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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.

Prime Time observing for October 2009

Seeing a bow, a demon,  and a few hundred billion stars  – meanwhile, Jupiter slams it into forward!

Please note: All charts with this post are for observers in mid-nothern latitudes centered on 40° N. If you are 10 or more degrees south or north of that – or if you’re not sure of your latitude – please go here to make your own custom star charts.

On tap this month is a new asterism,  the bow; a variable, Algol, the “demon star;” a neighboring galaxy you can see with the naked eye or binoculars; and yes, Jupiter, which appears to abruptly change directions as it moves against the background stars.

To begin our monthly exploration of the night sky, you can take a slide down Andromeda’s Couch to Mirfak and the Bow of Perseus in the northeast, assuming you learned these last month. If these are new to you, simply start by looking for the rising low in the northeast.

As usual, go out 45 minutes after sunset and watch the stars emerge. It may take another 15 minutes for  to see the bow  clearly, but what you are looking for is three stars in a vertical arc, with the middle one – Mirfak –  the brightest. How big an arc are we talking about? Just make a fist and hold it vertically at arm’s length, and your fist should just cover these three stars. How high? The bottom one should be about a first above the horizon.  Here’s a chart modified from Starry Nights Pro software..

Click chart for much larger view. If you observed last month you know the Great Square and Andromeda's Couch and can slide down the "Couch" to Mirfak, the brightest star in the bow of Hercules. If this is your first month of learning the sky, simply look to the northeast and find the bow.

Click chart for much larger view. If you observed last month you know the Great Square and Andromeda's Couch and can slide down the "Couch" to Mirfak, the brightest star in the bow of Perseus. If this is your first month of learning the sky, simply look to the northeast and find the bow.

Now if you want to be a stickler about mythology, Perseus doesn’t carry a bow – he wields a sword instead, which he is holding in his right hand high over his head, while in the left hand he holds the severed head of Medusa. Here’s how the 1822 “Urania’s Mirror” depicted it.

perseus

Perseus - click for alarger version.

Oh boy – and if you can see all that in these stars, then you have a very vivid imagination. I never would have learned the night sky if I had to try to trace out these complex constellations as imagined by ancient cultures and depicted in star guides up until fairly recently.  And for the purposes of helping you find your way around the night sky I think remembering the Bow of Perseus is easier. Mirfak, is just a tad dim to serve as one of our guidepost stars, but it does come in handy when identifying the “Demon Star,” whose proper name is Algol.

Getting sharp about brightness

As you start to learn the stars, it may surprise you how precise you can be about their brightness.  At first you may have difficulty just telling a first magnitude star from a second, but if you get to know Algol, the “Demon Star,” I bet you’ll find that you can quickly become quite sophisticated in assessing brightness and shaving your estimates down to a tenth of a magnitude.

Imagine a star that regularly varies in brightness every few days – that’s what Algol does. Exactly every 2 days, 20 hours and 49 minutes it begins a 10 hour period where its brightness dims more than a full magnitude. If you look during the right two hours, you’ll catch it at or near its dimmest – and most of the rest of the time you’ll catch it at peak brightness. And it’s quite easy to judge. But first let’s find it. Here’s the chart we’ll use.

algol_no_mags_web

Notice how Algol makes a very nice triangle with two companions, and all three stars are close to the same brightness – Almach, the bottom star in Andromeda’s Couch; Mirfak, the central star in the Bow of Perseus; and Algol. That brings us to our first challenge: Go out any clear night and study these three stars and decide which is the brightest. Two are equal in brightness, but one is a tad brighter than the other two. Which is it? Algol? Mirfak? Almach? (The answer is at the end of this text so you can ignore that answer until you actually have an opportunity to test yourself.)

However . . .

Because Algol is a variable, sometimes when you look at it, Algol will actually be significantly dimmer than either Mirfak or Almach. In fact, there’s a reasonable chance it will be dimmer than either of Mirfak’s two fainter companions that make up the Bow of Perseus. If when you test yourself, this is the case, congratulations! Make note of the date and time.

Algol is a special kind of variable star known as an eclipsing binary. That is, what looks like one star to us is really two stars, and when we see Algol’s light start to dim it means its companion is passing between Algol and us causing an eclipse. Since the stars are locked in orbit around one another this happens with clockwork regularity.

algol_edu

The above diagram came from this astronomy class web site which includes amore detailed scientific explanation.

Since either star of the pair can cause an eclipse, there is a much fainter, secondary eclipse of Algol – really too faint to be noticed by most observers. Why is one eclipse fainter – because one star is blue, Class B – and much hotter/brighter than the other star which is “K” class. (Remember – OBAFGKM.)  It is when the cooler star is in front that we see the dramatic change in light.

It’s fun to catch Algol in mid eclipse, but I suggest you not read about when to do this right now. Instead, do the little challenge first. Then when you’re ready, go to the final item in this, which explains how and when to catch Algol in eclipse and in the process, tells you the brightness of its companions.

OK – second project – Jupiter changes direction!

I described this in an earlier post an am quotingit in its entirety here.

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset as seen from latitude 42 degrees north and longitude 71 degrees west. Chart from StrayyN oghts Pro software. Click for larger image.  .
On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset. (Jupiter is made large to indicate its relative brightness – ut it will look like a very bright star – not a small moon!) This is how the sky appears from latitude 42 degrees north and longitude 71 degrees west. Chart from Starry Nights Pro software. Click for larger image.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

With just a few quick checks with binoculars we should be able to track the movement of Jupiter in relation to a bright, nearby star. You should start this project on or before October 1, 2009 if at all possible and plan to observe two or more nights between your start time and October 13. Then observe again in about a week and again near the end of the month.Your first couple of checks should show Jupiter in “retrograde” moving westward among the background stars. Your next two checks should show Juputer has resumed it’s normal eastward movement.

Use the following chart as both your guide and your log. That is, click on it to get a version you can print, take out under the stars, and record your observations on with a pencil.

Click for larger version, suitable for printing.
Click for larger version, suitable for printing.

So why does Jupiter appear to first go one way, then the other? Afterall, it isn’t really doing that, is it? Like the other planets – and us – it’s simply continuing a steady, eastward journey around the Sun. But so are we – and we are moving much faster because we’re much closer to the Sun. So what you are seeing is partly the movement of Jupiter – but also the apparent change in its position caused by our rapidly changing position.

I made the following animation from Solar System Live charts. It shows how Jupiter’s position changes slowly in relation to Earth and the other planets, particularly Neptune. The animation starts with September 1, 2009  and moves a month at a time for six months. The arrow shows our changing view of Jupiter with relation to Neptune, a much more distant – and even more slowly moving, planet. Notice that in late December Jupiter makes another close approach to Neptune – the third this year – which will make especially easy at that time to find this distant and faint planet. Right now you can use the chart above to track it down – it would be just visible in binoculars on a moonless night.

picasion.com_8320c15f05e4065bb6a5159017c4c205

So let’s review the movements we’re dealing with here.

1. The daily rotation of the Earth causes Jupiter to appear to rise inthe east and move westward as the night progresses.

2. The revolution of the Eartha round the sun at a much higher speed than Jupiter makes it so that for some time the huge planet appears to be moving westward in relation to background stars and the more distant planet Jupiter. That apparent westward motion comes to a stop October 13, 2009.

3. Jupiter’s own motion is more apparent after October 13, as it appears to move eastward against the background stars. This general motion will carry it about 30 degrees eastward – very close to where Uranus can be found now – in about a year. It takes Jupiter almost 12 of our years to make a complete circuit of the sky.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

See a few hundred billion stars at one glance!

Yes, you can do it if you have good dark skies, you have allowed your eyes to dark adapt, and you are looking at the right place.  Once again, Andromeda’s Couch is our guide, and what we are looking for this time is the Great Andromeda Galaxy aka M31.

This is our “neighbor” in space if you can wrap your mind around the idea that something “just” 2.5 million light years away is a “neighbor.” ( As you try to do that remind yourself that a single light year is about 6 trillion miles – of course, good luck if you can imagine a trillion of anything!)

But seriously, you can see this with  your naked eye – and even in normal, light-polluted skies, you can see it with binoculars. In fact, this is one object where the binocular view can be almost as rewarding as the view through a telescope. Here’s a wide field chart for mid-month and about 90 minutes after sunset. At that point the galaxy should be roughly half way up your eastern sky.

m31_finder

Click image for larger version.

Starting with the preceding chart – and moving to the chart below:

  1. Locate the Great Square
  2. Locate Andromeda’s Couch off the northeast corner of the Square.
  3. Go down to the middle star in the couch, then count up two stars and bingo!
  4. You can also find the general vicinity by using the western end of the “W” of Cassiopeia as if it were a huge arrow head pointing right at the Andromeda Galaxy.
Click image for larger chart.

Click image for larger chart.

Well, “bingo” if you have been doing this with binoculars. With the naked eye it’s more an “oh yeah – I see it – I think!” But what do you expect? Think about it. The light from the near side of this object started its journey about 150,000 years before the light from the more distant side did! And think of where the human race was 2.5 million years ago when these photons began their journey – or for that matter, where all these stars really are today! Nothing is really standing still -everything is in motion.

You might also want to think about the folks who are on a planet orbiting one of those stars in the Andromeda Galaxy and looking off in our direction. What will they see? A very faint patch – probably fainter than what we see when we look at the Andromeda Galaxy, but in binoculars and telescopes roughly similar in size and shape.  Both Andromeda and the Milky Way Galaxy we inhabit are huge conglomerations of stars. We’re about 100,000 light years in diameter – Andromeda is about 150,000 light years in diameter. The Milky Way contains perhaps 100 billion stars – the Andromeda Galaxy maybe 300 billion.  (Don’t quibble over the numbers – even the best estimates are just estimates. )

And yes, in a few billion years we will probably “collide” with the Andromeda Galaxy, for we are hurtling towards one another. Such galaxy collisions are not that unusual  and probably aren’t as violent as the word “collision” makes them sound – but they do, in slow motion, bring about radical changes.

But all that is for the professional astronomers to concern themselves with – for us, there’s the simple beauty and awe of knowing that with our naked eye – or modest binoculars – we can let the ancient photons from hundreds of billions of stars ping our brains after a journey of millions of years.

And now the truth about Algol and companions

Have you done the Algol test yet? Looked at Algol, Mirfak, and Almach and tried to decide which is brightest? If so, you can check your answer by continuing to read. If not, I suggest you first do that exercise, then come back to this.

Chances are that when you look at Algol, it will be at its brightest – but how can you tell? Well, as we mentioned, you can compare it to Mirfak – but there’s an even closer match with another nearby bright star – Almach.  That’s the third star in Andromeda’s Couch  – the one neareast Algol.

Mirfak is the brightest of the three at magnitude 1.8.

Almach is magnitude 2.1 – the exact brightness of Algol when Algol is at its brightest – which is most of the time. OK – for the hair splitters, Almach is a tad dimmer, but the difference is far too little to be able to tell with your eye.

Here’s a chart showing the magnitude of the stars near Algol that you can use to compare it to and see if it is going through an eclipse.  People who look at variable stars use charts like this, but with one important exception – the numbers are given like they were whole numbers so you will not confuse a decimal point with another star. Thus, a star like Mirfak, of magnitude 1.8, would have the number “18” next to it. I broke a convention here because there are just a few bright stars on the chart, so I didn’t worry about the possible confusion of a decimal point being another star.

algol_mag_color

So If Algol and Almach are the same, no eclipse is going on at the moment.  If Algol appears dimmer than Almach, then an eclipse is in progress. If it’s as dim or dimmer than either of the companions of Mirfak in the Bow, then you can be pretty sure you’ve caught Algol at or near  its darkest. In two hours – or less – it will start to brighten and will return to full brightness fairly quickly.

Catching Almach at its dimmest is fun, but not as easy as it may seem. Why? Because although  an eclipse happens every few days, it may happen during the daylight hours, or in the early morning, or some other time when it’s inconvenient. And, of course, you need clear skies.  So when I want to observe an Algol eclipse, I go to a handy predicting tool on the Web that you can find here.

I then note the dates and times and pick out only those dates when the times are convenient to me – that is, happening during my early evening observing sessions. Then, given the  iffiness of the weather, I usually find that there are only one or two times a month when I’ll get a good look at an eclipse of Algol.

If I do this for October I find that out of 11 Algol minima, just three hit at the right time for me. Those dates and times are:

  • 10/01/2009  9:09 pm EDT
  • 10/21/2009  10:50 pm EDT
  • 10/24/2009  07:39 pm EDT

Of course the dates and time may be different for you, depending on where you live, and none of us can escape the whims of the weather! So here’s hoping for clear skies for you so you can find a winking demon, follow the actions of Jupiter, and capture in your own eye the photos from a few hundred billion stars in the Andromeda Galaxy!

Jupiter’s back-and-forth wanderings

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset as seen from latitude 42 degrees north and longitude 71 degrees west. Chart from StrayyN oghts Pro software. Click for larger image.  .

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset. (Jupiter is made large to indicate its relative brightness - ut it will look like a very bright star - not a small moon!) This is how the sky appears from latitude 42 degrees north and longitude 71 degrees west. Chart from Starry Nights Pro software. Click for larger image.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

With just a few quick checks with binoculars we should be able to track the movement of Jupiter in relation to a bright, nearby star. You should start this project on or before October 1, 2009 if at all possible and plan to observe two or more nights between your start time and October 13. Then observe again in about a week and again near the end of the month.Your first couple of checks should show Jupiter in “retrograde” moving westward among the background stars. Your next two checks should show Juputer has resumed it’s normal eastward movement.

Use the following chart as both your guide and your log. That is, click on it to get a version you can print, take out under the stars, and record your observations on with a pencil.

Click for larger version, suitable for printing.

Click for larger version, suitable for printing.

So why does Jupiter appear to first go one way, then the other? Afterall, it isn’t really doing that, is it? Like the other planets – and us – it’s simply continuing a steady, eastward journey around the Sun. But so are we – and we are moving much faster because we’re much closer to the Sun. So what you are seeing is partly the movement of Jupiter – but also the apparent change in its position caused by our rapidly changing position.

I made the following animation from Solar System Live charts. It shows how Jupiter’s position changes slowly in relation to Earth and the other planets, particularly Neptune. The animation starts with September 1, 2009  and moves a month at a time for six months. The arrow shows our changing view of Jupiter with relation to Neptune, a much more distant – and even more slowly moving, planet. Notice that in late December Jupiter makes another close approach to Neptune – the third this year – which will make especially easy at that time to find this distant and faint planet. Right now you can use the chart above to track it down – it would be just visible in binoculars on a moonless night.

picasion.com_8320c15f05e4065bb6a5159017c4c205

So let’s review the movements we’re dealing with here.

1. The daily rotation of the Earth causes Jupiter to appear to rise inthe east and move westward as the night progresses.

2. The revolution of the Eartha round the sun at a much higher speed than Jupiter makes it so that for some time the huge planet appears to be moving westward in relation to background stars and the more distant planet Jupiter. That apparent westward motion comes to a stop October 13, 2009.

3. Jupiter’s own motion is more apparent after October 13, as it appears to move eastward against the background stars. This general motion will carry it about 30 degrees eastward – very close to where Uranus can be found now – in about a year. It takes Jupiter almost 12 of our years to make a complete circuit of the sky.

The incredible variety of stars – and how much we can learn from simply noticing their color!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Look at a star. Can you tell it’s color? Don’t be surprised if you can’t. At first all stars just seem white to many people.  Yet amateur astronomers joyfully describe them in exciting hues of red, orange, yellow, and blue.  The truth is, it would probably be more accurate to talk of these “colors” as “taints.”  A red star doesn’t look like a red Christmas tree light. But once you’ve taken a good close look – once you’ve made comparisons between key bright stars, such as Spica and Antares in the summer sky, or Rigel and Betelgeuse in the winter sky – well, you definitely should see that Antares and Betelgeuse are tainted red and Spica and Rigel are tainted an icy blue.

This drawing - details and copyright can be found here - does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system.

This drawing does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system. For more details and copyright information click image.

And from that single piece of information – the color –  you can make reasonable guesses about a star’s size, temperature, intrinsic brightness, life expectancy – even how rare it is and the way it is going to die. How? By knowing the secret of a wonderful little classification system that uses the sequence of letters OBAFGKM – a system built on more than 150 years of breaking star light into its constituent colors, studying the result, and comparing those results with what we can learn in the laboratory about light and its relationships to various elements.  Oh – and if you don’t mind risking a little political incorrectness, you can remember that sequence of letters by this wonderful little  mnemonic device – Oh Be A Fine Girl (Guy) Kiss Me.

And what all of this tells us about stargazing in general is that what we see when we look at the stars on a typical night are not the ordinary stars, but the extraordinary ones.  That is,  the stars we see with our naked eyes are a sampling of the unusually bright, unusually close – and in some cases – unusually large . With our naked eye we cannot see a single example of the most common type of star – not one.

From a BB shot to a mountain

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

And while we certainly notice the differences in brightness of the stars, and we may have now tuned ourselves to detect the subtle tints of color that separate red stars from yellow and orange from blue, there’s little we can discern with our naked eye that prepares us for the incredible variety of stars. Take one measure alone – spatial dimensions. All stars appear to us as a point source of light – they show no disk, except as an artifact of our telescopes. In short – they all look the same size – very, very small.

Yet this sameness is a far cry from reality. In reality stars have an incredible range of sizes. Let’s scale things down to the graspable. We’ll reduce our home planet to the size of a small bead, 2mm in diameter – less than one tenth of an inch. We’ll let that small bead represent the size of a white dwarf star. On that same scale, our star – the Sun – is about 9 inches in diameter. And on that scale the largest stars we know would be almost 2,000 feet in diameter.  Think of it! Stars range from something smaller than a BB shot to something larger than the tallest man-made structure. Then consider this – that BB shot represents something the size of the Earth, a typical size for a white dwarf star – but there’s actually a type of star much smaller than a white dwarf.  It’s called a “neutron star,” and while it may be as massive as our Sun, it’s about as big as a large city! I couldn’t figure out how to include that in our little model without using a powerful microscope.

(Magic interlude: to really get a quick handle on the size of planets and stars, go here (link opens in a new window) – then return 😉

This incredible size range alone should tell you that stars are quite different, one from the other. We first began to notice how different about 150 years ago when scientists combined a prism with a telescope and broke down the light from the stars into its various wavelengths, displaying a spectrum. In such spectra we see not only colors but thousands of dark lines at locations where specific wavelengths of light are absent, and these dark lines hold the key. From them, scientists can deduce chemical composition, temperature, and much more.

The Harvard connection

In the late 19th Century astronomers at Harvard University started to classify the spectrum of stars. There were obvious vast differences, and they applied an alphabetical system to rank stars by the strength of the lines that represented hydrogen.  At first it all made sense and the letters were in alphabetical order, but as their knowledge grew, the letters got scrambled until we finally ended up with a system that runs in this order:

O B A F G K M

So Astronomy students, in an age when we were less tuned in to sexism, learned the simple mnemonic Oh Be A Fine Girl Kiss Me.  More recently they have messed things up a bit with additional letters, L and T, and within each class there are nine different sub-classes, which are represented by numbers.  But for now let’s stick with the basic letters, as they cover the vast majority of stars.  Learn it anyway you like, but learn it. The order of the spectral letters is one of the few things I find worth memorizing, for it turns out to be an almost ideal classification system covering in a single order several major characteristics of the stars.

OBAFGKM

The mains pectral classes of stars with the class designation on the left, a represenative spectrum for that class of star, and then the temperature for that spectral class on the right in degrees Kelvin.Clcik on image for information about its source, copyright, etc.

First, these letters represent a temperature sequence. The hottest stars are at the top. and classified as “O” stars. “M” stars are the coolest common stars.

And OBAFGKM represents a mass sequence, with the stars at the top being the most massive.  (Don’t get mass and spatial dimensions confused, however.  A star can have the same mass, yet be as tiny as the Earth, or far, far larger than our Sun. )

But these same letters also represent a color sequence going from blue-white to red, with yellow stars in the middle. Our Sun, for example, is a “G” star – basic yellow.

The sequence also indicates something about life span. The hotter a star is, the shorter its life. “O” stars will live a few million years, furiously exhausting their nuclear fuel. “G” stars such as our Sun, are destined to live billions of years – about 10 billion for the Sun. And “M” stars may go on for trillions – much longer than the universe has existed to date.

And the sequence tells you something about how the stars will die – those near the top, the “O” stars, will go out with a bang, those near the bottom, a whimper.  So temperature, mass, color, life span, and the end game are all related.

They also represent a sort of frequency distribution. “O” stars are one in a million – very rare.  And “M” stars are most common. In fact, about 80 percent of the stellar population is believed to consist of “M” stars, yet we don’t see a single normal “M” star with our naked eye.  (Catch the hedging there?  We do see some abnormal “M” stars. More on that later.)

Know the spectral type, know the star

But the OBAFGKM sequence really hits the jackpot when it comes to dropping stars into convenient little boxes.  If you look up at Vega and note that it is blue/white, then you know it’s nearer the upper end of the list – probably a “B” or “A.” This means it is hotter than the Sun, more massive, and will live a much shorter time. That’s a lot to know just from a crude estimate of its color, but it’s pretty close to what astronomers have learned with more sophisticated techniques. Vega is classified as an “A” star which  has a temperature of 9600K compared to the 5777K of our Sun.  It has half again as much mass as the Sun and is expected to live less than a billion years, compared to the 10 billion for our Sun. In short, know the color – or better yet, the letter assigned to a star and you immediately know something about its color, temperature, mass, life expectancy,  and death,  as well as how common it is or isn’t.

But this really intersting thing come when you make a very simple graph that ranks stars by just two qualities – their actual luminosity  and their temperature/color. “Luminosity” means the actual brightness of a star independent of its distance from us. Color and temperature are indicators of the same thing. The result is what’s known as a Hertzprung-Russell diagram. Here’s an example with each dot representing a star.

526px-HRDiagram

What you should notice is the bulk of stars fall into three areas of the charts – the lower left quadrant where oyu find the white dqaefs, the upper right region where you find various giants, and a thick, swirling band that cuts across from one corner to the other. This band hold the most stars by far and it is known as the “main sequence.”

One thing I don’t like about the word “sequence,” however, is people assume that to be “on the main sequence” means a star is sort of sliding down it – that the sequence is a developmental or evolutionary sequence.  In fact, that was what scientists believed at first – that stars started out hot (top left), then got cooler as they aged, eventually ending up in th ebottom right.  Sounds logical. But the main sequence is not a developmental sequence.  Stars don’t start out life at one end and end up at the other. Instead they stay on the main sequence until their final stages of life, at which time they may move off the main sequence into another realm entirely.  Such moves signal dramatic changes in the star’s life, size, behavior, appearance, and life expectancy.

But start witht he idea that when a star starts life it does so at some particular point ont he mains equence and it stays at that point for most of its life. Where stars go as they leave the main sequence and exactly what happens to themt hen is a matter of mass. In fact, mass is probably the single most important characteristic of a star. You would think it would be chemical make up, but notice we haven’t talked much about star chemistry. That’s because for most stars its basically the same – 92 percent hydrogen, 8 percent helium, and everything else crammed into a fraction of one percent, if it’s there at all.  In a few cases, however, chemistry is significantly different and those stars get their own special classifications. Again, we’ll leave the exceptions to another time.

To appreciate the importance of mass you need to understand what a star is. Reaching for a definition that will cover the whole range of stars, James B. Kaler calls stars “self-luminous condensates of the fragmented dusty gases that fill interstellar space.” OK. As you get near the low mass end, stars do get a bit freaky and it’s difficult to fit them into the general picture. But let’s stick with the general picture where it’s easiest to think of a star as a huge ball of gas, expanding under the pressure of the nuclear furnace at its core, and held in together by the opposing inward pressure of its own gravity.

And that’s the key – gravity. There’s another word we could explore forever without getting an ultimate answer.  Gravity is the force that causes stuff to be attracted to other stuff resulting in more stuff. Honest. The interesting thing about gravity is it works on “the more the more.” That is, the more stuff you have, the more attractive the force of gravity, and so the more likely you are to have even more stuff.

In the ISM – the interstellar medium where stars are born – there’s a lot of stuff – mostly gas and dust – that is spread quite thinly. But various disturbances pass through the ISM, sort of priming the gravity pump, encouraging stuff to get together in clumps which then become bigger clumps until you finally have so much stuff in one spot that the inward pressure becomes crushing. At this point a star is born, for what happens is the hydrogen atoms start bumping into one another, and when they do this, they fuse together and form helium atoms.  At the same time, a small fraction of the matter in the hydrogen atoms gets consumed by the process – matter is turned directly into energy by that most wonderful formula E=MC square – and, of course, the amount of energy is tremendous as we found out when we built atomic bombs and later atomic power plants. A little matter goes a very long way, for we’re multiplying the converted mass by the speed of light (186,200) squared, a very big number. (Mind you, I am not a physicists. I try to recount these things as I understand them from my study. But having read a great book that traces the history and development of that equation it still leaves me completely bewildered why the energy produced should have anything to do with the speed of light. )

What’s so elegant about a star undergoing nuclear fusion is the wonderful way it counterbalances the force of gravity. Left on its own, gravity would have just kept squeezing the stuff tighter and tighter. But the very force of gravity causes the nuclear ignition and that creates the outward force to work against the inward force of gravity.

When we see a star we’re not really staring into a nuclear furnace.  The nuclear fusion in the core is a just the starting point for an exchange of energy that takes something in the order of a million years to reach the surface of the star and send out radiation in the form of the light that we see.  But oh my, what a mighty engine!  A star such as our sun “burns”  4 million tons of hydrogen a second, and according to Leon Golub and Jay Pasachoff in “Nearest Star,” the result is that in just one-thousandth of a second the Sun emits enough energy “to provide all of the world’s current energy needs for 5,000 years.”

And we look up there and it’s hard to not hear the echoes from our nursery –  “Twinkle, twinkle, little star.” Twinkle indeed!

OK – back to OBAFGKM.  How hot a star gets – and many other characteristics – depends entirely on how much stuff is gathered together in this ball of gas. The usual measure of this is to compare the amount with how much is in our Sun. Count the Sun as “one” and a typical “O” star is made of somewhere between 10 and 100 times as much stuff, while an “M” star may have just one tenth what our Sun has.

Other boxes for the stars

Without going into tremendous detail – it’s enough to try to swallow the main sequence in a single gulp – there are four other ways that stars are classified.  I want to mention them here because you’re bound to hear these terms, and some of them sound downright crazy – like calling our Sun, which is larger and brighter than most stars, a “dwarf.”

Stars fall into two broad evolutionary categories, with a third one that exists only in theory. Stars we see belong to either Population I or Population II.  Our Sun and most other stars we see are Population I. The oldest stars we see – such as the ones that are found in globular clusters – are Population II stars.  Population III stars were the first stars formed after the Big Bang. No one has yet seen a Population III star – they are a theoretical concept, but generally well accepted as such. A Population III star would be almost entirely hydrogen and helium. Population II stars were formed next – after the Population III stars had exploded and added “metals” to the universe – metals being elements other than hydrogen and helium.  So Population II stars have a more complex chemistry. Population I stars tend to be richer in metals.  They are the stars being formed today.

(Confused? Do you get the feeling astronomers count backwards?  Well stay tuned. )

Where dwarfs aren’t dwarfed by anything but giants

Another classification system looks at the size – the spatial dimension, not mass – of stars, and it would be confusing, if it weren’t so laughable. I’m not quite sure how astronomy ended up in this quandary, but I assume it’s another instance where new discoveries played havoc with the established naming process. Sort of like trying to straighten up a room as the kids are playing in it. Thus we have a size classification system that goes from smallest to largest stars and reads like this:

VII – white dwarfs
VI –  subdwarfs
V –  dwarfs – main sequence
IV –  subgiants
III –  giants
II –  bright giants
I –  supergiants, and yes
0 –  hypergiants

Oh boy! Wonder what happened to “normal?” Well it’s there. Normal is “dwarf.” In fact the main sequence holds about 95 percent of the stars, and you can consider “dwarf” and “main sequence” synonymous.  And that, of course, means our Sun is a “dwarf.”

OK – maybe I’m being too hard onthe astronomers. As mentioned earlier, the difference in the size of stars is mind-boggling. Here’s a good graphic thatc aptures only the top half od these differences – the difference between our “dwarf” Sun and the giants.

Star sizes 2

Remember, stars do not evolve along the main sequence, but they can fall  – or jump – off it. That’s where these classifications come in. Essentially, as a star nears its life’s end, it goes careening off the main sequence in what results in graceful curves when you start plotting temperature and mass on a graph.  The path to the end is complex and varies according to the initial mass of the star. In the later years of their lives, stars can swell up to incredible sizes and become red giants. An example is Betelgeuse, the star that marks the right (eastern) shoulder of Orion. Such a star doesn’t grow in mass, but it expands like a balloon and while its surface is relatively cool – thus the red color – it’s huge and so the star radiates a lot of light despite being relatively cool..

Stars actually can make the climb off the main sequence more than once and the second time they do this, they may turn into slowly pulsating giants, varying their output significantly over long periods. (Long, that is, from our viewpoint.) Such a star is Mira, “the wonderful.” It goes through an 11-month cycle where it reaches a peak making it easily visible to the naked eye, then it falls back to a point where it is so dim that’s it difficult to detect with binoculars. Chi Cygni is another such star, and there are many more.

In these end games, stars may go through a nearly explosive stage where they blow off a huge amount of their substance and create an expanding cloud that we see as a beautiful planetary nebula. The Ring Nebula in Lyra and the Dumbbell Nebula in Vulpecula are examples.

Stars can also undergo an incredible collapse where their core shrinks to the size of the Earth. Such stars still retain a significant amount of their mass, and they are known as “white dwarfs.” In this case the word “dwarf” makes sense, but the word “white” is another name that has been overtaken by new discoveries. White dwarfs can be red, or any other color normally associated with stars.

And, of course, stars can go out with a bang if they start life with enough mass. The result is a supernova that momentarily shines with incredible brightness, then leaves behind a ragged cloud such as we see with M1, the Crab Nebula.

This doesn’t mean, however, that the star was destroyed in the massive explosion.  In fact, although everyone seems to write about the life and “death” of stars – and I’ve fallen into that pattern as well – I’m not at all sure it’s appropriate. Stars don’t die. They go through incredible changes that may make them difficult or impossible for us to see – but some of the stuff of the star is still there and in most cases it still continues to radiate light. In the case of the Crab Nebula, the star that exploded left a significant core in the form of an incredibly compact neutron star – a star where all the atoms have been stripped of their electrons and protons and are crunched together so tightly that they are about the size of a large city.  Such a neutron star spins as it collapses, much as a figure skater does when she pulls in her arms. It also beams radio energy in one direction and this beam sweeps the heavens like a powerful lighthouse. When we’re aligned with such a neutron star, our radio telescopes pick up a regular pulse of energy, many times a second, from the rapidly spinning neutron star. We call this a pulsar, and again, many examples have been discovered.

In the case of our Sun the current betting is that it will “burp” a time or two creating a complex planetary nebula, then retreat to the white dwarf stage – and white dwarfs go out with a whimper, not a bang. But while theys top radiating, there is still star stuff there so I’m not sure if this really represents the detah of a star.

Whatever the evntual fate, however, it all depends on the initial mass. A star like our Sun ends up as a white dwarf. A star 2-to-3 times larger than our Sun ends up as a neutron star. And a star that starts out even bigger, ends up as a black hole.

The bigger they are, the farther they fall!

Summary

I know we’ve covered a lot of ground. There’s actually much, much more that could be said about stars.  But the basic message is simple. Most stars fit into a single classification system that tells us at a glance several major things about the star. Learning that sequence of letters – Oh Be A Fine Girl Kiss Me – goes a long way in helping you make some sense  out of those distant,  twinkling,  dots of light.

Here’s a summary of this system in table form

star_table

Click table for a larger version.

  • Ninety-five percent of all stars are on the main sequence.  Most of the stars that are not on the main sequence are white dwarfs. Roughly one percent of the stars fall into one of the giant categories.
  • Stars near the top of the main sequence are rare, as are giants of any spectral class, yet when we look at the constellations we are seeing mostly A and B main sequence stars, and a variety of giants, since these are the brighter stars.
  • Notice that stars in the last two categories,  L and T, are either barely visible to us as red stars, or not visible to our eyes at all because they shine only in the infrared. We don’t know how many of these stars there are, but they could be as common as the main sequence “M” stars.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter. The Kelvin temperature scale is the same as the Celsius one, except it starts at absolute zero. This adds 273 degrees to the Celsius scale, a minor consideration when you look at the typical temperature of a star.
  • Age – we can date star clusters by seeing what class of star remains. “O” stars die first, then “B,” etc. No dwarf “K” or “M” star has died yet – the universe isn’t old enough.
  • Understand in all of this we’re dealing with a continuum, so the numbers are just guides.

The stars are not a WYSIWYG world.  What you see is NOT what you get. But what you get is far more exciting, interesting, and elegant than what you see. That said, don’t forget that the goal here is to see, really see – to go out in the night and let the light from these unimaginably distance fires ping your brain, and when it does, mix that experiential knowledge with your abstract knowledge in the hopes of a greater awareness. Good luck!

Simply mind-boggling: Universal Measuring Sticks and Observing Logs

Measuring an 11-foot (meters) strip. (Click image for larger version.)

Measuring an 11-foot (3.4 meters) strip. (Click image for larger version.)

While simple, this project is next to impossible to depict well in a photograph because each “measuring stick” is just a few inches wide and more than 10-feet long. But build one and I bet you’ll find it a mind-bending experience!

I call this project the  Universal Measuring Sticks and Observing Logs and together these “measuring sticks” serve a simple function – they put into perspective the distance of each object you observe. And even if you don’t observe, they’ll help you get a handle on the incredible distances to the planets, stars, and other galaxies.

To do this our basic measuring unit will be the speed of light – 186,200 miles a second (300,000  kilometers a second). That, according to Einstein, is the speed limit for the universe. Nothing can go faster. So we simply ask ourselves how far will light go in a minute? An hour? A year?  Just starting with the distance travelled in a second boggles the mind – the distance that light travels in a single second would take it all the way around the Earth more than 7 times.  That is, it’s 24,902 miles around the Earth at the equator and if you divide that into 186,200 you get (rounded) 7.5 ( 40,076 km divided into 300,000 kmps gives you 7.5 as well). So our most basic unit, the light second, is already far larger than anything most of us have experienced. But at least it gives us a starting point to begin to get even more mind-numbing distances into perspective.

Materials needed for this project:
(2) 11-foot (or 3.3meter) lengths of adding machine tape*
ruler
pen or pencil(s)  – different colors helpful, but not necessary
calculator (helpful) or scrap paper

*You  can use any 11-foot strip of paper you have or create – but I found adding machine tape the easiest way to do this and it’s commonly available. Some might want to use four lengths and not use each side – or you might want to use a dozen sheets of ordinary paper. The goal is to make four scales each 10 feet (120-inches) long with little extra paper on each end to keep it neat.

We will actually make four measuring sticks, each a bit over 10 feet ( 3 meters) long. We need four because it is impossible to fit everything on a single scale and still have it readable.

Well, not absolutely impossible.  For example, the first and smallest scale used is for the solar system. On that scale, one inch  equals two light minutes. (If you’re using the metric system, then start with a scale of 25mm equals two light minutes – very nearly the same. ) That scale puts the moon just 1/100th  of an inch (a quarter of a millimeter) from Earth at one end with Neptune (and Pluto) near the other end.  But if we were to include the nearest star we observe in our northern hemisphere skies other than the Sun, it would  require a piece of adding machine tape 36 miles (57.9 kilometers) long! Possible – hardly practical. Oh – and were we to include the nearest galaxy we observe, the Andromeda Galaxy, we would need about 10 million miles (16 million kilometers)  of tape. Sort of defeats the purpose of a scale model! So it is for very practical reasons that we have created four measuring scales.

To use each scale start by putting a vertical line across your tape about 6-inches from the left hand edge. This is your starting point, which is, in all cases, Earth. To the left of this, put down the name of your measuring stick and the scale being used for that stick. To the right of this, calculate the distance to each item you observe, mark it and identify it on the scale. If you like, include the date observed.The result should look something like this measuring stick for the solar system – keep in mind this is just the first part – the whole “stick” goes onf or 10 feet.

This is the starting end of a solar system measuring stick.

This is the starting end of a solar system measuring stick.

General notes that apply to each stick

  • Light travels at 186,282 miles (300,000 km) a second. We use the speed of light as our measuring unit.
  • On the second and third scales in particular you may find that several objects are at, or near, the same distance, so to mark and identify them you will need to use the full width of the paper.
  • Distances up to 1,000 light years are pretty well known now and reasonably accurate because of measurements taken by the Hipparcos* satellite. Distances beyond this get increasingly fuzzy with many different indirect methods to determine them. For this reason you should regard all these distances as reasoned approximations. For close objects (within 1,000 light years) use sources written after the Hipparcos* measurements which were published in 1997.
  • Our distances are also an indicator of time – each distance tells us how long ago the light we see left an object. You may find it fun to mark all but the first scale with historical, evolutionary, and geological events on Earth. Such time references add to your perspective.

You can look up distances and calculate them to scale any time, but the real goal here is to reinforce the observing experience, so if you are observing , I suggest you use this more as a log and  mark your scale either when planning an observing session, or when reflecting on that session after observing. That way the abstract experience of learning and calculating distances is in your mind along with the real-life experience of observing the object.

The Scales – (If you prefer to work in metric, just change “1-inch” to 25mm – it will be close enough for these purposes.)

#1 The Solar System

Scale: 1-inch = 2 light minutes (or for those who want more precision, 120 light seconds*

Minimum distance in light hours, minutes, and seconds,  from the Earth to the moon and planets are:

  • Moon: 00:00:01.2  (that is 1.2 light seconds)
  • Venus 00:02:07  ( 2 minutes, 7 seconds)
  • Mars  00:03:02
  • Mercury  00:04:18
  • Sun 00:08:19
  • Jupiter  00:32:43
  • Saturn 01:06:28  ( 1 hour, 6 minutes, 28 seconds)
  • Uranus  02:23:35
  • Pluto 03:58:07**
  • Neptune 03:59:25

To calculate the distance on your Universal Measuring Stick, simply divide the time in minutes by 2, or the total time converted to seconds, by 120.

Example: Jupiter is 32 minutes, 43 seconds away. In seconds that is (32X60) + 43 or 1,920 seconds plus 43 which is 1,963 seconds. 1963/120 = 16, so Jupiter will be 16 inches away.

*Use 2 light minutes for reasonable approximations, or get more precise with seconds.

** yes, Pluto when closest to us is closer than Neptune when closest to us!

If you’ve made the solar system measuring stick, you should have the basic idea how and find the others easy.

#2 Our Stellar Neighborhood – to 2,600 light years

Scale: 1-inch = 21.6 light years

This scale covers most of what you can see with your naked eye –  as well as many things you can not see with the naked eye because they are too faint, but still fairly close to us. Well, close as astronomical objects go, but incredibly far away when it comes to what we’re used to.

(The entire solar system scale would be so small, it would be impractical to represent it on this scale with anything except the thinnest of lines right at the start.)

We’ll use some of our bright guidepost stars just for starters. Here are their distances in light years:

  • Polaris  430
  • Arcturus  37
  • Spica 262
  • Antares 600
  • Vega  25.3
  • Altair 16.8
  • Deneb 1,400
  • Big Dipper  80*

*This is an approximation covering the main stars of the Dipper which are really part of an open cluster.  With most asterisms the stars would be at various distances.

#3 Our home galaxy, the Milky Way – to 100,000 light years

Scale: 1-inch = 833 light years

(The previous scale would take up little more than the first three inches of this scale.)

This measuring stick takes us to some of the more distant open clusters, typical globular clusters, and some nebulae that are easy to observe with the naked eye, binoculars,or small telescopes.

Examples:

  • Pleiades M45 440
  • Dumbell Nebula M27  1250
  • Orion Nebula M42  1,300
  • Ring Nebula M57 2,300
  • Open Cluster M37 4,400
  • Globular Cluster M13 25,000

#4 Our observable universe – to 100 million light years

Scale: 1-inch = 833,000 light years

(The entire previous scale would take up about the first one eighth of an inch on this one.)

While the Andromeda galaxy can be detected with the naked eye and observed with ordinary binoculars, most of what we include on this measuring stick takes us to the limit of what we usually observe with a backyard observatory that includes at least a  6-inch telescope. We can reach farther into the universe than this, but with anything past the middle point on this scale you see very little – and most of what you see at these distances justifies the term amateur astronomers usually use for these objects – faint fuzzies!

Examples:

  • Andromeda Galaxy M31  2.5 million
  • Pair of galaxies beahind the Great Bear’s ears –  M81, M82  12 million
  • Whirlpool Galaxy M51 23 million
  • Leo Triplet Galaxies M65, M66, NGC 3628 35 million

Finally, getting the measure of the universe – here’s a brief tribute to the measurers – ancient and modern . . .

“HIPPARCHUS OF NICEA must have been an interesting fellow. He was a second-century B.C. mathematician, philosopher and astronomer. Using the only astronomical instrument available to him — his eyes — Hipparchus took on the daunting task of measuring the positions of the stars and planets as they passed overhead each night. He came up with a catalog of 1,080 stars, each of which he described simply as “bright” or “small.”
“Hipparchus wasn’t the first astronomer to pursue the science of astrometry, as the astronomical discipline of positional measurement is now called. However, his star catalog was the first of many compiled over the centuries by astronomers using ever-better instruments and techniques. From those measurements — all made from the Earth’s surface — astronomers have derived everything from basic stellar properties to estimates for the age of the universe.
“On August 8, 1989, the science of astrometry took a long-awaited leap to the stars. Riding aboard an Ariane rocket was the High Precision Parallax Collecting Satellite, otherwise known as Hipparcos. For the next three and a half years, Hipparchus’s 20th-century namesake measured the parallaxes and brightnesses of more than a million stars — despite a potentially crippling accident that sorely challenged the project’s architects.”

The above is quoted from this Web site: http://tinyurl.com/yurcq2 Go there for more details.

Prime Time observing for September 2009 – a square, a couch, dancing moons, and more!

Please note: All charts with this post are for observers in mid-nothern latitudes centered on 40° N. If you are 10 or more degrees south or north of that – or if you’re not sure of your latitude – please go here to make your own custom star charts.

Our focus as always is the eastern sky, 45 minutes after sunset, where in September 2009 we’ll find a brilliant Jupiter whose moons play a fascinating game of hide and seek. But our main goal will be to locate and remember  this month’s  two new asterisms the Great (empty) Square and Andromeda’s Couch.

Let’s start with Jupiter, though, because no prime time observer can fail to find Jupiter in the eastern sky starting about half an hour after sunset – there’s simply nothing brighter except the Sun and Moon – well nothing brighter in the eastern early evening sky.  Venus gets brighter than Jupiter, but it never appears in the eastern sky after sunset, though it is in the eastern sky these September mornings an hour or so before sunrise. If you’re one who likes to be up then, be sure to take a look – you can’t miss it!

Though not visible to the naked eye, what’s most fascinating about Jupiter is its four brightest moons. Yes, they look a lot like little stars, even in the telescope, but they are in a rough line with the planet’s mid-section and they continuously change positions around the planet from night to night. In fact these changes can be seen over the course of an hour or so, though at the least you need good binoculars that are held very steady in order to see them. Any small telescope, however, should reveal them easily. For an introduction to observing these four moons see the video and text here. This describes moon events for an extraordinary evening – September 2/3, 2009 – but at some time on many evenings you can observe one or two such events, so even if you miss the events of September 2/3, watching the animation and reading this should help you understand similar events that happen quite often whenever Jupiter is visible.

Of course Jupiter is not going to help you learn the rest of the night sky because like all planets it is constantly changing its position relative to the background stars. But our two bright asterisms for September will help and they are as simple as they come – a square with an arc of three bright stars attached to it.

Click chart for a much larger version.

Click chart for a much larger version.

The first is known as the “Great Square.” I call it the “Great (empty)  Square” because the area inside it is almost completely empty of other naked-eye stars.  The other asterism ties to it like the tail of a kite flying sideways.  It streams off one corner and I think of it as “Andromeda’s Couch.” Of course this 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 and their companions, however. Like most constellations, with Andromeda you need a huge imagination to see the figure these stars represented to the ancients. But knowing Andromeda was a lovely woman who was rescued by Perseus, I like to think of this graceful arc of stars as her couch.  That said, notice three things about it:

1. The bright star at the right – southern – end is also a corner of the Great Square. In fact, it is the brightest star of the four that make up the square, but only by a little.

2. The three stars are pretty equally spaced. Hold your fist at arms length and it should easily fit in the gap between the stars which means there are 10-15 degrees between each star. That’s similar to the spacing between stars inthe Square.

3. There’s another dimmer, but fairly bright star, between the first star ( the one at the corner of the Square)  and the middle one.

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.

Looking north

Meanwhile, for those in the northern hemisphere, the bright stars circling Polaris and always visible are well represented this month with the Big Dipper starting to move towards the horizon in the northwest and the “W” of Cassiopeia starting to take the dominant role in the northeast opposite it.

Click chart for a larger image. Northern skies as seen from about 40° N latitude in mid-September..

Click chart for a larger image. Northern skies as seen from about 40° N latitude in mid-September..

Our chart shows the northern celestial pole region about 90 minutes after sunset when skies are about as dark as they get. Will you see all these stars? Depends. First, on how much light pollution there is where you observe. Second, on how well your eyes are dark adapted. You must avoid white light for at least 15 minutes – better still, half an hour – if you wish to see the fainter stars. If you want to test how good your skies and night vision are, look at the Little Dipper. In light-polluted suburbs you will probably see just the three brightest stars. In good rural conditions you should see all seven.  And if you can see them, then this is a good opportunity to try to trace out Draco, one of a handful of constellations whose connect-the-dots pattern actually suggests the mythological figure of a dragon.  I love Draco, but quite honestly, I have to look for it – it doesn’t jump out at me the way the Big Dipper and the “W” do.  And as far as learning the sky – well, you learn the “W,” the Big Dipper and Polaris so you can then find stuff like Draco when you want to find it.

The arrows on the chart indicate the general direction in which the sky appears to move. Stay out an hour and this motion should become obvious to you.

. . . and the rest of the guideposts?

If you’ve located the new September asterisms then it’s time to check for the more familiar ones 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.

. . . our journey and September’s planets (2009)

In the course of a night you can still get a glimpse at all the planets – technically – but the truth is both Saturn and Mercury are very difficult to see this month, and Pluto is always just a faint speck visible in large amateur telescopes. Jupiter, as we’ve noted, dominates the evening sky in the southeast. Nearby – visible in binoculars or small telescopes – is Neptune. And an hour or so later, if you want to track it down with binoculars, Uranus will make a good test of your star-hopping skills.  In the morning sky both Mars and Venus are prominent, though Venus gets closer to the Sun throughout the month. At the start of the month Venus rises about three hours before the Sun – by the end of the month this is cut to about two hours – but even in twilight it is so bright it’s hard to miss.

Charts to help you find the  planets follow, but first, let’s look at the solar system from the perspective of someone in a spaceship hovering above it. This shows us where we are in our journey around the Sun and also gives us a chance to examine where the other planets are in relation to us. See if you can translate this perspective into what we see in our sky. The chart below was created with the Solar System Live capability found here. I added the arrows in Photoshop Elements simply to indicate the horizon and directions relating to the earth’s rotation on its axis.

Click image for larger view. Arrows indicate the western and eastern horizons at sunset on September 15, 2009. Smaller arrows show the direction these horizons move at the earth turns on its axis in the course of the night.

Click image for larger view. Arrows indicate the western and eastern horizons at sunset on September 15, 2009. Smaller arrows show the direction these horizons move as the earth turns on its axis in the course of the night. (Planets are not drawn to scale.)

Looking at the horizon line going out to the west – left – you can see that at sunset Saturn is nearly on the horizon.  Use the arrow going to the east (right) and you can see Uranus isn’t quite visible in our night sky at sunset, but Jupiter, Neptune, and Pluto are well beyond the eastern horizon.  Draw an imaginary line from Earth through Jupiter and you’ll see it comes near Neptune – which is why Neptune appears relatively close to Jupiter in our night sky this month, though you’ll need binoculars to find it. (Notice also that Neptune, while a giant planet, is more than twice the distance from the Sun as Jupiter – which is why it is so dim and small in our night sky while Jupiter is bright – and in a telescope – quite large. As these horizon lines rotate,  Saturn sets, followed by Mercury and  then several hours later Pluto and eventually Jupiter. Meanwhile, Uranus rises in the east, followed in the morning hours by Mars and Venus.  Notice also that Pluto is just a tad beyond Neptune these days, though the distance between them will slowly increase.  The chart does show, however, that for a while Neptune was our most distant planet. See how Pluto’s orbit was inside that of Neptune? Don’t forget, Pluto takes 248 earth years to get around the Sun once. These events hold generally true no matter where you are in the world, but they need to be fine tuned for your latitude. Folks in the southern hemisphere, for example, get a much better view of Saturn and Mercury early in the month, than those in the north.

Finding Uranus

Uranus can be found with binoculars – or in exceptional conditions the naked eye – but locating it is an advanced project for those already comfortable with finding the naked eye bright stars and asterisms. You need full darkness, your eyes should be dark adapted, and you should be in an area where light pollution isn’t a serious problem.  That said, finding this planet is relatively easy if you have a decent pair of binoculars and patience.  Here’s a chart to use. After reading the directions below, click on the chart to get a larger version.

This Uranus finder chart is meant to be used firstw ith the naked eye, then binoculars. The red circle represents the typical view with wide field 7X or 8X binoculars. See text for instructions. Click on chart for larger view.

This Uranus finder chart is for September, 2009, about two hours after sunset. It is meant to be used first with the naked eye, then binoculars. The red circle represents the typical view with wide field 7X or 8X binoculars. Included on this chart are many faint stars that can be seen only with binoculars. See text for instructions. Click on chart for larger view. (Made from Starry Nights Pro with modifications.)

Start your search by locating Jupiter and the Great Square. You may also see Fomalhaut, a first magnitude guidepost star that will be introduced in October.

Next look below the Great Square for the “Circlet.” This is a well-known asterism in the constellation Pisces – but in typical suburban skies it is a difficult object and you may be able to pick out just three of the brightest stars in it with your naked eye. In rural skies you should be able to see most of these stars with the naked eye, but try to locate them with binoculars. The entire Circlet probably will not fit in a single binocular field of view, but enough of it should so you know what you are seeing.

Now use your binoculars to try to locate the trapezoid of fainter stars below the Circlet. This little unnamed trapezoid will probably fit in your binocular field of view. The faintest star of these four is just a bit brighter than Uranus, so that gives you an idea of what you seek.

Finally, with your binoculars scan up and to the right (west)  of this trapezoid and you should pick up an arc of three stars all about the same brightness. The third – the highest – of these is Uranus. While you won’t see a disc, you may notice that it shines with a steadier light than the other two. This is typical of planets. In a good telescope Uranus will show a tiny disc and perhaps a greenish tinge, but to the casual observe may be easily mistaken for a star.

Finding Neptune

Neptune is both easier and harder to find than Uranus. Again, binoculars and a dark sky are needed. What makes it easier is it’s near Jupiter. What makes it harder, is it’s signifcantly fainter than Uranus – so faint that whether you see it or not will depend on how dark your skies are.  You will need this little finder chart, however, to pick it out of the starry background.

Finding Neptune requires binoculars, or a small telescope, and patience. Fortunately, Jupiter drops us right in the neighborhood! See text for complete directions - and click on chart to get an elarged version. (Made with screen shot from Starry Nights Pro. I added names and arrow.)

Finding Neptune requires binoculars, or a small telescope, and patience. Fortunately, Jupiter drops us right in the neighborhood! See text for complete directions - and click on chart to get an enlarged version. This charts is for mid-September, 2009, about two hours after sunset. Neptune will appear to move slightly towards Jupiter during the course of the month. (Made with screen shot from Starry Nights Pro. I added names and arrow.)

Step 1 – find Jupiter, the brightest “star” in the eastern sky. The red circle represents a widefield binocular view. Your binoculars may show a smaller field. Also see if you can spot the two bright stars in our chart that are to the left – east – of Jupiter. They are bright enough so you should be able to see them even in typical suburban skies. In any case, you certainly should be able to find them with binoculars by first locating Jupiter, then scanning to the left – eastward.

Step 2 – after locating the two bright stars, use binoculars to look for the arc of three dimmer stars above them. These three are about the same brightness as Uranus and just at the edge of naked eye visibility under excellent, dark skies. For most people this means they will be seen only in binoculars. Neptune is to their left – east – as indicated.

And for early risers – Venus and Mars!

A crescent moon and Venus dominate the morning sky in the east, along with Mars and half a dozen bright "winter" guidepost stars. Click chart for larger image. SLightly modified screen shot from Starry Nights Pro.

A crescent moon and Venus dominate the morning sky in the east, along with Mars and half a dozen bright "winter" guidepost stars. Click chart for larger image. SLightly modified screen shot from Starry Nights Pro.

Don’t miss the autumnal equinox!

OK – if you’re in the southern hemisphere, this marks the start of spring. In the northern hemisphere, it’s autumn. In either case, it’s when the Sun crosses the celestial equator and day and night are almost of equal length.

The autumnal equinox this year is on September 22, 2009, at  21:28 Universal Time.

So what? Well, if you’re just starting out in star gazing, this is a great time to get your bearings at your observing site. That is, on or about September 22 – a few days either way won’t matter much – note where the Sun either rises or sets. That marks the due east – or due west – point on your horizon and from that you can easily figure out where north and south are.

It’s also the day on which the reading of your equatorial sundial switches from one plate to the other. That is, in the north you go from the north-facing dial plate to the south-facing (underneath) one.  See our equatorial wrist dial project. if you want to know more about this.

And finally, I find it cool that day and night are nearly of equal length. For one thing, that means the stars get a break. For the next six months here in the north we’ll have longer nights and thus more time to enjoy the night sky.

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