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    My Journey through the Astronomical Year

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

Look North: May is the month the North Star gets two bright flankers!

Click for larger image. (Developed from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Is the North Star – Polaris – our brightest star? No! And it certainly won’t look that way this month as it shares the northern sky with two very bright stars. But, read on. Polaris is not nearly as dim as it looks!

If you have been learning your guidepost stars as they rise in the East, you won’t be surprised by the two bright stars which flank – and outshine – our pole star in May. To the northwest is Capella, a star we first met when it rose in the northeast in November. In May the northeast is dominated by a star that is almost Capella’s twin in brightness, Vega, a guidepost star we introduce in May. (See “Look East!” for more about Vega.) As a bonus we also have the twin guidepost stars, Castor and Pollux, making their way into the northern sky high above Capella. But let’s focus on Capella and Vega.

New star watchers frequently assume the North Star, Polaris, will be the brightest star in the sky. It isn’t even close! It is bright, but its fame comes because it’s very, very close to where the axis of the Earth points to the north celestial pole. So it serves anyone trying to find true north as a very good guide. But when it comes to brightness, it’s in the same league as the stars in the Big Dipper. Quite bright, but it can’t hold a candle to Capella and Vega. When you look at a list of the brightest stars, Vega is number 5 and Capella number 6. Polaris, our North Star, is number 48!

As simple as one, two, three!

That doesn’t mean Polaris is a slouch, though. First, in the eastern sky in May you meet Spica. (That’s on our chart for the east.) One distinction of Spica is that it’s as close to being magnitude 1 as any star gets. A distinction of Polaris is, as Spica defines magnitude 1, Polaris defines magnitude 2. (To be precise it’s magnitude 2.02.) Vega and Capella are extremely close to magnitude 0. Vega is 0.03 and Capella 0.08. Good luck on telling the difference! This month, if you look north 90 minutes after sunset, you may think Capella is a bit brighter actually – but if it appears that way it will be because it’s a bit higher in the sky and thus is not dimmed by having to fight its way through as much of our atmosphere as Vega is doing at the moment. So don’t try to split hairs. And yes, you’re right – they are NOT really as “simple as one, two, three” – on the magnitude scale they are as simple as zero, one, two – but that doesn’t sound as good! (Vega and Capella are zero; Spica is magnitude one, and Polaris, magnitude two.)

So which is really the brightest star of these four? Are you ready for this? Polaris! That’s right – if you put all four stars at the same distance, Polaris would appear to be the brightest. Remember, that the lower the magnitude number, the brighter the star. In absolute magnitude – the brightness we give to a star if they are all shining from the same distance  – these four stars line up this way:

  • Polaris -3.4
  • Spica -3.2
  • Capella 0.1
  • Vega 0.3

And those absolute magnitudes also reflect their order in distance from us.

  • Polaris 433 light years
  • Spica 250 light years
  • Capella 45 light years
  • Vega 25 light years

So sometimes a star is very bright because it’s – well, very bright. But sometimes it only appears to be very bright because it is very close to us. If you put our closest star into this group, our Sun – remember, it is just 8 light minutes from us – in absolute magnitude it would be by far the dimmest of this group – absolute magnitude 4.9! So while Polaris doesn’t look all that bright, it really is a very bright star! Another way to think about this is if you move our Sun out to where Polaris is, it would be about magnitude 10! You would need binoculars or a telescope to see it!

Click image for larger view of this chart. Yellow circle represents typical field of view for low power binoculars, such as 10X50.

To get an idea of the difference between Polaris and our Sun, point your binoculars towards Polaris.  You should be able to make out the “Engagement Ring” asterism – granted, a crude ring with Polaris as the diamond.  This asterism points you towards the true north celestial pole  – just avery short distance to the other side of Polaris –  and also gives you a good idea of about how far Polaris is from that pole.  Small binoculars will not show you the companion of Polaris, but to get an idea of how bright our Sun would be at the same distance, look for the star labelled 9.8 – and if you can’t see it, see if you can see the star that’s a bit brighter labelled “9.”  Don’t expect to see these instantly. Sit calmly, relax, and keep looking for at least a minute.

And here’s one more cool secret about Polaris. It has a companion that just happens to be quite dim – magnitude 9. It’s fun to see the two of them if you have a small telescope, though it’s not all that easy because Polaris is so much brighter than its companion. But if you get a chance to see Polaris and its companion in a telescope, remind yourself that the very faint companion is still a bit brighter than our Sun would look at this distance. This companion, known as Polaris B, was discovered in 1780 by William Herschel, and for many years Polaris was thought to be a binary star – that is, a system of two stars orbiting about a common center of gravity. But Polaris was holding one more surprise – it’s really a triple star.

The top image shows Polaris and its faint companion that can be seen in any decent backyard telescope. The bottom image shows the second companion, Polaris Ab, which has only been seen by using the Hubble Space Telescope.

This has been known for some time, but no one could see the third star until they turned the Hubble Space telescope on it in 2006. That’s when NASA released the first image of this third companion. The accompanying press release explained it this way:

By stretching the capabilities of NASA’s Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings  in a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C.

“The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,” said Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics). The companion proved to be less than two-tenths of an arc second from Polaris — an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away. At the system’s distance of 430 light years, that translates into a separation of about 2 billion miles.

“The brightness difference between the two stars made it even more difficult to resolve them,” stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. “With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight.”

So as I said, Polaris is no slouch. It not only is a very bright star, but it also has two companions, and scientists are still studying it because it is unusual in other respects. We’ll talk about those other differences another month.

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Look North in October 2013 – and find a really bright star!


Click image for larger view. (Prepared from Starry Nights Pro screen shot.)

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

When I say “really bright,” I mean Capella – and I say it because so many folks assume, quite understandably, that the North Star will be the brightest in that section of the sky. It isn’t.

Capella is one of our brightest stars, and you may catch it this month just peeping up over the northeastern horizon about an hour and a half after sunset. And it is brilliant!  When you see Capella,  compare it with the North Star  and you will get a good idea of what the word magnitude means. There are a solid two magnitudes difference between the two – or if you want to get technical, Capella is magnitude 0 and Polaris is magnitude 2.

Yes, the magnitude system goes backwards – the lower the number, the brighter the star. Each magnitude difference represents a change of about 2.5 times in brightness. So a zero magnitude star is 2.5 times as bright as a first magnitude star – and 2.5 x 2.5, or 6.25 times as bright as Polaris, a second magnitude star. And while we’re on this subject, the faintest star in the Little Dipper – it’s over in one corner of the cup – is just about magnitude five. That means Capella is 100 times as bright as that star. (OK – if you actually do the math it doesn’t come out because I rounded off the difference – it’s really 2.512 for those wanting more precision.)

In fact, there is another interesting way to look at that star – at magnitude 4.9 it is almost exactly what our Sun would look like if it were placed just 32.5 light years away. At that distance our sun would be magnitude 4.8 – and that distance is the distance we use to compare stars. That is, to get an “absolute magnitude” for them so we can compare apples with apples,  we ask ourselves how bright a star would be if it were 32.5 light years from us.

And while we’re on the subject of magnitude, the Little Dipper does give us a great range. Polaris, as we said, is magnitude 2. The other bright star in the Little Dipper is magnitude 2 also – it’s at the end of the cup away from Polaris. Sharing that end is a magnitude 3 star, and these two are known as the “Guardians of the Pole” as they are prominent, close, and like other stars, circle around the pole  every 24 hours. Most of the other stars in the Little Dipper are magnitude 4, with the one star in the corner of the cup being magnitude 5, as noted – well, actually 4.9. There’s a chart with these magnitudes in this post. Many people who live in light-polluted areas can see only three stars in the Little Dipper – Polaris and the two Guardians of the Pole.

How faint a star can you see? Depends upon your eye sight, the light pollution in your area, the transparency of the skies on any given night, how high the star is in the sky, and, of course, your vision and dark adaption. But as a general rule of thumb magnitude 6 has been accepted as the typical naked eye limit. However, if you live in a typically light polluted suburb you may see only to 4, or worse yet, magnitude 3. And observers on mountain tops with really clear skies reliably report seeing stars as faint as magnitude 8. All of this is with the naked eye. Add ordinary binoculars and you will certainly add three to five magnitudes to the faintest star you can see. In fact, you take a huge jump in the number of stars seen just by using binoculars with 50mm objective lenses. That’s why many amateur astronomers with fine telescopes also are likely to have a pair of 10X50 binoculars that they keep handy. I always did, but more recently I stumbled upon some very inexpensive Celestron 15X70 binoculars that I love for quick peaks, though they are two powerful and large to hold steady for serious observing.

Capella will be a “look east” guide star next month, but you can get to know it this month as you look north. And if you live in mid-northern latitudes, it will become a most familiar site. In fact, where I live – about latitude 42°N – it is in the night sky at some time every night of the year. This is because it is so far north that as it circles Polaris, it only dips below the horizon for a few hours at a time.

Look East! Drop off the slide to Spica and land on Saturn in May 2013!

If you followed “the arc” of the Big Dipper ‘s handle last month to find Arcturus, then you can do the same this month to find Spica – it’s like taking a long, cool slide from the Dipper – and if you hop off just before just before the end, this year you’ll land on Saturn!

Here’s how it looks – remember: look east  starting about an hour after Sunset.  But don’t wait too long – as the night goes on, everything will appear to rise and after a few hours this chart won’t be much help.

stuff

Start with the Big Dipper, high overhead tot he east. Following the arc of its handle, slide down to the brightest star int he east, Arcturus. Soften your slide and keep going and you’ll come to another bright star, Spica. Hopwever, if you hop off the slide just before getting to Spica, you’ll land on Saturn – which will be brighter than Spica, but not quite as bright as Arcturus.

Click here to download a printer-friendly version of this chart.

While the Dipper is easy to recognize, its stars are second magnitude – bright, but not the brightest. The brightest star in this section of the sky is Arcturus at magnitude zero. (Remember, the lower the magnitude number, the brighter the object.) The next brightest star you’ll see is over to your left, low in the northeast – Vega. In fact, Vega is magnitude  zero as well and the difference between it an Arcturus is next to impossible to detect with the eye. And this year we can say the same about Saturn  – it is over near  Spica and will outshine it by just a tad and be nearly as bright as Vega and Arcturus. All of these – Arcturus, Vega, Saturn, and Spica will be brighter than any of the Dipper stars.

As these stars get higher in the sky, you will notice that Spica is a rich blue, while Saturn has a yellow tint. About an hour after sunset Vega will be the lowest at about 20-degrees above the horizon – two fists held at arms length. Saturn will be about 23 degrees above the horizon, Spica about 30 degrees, Arcturus 47 degrees and Alkaid, the star at the end of the big Dipper’s handle, about 64 degrees.

We dealt with Arcturus last month. Saturn will be in our sky most of the night and as always is a treat for the small telescope user. From a naked eye perspective,  it’s fun to remember that the name “planet” means “wanderer” in Greek, but all “wanderers” are not created equal. Mars, Venus, and Mercury move  so quickly in our night sky that you can easily mark their changes over a period of a few days -certainly a week.  Saturn is much more sluggish.

Look at the chart and you’ll see how little Saturn changes position over the course of an entire year – it moves roughly 12 degrees.  To see this in the sky , find Saturn. Hold your fist at arms length so Saturn is just below it. Just above your fist is where Saturn was last year. Put Staurn on top of your fist and just below your fist is where it will be next year. So how long will it take Saturn to get around the sky to roughly the same position? Well, 360/12 = about 30 years!  Now if you think a moment, the Moon takes about 30 days to get around our sky – and that means the Moon moves each day about 12  degrees –  the same apparent distance covered by Saturn each year.  All of which should tell you that it would be reasonable to assume Saturn is much farther away from us than the Moon – which, of course, it is.

None of this is rocket science or in any way  profound, but I find it interesting to contemplate as I look up and see Saturn. I measure that distance it will travel in the next year and in my mind’s eye I perch above the Solar System and I see a long thin pie slice reaching from me to Saturn’s distance orbit and this helps me keep things in perspective – gives me a better intuitive feel for the neighborhood in which we live.  OK – for the record Saturn is moving at about 22,000 miles an hour, Mars about 54,000 miles an hour in a much shorter orbit, and we’re whipping right along close to 67,000 miles an hour – and we don’t even feel the wind in our face! Oh – and Saturn’s actual orbital period is 29.458 years.

On to this month’s new guidepost stars!

Vega and Spica are each fascinating stars, but let’s start with Vega. Shining brightly not far above the northeastern horizon as the evening begins, Vega comes about as close to defining the word “star” as you can get. In “The Hundred Greatest Stars” James Kaler calls it “the ultimate standard star” because its magnitude is about as close to zero as you can get (.03) and its color is about as close to white as you can get. (If you’re one of those who assumed all stars are white, you’re forgiven. Individuals vary in their ability to see different colors in stars and for everyone the color differences are subtle – in fact I think of them as tints rather than colors. )

It’s hard not to be attracted to Vega when you read Leslie Peltier’s wonderful autobiography, “Starlight Nights.” Vega was central to his astronomical observing throughout his career because he began with it when he first started reading the book from which I got the idea for this web site, “The Friendly Stars” by Martha Evans Martin. Peltier wrote:

According to the descriptive text Vega, at that very hour in the month of May, would be rising in the northeastern sky. I took the open book outside, walked around to the east side of the house, glanced once more at the diagram by the light that came through the east window of the kitchen, looked up towards the northeast and there, just above the plum tree blooming by the well, was Vega. And there she had been all the springtimes of my life, circling around the pole with her five attendant stars, fairly begging for attention, and I had never seen her.

Now I knew a star! It had been incredibly simple, and all the stars to follow were equally easy.

Vega went on to be the first target of the 2-inch telescope he bought with the $18 he made by raising and picking strawberries. (This was around 1915.) And Vega became the first target for every new telescope he owned until his death in 1980. If you still don’t know a star, go out and introduce yourself to Vega early on a May evening. Even without a plum tree to look over, you can’t miss her! And once you’ve done that you’re well on your way to making the night sky your own.  (And yes, Vega is the star from which the message comes in Carl Sagan’s book/movie “Contact.”)

Vital stats for Vega, also known as Alpha Lyrae:

• Brilliance: Magnitude .03 ; a standard among stars; total radiation is that of 54 Suns.
• Distance: 25 light years
• Spectral Type: A0 Dwarf
• Position: 18h:36m:56s, +38°:47′:01″

Spica, a really bright star – honest!

Spica is truly a very bright star, but the numbers you may read for its brightness can have you pulling your hair. That’s because there are at least four common ways to express the brightness of Spica and other stars, and writers don’t always tell you which way they’re using. So let’s look at these four ways and see what they mean for Spica.

The first is the most obvious. How bright does it look to you and me from our vantage point on Earth using our eyes alone? We then assign it a brightness using the magnitude system with the lower the number, the brighter star. (For full discussion of this system, see “How bright is that star?”)

By this measure Spica is 16th on the list of brightest stars and is about as close as you can come to being exactly magnitude 1. (Officially 1.04) Though I should add here that the number really marks the midpoint of a magnitude designation – that is, any star that is in the range of magnitude .5 to magnitude 1.5 is called “magnitude 1” and so on for the other numbers on the scale.

But that scale talks about what we see. It doesn’t account for distance. Obviously if you have two 60-watt light bulbs and one is shining 6 feet away from you and the other 1,000 feet away, they are not going to look the same brightness. But if we put them both at the same distance – say 100 feet – they would look the same. So it is with stars. To compare them we pretend they all were at the same distance – in this case 10 parsecs, which is about 32.6 light years. Put our Sun at that distance and it would be magnitude 4.83. (That’s about as faint as the fainest stars we see in the Little Dipper.) We call that its absolute magnitude.

The absolute magnitude for Spica is -3.55 – not quite as bright as dazzling Venus.

Wow! That’s pretty bright compared to our Sun! Yes it is. Sun 4.83; Spica -3.55. Don’t miss the “minus” sign in front of Spica’s number! That means there’s more than eight magnitudes difference between the Sun and Spica. And that relates to the next figure you are likely to see quoted. Something that is called its luminosity. Luminosity compares the brightness of a star to the brightness of our Sun. Unfortunately, the term is often misused – or poorly defined. Thus in the Wikipedia article I just read on Spica it said that “Spica has a luminosity about 2,300 times that of the Sun.” Yes, but what does that mean? It means that if we were to put the two side by side, Spica would appear to our eyes to be 2,300 times as bright as our Sun.

That is bright! But there’s more, much more. Spica is also a very hot star – in fact one of the brightest hot stars that we see with our naked eyes. But we miss most of that brightness because most of it is being radiated in forms of energy that our eyes don’t detect. In the case of Spica, that is largely ultraviolet energy. The Wikipedia article actually listed Spica’s luminosity twice, and the second time it gave it as “13,400/1,700.”

Oh boy – now we have Spica not 2,300 times as bright as the Sun, but more than 13,000 times as bright. Now that IS bright – but is it right? Yes! So why the difference? Again, the first “luminosity” given – 2,300 times that of the Sun – is measuring only what we can see with our eyes. The second is measuring total amount of electromagnetic radiation a star radiates and is properly called the “bolometric luminosity.” And why two numbers for that last figure? 13,400/1,700? Because while Spica looks like one star to us, it is really two stars that are very close together and one is much brighter than the other. So what we see as one star is really putting out energy in the neighborhood of 15,100 times as much as our Sun.

This can get confusing, so I suggest you remember three things about Spica.

1. It defines first magnitude, having a brightness as it appears to us of 0.98 – closer than any other star to magnitude 1.

2. It is really far brighter (magnitude -3.55), but appears dim because it is far away – about 250 light years by the most recent measurements.

3. It is very hot – appearing blue to our eyes – and because it is very hot it is actually radiating a lot more energy in wavelengths we don’t see, so it is far, far brighter than our Sun.

Spica is the brightest star in the constellation Virgo, one of those constellations where you can not really connect the dots and form a picture of a virgin unless you have an over abundance of imagination. Besides, the remaining stars are relatively faint. That’s why we focus on the bright stars and sometimes those simple patterns known as “asterisms” and use them as our guides.

Vital stats for Spica, also known as Alpha Virgo:

• Brilliance: Magnitude .98 ; as close to magnitude 1 as any star gets; a close double whose combined radiation is the equal of 15,100 Suns.
• Distance: 250 light years
• Spectral Types: B1,B4 Dwarfs
• Position: 13h:25m:12s, -11°:09′:41″

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” 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. If you have been reading these posts for several months, you may want to relate Spica to two earlier guidepost stars with which it forms a right triangle, Arcturus and Regulus. Here’s what that triangle looks like.

Click image for larger view. (Created, with modifications, from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Once you have identified the Right Triangle, note carefully the positions of Spica and Regulus. They pretty much mark the “ecliptic.” This is the path followed by the Sun. Also, within about 9 degrees north or south of it, you will find the planets and the Moon. That’s well illustrated in 2012 by the presence of both Saturn and Mars, very near the ecliptic, as noted on our chart.

Arcturus and Regulus are not the only guidepost stars and asterisms in the May sky. Again, if you have been reading these posts for several months, be sure to find the stars and asterisms you found in earlier months. Early on a May evening these will include, from east to west, the following: Arcturus, Spica, Saturn, Leo’s Rump (triangle), The Sickle,  Mars, Regulus, the Beehive, Procyon, Sirius, Pollux, Castor, and in the northwest near the horizon, Capella, and the Kite. Venus will be a bright evening “star” in the west, and if you look early in the month you may catch a glimpse of Sirius and Betelgeuse before they set.

Look North: May is the month Polaris ( the North Star) gets two bright flankers!

Click for larger image. (Developed from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Is the North Star – Polaris – our brightest star? No! And it certainly won’t look that way this month as it shares the northern sky with two very bright stars. But, read on. Polaris is not nearly as dim as it looks!

If you have been learning your guidepost stars as they rise in the East, you won’t be surprised by the two bright stars which flank – and outshine – our pole star in May. To the northwest is Capella, a star we first met when it rose in the northeast in November. In May the northeast is dominated by a star that is almost Capella’s twin in brightness, Vega, a guidepost star we introduce in May. (See “Look East!” for more about Vega.) As a bonus we also have the twin guidepost stars, Castor and Pollux, making their way into the northern sky high above Capella. But let’s focus on Capella and Vega.

New star watchers frequently assume the North Star, Polaris, will be the brightest star in the sky. It isn’t even close! It is bright, but its fame comes because it’s very, very close to where the axis of the Earth points to the north celestial pole. So it serves anyone trying to find true north as a very good guide. But when it comes to brightness, it’s in the same league as the stars in the Big Dipper. Quite bright, but it can’t hold a candle to Capella and Vega. When you look at a list of the brightest stars, Vega is number 5 and Capella number 6. Polaris, our North Star, is number 48!

As simple as one, two, three!

That doesn’t mean Polaris is a slouch, though. First, in the eastern sky in May you meet Spica. (That’s on our chart for the east.) One distinction of Spica is that it’s as close to being magnitude 1 as any star gets. A distinction of Polaris is, as Spica defines magnitude 1, Polaris defines magnitude 2. (To be precise it’s magnitude 2.02.) Vega and Capella are extremely close to magnitude 0. Vega is 0.03 and Capella 0.08. Good luck on telling the difference! This month, if you look north 90 minutes after sunset, you may think Capella is a bit brighter actually – but if it appears that way it will be because it’s a bit higher in the sky and thus is not dimmed by having to fight its way through as much of our atmosphere as Vega is doing at the moment. So don’t try to split hairs. And yes, you’re right – they are NOT really as “simple as one, two, three” – on the magnitude scale they are as simple as zero, one, two – but that doesn’t sound as good! (Vega and Capella are zero; Spica is magnitude one, and Polaris, magnitude two.)

So which is really the brightest star of these four? Are you ready for this? Polaris! That’s right – if you put all four stars at the same distance, Polaris would appear to be the brightest. Remember, that the lower the magnitude number, the brighter the star. In absolute magnitude – the brightness we give to a star if they are all shining fromt he same distance  -these four stars line up this way:

  • Polaris -3.4
  • Spica -3.2
  • Capella 0.1
  • Vega 0.3

And those absolute magnitudes also reflect their order in distance from us.

  • Polaris 433 light years
  • Spica 250 light years
  • Capella 45 light years
  • Vega 25 light years

So sometimes a star is very bright because it’s – well, very bright. But sometimes it only appears to be very bright because it is very close to us. If you put our closest star into this group, our Sun – remember, it is just 8 light minutes from us – in absolute magnitude it would be by far the dimmest of this group – absolute magnitude 4.9! So while Polaris doesn’t look all that bright, it really is a very bright star! Another way to think about this is if you move our Sun out to where Polaris is, it would be about magnitude 10! You would need binoculars or a telescope to see it!

Click image for larger view of this chart. Yellow circle represents typical field of view for low power binoculars, such as 10X50.

To get an idea of the difference between Polaris and our Sun, point your binoculars towards Polaris.  You should be able to make out the “Engagement Ring” asterism – granted, a crude ring with Polaris as the diamond.  This asterism points you towards the true north celestial pole  – just avery short distance to the other side of Polaris –  and also gives you a good idea of about how far Polaris is from that pole.  Small binoculars will not show you the companion of Polaris, but to get an idea of how bright our Sun would be at the same distance, look for the star labelled 9.8 – and if you can’t see it, see if you can see the star that’s a bit brighter labelled “9.”  Don’t expect to see these instantly. Sit calmly, relax, and keep looking for at least a minute.

And here’s one more cool secret about Polaris. It has a companion that just happens to be quite dim – magnitude 9. It’s fun to see the two of them if you have a small telescope, though it’s not all that easy because Polaris is so much brighter than its companion. But if you get a chance to see Polaris and its companion in a telescope, remind yourself that the very faint companion is still a bit brighter than our Sun would look at this distance. This companion, known as Polaris B, was discovered in 1780 by William Herschel, and for many years Polaris was thought to be a binary star – that is, a system of two stars orbiting about a common center of gravity. But Polaris was holding one more surprise – it’s really a triple star.

The top image shows Polaris and its faint companion that can be seen in any decent backyard telescope. The bottom image shows the second companion, Polaris Ab, which has only been seen by using the Hubble Space Telescope.

This has been known for some time, but no one could see the third star until they turned the Hubble Space telescope on it in 2006. That’s when NASA released the first image of this third companion. The accompanying press release explained it this way:

By stretching the capabilities of NASA’s Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings  in a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C.

“The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,” said Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics). The companion proved to be less than two-tenths of an arc second from Polaris — an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away. At the system’s distance of 430 light years, that translates into a separation of about 2 billion miles.

“The brightness difference between the two stars made it even more difficult to resolve them,” stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. “With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight.”

So as I said, Polaris is no slouch. It not only is a very bright star, but it also has two companions, and scientists are still studying it because it is unusual in other respects. We’ll talk about those other differences another month.

Look North in October 2012 – and find a really bright star!


Click image for larger view. (Prepared from Starry Nights Pro screen shot.)

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When I say “really bright,” I mean Capella – and I say it because so many folks assume, quite understandably, that the North Star will be the brightest in that section of the sky. It isn’t.

Capella is one of our brightest stars, and you may catch it this month just peeping up over the northeastern horizon about an hour and a half after sunset. And it is brilliant!  When you see Capella,  compare it with the North Star  and you will get a good idea of what the word magnitude means. There are a solid two magnitudes difference between the two – or if you want to get technical, Capella is magnitude 0 and Polaris is magnitude 2.

Yes, the magnitude system goes backwards – the lower the number, the brighter the star. Each magnitude difference represents a change of about 2.5 times in brightness. So a zero magnitude star is 2.5 times as bright as a first magnitude star – and 2.5 x 2.5, or 6.25 times as bright as Polaris, a second magnitude star. And while we’re on this subject, the faintest star in the Little Dipper – it’s over in one corner of the cup – is just about magnitude five. That means Capella is 100 times as bright as that star. (OK – if you actually do the math it doesn’t come out because I rounded off the difference – it’s really 2.512 for those wanting more precision.)

In fact, there another interesting way to look at that star – at magnitude 4.9 it is almost exactly what our Sun would look like if it were place just 32.5 light years away. At that distance our sun would be magnitude 4.8 – and that distance is the distance we use to compare stars. That is, to get an absolute magnitude for them so we can compare apples with apples,  we ask ourselves how bright a star would be if it were 32.5 light years from us.

And while we’re on the subject of magnitude, the Little Dipper does give us a great range. Polaris, as we said, is magnitude 2. The other bright star in the Little Dipper is magnitude 2 also – it’s at the end of the cup away from Polaris. Sharing that end is a magnitude 3 star, and these two are known as the “Guardians of the Pole” as they are prominent, close, and like other stars, circle around it every 24 hours. Most of the other stars in the Little Dipper are magnitude 4, with the one star in the corner of the cup being magnitude 5, as noted – well, actually 4.9. There’s a chart with these magnitudes in this post. Many people who live in light-polluted areas can see only three stars in the Little Dipper – Polaris and the two Guardians of the Pole.

How faint a star can you see? Depends upon your eye sight, the light pollution in your area, the transparency of the skies on any given night, how high the star is in the sky, and, of course, your vision and dark adaption. But as a general rule of thumb magnitude 6 has been accepted as the typical naked eye limit. However, if you live in a typically light polluted suburb you may see only to 4, or worse yet, magnitude 3. And observers on mountain tops with really clear skies reliably report seeing stars as faint as magnitude 8. All of this is with the naked eye. Add ordinary binoculars and you will certainly add three to five magnitudes to the faintest star you can see. In fact, you take a huge jump in the number of stars seen just by using binoculars with 50mm objective lenses. That’s why many amateur astronomers with fine telescopes also are likely to have a pair of 10X50 binoculars that they keep handy. I always did, but more recently I stumbled upon some very inexpensive Celestron 15X70 binoculars that I love for quick peaks, though they are two powerful and large to hold steady for serious observing.

Capella will be a “look east” guide star next month, but you can get to know it this month as you look north. And if you live in mid-northern latitudes, it will become a most familiar site. In fact, where I live – about latitude 42°N – it is in the night sky at some time every night of the year. This is because it is so far north and as it circles Polaris only dips below the horizon for a few hours at a time.

Sliding south to Spica!

Prime time in May finds the Big Dipper high in the north east. Take a slide down its gracefully curved handle, first to  our April guidepost star – Arcturus – and then on to Spica, our May guidepost star, which happens to be  as close to first magnitude as any star in our sky.

If you thought all our guidepost stars were first magnitude, think again. Technically, to be called “first magnitude” a star should be in the range of .5 to 1.5.  But to be a “guidepost” star we use a broader definition. The star must be very bright and help us anchor an important section of sky. So we call Polaris a “guidepost,” yet it is almost exactly magnitude 2. But being almost at the north celestial pole, it certainly is a useful “anchor” for us. And, of course, we call  Sirius a “guidepost” star and it is much brighter than first magnitude at -1.46. In fact, several of the guidepost stars are brighter than first magnitude, including Arcturus which is -.07. .Spica, though is right on target. It’s official magnitude is .96, so it’s as good a representative of a first magnitude star as any. Look at it – then look at Polaris. Spica is about 2.5 times as bright as Polaris – that’s how much difference there is between magnitudes. OK – a difference of one magnitude is actually 2.512. Why? read “Step 6 – How bright is that star?”   for a complete explanation.

A couple notes about the chart below. First, it is adapted from Starry Nights Pro software and when showing this much sky, the horizon is represented as curved upward. This may catch you unawares, for Spica is closer to the horizon than it may look at first glance. Arcturus is about 52 degrees above the eastern horizon while Spica is nearly 20 degrees lower in the southeast.

Second, Alkaid, the last star in the handle of the Dipper, is named because it too is a second magnitude star like Polaris, though Alkaid is a tad brighter than Polaris. Still, you get a good representation of the magnitude system by looking at these three stars – Alkaid, Arcturus, and Spica, since they are magnitude 2, magnitude 0, and magnitude 1 respectively. Once you have located Spica, study the three stars with that in mind – but at the same time, don’t forget that any star lower int he sky will look a bit dimmer than it should simply because you’re looking through a lot more atmosphere than you are when looking at a star that is high.

slide_to_spica

Start your slide on the handle of the Big Dipper, continue down and past Arcturus, and down more to Spica. (Click on image for larger version.)

Step 7: How bright is that star?

magnitude_scale

We’ve been rating stars by their brightness for at least 2,000 years and the operation is really quite simple, though the whole system may seem backwards to you. In any event, it’s worth learning because the different brightness of stars is one of the main ways we can tell one from the other and make some sense out of the night sky.

Stars are ranked in brightness by a numbering system called “magnitude” – the lower the magnitude number, the brighter the star. Generally, the brightest star we see is magnitude 0 and the dimmest we see is magnitude 6. If that feels backwards to you, think of it this way. Stars of magnitude 1 – very bright – are “first class” stars. Dimmer stars are magnitude 2 – “second class” stars, etc.

Simple – yes, but of course there are exceptions and several ifs, ands, and buts that make the whole business of brightness quite fascinating!

First, 10 stars ( if you include our Sun) are brighter than magnitude 1 – so they are magnitude zero and if brighter than that,  we go into a minus system. Magnitude -1 is brighter than 0, which is brighter than one.

The brightest star we see is our Sun. It is magnitude -26.7! The next brightest object is the full Moon, at magnitude -12. Then comes the planet Venus at -4.7 (sometimes) and Jupiter, usually about -2.3 or -2.4.  Other than the Sun, the  brightest star in our sky is Sirius, at magnitude -1.46. We don’t get to an actual  magnitude 1 star until Betelgeuse, which is magnitude .5.  Confused? Don’t be. We call Betelgeuse first magnitude because it falls in the range of .5 to 1.5. No star is exactly a “1” in brightness, though Spica comes very, very close at .96.

Of approximately 6,000 stars bright enough to be seen by the average person from some spot on Earth, only 22 are of first magnitude or brighter. Quite an exclusive club!  These include all the ones we learn as signpost stars and, of course, some of them are visible only from the Southern hemisphere.

While most books will tell you that the faintest star we see with the unaided eye is magnitude 6, this too is only an approximation and doesn’t really represent today’s  world that well.  Generally speaking, in our light-polluted regions, the faintest star we are likely to see is magnitude 4. If you live well away from city lights, you probably see to magnitude 5.   On mountain tops, well above the thicker parts of our atmosphere and free of light pollution, some observers with excellent eyes and skills can see stars as faint as magnitude 8.5 with the unaided eye. In my own area, which is semi-rural, I’m very happy when I can see a magnitude 5 star with my naked eye.

With binoculars or telescopes we see stars that are much fainter, but that’s another story. Here we’re focused on what we can see with no optical aid.

Many familiar star patterns are made up primarily of second magnitude stars, such as Orion’s Belt and most of the stars of the Big Dipper.  The North Star (Polaris) is second magnitude as well as one other star in this asterism. But the other five stars in the Little Dipper  are magnitudes 3, 4 and 5. This makes the Little Dipper a good object to use to get a sense of magnitude.

On a star chart it is common to represent magnitude by making circles of difference size to represent the star and its brightness. That really isn’t the same as brightness, but it does give us an idea of what to expect. So, for example, here’s how the Starry Nights Software represents the stars of the Little Dipper.

Study the four stars that make up the cup of the Little Dipper for agood idea of the magnitude scale. It includes magnitude 2,3,4, and 5 stars.

Study the four stars that make up the cup of the Little Dipper for agood idea of the magnitude scale. It includes magnitude 2,3,4, and 5 stars.

You’ll find that when you get to know stars of different brightness, the magnitude system makes general sense to our eyes. That is, there seems to be about the same gap in  brightness between a magnitude 1 and a magnitude 2 star as there is between a magnitude 2 and magnitude 3 star.

While the magnitude system has been around since the early Greeks, it was given a mathematical foundation about 150 years ago when it was decided that there would be a gap of 2.512 times between each level of brightness. That means a first magnitude star is 2.5 times as bright as a second one – and exactly 100 times as bright as a star at magnitude six – five magnitudes fainter.

At that time it was thought this logarithmic progression matched the way the human eye responded to brightness. It does so, but only approximately.  Still, the system is convenient, stars are routinely listed this way, and the system easy enough to become acquainted with while observing.

One last note – don’t expect to simply walk out of your nicely lit house and see faint stars. At first only the brightest will be visible to you. Your eyes need time to adjust to the dark. After about 15 minutes you’ll see much fainter stars and after about half an hour you can assume that the faintest star you see is the faintest you will see that night.

But also keep in mind that what you can see at any given moment varies in a number of ways, including:

  1. How good your eyes are at detecting faint objects.
  2. How clear your skies really are – astronomers quickly learn that all “clear” nights are not created equal!
  3. Your physical condition at the moment, including how tired you are and whether or not you have been recently drinking alcohol or smoking, both of which can impair night vision.
  4. The height of the star above the horizon – when you look at stars near the horizon, you are looking through a lot more air then when you are looking at stars directly overhead.
  5. Your experience in looking at faint objects   – yes, you can actually learn to see dim objects by looking slightly to one side of them, a technique called “averted vision” that takes practice to do well.

All of this helps make star-gazing more fun. Variety is, indeed, a tasty spice, and no two nights of observing are exactly the same.  Nor is it all as cut and dry as it seems.  An experienced, older observer can actually see more, in some instances, than a younger, inexperienced observer whose eyesight is technically better.

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