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  • Rapt in Awe

    My Journey through the Astronomical Year

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

Look East in April 2013 – take a simple slide to the World’s Fair Star!

The name"Arcturus" derives from Ancient Greek and means "Guardian of the Bear." It is the brightest star in the constellation Boötes. Click image for a much larger version. (Prepared from Starry Nights Pro screen shot.)

The name”Arcturus” derives from Ancient Greek and means “Guardian of the Bear.” It is the brightest star in the constellation Boötes. Click image for a much larger version. (Prepared from Starry Nights Pro screen shot.)

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

Arcturus isn’t universally known as the “World’s Fair Star,”  but  it should be.  Its light bridged two World’s Fairs, making an astronomical link between the one in 1893  and a second in 1933 – both held in Chicago.  It’s intriguing that  the general public was excited enough about science – in the middle of the Great Depression – to make such a link attractive to the Fair’s promoters. Light from Arcturus  – believed at that time to be 40 light years away – was captured by the 40-inch refractor at Yerkes Observatory and its energy used to turn on the lights for the 1933 Fair.

This put the public spotlight not only on Arcturus, but it raised consciousness about the vast distance between us and that star, since the light being used had started its journey during the 1893 Fair and arrived just in time to start the next Fair. When you know light can circle the Earth more than seven times in a single second, you start to understand just what an incredible journey that was.

Of course Arcturus has many other distinctions. For one thing, it makes a perfect connection with the best known asterism in the sky, the Big Dipper.  To find it, all you have to remember is “follow the arc to Arcturus.

Another way to remember where to find Arcturus is its name, derived from ancient Greek, which can be translated as “Bear Watcher.”  That’s because Arcturus looks like it’s keeping an eye on the “Great Bear,” Ursa Major, as both circle the northern pole.

You can also think of the magnitude system by which we rate the brightness of stars as starting near Arcturus. At magnitude -.04 it’s about as close to zero as you can get – the minus sign indicating it is a tad brighter than zero.  Its absolute magnitude is also pretty close to zero since absolute magnitude is defined as how bright a star would be if it were about 33 light years from us, and by modern measurement Arcturus is now believed to be about 37.6 light years from us.  That makes its absolute magnitude -.29.

Arcturus has the distinction of being the brightest star in the northern celestial hemisphere, but this is splitting hairs in several ways. It means it’s the brightest star north of the celestial equator. Sirius, now over in the southwest, is obviously  brighter. But Sirius is south of the celestial equator. Both stars are located close enough to the celestial equator so they can be seen from most places on Earth.

But Arcturus (-.04) also wins this “brightest star in the northern hemisphere” distinction by another hair. Remember that the lower the magnitude number, the brighter the star. Both Vega (.03) and Capella (.08) are north of the celestial equator, and the difference in brightness between Arcturus (-.04), Vega (.03), and Capella (.08) is roughly a tenth of a magnitude.  For practical purposes, they all look the same.  But in practical terms, making the comparison by naked eye is – well –  very impractical. Capella is currently fairly high in the northwest. But next month, when Vega is high enough in the east to see well,  Capella will be rather low in the northwest. At that time Arcturus should look brighter – but its actual brightness will be aided by the fact that it is high over head at that time, so you are seeing it while looking through a lot less air than you will be when looking at Vega or Capella. Besides, visually trying to compare stars that are this far apart in our sky is next to impossible since you have to look away from one to see the other. I simply think of all three as magnitude zero and leave the hair splitting to the scientists and their instruments.

Oops – we interrupt this program for a bulletin from 1907!

Yes, having just written how impractical the naked eye comparison is, I found this passage in “The Friendly Stars” by Martha Evans Martin, a book that was published more than a century ago:

Arcturus and Capella are so nearly equal in brightness that astronomers differ as to which outranks the other, even when they measure  their light with a supposedly accurate  instrument and a trained eye. To my own eye Arcturus outshines Capella, and on asking various inexperienced persons for off-hand opinions as to the relative brightness of the two stars, I have invariably had an answer in favor of Arcturus. The best authorities, however, make Capella a shade brighter.

Oh my! And now with 100 years of scientific progress, the verdict is that Martha Evans Martin and her causal observer friends were correct – and the “best authorities”  wrong. Arcturus is the brightest.  So much for my idea that you can’t tell the difference with the naked eye! Give it a try and see what you think. (You can find a chart for Capella and more details about that star  in this post.) Since we’re ranking stars, however, Arcturus is actually fourth on the list of brightest stars – two others that are ahead of it, Canopus and Rigel Kentaurus, are not seen by observers in mid-northern latitudes.Sirius, of course, is.

While Arcturus radiates a lot of energy, much of it is not in the form of visible light. It has what’s known as a “peculiar spectrum” and radiates much of its energy in the infrared portion of the spectrum.  This means that to our eyes it doesn’t look as bright as it really is.

Orange-ish Arcturus is 215 times as bright as our Sun and 25 times the Sun’s diameter. (Image courtesy of  Windows of the Universe.)

One more deception of sorts: This brightness is not because Arcturus is so big – well , yes it is, but not big in terms of the amount of stuff in it, but big in terms of surface area. If you’re measuring the amount of stuff that makes up Arcturus – its mass – it is about the same size as our Sun. But Arcturus has a much greater surface area, so think of it as a hugely bloated version of our Sun. (Keep that in mind when you look at the comparison sketch above.) It is a much older star and is now going through its red giant phase, something our Sun will probably do several billion years from now, burning the Earth to a cinder in the process.

Hmmm . . . to get an idea of how much impact that large surface area has, if you put our Sun out near Arcturus it would be barely visible to the naked eye – and then under truly dark –  not light polluted – skies.

Vital stats for Arcturus, also  known as Alpha Bootes:

•    Brilliance: Magnitude  -.04, brightest star in the celestial northern hemisphere; shines with the luminosity of 215 Suns.

•    Distance: 37 light years

•    Spectral Type: K1 Giant

•    Position: 14h:15m:38s, +19°:10′:5

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” stars and asterisms 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. So if you have been following – even if this is just your second month – look for the previous guidepost stars and asterisms that you have learned and that are still with us in April. Here’s the list from east to west.

  • Arcturus
  • Leo’s Rump  (triangle)
  • The Sickle
  • Regulus
  • the Beehive
  • Procyon
  • Sirius
  • Pollux
  • Castor
  • Betelegeuse
  • Orion’s Belt
  • Rigel
  • Capella
  • the Kite
  • Aldebaran
  • the Winter Hexagon
  • the Pleiades 

Look east in November 2011 for the “Eye of Sauron” star and Capella

November brings us our southernmost – and northernmost – guidepost stars, Fomalhaut and Capella. Their positions in the sky mean that for Northern Hemisphere observers Fomalhaut is the guidepost star we see for the shortest amount of time – and Capella is the one we see the longest.

In fact, for many, Capella is visible during some hour every night of the year – and for those north of latitude 45 degrees, it is circumpolar – that is, it never sets. But lonely – and freshly fascinating – Fomalhaut just puts in a relatively brief appearance low to the south.

From NASA:”This image, taken with the Advanced Camera for Surveys aboard NASA’s Hubble Space Telescope, shows the newly discovered planet, Fomalhaut b, orbiting its parent star, Fomalhaut. The small white box at lower right pinpoints the planet’s location. Fomalhaut b has carved a path along the inner edge of a vast, dusty debris ring encircling Fomalhaut that is 21.5 billion miles across. Fomalhaut b lies 1.8 billion miles inside the ring’s inner edge and orbits 10.7 billion miles from its star.” Click image for larger version.

Fomalhaut is “lonely” because there are few bright stars in its vicinity. It is “freshly fascinating” because early in this century the Hubble Space Telescope got a fantastic picture of a disc of “debris” surrounding it, showing this young star to be in the throes of forming planets. Then in 2008 scientists announced they had actually found a planet circling Fomalhaut (see photo above), the first planet outside our Solar System to be seen with visible light. Cool! Add to this the fact that the Hubble photograph of Fomalhaut could be easily mistaken for the Eye of Sauron, and for fans of the Lord of the Rings movie triology, Fomalhaut becomes especially memorable. (For more on the “Eye of Sauron” go here.)

Vital stats for Fomalhaut (FO-mal-ought)

• Brilliance: Magnitude 1.16; its luminosity is the equal of 16 Suns.
• Distance: 25 light years
• Spectral Types: A3V
• Position: 22:57:39, -29:37:20°

Brilliant Jupiter is by far the brightest "star" in the east on November evenings in 2011. To find Fomalhaut look southeast. Click chart for a larger image. (Chart modified from screen shot of Starry Nights Pro software.)

Click here to download a printer friendly version of the above chart.

Finding Fomalhaut

As always, it’s easiest if you start looking in the east 45 minutes after sunset when in the twilight only the brightest stars are visible as shown on our chart. Fomalhaut is the brightest star south of southeast and about a fist and a half above the horizon 45 minutes after sunset. I emphasize “star” because in 2010 Jupiter is in the general vicinity, but significantly brighter, and about three times as high. Trailing well behind Fomalhaut – to the east – and lower still is a second magnitude star (same brightness as the North Star) called Deneb Kaitos. It’s about the same distance from Fomalhaut as Fomalhaut is from Jupiter, and I mention it only so you won’t mistake it for Fomalhaut.

If you have learned the Great Square – see this post – then the two stars that form the western edge of that square can be used, as shown in our chart, as pointer stars. Drawing an arrow through those two stars leads you to Fomalhaut. You could also wait until a couple of hours after sunset when you would find Fomalhaut very close to due south. Even then, from my latitude of 41.5° N it is not quite two fists (19°) above the southern horizon.

Ahhh Capella!

Brilliant Jupiter again is by far the brightest "star" in the east on November evenings in 2011. To find Capella look northeast. Click chart for a larger image. (Chart modified from screen shot of Starry Nights Pro software.)

Click here to download a printer friendly version of the above chart.

Capella is distinctive because it’s not “a” star – it’s two! But these two, bright, yellow suns are so close together that you’ll always see them as one, even if you use a large telescope. Together they make a star that rivals Vega and Altair, now well into our western sky, in brightness. (See Summer Triangle chart here.) In fact Capella is the third brightest star in the Northern Hemisphere – but that’s a tad deceptive because it doesn’t count Sirius – the brightest star that most Northern Hemisphere observers can see, because technically Sirius is in the Southern Celestial Hemisphere, since it is below the celestial equator. But you don’t have to worry about such technicalities to enjoy a view of Capella. Just look near the horizon to the northeast. You will need a very clear horizon, however, especially at the start of the month because then Capella will not even be one fist above the horizon.

Just as Fomalhaut is a bit south of southeast, Capella is a bit north of northeast.

It’s easiest to find Capella if you start 45 minutes to an hour after sunset. Choose a spot with a clear horizon to the northeast and watch for a bright star to appear very near the horizon. Like all bright stars near the horizon, Capella will twinkle and flash in different colors because you are seeing it through a lot of atmosphere. It won’t show its soft, golden hue until it is much higher in the sky. Even a veteran skywatcher can be fooled by this. Recently my wife was looking to the northeast on a fall evening and saw what she thought was Capella. But it was so bright and blinking red and green so distinctly, that she changed her mind and decided it was an airplane! (There’s an airport off in that general direction.) When after a minute or so it hadn’t moved, she knew her first thought was correct – but boy it made a convincing airplane!

For me, Capella marks a graceful arc of bright stars and asterisms that circle the north celestial pole. If you have been following these directions for a few months, look at Capella, the Bow of Perseus, and the “W” of Cassiopeia to see what I mean. Watching these move in the course of a single night – or from month to month – always gives me a real sense of how, from our vantage point, all the stars appear to circle Polaris.

As mentioned, Capella is really a complex multiple star. Its two main components are both yellow giants dubbed Aa and Ab, but there are two more stars in this family. However, they are a pair of red dwarfs only visible in a telescope and are so far away from the two bright stars that they take more than 1,000 years to complete an orbit. The two bright stars orbit in just 104 days. James B. Kaler, in his book The Hundred Greatest Stars, says this about the Capella twins:

These two magnificent giants are separated by about the distance between Venus and the Sun. A resident on a ‘Jupiter’ ten times further out would see two ‘Suns’ about half a degree across (similar to the Sun in our own sky), separated at maximum by some 6 degrees, one setting right behind the other.
So when you find Capella, pause – picture yourself on the Jupiter-like planet with these twin yellow Suns in your sky!

Vital stats for Capella (kah-PEL-ah)

• Brilliance: Magnitude .08; its luminosity is the equal of 16 Suns.
• Distance: 42 light years
• Spectral Types: G8/G0
• Position: 05:16:41, +45:59:53

Still with us!

Other bright guide stars and asterisms introduced in previous months that are still readily seen include the Summer Triangle of Altair, Deneb, and Vega, which is high over head and crossing into the western sky. Arcturus is just above the horizon in the west, the Big Dipper just west of north and hugging the horizon, and the Teapot is diving into the ground in the southwest. And, of course, we have the “Bow” of Perseus with “Algol” the “Demon” star, the “W” of Cassiopeia, the “home plate” of Cepheus, Andromeda’s Couch, and the Great Square.

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