Look East in July! 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!

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 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 put the answers at the bottom of this post, but really – this is simple, and I think you would appreciate the size much more if you did the calculations yourself rather than have me tell 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. That gives it a radius of 1 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. My problem is, these linear measures don’t give us a good sense of the size difference. We need to visualize in terms of volume. We can approximate volume 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 calculate the volume of our scale Sun – in cubic inches, Just multiply 109 x 109 x 109, then multiply that by 4. Wow! Well, I hope you said “Wow!”

The Sun is 109 times the diameter of the Earth – which to me doesn’t sound like much, but in volume it is – well, you’ve done the calculation. (Or look for the answer at the bottom of the post if you haven’t.)

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 is 9 feet – our scale model Antares will have a diameter in feet of 9 x 800. Man, that’s big. (Just remind yourself that a mile is 5,280-feet.) 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 big. And don’t forget – on this same scale the huge planet you are standing on is just 2-inches in diameter. Compare that to your calculation regarding Antares!

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″

Calculator answers:

1.Now calculate the volume of our scale Sun – in cubic inches, Just multiply 109 x 109 x 109, then multiply that by 4. Wow! Well, I hope you said “wow!” The answer is 5,180,116 cubic inches – compared to 4 cubic inches for the Earth,

2. To really drive the difference home, divide 5,180,116 by 4 – that gives 1,295,029. So when seen as a volume, you could fit almost 1.3 million Earths inside the Sun.

3. Our scale model Sun is 9 feet – our scale model Antares will have a diameter in feet of 9 x 800 – 7.200 feet or 1.36 miles – so you’re comparing 9 feet with 1.36 miles – and that’s simply the linear diameter. In volume, Antares would be as much larger than the Sun, as the sun is to the Earth – roughly 1.3 million times the volume. That’s a lot of space to be occupied by one of nature’s nuclear reactors.

July 2010 Events – a really cool gathering of planets – where, when, and why

This is how the western sky should look on July 27, 2010, about forty minutes after sunset, from mid-northern latitudes. Venus should be easy. Use binoculars to help spot Mars and Mercury. On this date Mercury will make a neat double "star" with Regulus - the planet being brighter by about a full magnitude. But don't wait until July 27th - these planets put on a good show all month long! (Click image for larger view. Prepared from Starry Nights screen shot.)

• How to find the planets
• Planets in early July
• Planets in the middle of July
• Planets at the end of the month
• July calendar

This bright grouping of planets will make a great display in our western sky near the end of the month – but don’t wait until then to look for them. The fun is seeing them majestically draw together and that’s an all-month event! What’s more, it should give you an excellent sense of the plane of our solar system! I love events like this because they’re accessible to everyone, and if you approach them correctly you can almost hear the proverbial music of the spheres as gravity does its magic and the sky presents us with a changing tableau in four-part harmony. In July that tableau will include close encounters between planets and a bright star, as well as other planets and the Moon. Essentially, this is a show to enjoy any evening this month by stepping out about 45 minutes after sunset and looking west. By the end of the month the planets will be so low, however, you’ll need an unobstructed western horizon and clear skies to pick them out, even though they are all reasonably bright. Binoculars will help!

Unless the Moon happens to be in that section of sky – as it will in mid-month, the first thing that should catch your eye is brilliant Venus. Nothing outshines it except the Moon and Sun, and it should come into view half an hour or so after sunset. At this point sit back in your lawn chair and enjoy. As the sky continues to darken, Saturn will probably be the next to pop into view, followed by Mars. (Scanning for it between Venus and Saturn with binoculars will help.) At the beginning of the month all three are in a line that stretches over almost 40 degrees (four fists) of sky. All month these three will be drawing closer to one another until near the end of the month they are less than 10 degrees apart – a gathering so tight you could hold it in the bowl of the Big Dipper.

Enter Mercury

But wait – there’s more. In the last half of the month you should also scan near the horizon for Mercury. It will be quite bright – magnitude 0 – but despite its brightness may be difficult to see because it is so low and getting lost in the strong twilight. As twilight deepens it will be easier to see, but it will also be getting lower – so consider finding it a challenge. (This time around southern hemisphere observers have a better angle on Mercury and should find the fleet-footed planet easier to find. Just identifying these “wandering stars” is an accomplishment, but what makes the experience richer is to be able to envision why we are seeing them this way. It helps to picture the solar system as a disc, with all the planets on roughly the same plane. The line you see them in slanting towards the horizon is that plane. But when you take a look at the solar system from above it all becomes clearer. So in the charts below I’ve combined the sky view with an inset that shows you a view of the bright planets as seen from an imaginary vantage point above our solar system. (These inset charts are derived from online Orrery at Solar System Live, a web site I urge you to visit.) If you can see the relationship between the two charts – the one showing what we see and the other why we see it this way – try to carry that mental image out with you as you look at the real thing. Imagine where the Sun is and each planet – including Earth – are. In this way you can take your observing to a new level. OK – one essential – after reading the caption for each chart, click on the image and get a much larger view. It’s next to impossible to tell much from these small images.

Early July - Start your search about 45 minutes after sunset. The first planet you'll pick up is Venus, at magnitude -4 and about a fist and a half above the western horizon. Saturn should be the next easiest at magnitude 1.1 and about twice as high as Venus. Mars is the dimmest of the three, but you should be able to see it. Binoculars will help. Once you locate Venus just sweep up and to the south. Halfway between Venus and Mars you should see the guidepost star, Regulus, almost exactly as bright as Mars, but appearing a tad dimmer because the twilight there will be stronger. Be sure to click on the above image to see a much larger chart. Note in the inset how Venus, Mars, and Saturn are in a straight line, but Mercury is still quite far away. Jupiter is a late night/early morning event this month, rising in the east after midnight at the start of the month, but about two and a half hours after sunset by the end of the month. It shows in the Orrery view but is not part of the western sky show. (Chart prepared from Starry Nights Pro Screen shot - inset from the Solar System Live.)

Mid July - Again, start your search about 45 minutes after sunset. The first planet you'll pick up is Venus, at magnitude -4. Saturn should be the next easiest and with the crescent Moon and Venus should form a shallow triangle. Mars should be nearest the Moon. Binoculars will help. Be sure to click on the above image to see a much larger chart. Note in the inset how Mercury is getting closer to Venus, but it is still very difficult to see in the evening sky. However, it will get easier day by day as we move towards the end of the month. (Chart prepared from Starry Nights Pro screen shot - inset from the Solar System Live.)

Late July - Again, start your search about 45 minutes after sunset. The first planet you'll pick up is Venus, at magnitude -4. Look down and to the right for Mercury. At this stage binoculars will be a big help. Saturn and Mars are now close together and both have drawn closer to Venus. The inset shows the relationship from a perspective above the Solar System. Notice that speedy Mercury - with its smaller orbit - is now very close to Venus. Be sure to click on the above image to see a much larger chart. (Chart prepared from Starry Nights Pro screen shot - inset from the Solar System Live.)

I think that checking these planets any evening this month will be fun, but there are some special dates when they have close encounters with bright stars that are particularly interesting. You’ll find these in the chronological event summary that follows.

July 4 – Last quarter Moon

July 6 – Earth at greatest distance from Sun – 94,508,000 miles – feel any cooler? (No, distance has nothing to do with seasons 😉

July 8 – Do the Pleiades look small to you? Go out about 3:30 in the morning and look at the crescent Moon low in the east. That’s the Pleiades just above it and the two should show really nicely in the same binocular field. Which is larger? The little star cluster, or the Moon?

July 9, 10 – Ever wonder what a difference of about five magnitudes amounts to? Take a look at Venus about an hour or so after sunset tonight. It has a close encounter with Regulus. Venus is -4, Regulus 1.3 – they’ll both fit easily in the same binocular field.

July 11 – New Moon

July 15 – Crescent Moon forms a nice triangle with Saturn and Venus, while being nearest to Mars.

July 18 – First quarter Moon

July 26 – Full Moon

July 27 – Mercury squeaks by Regulus – less than half a degree separates them – making a nice binocular “double star.”

July 29, 30 – Mars and Saturn should make a real nice site in binoculars. Being so close to the horizon both may show a lot of false color, but do you notice the color difference of the two planets?

Look north in July and take the measure of your skies

Light pollution is a big issue these days. How does it impact you? Summer is a good time to check by looking north about two hours after sunset and seeing what stars you can see in and near the Little Dipper. Why summer? Because this is when the Little Dipper should be highest in your sky – standing upwards from Polaris, the North Star. Here’s what you should see on a typical July evening when you look north from mid-northern latitudes.

In Summer the faint stars of the Little Dipper are high above the North Star. Click image for larger view. (Derived from Starry Nights Pro screen shot.)

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

The Big Dipper is diving downward in the northwest but is still very high, and its handy “pointers” should get you quickly to the North Star, Polaris. Roughly opposite the Big Dipper you should see the “W” of Cassiopeia starting to make its way upward in the northeast. And unless you suffer from really terrible light pollution, you should see the two “Guardians of the Pole” – the second and third magnitude stars that mark the end of the Little Dipper. The brighter of these two is just a tad dimmer than Polaris, but since it’s higher in the sky right now and thus shining through less air to get to you, it will probably look just the same as the North Star in brightness.

To do this test you first have to wait until it is genuinely dark, and in summer that’s a bit longer than in winter. Twilight actually is divided into three steps. We have civil twilight which goes from sunset until when the sun is six degrees below the horizon. Nautical twilight is the next period, which continues until the Sun is 12 degrees below the horizon. Then you have Astronomical Twilight until the Sun is 18 degrees below the horizon. At that point it is as dark as it will get and will remain that dark until we run the sequence in reverse as the eastern horizon nears the Sun. As a general rule of thumb, you can consider each twilight period to last half an hour – but the exact length depends on where you are on Earth and the time of year. If you want to get precise, go to the U.S. Naval Observatory site, fill in the form you’ll find there, and you can get a table that will give you the start and end of these twilight times – or for that matter when the Moon rises, or the Sun sets. It’s very handy. (Note: the preceding link takes you to a page for US cities and towns – but there’s a second page here where you can put in the latitude and longitude for any location in the world, including in the US. )

The second thing you need to do is make sure your eyes are dark adapted. They are casually dark adapted after you have been out for 15 minutes and have not looked at any white lights. But it can take from half an hour to an hour of protecting your eyes from any white light, for them to become fully dark adapted. That doesn’t mean you have to sit around in the dark doing nothing waiting for this to happen. In the last hour or so before full darkness there are plenty of things to see – just avoid bright lights. That also means moonlight. You’re going to want to do this when the Moon is not in the sky, for it will make it difficult to see faint objects anywhere near it. In July of 2010 that means that a good time to do this test is between July 2 and July 13. (That’s why the table from the Naval Observatory for local Moon rise is also handy!)

So here’s the test:

How many stars can you see in the Little Dipper?

The Little Dipper consists of seven stars. Three are easy – Polaris and the two “Guardians” marked “21” and “30” on the chart below. If you can see only one of the “Guardians,” then your skies are limited to magnitude 2 stars and brighter – very poor. If you see both, but no other stars in the Little Dipper, then your limit is magnitude 3.

On our chart below, the magnitude of each star is listed as a whole number so as not to put decimal points on the chart that might then be confused for faint stars! So when you see a star listed as “21” that means “magnitude 2.1.”

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

Even in good, dark skies the other four stars in the Little Dipper may not be that easy to see – and the faintest ones may require averted vision – that is, don’t look exactly where the star should be. Instead, look a little to one side or the other and the star may pop into view. That’s because the center of your eyes are not as sensitive to faint light as the outer regions of your eyes.

Here’s another little trick that may help you locate these faint stars – use binoculars. With typical, hand-held binoculars you may be able to fit all four stars of the Little Dipper’s “cup” into the same field of view. If not, get the “Guardians” in your field of view, then move just a little to where the other two stars of the “cup” should be. This does not count, of course, for the light pollution test. For that test we’re trying to determine the faintest star you can see with the naked eye. But looking first at the stars with binoculars helps assure you that they really are there! You also can trace out the handle this way, though you will have to move your binoculars to do so.

If you can locate all the stars in the Little Dipper with your naked eye, you have very dark skies – congratulations. To see how good they are – and continue to test your eyesight and dark adaption – look for the stars marked “55” and “60” on our chart.

The star marked “60” is traditionally thought of as the faintest you can see with your naked eye. That’s a magnitude six star. In really pristine skies, such as those over Mauna Kea in Hawaii, experienced observers with excellent eyes can detect stars down to magnitude 8 with the naked eye. Personally, I’m happy when I can see all the stars in the Little Dipper and especially happy if I can get that “55” star – I’ve never seen the “60” one with my naked eye. But relative to the heavily light-polluted eastern seaboard of the US, I have dark skies.

This is not simply a good guide to light pollution in your area. It also is a handy guide to tell you just how good the skies are on any given night – and to show you how well you have dark adapted at any given moment. So whenever I go out to observe I frequently glance at the Little Dipper to test both my developing night vision and the clarity of the skies. (It never fails to amaze me how much and how quickly my night vision changes. )

To the casual observer all clear nights are equal. But the experienced star gazer knows they are not, and the stars in and about the Little Dipper are a good guide, especially in the summer months when they are so high in the sky.