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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.

August Guideposts: Asterisms guide you along the Milky Way

Editor’s note: There’s a companion project to this post  – build a scale model of the Milky Way – here.

One of the most beautiful and awe-inspiring astronomical sights is the Milky Way. Sadly, most people today are denied this sight because of light pollution, but don’t despair! With a little knowledge and just about any pair of binoculars, you should be able to easily spot our galaxy – the Milky Way –  and yes, even travel (with your eyes and binoculars)  toward its core.

We’re going to use our guidepost stars and three easily recognizable asterisms to do just that. Of course, if you can get out, away from light pollution, do so.  I live in one of the most light-polluted parts of the world, the northern half of the Eastern Seaboard of the United States, yet I have just enough rural countryside and ocean nearby to allow me to  regularly enjoy the Milky Way from my backyard, and I see it even better if I take a 15-minute drive to a site nearer the ocean. So this guide is for those at dark sites using just the naked eye, as well as for those who live in more light-polluted suburbs.

Here’s when and where and how to look.

When? Early on a moonless, August evening when the skies are crystal clear – frequently this comes right after a cold front passes. Although  the Milky Way can be seen many months of the year,  one of the best times is in August, about two hours after sunset.

Where? I would say anywhere except the city – and if there is a city nearby, try to orient yourself so it is to your north. Do your best to leave your southern sky free of light pollution.  The more free of light pollution, the better – but you don’t have to make a big expedition in order to do this.  While I can see the Milky Way from my backyard, my favorite viewing location is at a bird sanctuary with the ocean to its south about 10 minutes drive away. This spot also has the advantage of an almost clear horizon for 360 degrees!

How? Lie down, look up, nurture your night vision, and put your best binoculars to work.  A blanket, or better yet, a lawn lounge chair will help make this comfortable; otherwise, you’ll find the Milky Way a pain in the neck, and there’s no need for that! “Best binoculars” for this purpose means the ones with the largest diameter lens that you can comfortably hand hold, and that usually means something in the order of 50 mm. But smaller will do the job too. After all, Galileo was the one who showed us the real nature of the Milky Way – the fact that it was actually millions of distant stars and not a faint cloud – and just about any binocular you have will do a better job than the telescope he used. (For more on binoculars for astronomy see this post.)

Most important – guard your night vision! I’m going to suggest you start this exploration early – about 45 minutes after sunset, then stay at it for the next hour or more as the sky darkens above you. If you do this – and if you avoid white light, such as the headlights of cars and the beam of a flashlight – your eyes will naturally dark adapt, and about 45 minutes after you began you should start to pick up the Milky Way, though it may take another 15-30 minutes for you to be able to see it in its full glory.

One more critical “when” note. Choose a time several days after full Moon. When the Moon is waxing – getting larger and brighter each night – it will drown out the Milky Way.  You want to do your early evening Milky Way observing when the moon is waning. This is the couple of weeks after full and really up to the point where the Moon is about two  or three days  old. You need to wait five or six days after full Moon because you don’t want the Moon rising just as full darkness sets in and thus spoiling your view. (In 2009 full Moon is August 5 and good Milky Way observing begins August 11 and continues to about August 23, 2009.)

Where do you look? Up, in a word.

When you set up your blanket or lounge chair do your best to align it on a north-south axis with your head to the north and feet to the south.

What you want to see as you start out is the familiar guidepost stars of the Summer TriangleVega, Deneb, and Altair. These should be familiar because you learned them in June or July. If not, you can learn them now.  They are easy to pick out because as the sky starts to darken they will be the first stars visible, and they will be high in the east. Here’s a chart of what you will see.

Orient yourself north-south, then locate the Summer Triangle high in the east. Click for larger charts. (All charts use screen shots of Starry Nights software which I have then annotated.)

Orient yourself north-south, then locate the Summer Triangle high in the east. Click for larger chart. (All charts use screen shots of Starry Nights software which I have then annotated.)

The brightest – and highest – of the three will be Vega which will be approaching a point overhead. There’s roughly two fists (24 degrees) between Vega and Deneb and nearly four fists (39 degrees) between  Deneb and Altair, so the Triangle is huge. (Our chart is about 90 degrees wide covering from northeast on the left, to southeast on the right. And that real bright “star” near the horizon? That’s the planet Jupiter – at least in 2009. It won’t be in this position other years in August.)

These three Summer Triangle stars roughly bracket the Milky Way – that is Vega roughly marks the western border, Altair roughly denotes the eastern border, and Deneb is about at midstream.  But you need to wait, of course, for it to get darker before you can see the Milky Way.   Right now you’re learning where to look. Do keep in mind that the boundaries of the Milky Way, as with any stream, are not sharp and regular. It tends to meander a bit with little pools of light and some deep, dark areas as well.

But it is early yet. At this point the Milky Way is beyond your reach and probably your binoculars as well. If you have located the Summer Triangle, though, it is time to find the first of our three August asterisms. Remember that an asterism, while it may be part of a constellation, is different. It is a small collection of bright stars and unlike a constellation, it actually will look like the object its name implies.

Use Deneb to start your identification of the Northern Cross. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

Use Deneb to start your identification of the Northern Cross. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

The first of these asterisms we’ll use is the Northern Cross, which also serves as a good rough outline of Cygnus the Swan, the constellation which it inhabits.  You find it by starting with Deneb, the middle – and faintest – star in the Summer Triangle. I am assuming it is now more than an hour after sunset and other bright stars are starting to appear. Our Summer Triangle stars are magnitude zero and one – very bright. The Northern Cross stars are magnitude 2 and 3 – dimmer, but about as bright as the stars in the familiar Big Dipper. If you don’t find all these stars immediately, be patient. It may just need to be a little darker. But start with Deneb. It marks the top of the cross, which lies in a general north-south direction. Here’s a chart of the bright stars in this asterism.

Deneb, and the three stars that form the cross bar, should be easy to see.  The fifth star, Albireo, is the dimmest of the group, but note how it is roughly on a line halfway between Vega and Altair, and you should be able to locate it using those two as a guide.  Albireo is magnitude 3 and marks the tip of the cross and even if it is now quite dark, you may have difficulty seeing it in light polluted skies. If so, use your binoculars – and do notice that it does not quite make a straight line with the center star and Deneb.  Also, almost halfway between the center star and Albireo you may notice another, still fainter star that’s a little off this line.

If you want to see this as the swan of the constellation Cygnus, that’s easy enough. Deneb is Arabic for “tail” and marks the tail of the swan. The three bright stars of the cross-bar are the Swan’s wings and the other two stars extending to the south are the Swan’s long neck. Deneb is flying right down the Milky Way – which is what we will do as well, after we locate two more helpful asterisms.

For the first of these next two asterisms, reorient yourself so you are now looking north.

We want to locate  the W of Cassiopeia as it rises in the northeast.  This is a very helpful asterism that also will help you locate the North Star.  Much of the time, of course, we use the Dipper and its “Pointer” stars to find Polaris, the North Star. But as we move into the fall the Dipper gets lower and lower in the northwest. In my backyard it eventually sinks beneath trees. But no worry – as the Dipper goes down the W  of Cassiopeia rises and it is a perfect counterpoise to the Dipper. Like the Pointer stars, it is about 30  degrees –  roughly three fists – from Polaris, so I think of it as marking the other side of a huge circle going around the North Star. The Dipper marks one side of that circle, the W the other. Here’s the appropriate chart. (To see an animation of how these stars move around the North Star, see this post.)

Look north and you'll see the W of Cassiopeia rising in the northeast. The last star of the W - the one closest to the horizon - is the faintest and may no tbe visible at first. Click to enlarge this chart. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

Look north and you'll see the W of Cassiopeia rising in the northeast. The last star of the W - the one closest to the horizon - is the faintest and may no tbe visible at first. Click to enlarge this chart. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

But for our purposes on this August night we shall see the W as marking the northern extremity of the Milky Way. Oh the Milky Way  continues up here – but it does get fainter and a little harder to pick out.  Still, you should think in your mind of a river running through the W of Cassiopeia through the Northern Cross – or Swan – and on southward past Altair, the southern-most star in the Summer Triangle.

And where does it go? Why into the Teapot of course!  For those in mid-northern latitudes, which is where I am, the Teapot asterism tends to sit on our southern horizon – and some of the best and brightest parts of the Milky Way look like steam coming out of its spout! So turn around now and face south. For me, the Teapot is just 15-20 degrees off the horizon – roughly two fists – but if you live farther south, say about latitude 25 degrees, then it will be significantly higher.

Here’s what you should be  looking for:

You canbt hink of the Milky Way as streaming into the Teapot - or you can think of it as the steam coming out of the Teapot - which ever helps you remember best where to look.  Click for a larger chart. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

You can think of the Milky Way as a river streaming into the Teapot - or you can think of it as the steam coming out of the Teapot - which ever helps you remember best where to look. Click for a larger chart. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

The Teapot is the core of the constellation Sagittarius, and it follows the familiar scorpion of Scorpius. You may have found that last month when you located another of our guidepost stars, Antares. Now, as you look south –  it also should be quite dark by now – you should have no trouble seeing the second and third magnitude stars that form the Teapot, or for that matter, the wonderful curves of the scorpion to its west.

Now, if it is more than 90 minutes after sunset and if you are in a location away from light pollution and, of course, are enjoying one of those crystal clear nights with dark-adapted eyes, then you also should be seeing the Milky Way. It only takes time and patience for you to trace it out – to see areas that are brighter than others – as well as some dark patches that don’t mean the absence of stars, but the presence of obscuring dust. But don’t think of the dust as getting in the way – think of it as star stuff – for what you are seeing in many sections of the Milky Way are the parts of our galaxy where new stars are being born. Relax and explore with your binoculars – start to absorb the majesty of millions – no billions – of stars! Here’s an overview from the W through the Northern Cross to the Teapot. If conditions are right – and you have a dark sky – it will look to the naked eye like faint clouds that get brighter as your eye traces them out from north to south.

The August Milky Way appears as a cloudy area in this sweeping overview  showing a fully dark sky. By all means, click for a larger image of this chart.  (This chart use

The August Milky Way appears as a cloudy area in this sweeping overview showing a fully dark sky. By all means, click for a larger image of this chart. (This chart use

But what if you are in a typical, light-polluted, suburb? You’ re looking in the right spot, but you see no star clouds, only a handful of bright stars. Don’t despair. If you have located  the three asterisms – the W to the north, the Northern Cross overhead, and the Teapot to the south, then you are looking at the Milky Way even if you can’t see it.  Assuming it is at least 90 minutes after sunset, now is the time to start exploring with your binoculars. (You should do this even if you can see the Milky Way well with your naked eye.)

Start with the center of the Northern Cross and sweep from left to right – east to west – across the three stars that mark its cross arm.  When you’re on the center star of these three you should see lots of fainter stars in your binoculars – but as you get out beyond the stars that mark the ends of the crossbar, you should see fewer. Do broard sweeps with your binoculars being conscious of the background hue.  Away from the Milky Way it should be darker – if you notice it lightening as you move across the Milky Way – wall, that’s the Milky Way.  Remember to think of the Milky Way as a river running roughly north to south from the W to the Teapot – to find its borders, keep sweeping across it in an east-to-west direction as you also move slowly southward. (Looking at the background hue, however can be deceptive. You have to separate the change caused by the Milky Way from the change of say the light dome of a nearby city, or other source of bright light. The sky, for example, will appear lighter as you approach the moon. But then, trying to find the Milky Way with the moon in the sky isn’t a real good idea as mentioned earlier.

And what is it you are seeing and why does it appear this way to you? That’s the important question. And this is where you have to do some mental gymnastics.

Think of our galaxy as a large pizza pie with extra cheese and goodies heaped in the center.  Now put yourself away from that center – perhaps one-half of the way towards one edge and buried down at the level of the crust. That’s a pretty good simulation of our galaxy and our place in it. You really need to get outside it – we can only do this in our imaginations – and look at it from that perspective. If we could get outside it, here’s approximately what we would see:

Two view of our Galacy, the Milky Way. The one on the left is from aposition above it, the one on the right shopws you the galaxy edge-on. This is a screen shot from the wonderful, free software, "Where is M13."

Two view of our Galacy, the Milky Way. The one on the left is from aposition above it, the one on the right shopws you the galaxy edge-on. This is a screen shot from the wonderful, free software, "Where is M13."

The image on the left is how we think our galaxy would look if we could get above it and look down on it – like a big pinwheel of stars.  And what if you could see it edge on? Well, that’s the picture on the right. (This is a screen shot  from a wonderful – and free – software program called “Where is M13” that helps you understand where various objects really are in relation to us and the rest of the galaxy.)

OK – focus on the edge-on image – and note how really thin most of the galaxy is. It is about 100,000 light years across, but on average just 1,000 light years thick.

plane_view_MW

Now imagine yourself on a small dot (the Earth) rotating around that small dot in our image – the Sun. Do you see a lot of stars when you look “up” – that is, look in the direction of the words  “The Sun.”

No – in fact, if you look down, you don’t see many stars either – or for that matter, if you look in just about any direction there are relatively few stars visible to you. Why? Because the disc is just 1,000 light years thick, and most of the time you’re looking right through it the short way.  But  look along the plane of the galaxy – say  directly to the right or left – and what a difference!

Looking to the left you see many stars – in fact, a thin river of stars. Looking this direction, you’re looking through about 20,000 light years of star-filled space. We are looking along the plane, generally towards the outer rim, when we look at the W of Cassiopeia. Look along the plane to the right, and you see even more stars in a much wider river. Now you’re looking through about 30,000 light years of star-filled space and then right at the star-rich, galaxy core. And this, in a general way, is what we are doing when we look toward the Teapot of Sagittarius. That’s why the Milky Way is so much brighter and denser in that direction.

Not too difficult to understand – but this is only a rough sketch. As recently as 2008 scientists came up with a much different perspective of our galaxy than we had had up until then. Prior to the latest study, we thought the galaxy was a spiral with a bulge in the center and four main arms. Now we see it as a barred spiral – that is, the bulge in the center looks more like a bar that spills into two – not four – main spiral arms. There are other smaller arms in the spiral, and it all gets quite complex.

The problem, of course, is there is no way we can get outside our galaxy and look in. The distances are incredibly vast. Even if we could send a space probe at the speed of light, it would be thousands of years before it got outside our galaxy, took some pictures of us, and sent those pictures back. So we have to try to decide what the galaxy really looks like from the outside by studying it from the inside. Imagine, for a moment, being inside your body and trying to figure out what you look  like by what you can see from the inside, and you get an idea of the problem. Fortunately we can see other galaxies and in later months we’ll be looking at one that looks a lot like what we think ours would look like if we could only get outside it and look back.

Meanwhile, relax – look up – and dream of all  the wonders that are out there and sending their messages back to you in the form of millions of tireless photons that have travelled thousands of year to reach your eyes and ping you brain on this dreamy August evening.  Harvest some of those photons by surfing the Milky Way with your binoculars. You will notice that in some areas it is quite dense and you may even discover some tiny, tight clusters of new stars – or a globular cluster of old stars, or even a little hazy patch where new stars are being born.  You need a telescope to see these well, but you can just discern some of them with binoculars, and with telescope or binoculars, what you really need to see with is your mind’s eye. Knowing what you are looking at is what brings this faint cloud alive and turns it into the awesome collection of billions of stars – and more billions of planets –  that it is.

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2 Responses

  1. […] among many, many other faint dots. Better to spend your time exploring the Milky Way itself. See: August Guideposts: Asterisms guide you along the Milky Way. This is the general area within which you can find Pluto this month. But theplanet is faint and […]

  2. […] Be the first on your block to build your very own Milky Way Galaxy! Posted on August 8, 2009 by Greg Stone Editors note: This is a companion project to the post on viewing the summer Milky Way found here. […]

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