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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 East In January 2014 – King Jupiter, plus a trio of twins; Orion, man of the world; Betelgeuse, giant among giants!

January 2014 brings the usual host of bright and wondrous winter stars – and one star that isn’t  a star, but outshines them all in our skies – Jupiter. The “king of the planets” is absolutely dominant in the eastern sky this month, even though it gets a tad dimmer as the month goes on.  As we put some more distance between us and it, it drops from magnitude -2.7  at the start to -2.6 at the end of the month –  good luck on even being able to notice the change!  At magnitude 0 Capella is the brightest star we see, though in an hour or two, Sirius at Magnitude -1.5 will be up and come significantly closer to Jupiter’s brightness – but Jupiter will still dominate.

There are four new guidepost stars to meet this month and one new guidepost asterism, Orion. Orion is probably the best known figure in the heavens because it actually looks like a person and can be seen from most locations in the world since it’s centered on the celestial equator. That’s a lot for one month, but fun to think about on a dreary winter day and more fun to observe on a brilliant, winter evening. Here’s the chart for the eastern sky one hour after sunset for mid-northern latitudes. Remember, going out about 45 minutes to an hour  after sunset and looking east, you’ll see only the brightest stars as they come out. This makes it easier to identify and learn our guidepost stars. Our guidepost asterisms may not be as readily seen until a little later as the sky gets darker and more of the fainter stars come out.

The eastern sky as seen on a January evening about one hour after sunset. Click image for larger version. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

The eastern sky as seen on a January 2014 evening about one hour after sunset. Click image for larger version. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

Click here to download a black-on-white (printer-friendly) version of this chart

The January eastern sky – what to remember

 Castor – A trio of twins When you see Castor, think “twins” – a trio of twins. Well, in a sense there are really four pairs!

Castor is one of the Gemini Twins (Castor and Pollux), but in a small telescope we see it really is three stars, Castor A, Castor B, and Castor C - and though we can't see this in our telescopes, each of these stars is really a pair, making six stars in all!
When you spot Gemini in 2014 you will notice a brilliant Jupiter is right int he middle of it – this is shown on our “look east” chart at the beginning of this post. Click image for larger view.

But the fourth pair is just mythological – Castor is one of the “heavenly twins” of the constellation Gemini – the other twin being Pollux. This is nothing but a fanciful relationship, though, based on how the stars appear to us – and appeared to ancient cultures as well. But there is more, much more, to Castor. And, it’s what we don’t see that makes this bright star so fascinating. And seeing with your mind’s eye – your knowledge of what you are seeing – always enhances your experience under the night sky. So were you to look at Castor in a backyard telescope, you would see it has a twin – another bright star that appears quite close –  the two are known simply as Castor A and B. These two are related, orbiting one another about every 400 years.

But there’s more. Each of these two are twins! However, you can’t see this in a small telescope because in both cases the pairs of stars are extremely close to one another, orbiting one another in periods of less than 10 days. And as noted, each pair orbits the other pair in about 400 years.

But there’s more. Returning to that backyard telescope you may notice a third star, Castor C, quite a distance from the first two and significantly dimmer. This star is also part of the Castor family and it too has a twin that also is so close we can’t detect it without special instruments. In fact, Castor C consists of the closest pair of all, orbiting one another in less than a day! This pair, in turn, orbits the other four stars in the system once every 10,000 years or so. So when you look at Castor, remember that in classic mythology it has a twin, Pollux – and remember that what looks to you like a single bright star is really the combined light from six stars, all held together in one of the most complex star systems we know. (I wrote much more about the Castor system on the double-star blog. That post includes a scale model that puts Castor and company into perspective with the Earth and Sun. You’ll find it here. )

Vital stats (for just the brightest star in the Castor system):

  • Brilliance: Magnitude 1.58, the 23rd brightest star in our sky and the brightest second magnitude star. Absolute magnitude is 0.9. (Yes, we call a star “second magnitude” if it’s magnitude is between 1.5 and 2.5 – so you can see castor just slips into this category.)
  • Distance: 50 light years (not among the 200 nearest stars)
  • Spectral Type: A
  • Position: 07h:34m:36s, +31°:53′:18″
  • Compared to the Sun: Castor radiates 14 times as much energy as our Sun.

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Getting to know Pollux – the bigger, brighter twin

How Bayer saw the Gemini Twins in his 1603 atlas. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Pollux should feel a little cheated because it’s the brightest star in the constellation of Gemini and usually the brightest star was given the designation “alpha”  by the early chart maker, Bayer. Not Pollux. It is designated “Beta Geminorum” and follows its slightly dimmer twin brother around the sky. But Pollux has its own way of standing out: It has a slight edge in brilliance in our skyit is a tad closer to us; and it is an orange giant. What’s more, in 2008 it was confirmed to have a planet orbiting it. As an orange giant, it has moved off the “main sequence,” and instead of fusing hydrogen into helium, as our Sun does, it is fusing helium into carbon and oxygen. It will eventually blow off a lot of its substance becoming a planetary nebula. It is currently about eight times the diameter of our Sun – that’s huge, but nowhere near as large as our next star, Betelgeuse. The planet that is circling Pollux is also large – “Jupiter class” – and was first detected in 1993, but not confirmed until 2008.

Vital stats:

  • Brilliance: Magnitude 1.14, the 17th brightest star in our sky. Absolute magnitude is 0.7 .
  • Distance: 34 light years (not among the 200 nearest stars)
  • Spectral Type: K
  • Position: 07h:45m:19s, +28°:01′:35″

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 Orion – A man for all to see

If you’re in the same general latitude as I am in Westport, MA, then you see Orion like this as it rises in the east on a January evening.

Orion – as seen when rising in mid-northern latitudes. (Click for larger image.)

What always sticks with me about Orion is how Robert Frost described him in his wonderful poem, “The Star Splitter.”

‘You know Orion always comes up sideways. Throwing a leg up over our fence of mountains, And rising on his hands, he looks in on me . . .

But if I lived in Sydney, Australia, I wouldn’t see it this way. What I would see is a man standing on his head!

Orion, as seen when rising in the east from Sydney, Australia. (Click image for larger version.)

The real point here is that these stars do look like a man, and they can be seen from deep into both the southern and northern hemispheres. What’s more, the three distinctive stars that form Orion’s belt also mark the approximate position of the celestial equator in your sky, a handy thing to know. Of course, if you’re in the southern hemisphere, the celestial equator appears to make an arc across your sky to your north. In the northern hemisphere it appears to make an east-west arc across the sky to the south. But in either case the belt stars of Orion will rise just about due east and set due west. How high they get in your sky is calculated simply by subtracting your latitude from 90. That is, if your latitude is 42 degrees, as mine is, then Orion’s belt will be, at its highest, about 48 degrees above the horizon when it passes due south. From Sydney, Australia, the stars in the belt will cross about 56 degrees above the horizon as they pass due north. And yes, if you live on the equator these stars will cross directly over head. Anyway you look at it, Orion is a man for all latitudes – well, almost. At the north pole you would only see his top half, and at the south pole, only his feet! Return to Menu

Betelgeuse – giant among giants – and Rigel’s pretty large as well!

When you look at the eastern sky early on a January evening, get this picture in your head!

Here’s what our eastern sky would look like on a January evening if Adebaran and Rigel, two genuine giants, were as near to us as our Sun. The Sun, to scale, is also shown. Betelgeuse is NOT shown to scale.

Yes, that’s Rigel represented in the illustration, not Betelgeuse. Classified as a red supergiant, Betelgeuse is one of the largest stars you can see – and certainly up there with the biggest of all stars – yet it doesn’t look any bigger in our sky than other stars because all stars, except the Sun, are so far away they appear only as a point source of light to our eyes. Last month we showed what Aldebaran would look like if it were in our sky and the same distance from us as the Sun, and this month we’ve added Rigel to the picture. But we can’t do a similar thing with Betelgeuse – it wouldn’t be in our sky – it’s so large we would be in it if it were located where our Sun is! What’s more, it’s hard to put a number to the size of Betelgeuse, not because it can’t be measured, but because it’s hard to decide exactly what you want to measure when you’re dealing with a ball of gas – especially one like Betelgeuse. Our Sun is a little easier case. While it does not have a surface, it does appear to us to have an edge that’s fairly easy to define – it’s the place where its gases are dense enough to be opaque to our vision. Exactly how we define the size of Betelgeuse is a bit more difficult. I rely on James B. Kaler as my stellar authority. I love his books, and in one, “The Hundred Greatest Stars,” he describes the size of Betelgeuse variably as:

  • 650 times that of the Sun, or 2.8 AU (Astronomical Units – an Astronomical Unit is the distance between the Earth and the Sun – roughly 93 million miles)
  • 800 times the diameter of the Sun, or about 4 AU
  • 1600 times the Sun – about 8 AU when measured by modern observation in ultraviolet light

And on his Web site, after opting for a figure of around 8-9 AU, he writes:

However, the star is surrounded by a huge complex pattern of nested dust and gas shells, the result of aeons of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far having lost over a solar mass). That, an extended atmosphere, and the pulsations make it difficult to locate an actual “surface” to tell just how large the star actually is. Moreover, because of changes in gaseous transparency, the “size” of the star depends on the color of observation.

Betelgeuse has other problems. The pulsations he refers to are a sort of puffing up that occurs from time to time and changes both size and brightness significantly. Betelgeuse is usually thought of as about magnitude 0.55, but it can be as bright as 0.3, or as dim as 1.1. All this huffing and puffing will soon lead to an explosion, and Kaler says it will then be as bright as a crescent moon! But don’t hold your breath. “Soon” in astronomical terms means sometime in the next million years or so! Its distance, too, is uncertain, but 500 light years is a good ballpark figure. Let’s focus on that 8 AU size for a moment. When we build a scale model of our solar system and reduce the Sun to something about the size of a volleyball, the tiny speck of the Earth orbits at around 75 feet away. But at 8 AU Betelgeuse would be more like 600 feet in diameter. So pause for a moment as you look at Betelgeuse on a winter evening. Imagine yourself holding an 8-inch volleyball in one hand – our Sun – while you stand next to a red, raging, unstable monster ball that is 600 feet in diameter!

Vital stats:

  • Brilliance: Magnitude 0.3 – 1.1, the 10th brightest star in our sky (sometimes). Shines with the luminosity of about 90,000 Suns.
  • Distance: 570 light years
  • Spectral Type: M
  • Position: 05h:55m:10s, +7°:24′:25″

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Rigel – Blue and brilliant

Here we go again! Like Pollux, it looks like Rigel was short-changed having been designated the “Beta” star of the constellation Orion while dimmer Betelgeuse is the Alpha. Of course, Betelgeuse, being variable, may have been brighter when Johann Bayer made his designations in 1603. Bayer’s “system” is inconsistent, however, to say the least, so there’s no sense getting too worried about this. Like Betelgeuse, Rigel is a supergiant. It’s huge and it’s brilliant too – and since it is more distant (860 light years), it is intrinsically more brilliant than Betelgeuse. Jim Kaler writes: “Only about 10 million years old, Rigel should eventually expand to become a red supergiant very much like Betelgeuse is today, by which time it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova.” Rigel’s radius is 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury. Rigel is a challenging double star for amateurs with moderate-sized telescopes.

Vital stats:

  • Brilliance: Magnitude 0.12, the 7th brightest star in our sky. Shines with the luminosity of about 90,000 Suns.
  • Distance: 860 light years
  • Spectral Type: B
  • Position: 05h:55m:10s, +7°:24′:25″

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Look East In January 2013 – King Jupiter, plus a trio of twins; Orion, man of the world; Betelgeuse, giant among giants!

January 2013 brings the usual host of bright and wondrous winter stars – and one star that isn’t, but outshines them all – Jupiter. The “king of the planets” is absolutely dominant in the eastern sky this month, even though it gets a tad dimmer as the month goes on.  As we put some more distance between us and it, it drops from -2.7  at the start to -2.5 at the end of the month –  good luck on even being able to notice the change!  At magnitude 0 Capella is the brightest star we see, though in an hour or two, Sirius at Magnitude -1.5 will be up and come significantly closer to Jupiter’s brightness – but Jupiter will still dominate.

There are four new guidepost stars to meet this month and one new guidepost asterism, Orion. Orion is probably the best known figure in the heavens because it actually looks like a person and can be seen from most locations in the world since it’s centered on the celestial equator. That’s a lot for one month, but fun to think about on a dreary winter day and more fun to observe on a brilliant, winter evening. Here’s the chart for the eastern sky one hour after sunset for mid-northern latitudes. Remember, going out about 45 minutes to an hour  after sunset and looking east, you’ll see only the brightest stars as they come out. This makes it easier to identify and learn our guidepost stars. Our guidepost asterisms may not be as readily seen until a little later as the sky gets darker and more of the fainter stars come out.

lookeast
The eastern sky as seen on a January evening about one hour after sunset. Click image for larger version. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

Click here to download a black-on-white (printer-friendly) version of this chart

The January eastern sky – what to remember

 Castor – A trio of twins When you see Castor, think “twins” – a trio of twins. Well, in a sense there are really four pairs!

Castor is one of the Gemini Twins (Castor and Pollux), but in a small telescope we see it really is three stars, Castor A, Castor B, and Castor C - and though we can't see this in our telescopes, each of these stars is really a pair, making six stars in all!
Click image for larger view.

But the fourth pair is just mythological – Castor is one of the “heavenly twins” of the constellation Gemini – the other twin being Pollux. This is nothing but a fanciful relationship, though, based on how the stars appear to us – and appeared to ancient cultures as well. But there is more, much more, to Castor. And, it’s what we don’t see that makes this bright star so fascinating. And seeing withy our mind’s eye – your knowledge of what you are seeing – always enhances your experience under the night sky. So were you to look at Castor in a backyard telescope, you would see it has a twin – another bright star that appears quite close –  the two are known simply as Castor A and B. These two are related, orbiting one another about every 400 years. But there’s more. Each of these two are twins! However, you can’t see this in a telescope because in both cases the pairs of stars are extremely close to one another, orbiting one another in periods of less than 10 days. And as noted, each pair orbits the other pair in about 400 years. But there’s more. Returning to that backyard telescope you may notice a third star, Castor C, quite a distance from the first two and significantly dimmer. This star is also part of the Castor family and it too has a twin that also is so close we can’t detect it without special instruments. In fact, Castor C consists of the closest pair of all, orbiting one another in less than a day! This pair, in turn, orbits the other four stars in the system once every 10,000 years or so. So when you look at Castor, remember that in classic mythology it has a twin, Pollux – and remember that what looks to you like a single bright star is really the combined light from six stars, all held together in one of the most complex star systems we know. (I wrote much more about the Castor system on the double-star blog. That post includes a scale model that puts Castor and company into perspective with the Earth and Sun. You’ll find it here. )

Vital stats (for just the brightest star in the Castor system):

  • Brilliance: Magnitude 1.58, the 23rd brightest star in our sky and the brightest second magnitude star. Absolute magnitude is 0.9. (Yes, we call a star “second magnitude” if it’s magnitude is between 1.5 and 2.5 – so you can see castor just slips into this category.)
  • Distance: 50 light years (not among the 200 nearest stars)
  • Spectral Type: A
  • Position: 07h:34m:36s, +31°:53′:18″
  • Compared to the Sun: Castor radiates 14 times as much energy as our Sun.

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Getting to know Pollux – the bigger, brighter twin

How Bayer saw the Gemini Twins in his 1603 atlas. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Pollux should feel a little cheated because it’s the brightest star in the constellation of Gemini and usually the brightest star was given the designation “alpha”  by the early chart maker, Bayer. Not Pollux. It is designated “Beta Geminorum” and follows its slightly dimmer twin brother around the sky. But Pollux has its own way of standing out: It has a slight edge in brilliance in our skyit is a tad closer to us; and it is an orange giant. What’s more, in 2008 it was confirmed to have a planet orbiting it. As an orange giant, it has moved off the “main sequence,” and instead of fusing hydrogen into helium, as our Sun does, it is fusing helium into carbon and oxygen. It will eventually blow off a lot of its substance becoming a planetary nebula. It is currently about eight times the diameter of our Sun – that’s huge, but nowhere near as large as our next star, Betelgeuse. The planet circling Pollux is also large – “Jupiter class” – and was first detected in 1993, but not confirmed until 2008.

Vital stats:

  • Brilliance: Magnitude 1.14, the 17th brightest star in our sky. Absolute magnitude is 0.7 .
  • Distance: 34 light years (not among the 200 nearest stars)
  • Spectral Type: K
  • Position: 07h:45m:19s, +28°:01′:35″

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 Orion – A man for all to see

If you’re in the same general latitude as I am in Westport, MA, then you see Orion like this as it rises in the east on a January evening.

Orion – as seen when rising in mid-northern latitudes. (Click for larger image.)

What always sticks with me about Orion is how Robert Frost described him in his wonderful poem, “The Star Splitter.”

‘You know Orion always comes up sideways. Throwing a leg up over our fence of mountains, And rising on his hands, he looks in on me . . .

But if I lived in Sydney, Australia, I wouldn’t see it this way. What I would see is a man standing on his head!

Orion, as seen when rising in the east from Sydney, Australia. (Click image for larger version.)

The real point here is that these stars do look like a man, and they can be seen from deep into both the southern and northern hemispheres. What’s more, the three distinctive stars that form Orion’s belt also mark the approximate position of the celestial equator in your sky, a handy thing to know. Of course, if you’re in the southern hemisphere, the celestial equator appears to make an arc across your sky to your north. In the northern hemisphere it appears to make an east-west arc across the sky to the south. But in either case the belt stars of Orion will rise just about due east and set due west. How high they get in your sky is calculated simply by subtracting your latitude from 90. That is, if your latitude is 42 degrees, as mine is, then Orion’s belt will be, at its highest, about 48 degrees above the horizon when it passes due south. From Sydney, Australia, the stars in the belt will cross about 56 degrees above the horizon as they pass due north. And yes, if you live on the equator these stars will cross directly over head. Anyway you look at it, Orion is a man for all latitudes – well, almost. At the north pole you would only see his top half, and at the south pole, only his feet! Return to Menu

Betelgeuse – giant among giants – and Rigel’s pretty large as well!

When you look at the eastern sky early on a January evening, get this picture in your head!

Here’s what our eastern sky would look like on a January evening if Arcturus and Rigel, two genuine giants, were as near to us as our Sun. The Sun, to scale, is also shown. Betelgeuse is NOT show to scale.

Yes, that’s rigel represented inthe illustration, not Betelgeuse. Classified as a red supergiant, Betelgeuse is one of the largest stars you can see – and certainly up there with the biggest of all stars – yet it doesn’t look any bigger in our sky than other stars because all stars, except the Sun, are so far away they appear only as a point source of light to our eyes. Last month we showed what Aldebaran would look like if it were in our sky and the same distance from us as the Sun, and this month we’ve added Rigel to the picture. But we can’t do a similar thing with Betelgeuse – it wouldn’t be in our sky – it’s so large we would be in it if it were located where our Sun is! What’s more, it’s hard to put a number to the size of Betelgeuse, not because it can’t be measured, but because it’s hard to decide exactly what you want to measure when you’re dealing with a ball of gas – especially one like Betelgeuse. Our Sun is a little easier case. While it does not have a surface, it does appear to us to have an edge that’s fairly easy to define – it’s the place where its gases are dense enough to be opaque to our vision. Exactly how we define the size of Betelgeuse is a bit more difficult. I rely on James B. Kaler as my stellar authority. I love his books, and in one, “The Hundred Greatest Stars,” he describes the size of Betelgeuse variably as:

  • 650 times that of the Sun, or 2.8 AU (Astronomical Units – an Astronomical Unit is the distance between the Earth and the Sun – roughly 93 million miles)
  • 800 times the diameter of the Sun, or about 4 AU
  • 1600 times the Sun – about 8 AU when measured by modern observation in ultraviolet light

And on his Web site, after opting for a figure of around 8-9 AU, he writes:

However, the star is surrounded by a huge complex pattern of nested dust and gas shells, the result of aeons of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far having lost over a solar mass). That, an extended atmosphere, and the pulsations make it difficult to locate an actual “surface” to tell just how large the star actually is. Moreover, because of changes in gaseous transparency, the “size” of the star depends on the color of observation.

Betelgeuse has other problems. The pulsations he refers to are a sort of puffing up that occurs from time to time and changes both size and brightness significantly. Betelgeuse is usually thought of as about magnitude 0.55, but it can be as bright as 0.3, or as dim as 1.1. All this huffing and puffing will soon lead to an explosion, and Kaler says it will then be as bright as a crescent moon! But don’t hold your breath. “Soon” in astronomical terms means sometime in the next million years or so! Its distance, too, is uncertain, but 500 light years is a good ballpark figure. Let’s focus on that 8 AU size for a moment. When we build a scale model of our solar system and reduce the Sun to something about the size of a volleyball, the tiny speck of the Earth orbits at around 75 feet away. But at 8 AU Betelgeuse would be more like 600 feet in diameter. So pause for a moment as you look at Betelgeuse on a winter evening. Imagine yourself holding an 8-inch volleyball in one hand – our Sun – while you stand next to a red, raging, unstable monster ball that is 600 feet in diameter!

Vital stats:

  • Brilliance: Magnitude 0.3 – 1.1, the 10th brightest star in our sky (sometimes). Shines with the luminosity of about 90,000 Suns.
  • Distance: 570 light years
  • Spectral Type: M
  • Position: 05h:55m:10s, +7°:24′:25″

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Rigel – Blue and brilliant

Here we go again! Like Pollux, it looks like Rigel was short-changed having been designated the “Beta” star of the constellation Orion while dimmer Betelgeuse is the Alpha. Of course, Betelgeuse, being variable, may have been brighter when Johann Bayer made his designations in 1603. Bayer’s “system” is inconsistent, however, to say the least, so there’s no sense getting too worried about this. Like Betelgeuse, Rigel is a supergiant. It’s huge and it’s brilliant too – and since it is more distant (860 light years), it is intrinsically more brilliant than Betelgeuse. Jim Kaler writes: “Only about 10 million years old, Rigel should eventually expand to become a red supergiant very much like Betelgeuse is today, by which time it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova.” Rigel’s radius is 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury. Rigel is a challenging double star for amateurs with moderate-sized telescopes.

Vital stats:

  • Brilliance: Magnitude 0.12, the 7th brightest star in our sky. Shines with the luminosity of about 90,000 Suns.
  • Distance: 860 light years
  • Spectral Type: B
  • Position: 05h:55m:10s, +7°:24′:25″

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Look East In January 2012 – a trio of twins; Orion, man of the world; Betelgeuse, giant among giants!

January brings a host of bright and wondrous winter stars. There are four new guidepost stars to meet this month and one new guidepost asterism, Orion. Orion is probably the best known figure in the heavens because it actually looks like a person and can be seen from most locations in the world since it’s centered on the celestial equator. That’s a lot for one month, but fun to think about on a dreary winter day and more fun to observe on a brilliant, winter evening.

Here’s the chart for the eastern sky one hour after sunset for mid-northern latitudes. Remember, going out about 45 minutes after sunset and looking east, you’ll see only the brightest stars as they come out. This makes it easier to identify and learn our guidepost stars. Our guidepost asterisms may not be as readily seen until a little later as the sky gets darker and more of the fainter stars come out.

The eastern sky as seen on a January evening about one hour after sunset. Click image for larger version. Brilliant Jupiter is just off the chart, above and to the right, but still dominant in the early evening eastern sky in 2012. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

Click here to download a black-on-white (printer-friendly) version of this chart.

The January eastern sky – what to remember

 Castor – A trio of twins

When you see Castor, think “twins” – a trio of twins. Well, in a sense there are really four pairs!

Castor is one of the Gemini Twins (Castor and Pollux), but in a small telescope we see it really is three stars, Castor A, Castor B, and Castor C - and though we can't see this in our telescopes, each of these stars is really a pair, making six stars in all!
Click image for larger view.

But the fourth pair is just mythological – Castor is one of the “heavenly twins” of the constellation Gemini – the other twin being Pollux. This is nothing but a fanciful relationship, though, based on how the stars appear to us – and appeared to ancient cultures as well. But there is more, much more, to Castor. And, it’s what we don’t see that makes this bright star so fascinating. And seeing withy our mind’s eye – your knowledge of what you are seeing – always enhances your experience under the night sky.

So were you to look at Castor in a backyard telescope, you would see it has a twin – another bright star that appears quite close –  the two are known simply as Castor A and B. These two are related, orbiting one another about every 400 years. But there’s more. Each of these two are twins! However, you can’t see this in a telescope because in both cases the pairs of stars are extremely close to one another, orbiting one another in periods of less than 10 days. And as noted, each pair orbits the other pair in about 400 years. But there’s more.

Returning to that backyard telescope you may notice a third star, Castor C, quite a distance from the first two and significantly dimmer. This star is also part of the Castor family and it too has a twin that also is so close we can’t detect it without special instruments. In fact, Castor C consists of the closest pair of all, orbiting one another in less than a day! This pair, in turn, orbits the other four stars in the system once every 10,000 years or so.

So when you look at Castor, remember that in classic mythology it has a twin, Pollux – and remember that what looks to you like a single bright star is really the combined light from six stars, all held together in one of the most complex star systems we know. (I wrote much more about the Castor system on the double-star blog. That post includes a scale model that puts Castor and company into perspective with the Earth and Sun. You’ll find it here. )

Vital stats (for just the brightest star in the Castor system):

  • Brilliance: Magnitude 1.58, the 23rd brightest star in our sky and the brightest second magnitude star. Absolute magnitude is 0.9. (Yes, we call a star “second magnitude” if it’s magnitude is between 1.5 and 2.5 – so you can see castor just slips into this category.)
  • Distance: 50 light years (not among the 200 nearest stars)
  • Spectral Type: A
  • Position: 07h:34m:36s, +31°:53′:18″
  • Compared to the Sun: Castor radiates 14 times as much energy as our Sun.

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Getting to know Pollux – the bigger, brighter twin

How Bayer saw the Gemini Twins in his 1603 atlas. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Pollux should feel a little cheated because it’s the brightest star in the constellation of Gemini and usually the brightest star was given the designation “alpha”  by the early chart maker, Bayer. Not Pollux. It is designated “Beta Geminorum” and follows its slightly dimmer twin brother around the sky. But Pollux has its own way of standing out: It has a slight edge in brilliance in our skyit is a tad closer to us; and it is an orange giant. What’s more, in 2008 it was confirmed to have a planet orbiting it.

As an orange giant, it has moved off the “main sequence,” and instead of fusing hydrogen into helium, as our Sun does, it is fusing helium into carbon and oxygen. It will eventually blow off a lot of its substance becoming a planetary nebula. It is currently about eight times the diameter of our Sun – that’s huge, but nowhere near as large as our next star, Betelgeuse. The planet circling Pollux is also large – “Jupiter class” – and was first detected in 1993, but not confirmed until 2008.

Vital stats:

  • Brilliance: Magnitude 1.14, the 17th brightest star in our sky. Absolute magnitude is 0.7 .
  • Distance: 34 light years (not among the 200 nearest stars)
  • Spectral Type: K
  • Position: 07h:45m:19s, +28°:01′:35″

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 Orion – A man for all to see

If you’re in the same general latitude as I am in Westport, MA, then you see Orion like this as it rises in the east on a January evening.

Orion – as seen when rising in mid-northern latitudes. (Click for larger image.)

What always sticks with me about Orion is how Robert Frost described him in his wonderful poem, “The Star Splitter.”

‘You know Orion always comes up sideways.

Throwing a leg up over our fence of mountains,

And rising on his hands, he looks in on me . . .

But if I lived in Sydney, Australia, I wouldn’t see it this way. What I would see is a man standing on his head!

Orion, as seen when rising in the east from Sydney, Australia. (Click image for larger version.)

The real point here is that these stars do look like a man, and they can be seen from deep into both the southern and northern hemispheres. What’s more, the three distinctive stars that form Orion’s belt also mark the approximate position of the celestial equator in your sky, a handy thing to know. Of course, if you’re in the southern hemisphere, the celestial equator appears to make an arc across your sky to your north. In the northern hemisphere it appears to make an east-west arc across the sky to the south.

But in either case the belt stars of Orion will rise just about due east and set due west. How high they get in your sky is calculated simply by subtracting your latitude from 90. That is, if your latitude is 42 degrees, as mine is, then Orion’s belt will be, at its highest, about 48 degrees above the horizon when it passes due south. From Sydney, Australia, the stars in the belt will cross about 56 degrees above the horizon as they pass due north. And yes, if you live on the equator these stars will cross directly over head. Anyway you look at it, Orion is a man for all latitudes – well, almost. At the north pole you would only see his top half, and at the south pole, only his feet! Return to Menu

Betelgeuse – giant among giants – and Rigel’s pretty large as well!

When you look at the eastern sky early on a January evening, get this picture in your head!

Here’s what our eastern sky would look like on a January evening if Arcturus and Rigel, two genuine giants, were as near to us as our Sun. The Sun, to scale, is also shown. Betelgeuse is NOT show to scale.

Yes, that’s rigel represented inthe illustration, not Betelgeuse. Classified as a red supergiant, Betelgeuse is one of the largest stars you can see – and certainly up there with the biggest of all stars – yet it doesn’t look any bigger in our sky than other stars because all stars, except the Sun, are so far away they appear only as a point source of light to our eyes. Last month we showed what Aldebaran would look like if it were in our sky and the same distance from us as the Sun, and this month we’ve added Rigel to the picture. But we can’t do a similar thing with Betelgeuse – it wouldn’t be in our sky – it’s so large we would be in it if it were located where our Sun is!

What’s more, it’s hard to put a number to the size of Betelgeuse, not because it can’t be measured, but because it’s hard to decide exactly what you want to measure when you’re dealing with a ball of gas – especially one like Betelgeuse. Our Sun is a little easier case. While it does not have a surface, it does appear to us to have an edge that’s fairly easy to define – it’s the place where its gases are dense enough to be opaque to our vision.

Exactly how we define the size of Betelgeuse is a bit more difficult. I rely on James B. Kaler as my stellar authority. I love his books, and in one, “The Hundred Greatest Stars,” he describes the size of Betelgeuse variably as:

  • 650 times that of the Sun, or 2.8 AU (Astronomical Units – an Astronomical Unit is the distance between the Earth and the Sun – roughly 93 million miles)
  • 800 times the diameter of the Sun, or about 4 AU
  • 1600 times the Sun – about 8 AU when measured by modern observation in ultraviolet light

And on his Web site, after opting for a figure of around 8-9 AU, he writes:

However, the star is surrounded by a huge complex pattern of nested dust and gas shells, the result of aeons of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far having lost over a solar mass). That, an extended atmosphere, and the pulsations make it difficult to locate an actual “surface” to tell just how large the star actually is. Moreover, because of changes in gaseous transparency, the “size” of the star depends on the color of observation.

Betelgeuse has other problems. The pulsations he refers to are a sort of puffing up that occurs from time to time and changes both size and brightness significantly. Betelgeuse is usually thought of as about magnitude 0.55, but it can be as bright as 0.3, or as dim as 1.1. All this huffing and puffing will soon lead to an explosion, and Kaler says it will then be as bright as a crescent moon! But don’t hold your breath. “Soon” in astronomical terms means sometime in the next million years or so! Its distance, too, is uncertain, but 500 light years is a good ballpark figure.

Let’s focus on that 8 AU size for a moment. When we build a scale model of our solar system and reduce the Sun to something about the size of a volleyball, the tiny speck of the Earth orbits at around 75 feet away. But at 8 AU Betelgeuse would be more like 600 feet in diameter. So pause for a moment as you look at Betelgeuse on a winter evening. Imagine yourself holding an 8-inch volleyball in one hand – our Sun – while you stand next to a red, raging, unstable monster ball that is 600 feet in diameter!

Vital stats:

  • Brilliance: Magnitude 0.3 – 1.1, the 10th brightest star in our sky (sometimes). Shines with the luminosity of about 90,000 Suns.
  • Distance: 570 light years
  • Spectral Type: M
  • Position: 05h:55m:10s, +7°:24′:25″

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Rigel – Blue and brilliant

Here we go again! Like Pollux, it looks like Rigel was short-changed having been designated the “Beta” star of the constellation Orion while dimmer Betelgeuse is the Alpha. Of course, Betelgeuse, being variable, may have been brighter when Johann Bayer made his designations in 1603. Bayer’s “system” is inconsistent, however, to say the least, so there’s no sense getting too worried about this.

Like Betelgeuse, Rigel is a supergiant. It’s huge and it’s brilliant too – and since it is more distant (860 light years), it is intrinsically more brilliant than Betelgeuse. Jim Kaler writes: “Only about 10 million years old, Rigel should eventually expand to become a red supergiant very much like Betelgeuse is today, by which time it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova.”

Rigel’s radius is 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury.

Rigel is a challenging double star for amateurs with moderate-sized telescopes.

Vital stats:

  • Brilliance: Magnitude 0.12, the 7th brightest star in our sky. Shines with the luminosity of about 90,000 Suns.
  • Distance: 860 light years
  • Spectral Type: B
  • Position: 05h:55m:10s, +7°:24′:25″

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Coloring the stars – an exercise for all seasons

Trying to identify the true colors of stars, as we see them, is fun, challenging, and instructive.

Your assignment, should you choose to accept it, is to develop accurate color swatches that represent the colors of the bright stars, including our Sun, as they actually are seen and in so doing learn:

  • How to see color in the stars
  • What the color tells us about each star

The chart shows the Winter Hexagon because many of the brightest stars can be seen there all at one time, but it also includes swatches for several bright stars that are prominent in the spring, summer, and fall.

This image of the Winter Hexagon was taken by Jimmy Westlake looking at the skies over Stagecoach, Colorado. Look carefully and you can see color in some of the stars - especially if you click on the image to see the larger version. (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Star colors are real. They relate to a star’s temperature and from them we can surmise much more about a star. But they also are very subtle. I think of them not as colors, but as tints. I see stars essentially as white lights to which a little color has been added to tint it one way or another. I believe most people don’t see the colors at all when they first look at the stars and this can be frustrating, especially if you’ve read that Betelgeuse, for example, is an “orange” star.

With the naked eye you only will see color on the brightest stars because our eye simply needs a lot of light to detect color. In fact, point your binoculars at those bright stars, and you should find it easier to detect the colors because the binoculars gather more light. You can train yourself to see star colors, though people do differ in this ability. But for most, the colors are really quite obvious on some of the brighter stars, once you know what you can expect to see. And that’s what this little exercise is for – learning what you can expect to see.

Your main tools will be the chart and the color table provided here. You’ll have to provide the primary colors (red, yellow, and blue), plus white in some easily blendable medium, such as common water, tempera, or poster paints. Nothing fancy needed and no special painting skills required.

First, here’s the chart you will be coloring.

You can download a version for printing by clicking here.

And here’s the color table you will use as your guide

Your task is simple.

Next to each star on the star chart is its spectral classification. This consists of a letter and number. The letters go from blue to red stars in this order: OBAFGKM. Each letter gets divided into a numerical sub-classification from 0-9. So a “B0” star would be just at the beginning of the “B” category. A “B9” star would be at the end of that category and almost into the next one. The star chart shows the spectral classification for each star. Match that with what you see in the color table. Then determine its color.

You will notice that there are two different color scales in the table. That’s because the way we see color depends in part upon the environment in which we see it. The “conventional color” is what would be seen if the star were put under high magnification and projected onto a white sheet of paper in the daylight. The “apparent color” is what is seen by the naked eye in a dark sky. That is the color you want. That’s what you’ll try to duplicate by mixing your water colors and painting the swatch next to each star so it matches its classification – and thus what you are likely to see in the night sky.

The result will be a chart that will help you know what color to expect to see when you look at the stars in the sky. I should add that if you see a photograph of these stars, the colors will be similar, but different. That’s because the colors in a photograph depend on the color sensitivity of the film or computer chip used to record them – which is not the same as your eye. So you cannot use a photograph as an absolute guide to what you will see. The chart you make, if done well, will be a much better guide.

When you look for star color, make sure the star is high in your sky – hopefully at least 30 degrees or more above the horizon. All bright stars near the horizon will appear to flash many brilliant colors. Those colors – like the colors in our sunrises and sunsets – are caused by the Earth’s atmosphere. When you are looking at an object near the horizon, you are looking through much more air than when you are looking at a star high overhead.

Of course, you are going to have to use your judgment in making the color swatches, and you might experiment a bit on another piece of paper. That’s why I recommend using some sort of water color for this activity – so it’s easy to blend and thin your colors to get the color you want – the one that is closest to what you actually see. Of course to get orange you mix the yellow and red – and white will come in handy to lighten any of the colors.

Get the idea? Will your colors be perfect? I doubt it. But experimenting this way will give you a much better feel for how subtle star colors are and exactly what you are looking for when you go out at night. Too often people are confused and disappointed because they read that Betelgeuse or Aldebaran is a red or orange star – and when they read that, they are thinking of the conventional red or orange – quite naturally – but look at the chart and look at the difference between conventional colors and apparent colors.

More about a star’s spectral class

OBAFGKM is certainly a crazy order, I know. It started out to be an alphabetical list more than a century ago. But as they learned more about the stars, the letters got scrambled. Here’s an easy way to remember the order:

Oh Be A Fine Girl/Guy Kiss Me

At first they thought letters would be enough, but the more they learned about stars, the more they saw there were many subtle variations that were important. So for each letter there is a sub-classification system that goes from 0-9. Thus an O9.5 star, such as Mintaka, is in the “O” spectral class (blue) but about as close as one can get to being a “B” (blue white) star. Don’t be too concerned about these numbers, however. You’ll find it difficult enough just to get colors that accurately match the letter classifications. Besides, I’ve found that different sources sometimes give different numbers for the spectral classification of a specific star, so I see them as a good rough guide as to how solidly a star is into a specific class but not something to take overly seriously in terms of what we can detect with our eyes.

Mintaka, incidentally, is included here because bright “O” stars are hard to find. Mintaka was one of the easiest “O” star to identify, being the western-most star in Orion’s Belt. But coincidentally, Ainitak, the star at the other end of the belt, is also an”O” and  a bit brighter. But ay O9.7 it, too, just makes it into the “O” class by the skin of its teeth. In fact, it’s a bit closer to being a “B” star than Mintaka – but I guarantee you won’t see any difference.

“M” stars are even more difficult to find. True, Betelgeuse is one in the Winter Hexagon, and  in the summer we have another brilliant “M” star – Antares, the brightest star in the Scorpion.  But these are special. They both are Supergiants – stars that are going through their death throes and have expanded tremendously.  The vast majority of “M” stars are of average size, and in fact, these average-sized “M” stars are the most common stars in the universe – yet there is not a single “normal” class “M” star visible to our naked eye, let alone as bright as the stars that form the Winter Hexagon.

I also added our Sun to the chart. DO NOT LOOK AT THE SUN TO TRY TO DETERMINE COLOR. YOU WILL DAMAGE YOUR EYES. We were all taught as children to color our Sun yellow – and this is correct if you are talking about conventional color. But the Sun is a class G2 star, and I suggest you color its swatch the “apparent” color it would appear to our naked eye were we seeing it as just another bright star in our night sky. This means it would appear the same as Capella.

Binocular and telescope users can see many double stars, and some of these provide striking color contrast, such as the blue and gold of Albireo. Seeing two stars close together that are of different colors makes it even easier to see star colors but also presents a whole new set of challenges, and experienced observers frequently differ on what the colors of the double stars are. John Nanson has explored this in an excellent post to the “Star Splitters” blog that we co-author. To learn more about these stars and the special challenges of determining their colors, read John’s post here.

What we can surmise from the colors

As you can see from the temperature scale, blue stars are hot – red stars are “cool.” Cool, that is, as far as star temperatures go. They are still very, very hot: 3700 Kelvin is about 6,200 degrees Fahrenheit! (Steel melts at about half that temperature.)

So once you notice a star’s color, what more can it tell you about the star? A detailed answer is beyond this exercise, but it means you can make a very good guess about some other important characteristics of the star.

Here’s a summary in table form of what the spectral classification tells about the size and life expectancy of a star, and even hints at how it will probably die.

  • Ninety-five percent of all stars are on what is called the “main sequence.” Most of the stars that are not on the main sequence are white dwarfs. But a few others are giants or supergiants. Roughly one percent of the stars fall into one of the giant categories, such as Antares.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter.
  • Age – “O” stars have short lives and thus die first, then “B,” etc. No “dwarf” “K” or “M” star has died yet – the universe isn’t old enough.

Look East In January 2011 – a trio of twins; Orion, man of the world; Betelgeuse, giant among giants!

January brings a host of bright and wondrous winter stars. There are four new guidepost stars to meet this month and one new guidepost asterism, Orion. Orion is probably the best known figure in the heavens because it actually looks like a person and can be seen from most locations in the world since it’s centered on the celestial equator. That’s a lot for one month, but fun to think about on a dreary winter day and more fun to observe on a brilliant, winter evening.

Here’s the chart for the eastern sky one hour after sunset for mid-northern latitudes. Remember, going out about 45 minutes after sunset and looking east, you’ll see only the brightest stars as they come out. This makes it easier to identify and learn our guidepost stars. Our guidepost asterisms may not be as readily seen until a little later as the sky gets darker and more of the fainter stars come out.

The eastern sky as seen on a January evening about one hour after sunset. Click image for larger version. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

Click here to download a black-on-white (printer-friendly) version of this chart.

The January eastern sky – what to remember

Castor – A trio of twins

When you see Castor, think “twins” – a trio of twins. Well, in a sense there are really four pairs!

Castor is one of the Gemini Twins (Castor and Pollux), but in a small telescope we see it really is three stars, Castor A, Castor B, and Castor C - and though we can't see this in our telescopes, each of these stars is really a pair, making six stars in all!
Click image for larger view.

But the fourth pair is just mythological – Castor is one of the “heavenly twins” of the constellation Gemini – the other twin being Pollux. This is nothing but a fanciful relationship, though, based on how the stars appear to us. But there is more, much more, to Castor. And, it’s what we don’t see that makes this bright star so fascinating.

Were you to look at Castor in a backyard telescope, you would see it has a twin – another bright star that appears quite close – so the two are Castor A and B. These two are related, orbiting one another about every 400 years. But there’s more. Each of these two are twins! However, you can’t see this in a telescope because in both cases the pairs of stars are extremely close to one another, orbiting one another in periods of less than 10 days. And as noted, each pair orbits the other pair in about 400 years. But there’s more.

Returning to that backyard telescope you may notice a third star, Castor C, quite a distance from the first two and significantly dimmer. This star is also part of the Castor family and it too has a twin that also is so close we can’t detect it without special instruments. In fact, Castor C consists of the closest pair of all, orbiting one another in less than a day! This pair, in turn, orbits the other four stars in the system once every 10,000 years or so.

So when you look at Castor, remember that in classic mythology it has a twin, Pollux – and remember that what looks to you like a single bright star is really the combined light from six stars, all held together in one of the most complex star systems we know. (I wrote much more about the Castor system on the double-star blog. That post includes a scale model that puts Castor and company into perspective with the Earth and Sun. You’ll find it here. )

Vital stats (for just the brightest star in the Castor system):

  • Brilliance: Magnitude 1.58, the 23rd brightest star in our sky and the brightest second magnitude star. Absolute magnitude is 0.9.
  • Distance: 50 light years (not among the 200 nearest stars)
  • Spectral Type: A
  • Position: 07h:34m:36s, +31°:53′:18″
  • Compared to the Sun: Castor radiates 14 times as much energy as our Sun.

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Getting to know Pollux – the bigger, brighter twin

Pollux should feel a little cheated because it’s the brightest star in the constellation of Gemini and usually the brightest star was given the designation “alpha.” Not Pollux. It is designated “Beta Geminorum” and follows its slightly dimmer twin brother around the sky. But Pollux has its own way of standing out: It has a slight edge in brilliance in our sky; it is a tad closer to us; and it is an orange giant. What’s more, in 2008 it was confirmed to have a planet orbiting it.

As an orange giant, it has moved off the “main sequence,” and instead of fusing hydrogen into helium, as our Sun does, it is fusing helium into carbon and oxygen. It will eventually blow off a lot of its substance becoming a planetary nebula. It is currently about eight times the diameter of our Sun – that’s huge, but nowhere near as large as our next star, Betelgeuse. The planet circling Pollux is also large – “Jupiter class” – and was first detected in 1993, but not confirmed until 2008.

Vital stats:

  • Brilliance: Magnitude 1.14, the 17th brightest star in our sky. Absolute magnitude is 0.7 .
  • Distance: 34 light years (not among the 200 nearest stars)
  • Spectral Type: K
  • Position: 07h:45m:19s, +28°:01′:35″

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Orion – A man for all to see

If you’re in the same general latitude as I am in Westport, MA, then you see Orion like this as it rises in the east on a January evening.

Orion – as seen when rising in mid-northern latitudes. (Click for larger image.)

What always sticks with me about Orion is how Robert Frost described him in his wonderful poem, “The Star Splitter.”

‘You know Orion always comes up sideways.

Throwing a leg up over our fence of mountains,

And rising on his hands, he looks in on me . . .

But if I lived in Sydney, Australia, I wouldn’t see it this way. What I would see is a man standing on his head!

Orion, as seen when rising in the east from Sydney, Australia. (Click image for larger version.)

The real point here is that these stars do look like a man, and they can be seen from deep into both the southern and northern hemispheres. What’s more, the three distinctive stars that form Orion’s belt also mark the approximate position of the celestial equator in your sky, a handy thing to know. Of course, if you’re in the southern hemisphere, the celestial equator appears to make an arc in your sky to your north. In the northern hemisphere it appears to make an arc in the sky to the south.

But in either case the belt stars of Orion will rise just about due east and set due west. How high they get in your sky is calculated simply by subtracting your latitude from 90. That is, if your latitude is 42 degrees, as mine is, then Orion’s belt will be, at its highest, about 48 degrees above the horizon when it passes due south. From Sydney, Australia, the stars in the belt will cross about 56 degrees above the horizon as they pass due north. And yes, if you live on the equator these stars will cross directly over head. Anyway you look at it, Orion is a man for all latitudes – well, almost. At the north pole you would only see his top half, and at the south pole, only his feet! Return to Menu

Betelgeuse – giant among giants

When you look at the eastern sky early on a January evening, get this picture in your head!

Here's what our eastern sky would look like on a January evening if Arcturus and Rigel, two genuine giants, were as near to us as our Sun. The Sun, to scale, is also shown. What isn't shown to scale is Betelgeuse. That's because we couldn't see it as a star if it were the same distance from us as the Sun - for we would be buried deep inside it, and Betelgeuse would be everywhere.

If the Sun looks smaller than you think it should in the above image. Classified as a red supergiant, Betelgeuse is one of the largest stars you can see – and certainly up there with the biggest of all stars – yet it won’t look any bigger in our sky than other stars because all stars, except the Sun, are so far away they appear only as a point source of light to our eyes. Last month we showed what Aldebaran would look like if it were in our sky and the same distance from us as the Sun, and this month we’ve added Rigel to the picture. We can’t do a similar thing with Betelgeuse – it wouldn’t be in our sky – we would be in it!

What’s more, it’s hard to put a number to the size of Betelgeuse, not because it can’t be measured, but because it’s hard to decide exactly what you want to measure when you’re dealing with a ball of gas – especially one like Betelgeuse. Our Sun is a little easier case. While it does not have a surface, it does appear to us to have an edge that’s fairly easy to define – it’s the place where its gases are dense enough to be opaque to our vision.

Exactly how we define the size of Betelgeuse is a bit more difficult. I rely on James B. Kaler as my stellar authority. I love his books, and in one, “The Hundred Greatest Stars,” he describes the size of Betelgeuse variably as:

  • 650 times that of the Sun, or 2.8 AU (Astronomical Units – an Astronomical Unit is the distance between the Earth and the Sun – roughly 93 million miles)
  • 800 times the diameter of the Sun, or about 4 AU
  • 1600 times the Sun – about 8 AU when measured by modern observation in ultraviolet light

And on his Web site, after opting for a figure of around 8-9 AU, he writes:

However, the star is surrounded by a huge complex pattern of nested dust and gas shells, the result of aeons of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far having lost over a solar mass). That, an extended atmosphere, and the pulsations make it difficult to locate an actual “surface” to tell just how large the star actually is. Moreover, because of changes in gaseous transparency, the “size” of the star depends on the color of observation.

Betelgeuse has other problems. The pulsations he refers to are a sort of puffing up that occurs from time to time and changes both size and brightness significantly. Betelgeuse is usually thought of as about magnitude 0.55, but it can be as bright as 0.3, or as dim as 1.1. All this huffing and puffing will soon lead to an explosion, and Kaler says it will then be as bright as a crescent moon! But don’t hold your breath. “Soon” in astronomical terms means sometime in the next million years or so! Its distance, too, is uncertain, but 500 light years is a good ballpark figure.

Let’s focus on that 8 AU size for a moment. When we build a scale model of our solar system and reduce the Sun to something about the size of a volleyball, the tiny speck of the Earth orbits at around 75 feet away. But at 8 AU Betelgeuse would be more like 600 feet in diameter. So pause for a moment as you look at Betelgeuse on a winter evening. Imagine yourself holding an 8-inch volleyball in one hand – our Sun – while you stand next to a red, raging, unstable monster ball that is 600 feet in diameter!

Vital stats:

  • Brilliance: Magnitude 0.3 – 1.1, the 10th brightest star in our sky (sometimes). Shines with the luminosity of about 90,000 Suns.
  • Distance: 570 light years
  • Spectral Type: M
  • Position: 05h:55m:10s, +7°:24′:25″

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Rigel – Blue and brilliant

Here we go again! Like Pollux, it looks like Rigel was short-changed having been designated the “Beta” star of the constellation Orion while dimmer Betelgeuse is the Alpha. Of course, Betelgeuse, being variable, may have been brighter when Johann Bayer made his designations in 1603. Bayer’s “system” is inconsistent, however, to say the least, so there’s no sense getting too worried about this.

Like Betelgeuse, Rigel is a supergiant. It’s huge and it’s brilliant too – and since it is more distant (860 light years), it is intrinsically more brilliant than Betelgeuse. Jim Kaler writes: “Only about 10 million years old, Rigel should eventually expand to become a red supergiant very much like Betelgeuse is today, by which time it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova.”

Rigel’s radius is 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury.

Rigel is a challenging double for amateurs with moderate-sized telescopes.

Vital stats:

  • Brilliance: Magnitude 0.12, the 7th brightest star in our sky. Shines with the luminosity of about 90,000 Suns.
  • Distance: 860 light years
  • Spectral Type: B
  • Position: 05h:55m:10s, +7°:24′:25″

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