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.

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.

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

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 in October 2012 – and find a really bright star!

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

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

When I say “really bright,” I mean Capella – and I say it because so many folks assume, quite understandably, that the North Star will be the brightest in that section of the sky. It isn’t.

Capella is one of our brightest stars, and you may catch it this month just peeping up over the northeastern horizon about an hour and a half after sunset. And it is brilliant!  When you see Capella,  compare it with the North Star  and you will get a good idea of what the word magnitude means. There are a solid two magnitudes difference between the two – or if you want to get technical, Capella is magnitude 0 and Polaris is magnitude 2.

Yes, the magnitude system goes backwards – the lower the number, the brighter the star. Each magnitude difference represents a change of about 2.5 times in brightness. So a zero magnitude star is 2.5 times as bright as a first magnitude star – and 2.5 x 2.5, or 6.25 times as bright as Polaris, a second magnitude star. And while we’re on this subject, the faintest star in the Little Dipper – it’s over in one corner of the cup – is just about magnitude five. That means Capella is 100 times as bright as that star. (OK – if you actually do the math it doesn’t come out because I rounded off the difference – it’s really 2.512 for those wanting more precision.)

In fact, there another interesting way to look at that star – at magnitude 4.9 it is almost exactly what our Sun would look like if it were place just 32.5 light years away. At that distance our sun would be magnitude 4.8 – and that distance is the distance we use to compare stars. That is, to get an absolute magnitude for them so we can compare apples with apples,  we ask ourselves how bright a star would be if it were 32.5 light years from us.

And while we’re on the subject of magnitude, the Little Dipper does give us a great range. Polaris, as we said, is magnitude 2. The other bright star in the Little Dipper is magnitude 2 also – it’s at the end of the cup away from Polaris. Sharing that end is a magnitude 3 star, and these two are known as the “Guardians of the Pole” as they are prominent, close, and like other stars, circle around it every 24 hours. Most of the other stars in the Little Dipper are magnitude 4, with the one star in the corner of the cup being magnitude 5, as noted – well, actually 4.9. There’s a chart with these magnitudes in this post. Many people who live in light-polluted areas can see only three stars in the Little Dipper – Polaris and the two Guardians of the Pole.

How faint a star can you see? Depends upon your eye sight, the light pollution in your area, the transparency of the skies on any given night, how high the star is in the sky, and, of course, your vision and dark adaption. But as a general rule of thumb magnitude 6 has been accepted as the typical naked eye limit. However, if you live in a typically light polluted suburb you may see only to 4, or worse yet, magnitude 3. And observers on mountain tops with really clear skies reliably report seeing stars as faint as magnitude 8. All of this is with the naked eye. Add ordinary binoculars and you will certainly add three to five magnitudes to the faintest star you can see. In fact, you take a huge jump in the number of stars seen just by using binoculars with 50mm objective lenses. That’s why many amateur astronomers with fine telescopes also are likely to have a pair of 10X50 binoculars that they keep handy. I always did, but more recently I stumbled upon some very inexpensive Celestron 15X70 binoculars that I love for quick peaks, though they are two powerful and large to hold steady for serious observing.

Capella will be a “look east” guide star next month, but you can get to know it this month as you look north. And if you live in mid-northern latitudes, it will become a most familiar site. In fact, where I live – about latitude 42°N – it is in the night sky at some time every night of the year. This is because it is so far north and as it circles Polaris only dips below the horizon for a few hours at a time.

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.

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. )

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.

Look North in April 2013! See Mizar – the best thing since – well, since sliced bread!

In April the Big Dipper is climbing high overhead in the northeast and starting to pour its contents into the Little Dipper – not a very good idea, but fun to contemplate. Meanwhile, the only double star pair where both stars have proper names – Mizar and Alcor – is high in the northeast and ready to challenge your eyesight and boggle your mind.

Mizar is the middle of the three stars that form the handle of the Big Dipper – the same three that we use as an arc to trace a path to Arcturus. (That reference is explained in this month’s “Look East” post.) Wait until an hour or more after sunset, then focus on that center star. Is it one star – or two? For my old eyes, it is one. And since my eyes are not that bad, I question those who say this is an “easy” test of eyesight. But lots of people do indeed see two stars there when they look carefully. Maybe you’re one of them. If you’re not sure, or can see just one, take a look with your binoculars. Now you certainly should see two.

The brighter of the two is Mizar, the fainter one Alcor. More on that in a minute. First, here’s our northern sky for this month.

Arrows indicate directions in the sky where north is always the direction towards the north celestial pole, and west is always the direction the stars appear to move. Click image for larger view. (Developed from Starry Nights Pro screen shot.)

Download a printer-friendly version of this chart here.

And here’s what you should see when you look with binoculars at the Big Dipper’s handle.

Zooming in on the center star in the Big Dipper’s handle using binoculars, you should see it is really two stars – Mizar and Alcor. Click image for larger view. (Developed from Starry Nights Pro screen shot.)

The words “double star” simply mean that a star that appears as one to our naked eyes, is seen as two when optical aid is used. But they may simply be two stars that are closely aligned, yet in reality very far apart and have no real connection to one another. “Binary star” is the term used for two stars that are gravitationally linked to one another. So here’s the double rub with Mizar:

• When you are looking at Mizar and Alcor, you probably are looking at six stars, not two!
• Scientists still dispute whether Mizar and Alcor are a true double, even though they have been observing this system with telescopes since 1650!

My “sliced bread” reference figures into the Mizar/Alcor picture in a roundabout way. I have trouble remembering things. So when I wanted to remember the approximate distance to Mizar – 80 light years – I asked myself what interesting thing was going on 80 years ago that can help me remember the distance to these stars? And the answer – given a little research – was that about 80 years ago America was introduced to sliced bread all packaged neatly. Actually, sliced bread was first introduced in 1928, according to Wikipedia, but it was in 1930 that the first national marketing campaign began for “Wonder Bread.” Wonderful. So about 80 years ago the light you see left Mizar and Alcor to begin its journey to your eye.  Don’t let the different dates bother you because an approximation is close enough.

And Mizar alone is a lot more interesting than sliced bread.

Even a small telescope reveals that Mizar itself is a beautiful double! That’s what was revealed when a telescope was turned on it in 1650. But no telescope can reveal to the eye that these two stars are in fact, each a double! The stars in each pair are so close to one another that only an instrument known as an interferometer can reveal them. So what we see as Mizar is in fact four stars. (Double stars are a special love of mine, and I wrote about observing Mizar  in the double star blog I share with John Nanson here.)

But what about Alcor? The Hipparchos satellite, the best modern source for star distances, found Mizar to be 78.1 light years away and Alcor to be 81.1. Those are great ball park figures and good enough for the sliced bread reference. But they may be wrong. The astronomer James Kaler wrote a few years ago in his book “The Hundred Greatest Stars” that these distances may be wrong – in fact, some evidence suggested then that Mizar was actually farther away than Alcor. Kaler concluded in his book that they are “probably paired.”

But now comes more evidence as reported in the current (2010) Wikipedia reference to Mizar:

. . . in 2009, it was reported by astronomer Eric Mamajek and collaborators that Alcor actually is itself a binary, consisting of Alcor A and Alcor B, and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six.

So what our naked eye reveals as one or two stars, may indeed be a complex system of six stars! Which in my mind says that slicing up Mizar and Alcor this way may be – well, may be the best thing since sliced bread and just the sort of thing that makes observing the stars such a treat for the eye and mind!

Look East in April 2013 – take a simple slide to the World’s Fair Star!

The name”Arcturus” derives from Ancient Greek and means “Guardian of the Bear.” It is the brightest star in the constellation Boötes. Click image for a much larger version. (Prepared from Starry Nights Pro screen shot.)

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

Arcturus isn’t universally known as the “World’s Fair Star,”  but  it should be.  Its light bridged two World’s Fairs, making an astronomical link between the one in 1893  and a second in 1933 – both held in Chicago.  It’s intriguing that  the general public was excited enough about science – in the middle of the Great Depression – to make such a link attractive to the Fair’s promoters. Light from Arcturus  - believed at that time to be 40 light years away – was captured by the 40-inch refractor at Yerkes Observatory and its energy used to turn on the lights for the 1933 Fair.

This put the public spotlight not only on Arcturus, but it raised consciousness about the vast distance between us and that star, since the light being used had started its journey during the 1893 Fair and arrived just in time to start the next Fair. When you know light can circle the Earth more than seven times in a single second, you start to understand just what an incredible journey that was.

Of course Arcturus has many other distinctions. For one thing, it makes a perfect connection with the best known asterism in the sky, the Big Dipper.  To find it, all you have to remember is “follow the arc to Arcturus.”

Another way to remember where to find Arcturus is its name, derived from ancient Greek, which can be translated as “Bear Watcher.”  That’s because Arcturus looks like it’s keeping an eye on the “Great Bear,” Ursa Major, as both circle the northern pole.

You can also think of the magnitude system by which we rate the brightness of stars as starting near Arcturus. At magnitude -.04 it’s about as close to zero as you can get – the minus sign indicating it is a tad brighter than zero.  Its absolute magnitude is also pretty close to zero since absolute magnitude is defined as how bright a star would be if it were about 33 light years from us, and by modern measurement Arcturus is now believed to be about 37.6 light years from us.  That makes its absolute magnitude -.29.

Arcturus has the distinction of being the brightest star in the northern celestial hemisphere, but this is splitting hairs in several ways. It means it’s the brightest star north of the celestial equator. Sirius, now over in the southwest, is obviously  brighter. But Sirius is south of the celestial equator. Both stars are located close enough to the celestial equator so they can be seen from most places on Earth.

But Arcturus (-.04) also wins this “brightest star in the northern hemisphere” distinction by another hair. Remember that the lower the magnitude number, the brighter the star. Both Vega (.03) and Capella (.08) are north of the celestial equator, and the difference in brightness between Arcturus (-.04), Vega (.03), and Capella (.08) is roughly a tenth of a magnitude.  For practical purposes, they all look the same.  But in practical terms, making the comparison by naked eye is – well –  very impractical. Capella is currently fairly high in the northwest. But next month, when Vega is high enough in the east to see well,  Capella will be rather low in the northwest. At that time Arcturus should look brighter – but its actual brightness will be aided by the fact that it is high over head at that time, so you are seeing it while looking through a lot less air than you will be when looking at Vega or Capella. Besides, visually trying to compare stars that are this far apart in our sky is next to impossible since you have to look away from one to see the other. I simply think of all three as magnitude zero and leave the hair splitting to the scientists and their instruments.

Oops – we interrupt this program for a bulletin from 1907!

Yes, having just written how impractical the naked eye comparison is, I found this passage in “The Friendly Stars” by Martha Evans Martin, a book that was published more than a century ago:

Arcturus and Capella are so nearly equal in brightness that astronomers differ as to which outranks the other, even when they measure  their light with a supposedly accurate  instrument and a trained eye. To my own eye Arcturus outshines Capella, and on asking various inexperienced persons for off-hand opinions as to the relative brightness of the two stars, I have invariably had an answer in favor of Arcturus. The best authorities, however, make Capella a shade brighter.

Oh my! And now with 100 years of scientific progress, the verdict is that Martha Evans Martin and her causal observer friends were correct – and the “best authorities”  wrong. Arcturus is the brightest.  So much for my idea that you can’t tell the difference with the naked eye! Give it a try and see what you think. (You can find a chart for Capella and more details about that star  in this post.) Since we’re ranking stars, however, Arcturus is actually fourth on the list of brightest stars – two others that are ahead of it, Canopus and Rigel Kentaurus, are not seen by observers in mid-northern latitudes.Sirius, of course, is.

While Arcturus radiates a lot of energy, much of it is not in the form of visible light. It has what’s known as a “peculiar spectrum” and radiates much of its energy in the infrared portion of the spectrum.  This means that to our eyes it doesn’t look as bright as it really is.

Orange-ish Arcturus is 215 times as bright as our Sun and 25 times the Sun’s diameter. (Image courtesy of  Windows of the Universe.)

One more deception of sorts: This brightness is not because Arcturus is so big – well , yes it is, but not big in terms of the amount of stuff in it, but big in terms of surface area. If you’re measuring the amount of stuff that makes up Arcturus – its mass – it is about the same size as our Sun. But Arcturus has a much greater surface area, so think of it as a hugely bloated version of our Sun. (Keep that in mind when you look at the comparison sketch above.) It is a much older star and is now going through its red giant phase, something our Sun will probably do several billion years from now, burning the Earth to a cinder in the process.

Hmmm . . . to get an idea of how much impact that large surface area has, if you put our Sun out near Arcturus it would be barely visible to the naked eye – and then under truly dark –  not light polluted – skies.

Vital stats for Arcturus, also  known as Alpha Bootes:

•    Brilliance: Magnitude  -.04, brightest star in the celestial northern hemisphere; shines with the luminosity of 215 Suns.

•    Distance: 37 light years

•    Spectral Type: K1 Giant

•    Position: 14h:15m:38s, +19°:10′:5

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” stars and asterisms 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. So if you have been following – even if this is just your second month – look for the previous guidepost stars and asterisms that you have learned and that are still with us in April. Here’s the list from east to west.

• Arcturus
• Leo’s Rump  (triangle)
• The Sickle
• Regulus
• the Beehive
• Procyon
• Sirius
• Pollux
• Castor
• Betelegeuse
• Orion’s Belt
• Rigel
• Capella
• the Kite
• Aldebaran
• the Winter Hexagon
• the Pleiades 

Look East: March 2013 Roars in like a sickle and triangle! (Huh?)

Sure, I’d like to tell you March roars in like a lion – but honestly it’s easier to point to the sickle and the triangle and the “Little King” we call Regulus, this last being the new guidepost star for March. But there is a lion there, too. Let’s look at the sickle and triangle first, though, because they’re two very easy asterisms you’ll see in the east about an hour or so after sunset. The Big Dipper off to the northeast gives you an idea of size for comparison.

This is the eastern sky as it will appear about an hour after sunset from mid-northern latitudes. Click image for larger view. Prepared from Starry Nights Pro screen shot.

Click for printer friendly version of the above chart.

OK – so can you make this into a lion? I find it fairly easy if I consider the sickle his head and mane – and I consider the triangle his rear haunches. I leave the rest to my imagination and don’t really attempt to connect the dots.

The stars of Leo do indeed trace out some key parts of the Lion depicted in this plate from the 1603 Bayer atlas. (Click image for larger view.) Note that the bright star that marks the tail is named “Denobola,” which in Arabic really does mean “tail.” We encounter this also in the tail of Cygnus the Swan where the bright star is named “Deneb.” The Arabic star names are frequently descriptive. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Regulus, our new bright guidepost star for this month, means “little king,” or “prince,” in Latin. That fits right in with the lion‘s reputation as King of the Beasts. And what a lovely image to have a prince leading a lion onto the night-time stage this month!

Is Regulus memorable in its own right? Well yes. It’s a star that is spinning so fast that if we could see its disc, it would look like a beach ball that someone sat on. It takes Regulus about 16 hours to make one rotation – in comparison, our Sun, a smaller star, takes about a month to rotate. In fact, if Regulus were spinning just a bit faster, it would spin itself apart!

The rapid spinning gives Regulus an equatorial diameter that is about one-third bigger than its polar diameter. This also results in the polar regions of Regulus being much hotter than its equator.

Regulus is also a multiple star system, but as such rather dull visually. The second star in the system is much fainter, so it can barely be detected by a skilled observer using binoculars – and in a telescope it’s so far away from the primary star that the two stars don’t seem like a pair at all. Both these stars are spectroscopic doubles – meaning the companions are so close we can’t see them with a telescope.

Though a relatively young star – about 250 million years as compared to the five-billion-year age of our Sun – Regulus is apparently nearing the end of its normal life as a “main sequence” star. That is, it’s about to finish burning hydrogen, which means it will soon go into the last stages of its life. But according to Jim Kaler, Regulus is also a curious case. It appears to have a very close white dwarf companion which scientists believe once was much larger and brighter than Regulus. But the gases were drawn from the white dwarf into Regulus, making Regulus both huge and bright and causing it to spin the way it does.

In total, Regulus is another example of how what looks like a common star to us, is quite fascinating when seen in the light of modern science.

Vital stats for Regulus:

• Brilliance: Magnitude 1.35, 22nd among the brightest stars in our sky; shines with the luminosity of about 150 Suns.
• Distance: 77 light years
• Spectral Type: B7V
• Position: 10h:08m:22s, +11°:58′:02

The buzz about the Beehive (M44) and Leo’s whiskers – a binocular treat!

In ancient times the constellation Leo extended much farther east and west, and M44 was considered to be its whiskers.

from “The Next Step – Finding and Viewing Messier Object” by Ken Graun

Whiskers indeed! I like that. It’s a great way to remember where to look for M44, for if you can find the Sickle – the huge head and mane of Leo – then all you have to think is “now where would his whiskers be?” Scan 2-3 binocular fields in that direction – westward – and you should soon stumble upon M44, the Beehive. Here is a chart you can use to find it. Do wait  until about two hours after sunset when it is really dark and M44 is well up in the sky.

Click image for larger view. Prepared from Starry Nights Pro screen shot.

Click here to download a black-on-white (printer-friendly) version of this chart.

M44 also is known as “the Beehive,” and Praesepe, which is Latin for manger. And if you have dark skies, away from light pollution, you will see this as a small, wispy cloud, perhaps suggestive of Leo’s whiskers. It is, in fact, a beautiful star cluster as binoculars or a small telescope will reveal. Galileo first discovered its true nature, and in this hazy patch counted more than 40 stars. You should see about that many with your binoculars. This is one of the nearest star clusters to us, and although there is still debate over its exact distance, it is around 580 light years. That compares with about 400 light years for the Pleiades. The two clusters are pretty close to the same size, but M44 is considered much older. M45 – the Pleiades – is estimated to be 78 million years old, while M44 is thought to be about 660 million years old. As star ages go, they’re both quite young. But open clusters, such as these, do not last too long – the members stars tend to get drawn off by close encounters with other stars as the whole clusters moves about our Milky Way galaxy.

The Latin name, Praesepe, is worth examining because it explains the names of two relatively bright stars which flank it – Asellus Borealis and Asellus Australis. Borealis means “northern” and Australis means “southern.” Asellus means “ass” – as in donkey – and Praesepe means “crib” or “manger.” In other words, the Beehive apparently looked to some like a pile of hay in a manger, and these two flanking stars were donkeys eating that hay, one to the north and one to the south. In binoculars the scene should look something like this.

M44 and surroundings as it would appear in binoculars with a 5-degree field of view. Click image for larger view. (Chart derived from Starry Nights software screen shot.)

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

The two donkeys are about as bright as the stars in the handle of the Little Dipper, so under dark skies should be faintly visible to the naked eye with the northern one the dimmest. The third star, Eta Cancri, is dimmer still. Its name, however, indicates that it, the Beehive, and other stars shown here are all part of the rather obscure constellation known as Cancer, the crab.

Look North in March 2013 – Oops, there’s a big hole in the sky!

Well, not really – but unless you live in an area with very dark skies, free of light pollution, you’re going to have a hard time seeing the faint stars above Polaris, the North Star, at this time of year. Here’s what our March north sky star chart looks like.

Our northern sky is quite dark above Polaris, but the Big Dipper is prominent in the northeast and serves as our primary guide to finding the North Star. Click image for larger view. (Prepared from a screen shot of Starry Nights software.)

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

Notice the emptiness? The area labeled “Dark Hole?” Mind you, this is not a black hole – just an area of our sky that looks quite empty – unless your viewing location is free of light pollution and your eyes are thoroughly dark adapted. If you can see all seven stars of the Little Dipper, then you should see several stars in this area. But even then I doubt if you will be able to trace out the constellation which goes there. It’s known as Camelopardalis. My copy of Urania’s Mirror, published in 1832, says Camelopardalis consists:

. . . of 58 stars, but none larger than the fourth magnitude. . . .The Camelopard is an Abysinian animal, taller than the elephant, but not so thick. He is so named because he has a head and neck like a camel, and is spotted like a leopard; but his spots are white upon a reddish brown ground. The Italians call him giraffa. To Hevelius, who formed the constellation, he owes his celestial honors.

Ah, giraffe! Thank you, Italians. Here’s how he is pictured in full color on one of the constellation cards that came with Urania’s Mirror (The scan is © Ian Ridpath.)

Camelopardalis as depicted on the card from Urania’s Mirror, 1832. Notice the Pointer Stars of the Big Dipper are near the upper left and Polaris is just to the right of the giraffe’s head, so at this time of year the giraffe would appear upside down in our northern skies.

If you put him in the sky at this time of year his head would be down near Polaris. . . . Hmmm… the illustrator seems to have forgotten the spots mentioned in the text, and the animal’s neck got a bit longer than a camel’s. Ah well – while the 1830s had some advantages in terms of simplicity, I don’t think I would like to be trying to learn the night sky with Urania’s Mirror as my only guide.

Oh – but speaking of long necks, one of the things that has always fascinated me is some of the early attempts at astronomical telescopes and particularly the one in the following woodcut. This was an instrument built by Johann Hevelius in the mid-17th century at his observatory in Poland. The tube was about 150 feet long – befitting, in a strange way, for the man who put a giraffe in the northern sky!

Click image for larger view.

There was a logic to this giraffe-like telescope.

At the time a telescope’s lens could not bring the different colors of light to a single focus, so bright objects were always fringed with color and nothing was in really sharp focus. This negative effect, however, could be lessened by making the telescope’s focal length longer – so to get a really good telescope you had to go to these ridiculous extremes – which, of course, made it a nearly impossible telescope to use in any practical way.

Impressive to look at – difficult to aim and look through.

Fortunately the achromatic lens – combining two different types of glass – was invented and this reduced the problem considerably even in a relatively short telescope. We still use such achromatic lenses today ins mall refractor, though if you want to get a really sharp, color-free image you pay considerably more money for an apochromatic lens. Or, you listen to Newton who figured way back int he 1600s that the way around this was to design a telescope that used a mirror to collect the light rather than a lens. Trouble was, it took a long time to learn how to make mirrors that didn’t tarnish quickly when exposed to the night air. Nothings easy!

Now – about or “hole” in the northern sky. Get to a place where light pollution is at a minimum and it will fill with stars – relatively faint, but they are there. Just scan around with binoculars and you’ll find some even through the typical light pollution most people today are forced to endure.