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  • Rapt in Awe

    My Journey through the Astronomical Year

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

Look East in June 2014 and see if you can make the stars “pop!”

How can we make the stars pop out of the sky and into our mind’s eye?

That’s the perennial problem for me, for what we actually see is so much less than what is actually there that we can’t help but belittle the stars unintentionally. This month’s guide star, Deneb, is a prime example. It’s easy to spot using our chart as it rises in the northeast below and to the left of Vega. In terms of our bright guide star list, Deneb’s rather dim – 19th in the list of brightest stars we see with the naked eye. But that reveals much more about our point of view than about Deneb. Deneb, plain and simple, is one of the most luminous stars in our galaxy. Vega, just above and to the right of it in the northeast, looks so much brighter – but it isn’t. It’s simply so much closer. Vega is just 25 light years away.

Deneb, by one the most recent calculations, is 1,425 light years from us. (This is still open to debate and some put it nearly twice that far away!) But we’ll use the 1,425 light year figure. Put Deneb in Vega’s place – just 25 light years away –  and it would be visible in broad daylight! Does that help it “pop?”

When astronomers talk about how “luminous” a star is they don’t mean how bright it appears to us in our night sky. They mean how bright it actually is. In fact, frequently they use “luminous” to include all the radiation that comes from a star – even radiation in wavelengths that we don’t see, such as infrared and ultraviolet. They then compare a star’s luminosity with the luminosity of the Sun – the Sun being “1.” When they examine Deneb that way they get a luminosity of 54,400 Suns – awesome! (Popping yet? Can you imagine our Sun being twice as brght as it is? three times a bright? How about 54,400 times brighter?)  But when we look at Deneb we see a star that is just moderately bright – magnitude 1.25.

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

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

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

OK – let’s get serious about this popping business. When you look at Deneb, you have to use your mind’s eye to see it for what it really is, not just for what it appears to be. So what should we see when we look at Deneb? First we should see something huge. Deneb is classed as a “supergiant.” So sit back and try to imagine a star whose diameter is 108 times that of the Sun. No, wait! First imagine how big the Earth is. Then get in your mind the fact that the Sun is 109 times the diameter of the Earth. Got that? Now try to imagine that Deneb is to our Sun what our Sun is to the Earth. birdshotBut wait! I really do not want to talk about diameters. Those are for people who live in a flat world. Think in terms of volume, because that’s what a planet or star really is – a volume – a mass formed into a sphere. To get your mind around volume, picture the earth as a tiny bird shot just 2.5mm in diameter. Here’s one to give you the idea. Now picture a sphere about 10.5 inches in diameter – a basketball would be close, or this glass garden globe. See the difference? When talking diameters, the Sun is 109 Earths. But when you’re talking volume, you could fit well over a million Earths inside the Sun. sun_deneb Now think about the same thing in terms of Deneb. That little lead shot is our magnificent Sun. The blue globe is Deneb! That’s what you should see in your mind’s eye when you watch this month’s guide star rise in the northeast. Were Deneb our Sun, its surface would reach halfway to the orbit of Earth and needless to say, Earth would be in a hopelessly hot location. But there’s more, of course. Size is a great starting point, but it doesn’t equate with mass. A lot of stars are bloated – that is, their mass is spread out over a large area and they have a huge surface area from which to radiate a tremendous amount of energy. That is the case with Deneb. It is believed to be about 10-15 solar masses, but its total luminosity – the total amount of energy it radiates when compared to the Sun is a whopping 54,400 times that of the Sun! Wrap your mind’s eye around that! That’s why astronomer/author James Kaler writes that Deneb is

among the intrinsically brightest stars of its kind (that is, in its temperature or spectral class) in the Galaxy. If placed at the distance of Vega, Deneb would shine at magnitude – 7.8, 15 times more brightly than Venus at her best, be as bright as a well-developed crescent Moon, cast shadows on the ground, and easily be visible in broad daylight.

Deneb is unusual for supergiant stars for it is of spectral Class A – that means it’s your basic white star and very hot as stars go. Other very large stars, such as Betelgeuse, are in a different stage of development and quite cool and red to the eye. Deneb is believed to be just 10 million years old. That’s very young in terms of star ages. Our Sun is believed to be 5 billion years old. Deneb will never get to that ripe old age. Massive stars such as Deneb live in the fast lane, burning up their core hydrogen fuel relatively quickly. Kaler gives this analysis:

The star is evolving and has stopped fusing hydrogen in its core. However, it’s hard to know just what is going on. It might be expanding and cooling with a dead helium core and on its way to becoming a red supergiant, or it might have advanced to the state of core helium fusion. Having begun its life as a hot class B (or even class O) star of 15 to 16 solar masses just over 10 million years ago, its fate is almost certainly to explode sometime astronomically soon as a grand supernova.

Kaler certainly knows what he’s talking about, but don’t bother to keep a “death watch” on Deneb. “Astronomically soon” means some time in the next 100 million years or so 😉 Sherlock Holmes once chided his companion Watson saying “you see, but you do not observe.” With the stars, we have to take our cue from Holmes. We have to go beyond merely seeing. And in truth, we have to go beyond merely observing. We have to take the knowledge the scientists have given us and somehow apply it to what we see, so with our mind’s eye we truly observe. Only then can we pop Deneb out of that “twinkle, twinkle little star” category and see it for what it really is.

Vital stats for Deneb (DEN-ebb), also known as Alpha Cygni:

• Brilliance: Magnitude 1.24; its luminosity is the equal of 54,400 Suns. • Distance: 1,425 light years • Spectral Types: A2 supergiant • Position: 20h:41m:26s, +45°:16′:49″

Guide star reminder

Each month you’re encouraged to learn the new “guide” stars rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months. Deneb and the Northern Cross join several other guide stars and asterisms in the June sky. Again, if you have been reading these Posts for several months, be sure to find the stars, asterisms, and planets you found in earlier months. Early on a June evening these will include, from east to west, the following: Deneb, Vega, Arcturus, Spica, Saturn, Leo’s Rump (triangle),  the Sickle, Regulus, the Beehive, and in the northwest getting near the horizon, Pollux and Castor. You may also see Capella very near the horizon. For more experienced observers looking to extend their knowledge of the skies this month, I highly recommend trying to track down two more asterism – the Northern Crown and the KeystoneOK, technically the Northern Crown (Corona Borealis) is a constellation. But I always apply the name to the handful of moderately bright stars that look like a half circle – a crown. As the chart below shows, these two asterisms are located on a line between Arcturus and Vega and they sort of divide that line into thirds. As with our guide stars and other asterisms, they will help you if you advance to finding other more interesting objects int he night sky with binoculars and telescope.

keystone-crown

For a printer-friendly version of this chart, click here. The Crown itself can provide you with an interesting test of how dark your skies are since a couple hours after sunset on a June night it is well up in your eastern sky. It consists of a circlet of seven stars which can just fit within the field of view of wide-field binoculars – the example below shows an eight degree circle. It may be helpful to look at these stars with your binoculars, even if they don’t all fit in the same field of view at once. But to test how dark your skies are – and how transparent they are at the moment – wait until your vision is dark adapted, then see how many of these stars you can see. The numbers beside the stars are the magnitudes in decimals as given by Starry Nights software. However, I’ve followed the convention of not using a decimal point, since it might be mistaken for another faint star. So “41” means magnitude 4.1, for example. If you are seeing all seven stars you can be happy with your skies and these light-polluted times. In a truly dark location, however, this will be easy – but sadly such locations are rare these nights.

Read text above for explanation of how to use. Thenc lick on image to give you a larger view and luse the link below to download a printer friendly version. (Made from Starry Nights screen shot.)

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

Look east in August 2013 – kick back, lie back, look up and enjoy our home galaxy!

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. (OK – actually most of southern New England!)

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 –  all of which give increasingly good views of the Milky Way – 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 2013 your best views during that time period (two hours after sunset) will come between August 1st and 12th and again after August 26th  – on other dates in August the Moon is more likely to interfere. Of course, when the Moon is young and waxing you can always wait for it to set – and when it is old and waning  you need to make sure it hasn’t risen yet. For a Moon calendar for any month go here.  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:

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 pin wheel 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 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.

Look East in July 2012 – Great Stars, Great Asterisms – even a Great Constellation!

Well, a “great constellation” if you look southeast. I’m not a big fan of constellations. Most don’t look anything like their names imply; some are quite obscure; and many simply can’t be seen in typical suburban skies these days. Scorpius is an exception. It looks like the Scorpion of its name – a truly beautiful constellation with its graceful, curving tail. What’s more, many of its brighter stars actually do hang out together – they are not just an accident of our line of sight.

The Scorpion as Bayer saw him in his 1603 illustrated star atlas, Uranometria. Click for a much larger image. (Used by permission from the Linda Hall Library of Science, Engineering & Technology.)

It dominates our southeastern sky in July, just as the Summer Triangle – a terrific asterism, dominates our eastern sky this month. And we have two fascinating new “guide” stars – the intriguingly close and rapidly spinning Altair – and the incredibly huge and red Antares that is right at the heart of the Scorpion! Let’s take a look at the chart first, then examine these stars along with their quaint little companion, a very real looking teapot complete with “steam” coming out of its spout! Wow! Summer nights may be short, but they sure offer some nice visual treats!

Oh – about that “teapot.” We won’t discuss it, but you can clearly see it tagging behind the scorpion. If you have real clear skies, the Milky Way is beautiful in this area and looks like steam rising from the teapot. More on this next month. Meanwhile, click image for a larger version. (Developed from a Starry Nights Pro screen shot. )

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

First up is the Summer Triangle – it’s an asterism that you can’t miss, and it will grace our evening skies right up into early winter. If you’ve been following for a few months, you’ve already met its lead star, brilliant Vega. And last month we were introduced to Deneb on the other corner. In fact, we saw that we could make a quite impressive Northern Triangle out of Deneb, Vega, and Polaris. But better known than that asterism is the Summer Triangle shown above of Vega, Deneb, and Altair.

Altair is hard to miss. It is the brightest star low in the east early on a July evening, but it is also distinctive because it has two reasonably bright companions, close on either side, that form a straight line with it. This is appropriate because it’s not hard to see Altair and those two companions as representing an eagle in flight, and that’s good because they are the major stars in a constellation known as Aquilla, the Eagle.

Altair is white, much like Deneb and Vega, and is even closer to us than Vega. Vega is 25 light years away, Altair just 16. That’s in contrast to Deneb, which you may recall is an astounding 1,425 light years from us – astounding because even at that distance it is almost as bright as its much closer companions. Altair also distinguishes itself by spinning incredibly fast. It takes our Sun almost a month to complete a rotation on its axis. Altair, almost twice as large as our Sun, spins once on its axis in just 10 hours. Why, I don’t know, but it’s one more reminder of how these stars, which all look pretty much the same to us because they’re so far away, all have their special traits that distinguish them as individuals.

The most obvious special trait for Antares, our other new guide star this month, is its redness – and it’s one of only four guide stars that is quite close to the ecliptic – the path of the planets. That means that reddish Mars comes close, sometimes, to reddish Antares, and that’s appropriate because the name “Antares” actually means “like Mars.” However, science tells us something else about Antares. It is huge. I mean BIG.

Get out your calculator and do a little simple math. (OK, I’ll do the math, but really – this is simple, and I think you would appreciate the numbers much more if you did the calculations yourself.) One possible source of confusion:  To visualize a sphere I use its diameter. To actually calculate things I need the radius – since a radius is half of a diameter  you’ll find me jumping back and forth between these two terms – don’t let it confuse you.)

So try this. Start with something manageable, like the Earth. It’s about 8,000 miles in diameter and that’s a number that’s fairly easy to imagine. Let’s reduce Earth to a ball 2 inches in diameter. It would have a radius, then, of one inch.

Now let’s make a scale model Sun to go with our Earth. That’s easy. The radius of the Sun is 109 times the radius of the Earth. That means the Sun will have a radius of 109 inches – roughly 9 feet. So now we have a one-inch Earth and a 9-foot Sun. So our scale model has two balls – one two inches in diameter to represent the Earth and one 18 feet in diameter to represent the Sun.

That certainly should tell you that the Sun is a lot bigger than Earth, but my problem is, these linear measures don’t give us a really good sense of the size difference. We need to visualize spheres in terms of volume. We can get a rough approximation of the  volume  of a sphere by simply cubing the radius and multiplying it by 4. If we do this for our scale model Earth we have (1 x 1 x 1) x 4 – or four cubic inches. Now to calculate the volume of our scale model Sun – in cubic inches – we multiply 109 x 109 x 109, then multiply that by 4. Wow! Well, if you tried it on your calculator I hope you said “Wow!” You should get 5,180,116. That means you can fit well over one million Earths in our Sun! That to me is a lot more impressive than the linear measure where we find the diameter of the Sun is about 109 times the diameter of Earth.

Now let’s do a similar exercise with Antares. Antares has a radius more than 800 times the Sun. Do the math. Our scale model Sun has a radius of  9 feet – our scale model Antares will have a radius in feet of 9 x 800. Man, that’s big. About 7,200 feet!  (Just remind yourself that a mile is 5,280-feet.)  So now we have three models – a 2-inch diameter Earth, an 18-foot diameter Sun, and a 14,400-foot diameter Antares – that last is approaching three miles!

Don’t bother to calculate the volume. Unless you use scientific notation, your calculator probably won’t handle it. But you get the idea. That little dot of red light we call Antares is B-I-G. And don’t forget – on this same scale the huge planet you are standing on is just 2-inches in diameter.

Here’s a graphic representation courtesy of Sakurambo:

Notice the artist didn’t even attempt to represent the Earth on this scale!

Think of it this way. If Antares were our star, both the Earth and Mars would be orbiting inside it!

That’s huge – even bigger than Deneb – which we noted last month was a “supergiant” – the same class that Antares belongs in. But Deneb would only reach about halfway to Earth – Antares would go past both Earth and Mars. Deneb, however, is a very young, very bright, very hot star, which is why it shines so brightly from such a great distance. Antares is much closer – about 600 light years vs 1,425 for Deneb. But Antares is old – a star in its dying stages, and is large and bright because it is so bloated. It really is quite cool as stars go – that’s why it appears red to us. But it has such a huge surface area that even from a distance of 600 light years it appears bright to us – a bit brighter in our sky than Deneb, actually.

So let’s briefly consider these four guide stars together – Vega is our “standard” star – white, about the size of the Sun, and quite close at 25 light years. Altair has some unusual features, but is still rather normal as stars go. Deneb is distinguished by being large and hot; Antares by being even larger, but relatively cool.

Vital stats for Altair (AL-tair), also known as Alpha Aquilae:

• Brilliance: Magnitude .77; its luminosity is the equal of 11 Suns.
• Distance:16.8 light years
• Spectral Types: A, main sequence
• Position: 19h:50m:47s, +08°:52′:06″

Vital stats for Antares (an-TAIR-ease), also known as Alpha Scorpii:

• Brilliance: Magnitude 1.09; its luminosity is the equal of 65,000 Suns.
• Distance: 600 light years
• Spectral Types: M, supergiant
• Position: 16h:29m:24s, -26°:25′:55″

Look East in June 2012 and see if you can make the stars “pop!”

How can we make the stars pop out of the sky and into our mind’s eye? That’s the perennial problem for me, for what we actually see is so much less than what is actually there that we can’t help but to either ignore or belittle the stars unintentionally.

This month’s guide star, Deneb, is a prime example. It’s easy to spot using our chart as it rises in the northeast below and to the left of Vega. In terms of our bright guide star list, Deneb’s rather dim – 19th in the list of brightest stars we see with the naked eye. But that reveals much more about our point of view than about Deneb.

Deneb, plain and simple, is one of the most luminous stars in our galaxy. Vega, just above and to the right of it in the northeast, looks so much brighter – but it isn’t. It’s simply so much closer. Vega is just 25 light years away. Deneb, by the most recent calculations, is 1,425 light years from us. Put Deneb in Vega’s place and it would be visible in broad daylight! Does that help it “pop?”

When astronomers talk about how “luminous” a star is they don’t mean how bright it appears to us in our night sky. They mean how bright it actually is. In fact, frequently they use “luminous” to include all the radiation that comes from a star – even radiation in wavelengths that we don’t even see, such as infrared and ultraviolet. They then compare a star’s luminosity with the luminosity of the Sun – the Sun being “1.” When they examine Deneb that way they get a luminosity of 54,400 Suns – awesome! (Popping yet? Can you imagine our Sun being 54,400 times brighter?)  But when we look at Deneb we see a star that is just moderately bright – magnitude 1.25.

Click image for a larger version. The full “Summer Triangle” asterism will be easier to see as the month goes on, and the key stars get higher earlier in the evening. The key focus this month is the guide star Deneb.

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

OK – let’s get serious about this popping business. When you look at Deneb, you have to use your mind’s eye to see it for what it really is, not just for what it appears to be. So what should we see when we look at Deneb?

First we should see something huge. Deneb is classed as a “supergiant.” So sit back and try to imagine a star whose diameter is 108 times that of the Sun. No, wait! First imagine how big the Earth is. Then get in your mind the fact that the Sun is 109 times the diameter of the Earth. Got that? Now try to imagine that Deneb is to our Sun what our Sun is to the Earth.

birdshotBut wait! I really do not want to talk about diameters. Those are for people who live in a flat world. Think in terms of volume, because that’s what a planet or star really is – a volume – a mass formed into a sphere. To get your mind around volume, picture the earth as a tiny bird shot just 2.5mm in diameter. Here’s one to give you the idea.

Now picture a sphere about 10.5 inches in diameter – a basketball would be close, or this glass garden globe. See the difference? When talking diameters, the Sun is 109 Earths. But when you’re talking volume, you could fit well over a million Earths inside the Sun.

sun_deneb

Now think about the same thing in terms of Deneb. That little lead shot is our magnificent Sun. The blue globe is Deneb! That’s what you should see in your mind’s eye when you watch this month’s guide star rise in the northeast. Were Deneb our Sun, its surface would reach halfway to the orbit of Earth and needless to say, Earth would be in a hopelessly hot location.

But there’s more, of course. Size is a great starting point, but it doesn’t equate with mass. A lot of stars are bloated – that is, their mass is spread out over a large area and they have a huge surface area from which to radiate a tremendous amount of energy. That is the case with Deneb. It is believed to be about 10-15 solar masses, but its total luminosity – the total amount of energy it radiates when compared to the Sun is a whopping 54,400 times that of the Sun! Wrap your mind’s eye around that!

That’s why astronomer/author James Kaler writes that Deneb is

among the intrinsically brightest stars of its kind (that is, in its temperature or spectral class) in the Galaxy. If placed at the distance of Vega, Deneb would shine at magnitude – 7.8, 15 times more brightly than Venus at her best, be as bright as a well-developed crescent Moon, cast shadows on the ground, and easily be visible in broad daylight.

Deneb is unusual for supergiant stars for it is of spectral Class A – that means it’s your basic white star and very hot as stars go. Other very large stars, such as Betelgeuse, are in a different stage of development and quite cool and red to the eye. Deneb is believed to be just 10 million years old. That’s very young in terms of star ages. Our Sun is believed to be 5 billion years old. Deneb will never get to that ripe old age. Massive stars such as Deneb live in the fast lane, burning up their core hydrogen fuel relatively quickly.

Kaler gives this analysis:

The star is evolving and has stopped fusing hydrogen in its core. However, it’s hard to know just what is going on. It might be expanding and cooling with a dead helium core and on its way to becoming a red supergiant, or it might have advanced to the state of core helium fusion. Having begun its life as a hot class B (or even class O) star of 15 to 16 solar masses just over 10 million years ago, its fate is almost certainly to explode sometime astronomically soon as a grand supernova.

Kaler certainly knows what he’s talking about, but don’t bother to keep a “death watch” on Deneb. “Astronomically soon” means some time in the next 100 million years or so 😉

Sherlock Holmes once chided his companion Watson saying “you see, but you do not observe.” With the stars, we have to take our cue from Holmes. We have to go beyond merely seeing. And in truth, we have to go beyond merely observing. We have to take the knowledge the scientists have given us and somehow apply it to what we see, so with our mind’s eye we truly observe. Only then can we pop Deneb out of that “twinkle, twinkle little star” category and see it for what it really is.

Vital stats for Deneb (DEN-ebb), also known as Alpha Cygni:

• Brilliance: Magnitude 1.24; its luminosity is the equal of 54,400 Suns.
• Distance: 1,425 light years
• Spectral Types: A2 supergiant
• Position: 20h:41m:26s, +45°:16′:49″

Guide star reminder

Each month you’re encouraged to learn the new “guide” stars rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months.

Deneb and the Northern Cross join several other guide stars and asterisms in the June sky. Again, if you have been reading these Posts for several months, be sure to find the stars, asterisms, and planets you found in earlier months. Early on a June evening these will include, from east to west, the following: Deneb, Vega, Arcturus, Spica, Saturn, Leo’s Rump (triangle),  the Sickle, Regulus, the Beehive, and in the northwest getting near the horizon, Pollux and Castor. You may also see Capella very near the horizon.

For more experienced observers looking to extend their knowledge of the skies this month, I highly recommend trying to track down two more asterism – the Northern Crown and the KeystoneOK, technically the Northern Crown (Corona Borealis) is a constellation. But I always apply the name to the handful of moderately bright stars that look like a half circle – a crown. As the chart below shows, these two asterisms are located on a line between Arcturus and Vega and they sort of divide that line into thirds. As with our guide stars and other asterisms, they will help you if you advance to finding other more interesting objects int he night sky with binoculars and telescope.

keystone-crown

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

The Crown itself can provide you with an interesting test of how dark your skies are since a couple hours after sunset on a June night it is well up in your eastern sky. It consists of a circlet of seven stars which can just fit within the field of view of wide-field binoculars – the example below shows an eight degree circle. It may be helpful to look at these stars with your binoculars, even if they don’t all fit in the same field of view at once. But to test how dark your skies are – and how transparent they are at the moment – wait until your vision is dark adapted, then see how many of these stars you can see. The numbers beside the stars are the magnitudes in decimals as given by Starry Nights software. However, I’ve followed the convention of not using a decimal point, since it might be mistaken for another faint star. So “41” means magnitude 4.1, for example. If you are seeing all seven stars you can be happy with your skies and these light-polluted times. In a truly dark location, however, this will be easy – but sadly such locations are rare these nights.

Read text above for explanation of how to use. Thenc lick on image to give you a larger view and luse the link below to download a printer friendly version. (Made from Starry Nights screen shot.)

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

Look East in July 2011 – Great Stars, Great Asterisms – even a Great Constellation!

Well, a “great constellation” if you look southeast. I’m not a big fan of constellations. Most don’t look anything like their names imply; some are quite obscure; and many simply can’t be seen in typical suburban skies these days. Scorpius is an exception. It looks like the Scorpion of its name – a truly beautiful constellation. What’s more, many of its brighter stars actually do hang out together – they are not just an accident of our line of sight.

The Scorpion as Bayer saw him in his 1603 illustrated star atlas, Uranometria. Click for a much larger image. (Used by permission from the Linda Hall Library of Science, Engineering & Technology.)

It dominates our southeastern sky in July, just as the Summer Triangle – a terrific asterism, dominates our eastern sky this month. And we have two fascinating new “guide” stars – the intriguingly close and rapidly spinning Altair – and the incredibly huge and red Antares that is right at the heart of the Scorpion! Let’s take a look at the chart first, then examine these stars along with their quaint little companion, a very real looking teapot complete with “steam” coming out of its spout! Wow! Summer nights may be short, but they sure offer some nice visual treats!

Oh - about that "teapot." We won't discuss it, but you can clearly see it tagging behind the scorpion. If you have real clear skies, the Milky Way is beautiful in this area and looks like steam rising from the teapot. More on this next month. Meanwhile, click image for a larger version. (Developed from a Starry Nights Pro screen shot. )

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

First up is the Summer Triangle – it’s an asterism that you can’t miss, and it will grace our evening skies right up into early winter. If you’ve been following for a few months, you’ve already met its lead star, brilliant Vega. And last month we were introduced to Deneb on the other corner. In fact, we saw that we could make a quite impressive Northern Triangle out of Deneb, Vega, and Polaris. But better known than that asterism is the Summer Triangle shown above of Vega, Deneb, and Altair.

Altair is hard to miss. It is the brightest star low in the east early on a July evening, but it is also distinctive because it has two reasonably bright companions, close on either side, that form a straight line with it. This is appropriate because it’s not hard to see Altair and those two companions as representing an eagle in flight, and that’s good because they are the major stars in a constellation known as Aquilla, the Eagle.

Altair is white, much like Deneb and Vega, and is even closer to us than Vega. Vega is 25 light years away, Altair just 16. That’s in contrast to Deneb, which you may recall is an astounding 1,425 light years from us – astounding because even at that distance it is almost as bright as its much closer companions. Altair also distinguishes itself by spinning incredibly fast. It takes our Sun almost a month to complete a rotation on its axis. Altair, almost twice as large as our Sun, spins once on its axis in just 10 hours. Why, I don’t know, but it’s one more reminder of how these stars, which all look pretty much the same to us because they’re so far away, all have their special traits that distinguish them as individuals.

The most obvious special trait for Antares, our other guide star this month, is its redness – and it’s one of only four guide stars that is quite close to the ecliptic – the path of the planets. That means that reddish Mars comes close, sometimes, to reddish Antares, and that’s appropriate because the name “Antares” actually means “like Mars.” However, science tells us something else about Antares. It is huge. I mean BIG.

Get out your calculator and do a little simple math. (OK, I’ll do the math, but really – this is simple, and I think you would appreciate the numbers much more if you did the calculations yourself.) One possible source of confusion:  To visualize a sphere I use its diameter. To actually calculate things I need the radius – since a radius is half of a diameter  you’ll find me jumping back and forth between these two terms – don’t let it confuse you.)

So try this. Start with something manageable, like the Earth. It’s about 8,000 miles in diameter and that’s a number that’s fairly easy to imagine. Let’s reduce Earth to a ball 2 inches in diameter. It would have a radius, then, of one inch.

Now let’s make a scale model Sun to go with our Earth. That’s easy. The radius of the Sun is 109 times the radius of the Earth. That means the Sun will have a radius of 109 inches – roughly 9 feet. So now we have a one-inch Earth and a 9-foot Sun. So our scale model has two balls – one two inches in diameter to represent the Earth and one 18 feet in diameter to represent the Sun.

That certainly should tell you that the Sun is a lot bigger than Earth, but my problem is, these linear measures don’t give us a really good sense of the size difference. We need to visualize spheres in terms of volume. We can get a rough approximation of the  volume  of a sphere by simply cubing the radius and multiplying it by 4. If we do this for our scale model Earth we have (1 x 1 x 1) x 4 – or four cubic inches. Now to calculate the volume of our scale model Sun – in cubic inches – we multiply 109 x 109 x 109, then multiply that by 4. Wow! Well, if you tried it on your calculator I hope you said “Wow!” You should get 5,180,116. That means you can fit well over one million Earths in our Sun! That to me is a lot more impressive than the linear measure where we find the Sun is about 109 times the diameter (or 109 times the radius) of Earth.

Now let’s do a similar exercise with Antares. Antares has a radius more than 800 times the Sun. Do the math. Our scale model Sun has a radius of  9 feet – our scale model Antares will have a radius in feet of 9 x 800. Man, that’s big. About 7,200 feet!  (Just remind yourself that a mile is 5,280-feet.)  So now we have three models – a 2-inch diameter Earth, an 18-foot diameter Sun, and a 14,400-foot diameter Antares – that last is approaching three miles!

Don’t bother to calculate the volume. Unless you use scientific notation, your calculator probably won’t handle it. But you get the idea. That little dot of red light we call Antares is B-I-G. And don’t forget – on this same scale the huge planet you are standing on is just 2-inches in diameter.

Here’s a graphic representation courtesy of Sakurambo:

Notice the artist didn’t even attempt to represent the Earth on this scale!

Think of it this way. If Antares were our star, both the Earth and Mars would be orbiting inside it!

That’s huge – even bigger than Deneb – which we noted last month was a “supergiant” – the same class that Antares belongs in. But Deneb would only reach about halfway to Earth – Antares would go past both Earth and Mars. Deneb, however, is a very young, very bright, very hot star, which is why it shines so brightly from such a great distance. Antares is much closer – about 600 light years vs 1,425 for Deneb. But Antares is old – a star in its dying stages, and is large and bright because it is so bloated. It really is quite cool as stars go – that’s why it appears red to us. But it has such a huge surface area that even from a distance of 600 light years it appears bright to us – a bit brighter in our sky than Deneb, actually.

So let’s briefly consider these four guide stars together – Vega is our “standard” star – white, about the size of the Sun, and quite close at 25 light years. Altair has some unusual features, but is still rather normal as stars go. Deneb is distinguished by being large and hot; Antares by being even larger, but relatively cool.

Vital stats for Altair (AL-tair), also known as Alpha Aquilae:

• Brilliance: Magnitude .77; its luminosity is the equal of 11 Suns.
• Distance:16.8 light years
• Spectral Types: A, main sequence
• Position: 19h:50m:47s, +08°:52′:06″

Vital stats for Antares (an-TAIR-ease), also known as Alpha Scorpii:

• Brilliance: Magnitude 1.09; its luminosity is the equal of 65,000 Suns.
• Distance: 600 light years
• Spectral Types: M, supergiant
• Position: 16h:29m:24s, -26°:25′:55″

July’s Guidepost Stars: Look east – look south!

July offers an eyeful, and if you are just starting to learn the sky this month, you have four new guidepost stars to meet. However, those who were out in June already met Vega and perhaps the rest of the Summer Triangle – if not, by all means get out about 45 minutes after sunset on a July evening and look to the east. In the twilight you should see something like the simulation below with Vega the highest and brightest of the trio. Altair is next in brightest and the most southerly. Deneb is the least bright of the three and most northerly. This triangle is huge – roughly 20 degrees (two fists) in height and 40 degrees (four fists) in width. And you’ll have plenty of time to get familiar with it, for it’s prominent in northern latitudes right into winter.

 

Summer triangle in twilight, as depicted in Starry Nights software. Be sure to click for larger image.

Summer triangle in twilight, as depicted in Starry Nights software. Be sure to click for larger image.

 

 

But once you feel confident you have this triangle identified, face South and look for our other July guidepost star, Antares. The name “Antares” can be translated as “rival of Mars” and is an indication of the star’s redness. If this isn’t immediately apparent to you, compare it to blue/white Vega. Or, if you have been studying the sky for several months, compare it to the icy blue of Spica, now in the southwest sky. (Remember – you reach Spica by following the arc of the Big Dipper’s handle first to Arcturus, then continuing on to Spica. 

 

Stellarium's depiction of the twilight sky looking due south in July from mid-northern latitudes. Antares is the bright star to the left (east) and Spica the other one. Click for a much larger image.

Stellarium's depiction of the twilight sky looking due south in July from mid-northern latitudes. Antares is the bright star to the left (east) and Spica the other one. Click for a much larger image.

 

 

The image above shows how the twilight sky to the south looks in early July. The lower star on the left is Antares, the one to the right is Spica. There’s about 45 degrees between these two, so as it gets fully dark you’ll notice Antares is almost due South. In northern latitudes Antares is quite low – here in Westport, MA. it is only 22 degrees above the southern horizon when it transits (about 10 pm in early July) – just half as high as Polaris which, of course, anchors the northern sky. (No – Antares is not the southern pole star. It is a long way from it. In fact, there is no bright star marking the southern celestial pole, but if there were you would have to be in the southern hemisphere to see it. Antares is less than one third of the way to the South Celestial Pole. )

As the sky gets fully dark on a July evening, Antares will be due South and be the core of one of the constellations that really does resemble it’s name – Scorpius, the scorpion.

Antares at the heart of Scorpius.

Antares at the heart of Scorpius.

The stick figure charted at right doesn’t really do justice to the graceful curves of its body and tail. Sadly, for me this constellation is largely hidden by trees. My best view of Scorpius comes when I look to the south over the bay or ocean, though I can see it early on a July evening if I step out into the road in front of my house, for the road heads south and at that time Scorpius dominates my southern sky from the horizon to about one third of the way up.

Vega brings us the Summer Triangle – and continues to do so right into winter!

Vega is the guidepost star for June, heralding the rising of the Summer Triangle which stays with us right until winter! Look for it in the eastern sky about 45 minutes after sunset. (Click for much larger image.)

Vega is the guidepost star for June, heralding the rising of the Summer Triangle which stays with us right until winter! Look for it in the eastern sky about 45 minutes after sunset. (Click for much larger image.)

In these June nights when it isn’t really dark untila fter 10 pm  – at least for those of us on Daylight Savings Time – the Big Dipper pointing  to Arcturus and Spica are still high inthe sky, but brilliant Vega – magnitude zero – is well up in the east and bringing with it the other two stars of the Summer Triangle.  Deneb and Altair.  We’ll focus on those two next month. This month it’s enough to remind yourself of where to find Arcturus and Spica, then move on to Vega. And while you’re at it, see if you can notice the color difference between Arcturus and Vega – but wait until Vega is high enough so it now longer twinkles and changes color in the thick atmosphere near the horizon.

We call these three the “summer” triangle, but the truth is, they dominate our sky for a full six motnhs. You can see them in the east  on the night of the Summer Solstice – June 21 – and in the West around the Winter Solstice near Christmas. In fact, they’re stillt here a month later, but by then Altair is starting to get lost in the twilight, though Vega is still high enough in the northwest to see easily.

Add a couple of asterisms!

While I feel the guidepost stars are the most important to learn, if you really want to find your way about the night sky it’s also helpful to learn some key asterisms.  For June there are two to add to your memoru banks – the Keystone of Hercules and  the half circle of stars thata re the core of Coronoa Borealis – the Northern Crown.

Once the sky has really darkened on a June evening, look for the Keystone and the Crown on a line drawn between the two, bright guideposts stars of Arcturus and Vega.  (Click image for much larger chart.)

Once the sky has really darkened on a June evening, look for the Keystone and the Crown on a line drawn between the two, bright guideposts stars of Arcturus and Vega. (Click image for much larger chart.)

Once the sky has really darkened – between 10 and 10:30 on June evenings in Westport, MA  – draw an imaginary line between Vega and Arcturus.

Now look for our two helpful asterisms along this line.

The Keystone of Hercules is made up of four stars of magnitude 3-4 that form a perfect keystone, its narrow end to the south. (The star in the southeast corner is faintest – magnitude 4. The star in the southwest corner is the brightest.)

The second asterism to seek out, the Northern Crown, has one really bright star of second magnitude. Its other stars are magnitude four and five, so your eyes must be dark adapted to see this. If you live in suburban, light-polluted skies, try this. Find its brightest star, then with binoculars look first for the arc of three stars trailing off to the east, then the arc of two stars to the west of this dominant star.

Note: The typical binocular field will capture just half of this asterism at a time.

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