either side of the peak.

And yes, a Perseid can show up days either side of the peak.  How will you know it’s a Perseid? Draw a mental line extending the path of the meteor back towards the Perseid’s radiant point. If your line points back to that area of the sky – see map above – then you saw a Perseid. But there are always strays around – random meteors that have no connection to the shower – and at this time of year we have a couple weaker showers that may produce a few meteors going in other directions.

Meteors and meteor showers are fun if for no other reason than they are a chance to see something happening in the sky. Much of what we look at doesn’t change – or rather changes so slowly we don’t notice the change. Meteors, on the other hand, demand that you be looking in the right place at the right time. Only on the very rare, very bright meteors do we actually have time to alert others and have them turn their heads and see what we see.  And what we see is a space event happening closer to us than any other natural one. What’s more, meteors can have real scientific value.  They are viewed by some as our cheapest “space probe.” They are relatively pristine bits of matter left over from the early days of the solar system and so can tell a story to those who know how to read them.

Meteors – “falling stars ” – can be seen any time. You don’t have to wait for a “shower” like the Perseids; you just have to be lucky. But they are most frequent at certain times in the year when the Earth happens to be plowing through a meteoroid-rich area.  We call this occasion a meteor shower. (For your dictionary: A meteoroid is a small bit of space rock that becomes a meteor when it collides with our  atmosphere and heats to incandescence as it descends towards Earth. When it gets here – which is rarely as anything except fine, incinerated dust – it is a meteorite. )

The reason for a shower such as the Perseids is that we are passing through the debris trail of a comet. Think about it. The general model for a comet is a “dirty snowball,” and as that dirty snowball nears the Sun it melts, and as it melts it leaves a trail of dirt particles behind it – particles that remain in orbit until something like the Earth sweeps by and captures some of them with its gravity.

The comet itself can vanish entirely – but the result is a river of space dust – a river that is most intense nearest where the comet actually was.  That’s why there are some years – the 1990s in the case of the Perseids – when the meteor shower is more intense than others.  Now we are in a period when we are passing through the trail of the comet that creates the Perseids at a point where that trail is relatively sparse – so there will simply be fewer Perseids than there were  15-20 years ago.

That trail is not encountered all over the sky. It collides with our atmosphere near a particular point in our sky. That point is called the radiant – you might think of it as a hole through which the Perseids fall – and in the case of the Perseids, it appears to be in the constellation Perseus.  But we don’t see all the meteors at this point. We see a meteor only when its collision with our atmosphere is intense enough to make it burn up. The faint meteors we see are made by a speck of dirt about the diameter of a pencil lead. The brightest ones are caused by something about the diameter of the pencil’s eraser.  In either case it will, for all practical purposes, burn up entirely in our atmosphere – 50 to 75 miles up – and nothing significant will remain for anyone to find on Earth. But exactly where it burns up is another thing. That’s why we will see a sudden flare – a falling star – anywhere in the sky.

And that’s awesome! Consider this: If someone struck a match 50 miles away would you see it?  Yet a grain of sand, hurtling into the atmosphere, shows us such a brilliant light we can’t miss it! Why? Well, for one thing it is hitting our atmosphere at something in the order of 133,000 miles an hour – that makes a “speeding bullet” look like the proverbial turtle!

When you are watching for Perseids, you don’t have to look near the radiant point, though you will see more there.  A meteor can flare up suddenly anywhere and appear to draw a short (usually 5-10 degrees long) straight line across the dome of the sky. (Bright ones may actually leave a trail, which you can see for a few seconds with the naked eye or longer with binoculars.) If we trace a line backwards along the meteor’s trail we will see it comes from the area near the radiant point.

In the early evening, that Perseid radiant point is low in the northeast. That means nearly half the meteors that are radiating from it are happening below our eastern horizon. That’s why the shower is best in the early morning hours when the radiant is high in our sky. If the radiant is overhead, then we have nearly doubled our chances of seeing a meteor.

There are many meteor showers in the course of a year and some are better than others. The Perseids is one of the most reliable ones and happens to come at a convenient time for northern hemisphere observers when it is comfortable to be out at night, lying on the ground, and looking up.

In the final analysis there’s only so much time you can spend lying on your back gazing at the starry sky; though I very much enjoy that time, it’s made much more enjoyable by knowing that at any instant there’s a heightened likelihood that I will see a bright meteor.  That – and the summer Milky Way – make looking for Perseids in a dark and moonless sky always worth the effort for me.

Look North In August 2013 – All hail the Queen! (OK – the “W”)

Click image for larger view. (Derived from Starry Nights Pro screen shot.)

For printer friendly chart, download this.

The easily recognizable “W” of Cassiopeia (kass ee oh pee’ uh), the Queen, is well up in the northeast early on an August evening. Find it and you have a good starting point for tracing the Milky Way on south through Deneb.

When the “W” circles to a point high overhead, it will look like an “M,” of course, but that’s just part of the fun. Some people also see this asterism as forming the chair – or throne – for Cassiopeia. I like it because along with the Big Dipper, it nicely brackets the north celestial pole and provides another rough guide for finding Polaris. As the “W” rises, the Dipper plunges until it may be too close to the horizon for many to see. Both the stars of the Dipper and the stars of the “W” are 28 degrees from Polaris – roughly three fists.  When the Dipper gets on the horizon, the “W”  turns into an “M” directly above Polaris, so just measure three fists down from this “M” and you should be in the right region for finding the North Star.

Normally I do not find constellations or their associated myths too useful. Cassiopeia is an exception. Knowing the myth connected with this constellation will help you remember several important neighbors, and though we’ll meet these in the next two months, I’ll give you a “heads up” now and repeat the story when we meet the others. It goes like this:

Cepheus (King of Ethiopia)  and Cassiopeia (Queen of Ethiopia) have a beautiful daughter, Andromeda. Cassiopeia bragged so much about Andromeda’s beauty, that the sea nymphs got angry and convinced Poseidon to send a sea monster to ravage Ethiopia’s coast. To appease the monster, Cepheus and Cassiopeia  chained the poor child (Andromeda)  to a rock. But don’t worry. Perseus is nearby and comes to the rescue of the beautiful maiden, and they ride off into the sunset on Pegasus, Perseus’ flying horse! These five constellations – Cepheus, Cassiopeia, Andromeda, Perseus, and Pegasus – are all close to one another in the sky and all are visible in the fall, so we will meet them soon.

One of the bright stars of Cassiopeia is also a special aid to finding your way around the heavens, but in a more modern sense. It is part of an asterism known as the “Three Guides.”  These three bright stars are all very close to the Zero Hour Right Ascension circle in the equatorial coordinate system – the system that is roughly the celestial equivalent of latitude and longitude and is commonly used to give a permanent address to stars and other celestial objects. These three bright stars mark a great circle that goes through both celestial poles and the equinoxes and is known by the eminently forgettable name of  “equinoctial colure.”

Click image for larger view. (See note at end of post for source of this drawing.)

We’ll meet the other two stars in this asterism next month, but for now, simply take note of Beta Cassiopeia.It’s marked on our chart and is the bright star at that end of the “W” that is highest in the sky this month. Remember that this star is very near the “0” hour  circle, which you can visualize by drawing an imaginary line from Polaris through Beta Cassiopeia and eventually the south celestial pole. This line will cross the ecliptic at the equinoxes.  Of course, this helps only if you are familiar with the equatorial coordinate system! If that means nothing to you, then don’t clutter your mind with this right now.

The source for the drawing showing the equinoctial colure can be found here.

Look North in June 2013! We have a special ‘North Sky Triangle’ this month!

Click to enlarge. (Prepared from Starry Nights Pro screenshot.)

For a printer-friendly version of this chart, click here.

Yep, that’s Deneb, the guide star that is the subject of our “Look East” Post for June, gracing our “Look North” chart as well. In fact, besides Polaris we have three key guide stars in our northern sky every June, each of which is noted for, among other things, just how far north it is.

Of the three, Capella may be the hardest to find, for it is very near the horizon in the northwest around sunset. But if you have a clear horizon in that direction, you should still pick it up, especially at the start of the month. More prominent, however, are Deneb and Vega. These stars play the key role in one of the best known sky triangles – the Summer Triangle, but that triangle becomes easier to see next month and it will be in the eastern sky. For June it is fun to link Deneb and Vega with Polaris in what we’ll call the North Sky Triangle – and the linkage has some special meaning.

We just happen to be lucky to be living in an era when we have a bright star near the North Celestial Pole – Polaris. There’s no such bright star near the South Celestial Pole, and in other eras there is none near the North Pole either. But in the distant past – and in the distant future – Deneb actually will be the bright star nearest the pole – not as near as Polaris, but still a good general guide to it.

gyroscope precessionThat’s because the Earth wobbles as it spins on its axis in much the same way as a spinning top does. So the axis of the Earth doesn’t always point to the same place. It slowly makes a great circle around the northern sky, taking roughly 25,000 year to complete. Right now our axis is pointing to within a degree of Polaris. Not precise, but good enough so it is a ready indicator of true north. A mere 18,000 years ago Deneb was within 7 degrees of the pole and will be again around the year 9800!

This wobbling of the pole is really kind of mind boggling. I look at Polaris now, and it’s a bit short of 42 degrees above my northern horizon. But in a mere 14,000 years, Polaris will be almost straight over my head, and guess what will be the pole star then? Not Deneb, but its brighter – in our skies – companion, Vega. Of course none of us is likely to witness that event, but it’s still food for thought and gives us a sense of the majestic rhythms and time frame of the heavens.

And speaking of that time frame, as I write this it occurs to me that 14,000 years really feels like a very long time from now – while the 10-million-year age of Deneb doesn’t seem that long – and it isn’t, astronomically speaking. I’m not talking now about what your mind tells you about those numbers. I’m talking about your emotional reaction to them. I wonder if it’s similar to mine? This isn’t idle speculation. It’s central to our appreciating what we are seeing. But I think 14,000 sounds like a long time because it’s a number that fits into our day-to-day experience. Huge, but we can easily imagine 1,000 of something.  So imagining 14,000 of something comes easily to us. But few of us have any experience with one million. It would take about 11 days to count one million seconds and no one in his right mind is about to try it. So when we speak of the 10-million-year age of Deneb, or the five-billion-year age of the Sun, the numbers lose their emotional impact because they don’t relate easily to our experience. But 14,000 – well, we know, in an emotional sense, just how long that is!

Events – June 2013 – Venus chases Mercury, Saturn is due south, and the Summer Solstice arrives

This is a delightful time to find the ever elusive Mercury because through the first two weeks of June it puts on its best show of the year for those in the Northern Hemisphere and it’s relatively easy to find because brilliant Venus points the way.

Look west about 30 minutes after Sunset on June 1, 2013 and you can find three planets ina row. Venus should be obvious to the naked eye. Find it and put it int he bottom of your low-power binocular field and you should see Mercury near the top of the field of view. Move Venus to the top and you should pick up Jupiter near the bottom of the field of view. Jupiter is brighter than Mercury, but may not appear to be because it will belower and more impacted by looking through the atmosphere and the twilight which will be brightest near the horizon.

Look west about 30 minutes after sunset on June 1, 2013 and you can find three planets in a row. Venus should be obvious to the naked eye. Find it and put it in the bottom of your low-power binocular field and you should see Mercury near the top of the field of view. Move Venus to the top and you should pick up Jupiter near the bottom of the field of view. Jupiter is brighter than Mercury, but may not appear to be because it will be lower and more impacted by looking through the atmosphere and the twilight which will be brightest near the horizon. Jupiter will soon drop out of sight but for the next two weeks Venus and Mercury will make easy targets.

Special June 2013 dates for viewing Mercury, Venus, Jupiter and the Moon:

  • 1 – Mercury at its brightest and Jupiter still in view.
  • 9 – A very thin crescent Moon little more than one day old will be roughly 10 degrees – one fist – beneath the pair of planets – Venus and Mercury.
  • 10 – A much easier to see – and quite pleasing – will be a 2-day lunar crescent  beside the pair (just south)  and almost fitting in the same binocular field of view.
  • 11 – The 3-day crescent Moon will have climbed well past the pair, but still make a nice show.
  • 18 – Mercury has reached its peak and started back down and on this date drawing within a couple degrees of Venus – just south. The two will be quite high – about 11 degrees half an hour after sunset, but Mercury will have faded to magnitude 1.1 making it a challenge to pick out in the twilight.

Also this month:

But let’s start with Mercury because the speedy little planet is always a challenge to see. Why? First, because it is very fast. Earth chugs along at a “mere”  66,000 miles an hour in its annual journey around our star, the  Sun. (Doesn’t feel like we’re going that fast, does it?) But Mercury, being closer to the Sun, moves much faster –  it covers a much smaller orbit at a blazing 107,000 miles an hour. So that means when it is well placed for observing it doesn’t stay that way long and it’s easy for the weather and the rest of life to get in the way of seeing it.

What’s more, because it is so near the Sun we only see it as it pulls out to one side or the other  of the Sun and it does that for relatively brief intervals.

And even these quick glimpses vary considerably because it’s orbit is much more lopsided than most. At one point it can be as much as 43.6 million miles from the Sun – and at another it may be as little as 28.6 million miles. (In comparison, Earth can vary from roughly 94.5 million miles from the Sun to 91.4 million miles.)

How far it is from the Sun impacts how easy it is for us to see. If close to the Sun it either rises or sets in strong twilight – and since it seldom gets much brighter than magnitude -1, it can be  quite difficult to pick out in the twilight. And even when it gets pretty far away from the Sun, it’s so small that it never becomes as bright as Venus – in fact, in June it will be easily outshone by Jupiter and Venus.  This June, Mercury will reach a maximum brightness of -0.4 and that on the first day – it grows a bit dimmer each night thereafter, though this will be hard to judge because it also puts more distance between it and the Sun, so that means we see it against a darker background each night.

Cool, huh? I mean it moves a little bit more into darker sky each night – but at the same time it dims a little each night – doesn’t want to make things too easy for us!  😉

But sometimes several factors combine to give us an especially good look at Mercury and this June is such a case – with the added bonus that the much brighter Venus will be near it and thus point the way to finding Mercury.  The basic routine is simple. You want to start looking about 30 minutes after sunset and when you spot Venus, turn your binoculars on it – about any binoculars will do – and for much of the month Mercury will fit  in the same field of view looking like a significantly dimmer star.  As it gets darker you should be able to pick it out with your naked eye – though if you wait too long it will be too close to  the Western horizon – so timing really counts.

As the month progresses Mercury will be a bit higher each night 30 minutes after sunset and Venus will appear to chase it – but Jupiter will drop out of view in just a few days.

Your first challenge, of course,is to merely find the planets in your evening sky and that require an unobstructed western horizon, good clear skies, and appropriate timing – and binoculars sure help, but aren’t absolutely necessary.

But what are you really seeing?

Or maybe the better question is: Why do you see the planets this way?

For the answer we turn to an Orrery – and there’s one online that can be found here. I used it – and modified the view with labels and arrows – to produce the two images that follow.  Essentially this is a view from overhead showing the counter-clockwise motion of the planets around the Sun.  It is only very roughly proportional and your challenge is to look at the Orrery view, then mentally place yourself on the Earth and imagine what your view at sunset would look like. Remember – now you’re getting down in the plane of the solar system and looking outward and what you see is a two-dimensional view that  cancels out the huge distances between the planets.

This June 1 view holds true for Mercury and Venus formost of the month. Jupiter quickly gets too close to the Sun. What we see at sunset are theplanets to the left of the arrow pointing west. As the Earth rotates, the arrow sweeps to the left and theplanets vanish from our view - although Saturn, seen int he southeast at the start of the evening, appears to climn higher in our sky as we turn towards it.  But, of course, nothing stands still. The Planets also revolve around the Sun, so from night to night Venus and Mercury will close the distance between themselves and Earth. The result in 30 days is shown in the next image.

This June 1 view holds true for Mercury and Venus for much of the month. Jupiter quickly gets too close to the Sun. What we see at sunset are the planets to the left of the arrow pointing west from the Earth. As the Earth rotates, the arrow sweeps to the left and the planets set – although Saturn, seen in the southeast at the start of the evening, appears to climb higher in our sky as we turn towards it. But, of course, nothing stands still. The planets also revolve around the Sun, so from night to night Venus and Mercury will close the distance between themselves and Earth. The result in 30 days is shown in the next image.

By the end of the month our view tot he west at Sunset shows us only Venus. Jupiter has long vanished fromt he scene and infact, may just becoming visible inthe pre-dawn sky with Mars.  Venus remain in view,  and Staurn is the dominant planet for much of the Summer night in 2013.

By the end of the month our view to the west at Sunset shows us only Venus. Jupiter has long vanished from the scene and in fact, may just be coming visible in the pre-dawn sky with Mars. Venus remains in view, and Saturn is the dominant planet for much of the Summer night in 2013.

As I watch this wonderful dance of the planets from night to night – and the changes are especially obvious with the swiftly moving inner planets of Mercury and Venus –  I try to get a picture in my minds eye of what’s really happening. Do this enough and when you look at a planet or the Moon in the sky, you can easily sense exactly where they are  in their orbits around the Sun – and where you are in respect to them. I find this a very satisfying piece of mental gymnastics – that we little creatures on our tiny little spaceship Earth, whirling and hurtling about the Sun at incredible speeds, have been able to figure this out. Don’t get me wrong – I take no c edit for that – just one of those special moments when I feel proud to be one of the billions of homo sapiens  here and feel maybe we have earned that name – sapiens indeed! 😉

One last piece of dynamics at work. As mentioned, Mercury is at its brightest at the start of the month.  By the 14 it has dropped about one whole magnitude to 0.76, by the 21st it’s magnitude 1.5, and by the 28th magnitude 2.6. So it will be getting harder and harder to see as it drops more and more into the twilight zone and as it loses brightness. Why does it get dimmer? Look at the Orrery charts – it is moving to a position between us and the Sun and just like the Moon, as it gets between us and the Sun it goes through phases. By the 28th it is a thin crescent and so is reflecting very little light our direction. (These phases can be seen with a small telescope, but you will not detect them with binoculars.)

Mercury and Venus week by week

The following chart shows you the changing positions of Venus and Mercury during June 2013. All are for 30 minutes after sunset for mid-northern latitudes, and all are prepared from Starry Nights Pro screen shots with labels added. Venus never gets much higher than 10 degrees above the horizon – not only throughout June, but essentially for the rest of the year it will be a pretty constant western star – slipping southward somewhat and eventually, in late fall, rising some , before going off stage in January 2014. Mercury’s appearance int he West is strictly for June and as you can see, by the end of the month it’s heading quickly for the horizon and is quite faint.

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Saturn – a southern star!

Wait until about an hour after sunset during June 2013, then look due south.  Almost four fists above the horizon (from mid-northern latitudes)  and about one fist apart are two bright “stars.” The slightly brighter one on the left (east) is Saturn. The other – which should appear bluer – is the first magnitude guidepost star  Spica. Down to the  lower left is another star of about the same brightness, Antares. Compare these three – they are roughly the same brightness, but Antares is tinted red, Saturn yellow, and Spica blue.  They  make a good introduction to noting star colors – which to my eye are more tints, but certainly detectable.

saturn_1

Who can resist the Solstice magic?

We’re fascinated with extremes and since the Sun is responsible for maintaining all life on Earth it’s rather natural to want to track its movements in our sky and mark its extremes. There’s no better time to do so than when it reaches it’s most northern point – the Summer solstice. And if you want to be really accurate that happens at 1:04 am EDT on the morning of June 21. So the time to greet the astronomical start of Summer is to mark the sunrise on June 21.

That said, this is more for those who are fond of records. Truth is you will be hard pressed to tell the difference of where exactly where the sun rises or sets a couple days before that date, or a couple days afterwards. The changes are just too small for us.  So you have to take the word of those who track such things with sophisticated math and instruments – 1:04 am June 21 is the time and date when the Sun reaches its most northerly point.  And the shortest night of the year is June 20-21.

All of this, of course, is for northern hemisphere observers. Our friends Down Under are marking the start of winter.

And speaking of special events . . .

On the night of June 22-23 we have a full Moon – the largest full Moon of the year.

Why is this larger than other full Moons? Because it is closer to us at this particular full Moon. How much larger is it? Significantly – but not so much that you really can tell the difference. To do that you need to see a larger full moon next to a small full Moon – and you can do that by going to this web site which gives a wonderfully detailed explanation.

Meanwhile, just sit back and enjoy it – and don’t confuse this with the Moon illusion phenomena – that is simply our eyes and brain playing tricks on us to make the Moon (or the Sun) look much larger when it is near the horizon, than when it is high in the sky.

Look East! Drop off the slide to Spica and land on Saturn in May 2013!

If you followed “the arc” of the Big Dipper ‘s handle last month to find Arcturus, then you can do the same this month to find Spica – it’s like taking a long, cool slide from the Dipper – and if you hop off just before just before the end, this year you’ll land on Saturn!

Here’s how it looks – remember: look east  starting about an hour after Sunset.  But don’t wait too long – as the night goes on, everything will appear to rise and after a few hours this chart won’t be much help.

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Start with the Big Dipper, high overhead tot he east. Following the arc of its handle, slide down to the brightest star int he east, Arcturus. Soften your slide and keep going and you’ll come to another bright star, Spica. Hopwever, if you hop off the slide just before getting to Spica, you’ll land on Saturn – which will be brighter than Spica, but not quite as bright as Arcturus.

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While the Dipper is easy to recognize, its stars are second magnitude – bright, but not the brightest. The brightest star in this section of the sky is Arcturus at magnitude zero. (Remember, the lower the magnitude number, the brighter the object.) The next brightest star you’ll see is over to your left, low in the northeast – Vega. In fact, Vega is magnitude  zero as well and the difference between it an Arcturus is next to impossible to detect with the eye. And this year we can say the same about Saturn  – it is over near  Spica and will outshine it by just a tad and be nearly as bright as Vega and Arcturus. All of these – Arcturus, Vega, Saturn, and Spica will be brighter than any of the Dipper stars.

As these stars get higher in the sky, you will notice that Spica is a rich blue, while Saturn has a yellow tint. About an hour after sunset Vega will be the lowest at about 20-degrees above the horizon – two fists held at arms length. Saturn will be about 23 degrees above the horizon, Spica about 30 degrees, Arcturus 47 degrees and Alkaid, the star at the end of the big Dipper’s handle, about 64 degrees.

We dealt with Arcturus last month. Saturn will be in our sky most of the night and as always is a treat for the small telescope user. From a naked eye perspective,  it’s fun to remember that the name “planet” means “wanderer” in Greek, but all “wanderers” are not created equal. Mars, Venus, and Mercury move  so quickly in our night sky that you can easily mark their changes over a period of a few days -certainly a week.  Saturn is much more sluggish.

Look at the chart and you’ll see how little Saturn changes position over the course of an entire year – it moves roughly 12 degrees.  To see this in the sky , find Saturn. Hold your fist at arms length so Saturn is just below it. Just above your fist is where Saturn was last year. Put Staurn on top of your fist and just below your fist is where it will be next year. So how long will it take Saturn to get around the sky to roughly the same position? Well, 360/12 = about 30 years!  Now if you think a moment, the Moon takes about 30 days to get around our sky – and that means the Moon moves each day about 12  degrees –  the same apparent distance covered by Saturn each year.  All of which should tell you that it would be reasonable to assume Saturn is much farther away from us than the Moon – which, of course, it is.

None of this is rocket science or in any way  profound, but I find it interesting to contemplate as I look up and see Saturn. I measure that distance it will travel in the next year and in my mind’s eye I perch above the Solar System and I see a long thin pie slice reaching from me to Saturn’s distance orbit and this helps me keep things in perspective – gives me a better intuitive feel for the neighborhood in which we live.  OK – for the record Saturn is moving at about 22,000 miles an hour, Mars about 54,000 miles an hour in a much shorter orbit, and we’re whipping right along close to 67,000 miles an hour – and we don’t even feel the wind in our face! Oh – and Saturn’s actual orbital period is 29.458 years.

On to this month’s new guidepost stars!

Vega and Spica are each fascinating stars, but let’s start with Vega. Shining brightly not far above the northeastern horizon as the evening begins, Vega comes about as close to defining the word “star” as you can get. In “The Hundred Greatest Stars” James Kaler calls it “the ultimate standard star” because its magnitude is about as close to zero as you can get (.03) and its color is about as close to white as you can get. (If you’re one of those who assumed all stars are white, you’re forgiven. Individuals vary in their ability to see different colors in stars and for everyone the color differences are subtle – in fact I think of them as tints rather than colors. )

It’s hard not to be attracted to Vega when you read Leslie Peltier’s wonderful autobiography, “Starlight Nights.” Vega was central to his astronomical observing throughout his career because he began with it when he first started reading the book from which I got the idea for this web site, “The Friendly Stars” by Martha Evans Martin. Peltier wrote:

According to the descriptive text Vega, at that very hour in the month of May, would be rising in the northeastern sky. I took the open book outside, walked around to the east side of the house, glanced once more at the diagram by the light that came through the east window of the kitchen, looked up towards the northeast and there, just above the plum tree blooming by the well, was Vega. And there she had been all the springtimes of my life, circling around the pole with her five attendant stars, fairly begging for attention, and I had never seen her.

Now I knew a star! It had been incredibly simple, and all the stars to follow were equally easy.

Vega went on to be the first target of the 2-inch telescope he bought with the $18 he made by raising and picking strawberries. (This was around 1915.) And Vega became the first target for every new telescope he owned until his death in 1980. If you still don’t know a star, go out and introduce yourself to Vega early on a May evening. Even without a plum tree to look over, you can’t miss her! And once you’ve done that you’re well on your way to making the night sky your own.  (And yes, Vega is the star from which the message comes in Carl Sagan’s book/movie “Contact.”)

Vital stats for Vega, also known as Alpha Lyrae:

• Brilliance: Magnitude .03 ; a standard among stars; total radiation is that of 54 Suns.
• Distance: 25 light years
• Spectral Type: A0 Dwarf
• Position: 18h:36m:56s, +38°:47′:01″

Spica, a really bright star – honest!

Spica is truly a very bright star, but the numbers you may read for its brightness can have you pulling your hair. That’s because there are at least four common ways to express the brightness of Spica and other stars, and writers don’t always tell you which way they’re using. So let’s look at these four ways and see what they mean for Spica.

The first is the most obvious. How bright does it look to you and me from our vantage point on Earth using our eyes alone? We then assign it a brightness using the magnitude system with the lower the number, the brighter star. (For full discussion of this system, see “How bright is that star?”)

By this measure Spica is 16th on the list of brightest stars and is about as close as you can come to being exactly magnitude 1. (Officially 1.04) Though I should add here that the number really marks the midpoint of a magnitude designation – that is, any star that is in the range of magnitude .5 to magnitude 1.5 is called “magnitude 1” and so on for the other numbers on the scale.

But that scale talks about what we see. It doesn’t account for distance. Obviously if you have two 60-watt light bulbs and one is shining 6 feet away from you and the other 1,000 feet away, they are not going to look the same brightness. But if we put them both at the same distance – say 100 feet – they would look the same. So it is with stars. To compare them we pretend they all were at the same distance – in this case 10 parsecs, which is about 32.6 light years. Put our Sun at that distance and it would be magnitude 4.83. (That’s about as faint as the fainest stars we see in the Little Dipper.) We call that its absolute magnitude.

The absolute magnitude for Spica is -3.55 – not quite as bright as dazzling Venus.

Wow! That’s pretty bright compared to our Sun! Yes it is. Sun 4.83; Spica -3.55. Don’t miss the “minus” sign in front of Spica’s number! That means there’s more than eight magnitudes difference between the Sun and Spica. And that relates to the next figure you are likely to see quoted. Something that is called its luminosity. Luminosity compares the brightness of a star to the brightness of our Sun. Unfortunately, the term is often misused – or poorly defined. Thus in the Wikipedia article I just read on Spica it said that “Spica has a luminosity about 2,300 times that of the Sun.” Yes, but what does that mean? It means that if we were to put the two side by side, Spica would appear to our eyes to be 2,300 times as bright as our Sun.

That is bright! But there’s more, much more. Spica is also a very hot star – in fact one of the brightest hot stars that we see with our naked eyes. But we miss most of that brightness because most of it is being radiated in forms of energy that our eyes don’t detect. In the case of Spica, that is largely ultraviolet energy. The Wikipedia article actually listed Spica’s luminosity twice, and the second time it gave it as “13,400/1,700.”

Oh boy – now we have Spica not 2,300 times as bright as the Sun, but more than 13,000 times as bright. Now that IS bright – but is it right? Yes! So why the difference? Again, the first “luminosity” given – 2,300 times that of the Sun – is measuring only what we can see with our eyes. The second is measuring total amount of electromagnetic radiation a star radiates and is properly called the “bolometric luminosity.” And why two numbers for that last figure? 13,400/1,700? Because while Spica looks like one star to us, it is really two stars that are very close together and one is much brighter than the other. So what we see as one star is really putting out energy in the neighborhood of 15,100 times as much as our Sun.

This can get confusing, so I suggest you remember three things about Spica.

1. It defines first magnitude, having a brightness as it appears to us of 0.98 – closer than any other star to magnitude 1.

2. It is really far brighter (magnitude -3.55), but appears dim because it is far away – about 250 light years by the most recent measurements.

3. It is very hot – appearing blue to our eyes – and because it is very hot it is actually radiating a lot more energy in wavelengths we don’t see, so it is far, far brighter than our Sun.

Spica is the brightest star in the constellation Virgo, one of those constellations where you can not really connect the dots and form a picture of a virgin unless you have an over abundance of imagination. Besides, the remaining stars are relatively faint. That’s why we focus on the bright stars and sometimes those simple patterns known as “asterisms” and use them as our guides.

Vital stats for Spica, also known as Alpha Virgo:

• Brilliance: Magnitude .98 ; as close to magnitude 1 as any star gets; a close double whose combined radiation is the equal of 15,100 Suns.
• Distance: 250 light years
• Spectral Types: B1,B4 Dwarfs
• Position: 13h:25m:12s, -11°:09′:41″

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” stars rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months. If you have been reading these posts for several months, you may want to relate Spica to two earlier guidepost stars with which it forms a right triangle, Arcturus and Regulus. Here’s what that triangle looks like.

Click image for larger view. (Created, with modifications, from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Once you have identified the Right Triangle, note carefully the positions of Spica and Regulus. They pretty much mark the “ecliptic.” This is the path followed by the Sun. Also, within about 9 degrees north or south of it, you will find the planets and the Moon. That’s well illustrated in 2012 by the presence of both Saturn and Mars, very near the ecliptic, as noted on our chart.

Arcturus and Regulus are not the only guidepost stars and asterisms in the May sky. Again, if you have been reading these posts for several months, be sure to find the stars and asterisms you found in earlier months. Early on a May evening these will include, from east to west, the following: Arcturus, Spica, Saturn, Leo’s Rump (triangle), The Sickle,  Mars, Regulus, the Beehive, Procyon, Sirius, Pollux, Castor, and in the northwest near the horizon, Capella, and the Kite. Venus will be a bright evening “star” in the west, and if you look early in the month you may catch a glimpse of Sirius and Betelgeuse before they set.

Look North: May is the month Polaris ( the North Star) gets two bright flankers!

Click for larger image. (Developed from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Is the North Star – Polaris – our brightest star? No! And it certainly won’t look that way this month as it shares the northern sky with two very bright stars. But, read on. Polaris is not nearly as dim as it looks!

If you have been learning your guidepost stars as they rise in the East, you won’t be surprised by the two bright stars which flank – and outshine – our pole star in May. To the northwest is Capella, a star we first met when it rose in the northeast in November. In May the northeast is dominated by a star that is almost Capella’s twin in brightness, Vega, a guidepost star we introduce in May. (See “Look East!” for more about Vega.) As a bonus we also have the twin guidepost stars, Castor and Pollux, making their way into the northern sky high above Capella. But let’s focus on Capella and Vega.

New star watchers frequently assume the North Star, Polaris, will be the brightest star in the sky. It isn’t even close! It is bright, but its fame comes because it’s very, very close to where the axis of the Earth points to the north celestial pole. So it serves anyone trying to find true north as a very good guide. But when it comes to brightness, it’s in the same league as the stars in the Big Dipper. Quite bright, but it can’t hold a candle to Capella and Vega. When you look at a list of the brightest stars, Vega is number 5 and Capella number 6. Polaris, our North Star, is number 48!

As simple as one, two, three!

That doesn’t mean Polaris is a slouch, though. First, in the eastern sky in May you meet Spica. (That’s on our chart for the east.) One distinction of Spica is that it’s as close to being magnitude 1 as any star gets. A distinction of Polaris is, as Spica defines magnitude 1, Polaris defines magnitude 2. (To be precise it’s magnitude 2.02.) Vega and Capella are extremely close to magnitude 0. Vega is 0.03 and Capella 0.08. Good luck on telling the difference! This month, if you look north 90 minutes after sunset, you may think Capella is a bit brighter actually – but if it appears that way it will be because it’s a bit higher in the sky and thus is not dimmed by having to fight its way through as much of our atmosphere as Vega is doing at the moment. So don’t try to split hairs. And yes, you’re right – they are NOT really as “simple as one, two, three” – on the magnitude scale they are as simple as zero, one, two – but that doesn’t sound as good! (Vega and Capella are zero; Spica is magnitude one, and Polaris, magnitude two.)

So which is really the brightest star of these four? Are you ready for this? Polaris! That’s right – if you put all four stars at the same distance, Polaris would appear to be the brightest. Remember, that the lower the magnitude number, the brighter the star. In absolute magnitude – the brightness we give to a star if they are all shining fromt he same distance  -these four stars line up this way:

  • Polaris -3.4
  • Spica -3.2
  • Capella 0.1
  • Vega 0.3

And those absolute magnitudes also reflect their order in distance from us.

  • Polaris 433 light years
  • Spica 250 light years
  • Capella 45 light years
  • Vega 25 light years

So sometimes a star is very bright because it’s – well, very bright. But sometimes it only appears to be very bright because it is very close to us. If you put our closest star into this group, our Sun – remember, it is just 8 light minutes from us – in absolute magnitude it would be by far the dimmest of this group – absolute magnitude 4.9! So while Polaris doesn’t look all that bright, it really is a very bright star! Another way to think about this is if you move our Sun out to where Polaris is, it would be about magnitude 10! You would need binoculars or a telescope to see it!

Click image for larger view of this chart. Yellow circle represents typical field of view for low power binoculars, such as 10X50.

To get an idea of the difference between Polaris and our Sun, point your binoculars towards Polaris.  You should be able to make out the “Engagement Ring” asterism – granted, a crude ring with Polaris as the diamond.  This asterism points you towards the true north celestial pole  – just avery short distance to the other side of Polaris –  and also gives you a good idea of about how far Polaris is from that pole.  Small binoculars will not show you the companion of Polaris, but to get an idea of how bright our Sun would be at the same distance, look for the star labelled 9.8 – and if you can’t see it, see if you can see the star that’s a bit brighter labelled “9.”  Don’t expect to see these instantly. Sit calmly, relax, and keep looking for at least a minute.

And here’s one more cool secret about Polaris. It has a companion that just happens to be quite dim – magnitude 9. It’s fun to see the two of them if you have a small telescope, though it’s not all that easy because Polaris is so much brighter than its companion. But if you get a chance to see Polaris and its companion in a telescope, remind yourself that the very faint companion is still a bit brighter than our Sun would look at this distance. This companion, known as Polaris B, was discovered in 1780 by William Herschel, and for many years Polaris was thought to be a binary star – that is, a system of two stars orbiting about a common center of gravity. But Polaris was holding one more surprise – it’s really a triple star.

The top image shows Polaris and its faint companion that can be seen in any decent backyard telescope. The bottom image shows the second companion, Polaris Ab, which has only been seen by using the Hubble Space Telescope.

This has been known for some time, but no one could see the third star until they turned the Hubble Space telescope on it in 2006. That’s when NASA released the first image of this third companion. The accompanying press release explained it this way:

By stretching the capabilities of NASA’s Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings  in a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C.

“The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,” said Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics). The companion proved to be less than two-tenths of an arc second from Polaris — an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away. At the system’s distance of 430 light years, that translates into a separation of about 2 billion miles.

“The brightness difference between the two stars made it even more difficult to resolve them,” stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. “With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight.”

So as I said, Polaris is no slouch. It not only is a very bright star, but it also has two companions, and scientists are still studying it because it is unusual in other respects. We’ll talk about those other differences another month.

April (2013) events: The changing of the (planetary) Guard and a Comet – still

Ahhh . . . Saturn! We love you – afterall, you brought us Saturn-day! And before I get, you still have a chance to see Comet PanSTARRS in binoculars – mark April 4 on your calendar – though it is growing quite dim.

But let’s start with Saturn In  April 2013 we have Saturn taking over the dominant planet duties from Jupiter – though Jupiter will still be with us even next month, it will get lower and lower in the west, making observing it’s wonderful moons more and more difficult for the binocular user – though next month it will have an interesting naked-eye encounter with a couple other planets.

Saturn, which has been dominating morning skies for months, becomes seriously dominant in the evening sky this month. In fact on April 28th it is at “opposition,” one of those technical terms which is easy to remember because all it means is it will be the “opposite” the Sun in our sky. That is, as the Sun sets in the west, Saturn will rise in the east. But even on the first of April it put in an appearance low in the southeast within a few hours of sunset for a nice triangle of bright “stars” with  Arcturus (our guide star for this month), and the icy, blue Spica.

saturn_rising

Click image for a much larger version of this chart. (Prepared from Starry Nights Pro screen shot.)

Click this link for a version of this chart suitable for printing: saturn_rising

Saturn will be just a few magnitudes dimmer than brilliant, zero magnitude, Arcturus and brighter than Spica, though it will be interesting to do a color comparison between these last two. Wait until they’re both pretty high and Saturn should be a creamy yellow, Spica a very definite blue. (Near the horizon they will appear to twinkle madly and flash all sorts of colors due to  our atmosphere. )

Of course Saturn’s main appeal is in the telescope – even the smallest of scopes should reveal it’s beautiful ring system which this month is well placed for observing. (Some times the ring are almost edge-on from our point of view that that is not nearly so much fun. )

Comet Tails

I’m afraid that while you may pick up Comet PanSTARRS in binoculars this month, it will be much easier to follow in a small telescope. I suspect the highlight of the month will come April 4th when the comet, just a matter of a few light minutes from Earth, will appear to pass the Great Andromeda Galaxy (M31) – a whopping 2.5 million light years away.  (Actually, it will be quite close to the galaxy during the entire first week of April.) However, while you should be able to pick up this encounter in binoculars –  both galaxy and comet in the same field of view – it will be a more pleasing sight in a small, low-powered, telescope. Even then, both the comet and the galaxy will be competing with the thick atmosphere, low on the northwestern horizon, not to mention evening twilight.  (Pictures will make both appear brighter because of the sensitivity of long exposures. )

Click image for a much larger version of this chart. (Prepared from Starry Nights Pro screen shot.)

Don’t expect a tail any thing like this long – but you should detect an elongation of the comet in the general direction the tail depicted here is pointing.  M31 should be both bigger and brighter than the comet. Click image for a much larger version of this chart. (Prepared from Starry Nights Pro screen shot.)

Click this link for a version of this chart suitable for printing: comet

As a guide I suggest you wait until an hour after sunset, then scan about 10 degrees (one fist) above the northwestern horizon for the pair. The familiar “W” of Cassiopeia can also help. Use the lower half of this bright asterism as an arrowhead pointing you towards magnitude 2 Mirach.  That will help get you in the right vicinity – not an easy task when you are competing with the twilight. However, even 90 minutes after sunset – when it should be completely dark – this pair will still be more than 6  degrees above the horizon.  Whether you see it or not, I suggest you check Spaceweather.com for the latest photographs because you can be sure some enterprising amateur astronomers will capture the scene.

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