This is the month to meet your neighbors – a few billion of them at least!
In August we break our pattern of focusing on bright stars and instead focus on that ancient stream of stars known as the Milky Way – our own galaxy. This means observing a bit later than normal, and if you live within urban or suburban light pollution, going to where you have really dark skies. This does not mean you have to move to – or visit – Arizona. I live in one of the worst light pollution regions of the US, and I can see the Milky Way from my back yard – and see it even better if I take a 12-minute drive to a nearby wildlife sanctuary. But I do have significantly darker skies than people just a mile or two from me. You need a clear moonless night and your eyes need to be well dark adapted. Then you want to look up for a wide, faint “cloud” with a roughly north-to-south orientation.
I've reduced the brightness and contrast on this image in an attempt to approximate what can be seen from an area with light to moderate light pollution. Still, a photograph always shows more - but it just can't capture the magic of being there. In this case the photographer also caught a Perseid meteor. As you can see, the heart of the Milky Way is nicely framed by the bright Summer Triangle stars of Vega, Deneb, and Altair. Click image for larger version.
Seeing the Milky Way is worth the special effort. It is one of the most beautiful and awe-inspiring astronomical sights, and your naked eye is the best way to take it all in, though binoculars will provide a special treat as well. In what follows, we’ll focus on where you should be to observe the Milky Way, when you should look. and finally, where in the sky you should look.
1. Where you should be
Sadly, most people today are routinely denied this sight because of light pollution, but don’t despair! While the darker your skies are, the better, like me you may find that pretty dark skies are just a short drive away. There is an international guide to light pollution and here’s what it shows for light pollution in and around “Driftway Observatory,” my backyard.
On this map of light pollution for southeastern New England, Driftway Observatory is right in the center on the border of an orange/yellow area. Obviously black is the best. Blue is darned good. Green and yellow are desirable. Orange means getting poor; red and white are quite terrible. You should look for at least a yellow area - but to the south of a heavily light-polluted city if possible.
You can get a map for any region of the world. The simplest path is to go here. Scroll down, to the thumbnail maps and choose a region of the world that suits you and download the map for that region. Another path is limited to observers in the United States, Canada, and Mexico. For them there are “Clear Sky Charts” – astronomical viewing weather forecasts – for hundreds of locations. You can find a location near you by starting here. Underneath your regional Clear Sky Chart you will see a short list of “Nifty links.” The last one takes you to a light pollution map for that region. It may be helpful to know your latitude and longitude first, so If you don’t know what it is, you can find it here. All of this is useful information for any sky observer to have, so if you track down a Clear Sky Clock for your region,f or example, bookmark it.
Here’s how to make sense of the light pollution maps in terms of seeing the Milky Way.
Red – “Milky Way at best very faint at zenith.”
Orange – “Milky Way washed out at zenith and invisible at horizon.”
Yellow – “Some dark lanes in Milky Way but no bulge into Ophiuchus. Washed out Milky Way visible near horizon.”
Green – “Milky Way shows much dark lane structure with beginnings of faint bulge into Ophiuchus.”
If you can get into the blue, grey, or black areas – enjoy! I envy you
One critical point though: Pay attention to where there are cities. They will create light domes that will wash out at least areas fairly low in the sky. In my situation I have two small cities, Fall River to the northwest and New Bedford to the northeast. Both have populations of around 100,000 and both create light domes in those regions of the sky. Fortunately, the northern sky isn’t important for seeing the Milky Way, especially in August. But if you have a large city – or shopping mall, or anything that might create a light dome – it is better to look for an area south of it. In August in mid-northern latitudes the Milky Way is best from right overhead on down to the southern horizon. That’s why my best view is from a wildlife sanctuary just a few miles away and right on the north shore of Buzzards Bay and the ocean. It means when I’m looking at the southern Milky Way – towards the very center of our galaxy – I’m seeing it over a huge expanse of water where light pollution is the least.
2. When to look
Begin looking early on a moonless, August evening and ideally, 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 to see it is in August, about two hours after sunset. In 2010 your best views will come between August 1st and 15th – after that the Moon will offer more and more interference each night for the next two weeks. However, by the 31st, you should get in a solid hour of Milky Way treat before the waning, gibbous Moon rises. If you miss it in the first two weeks of August, try again the first two weeks of September - this guide will still be useful, though everything will have moved higher and to the west a bit.
I say two hours after sunset because it takes that long in mid-northern latitudes for it to get fully dark at this time of year, and you need full darkness. (You can find out the local time Astronomical Twilight ends – when it is fully dark – by going to this Web site. From the drop-down menu you’ll find there, choose “astronomical twilight.”) However, you can certainly start looking earlier. This is something where beach chairs or lounges are nice, and maybe even a blanket. You can start about an hour after sunset when the brightest stars are visible. This will help you get your bearings and you can dark adapt as the skies get darker.
Finally, you need to protect your eyes from white lights. It takes 10-15 minutes for your eyes to become about 50 percent dark adapted. At that point your color vision is as good as it will get, but your sensitivity to dim light will continue to increase. In another 15 minutes or so you will reach about 90 percent dark adaption. The remaining 10 percent can take as long as four hours. So I consider that after half an hour my eyes are about as good as I can expect them to be. During all this time and beyond you should avoid looking at white light. You can use a red light to check a chart if you like, but keep it dim and use it sparingly. If you’re in a location where automobiles drive by, don’t look at them – close your eyes and turn away.
Where to look
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. You may want to favor the east just a bit.
What you want to find as you start out is the familiar guidepost stars of the Summer Triangle – Vega, Deneb, and Altair. These were new guidepost stars in May, June, and July. If you are just starting this journey in August,they are still easy to pick out from our chart. As the sky in the east starts to darken they will be the first stars visible, 30-45 minutes after sunset.
Click image for a larger view. (Derived from a Starry Nights Pro screen shot.)
You can download a printer friendly version of this chart here.
The brightest – and highest – of the three will be Vega, which will be approaching a point overhead. There are roughly two fists (24 degrees) between Vega and Deneb and nearly four fists (39 degrees) between Deneb and Altair, so the Triangle is huge.
These three Summer Triangle stars roughly bracket the Milky Way – that is Vega is near the western border, Altair 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. 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.
As the skies darken and your eyes continue to dark adapt, you should try to find three distinctive asterisms that will anchor both ends of the Milky Way, plus the middle. If you have found Deneb, then you have the first star in the Northern Cross. In fact, you may want to see this as a stick figure of the constellation Cygnus the Swan. In that case, Deneb marks its tail; the bar of the cross, its wings, and its long neck stretch out to the south as if it were flying down the Milky Way. To the north you should locate the “W” of Cassiopeia described in detail in our “Look North” post this month. And to the south, find the “Teapot,” which we described in more detail last month. Here’s a chart showing the whole sweep of that section of sky.
Click image for larger view. (derived from Starry Nights Pro screens hot.)
You can download a printer friendly version of this chart here.
Now, if it is about two hours 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! 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.
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:
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.
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 they 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 traveled thousands of years to reach your eyes and ping your 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.
Filed under: 1. Month-by-month, h. August | Tagged: Cassiopeia, Cygnus, Light Pollution, Milky Way, Northern Cross, our galaxy, Teapot | Leave a comment »