Arcturus
Published on Jun 1, 2007 at 4:18 pm.
3 Comments.
Filed under stars.
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From mid-northern latitudes this time of year, if you go outside a little after sunset and look up, you’ll see a very bright star nearly overhead. That is the star Arcturus. It is actually the third brightest star in the sky, shining with magnitude -0.05. In fact, Arcturus is the brightest star north of the celestial equator. If you look carefully, you’ll also see that Arcturus isn’t exactly white. It is a more orange color. Technically, we call it a red giant. That is a star that has used up all of the hydrogen in its core. Stars fuse hydrogen into helium to support themselves against gravity. When the hydrogen burns out, the core collapses and gets hotter. This heats layers of gas above the core and they expand outward, pushing the outer layers of the star outward. The expanding outer layers become cooler, and the star as a whole appears brighter. But, this is only temporary. Eventually helium starts to fuse in the core of the star, and it begins to get smaller and hotter. Then, the helium runs out, and the star expands again to be an even bigger red giant. It then sheds its outer layers (which become a planetary nebula) and the core collapses to form what we call a white dwarf — essentially a large mass of mostly carbon. Arcturus appears to be in the early portions of its death throes.
The surface of Arcturus is about 4300K, compared with a temperature of about 5800K for the Sun. That makes a valid comparision, since Arcturus is believed to have only a little more mass than the Sun. That would mean that it would have been only a little hotter and brighter than the Sun when it was on the Main Sequence, fusing hydrogen like the Sun does. Now, Arcturus is cooler, and it is larger (nearly 16 times the diameter of the Sun) and brighter (about 110 times more luminous than the Sun). Being only a bit more massive than the Sun, you’d expect it to have a similar life span. The Sun, though is only halfway through its life, and Arcturus is at its end. That suggests that it is an older star. That supposition is further enforced because Arcturus contains less than half of the metals that the Sun has. Astronomers call anything other than hydrogen and helium a metal. Metals are only produced during fusion in stars. The earlier a star forms in the history of the galaxy, the fewer generations of stars have gone before to produce metals. So the lowere the metal content, generally the older that a star is.
Arcturus is only 37 light years away. It is moving very quickly, though, relative to the Sun (nearly 122 km/s). Arcturus’ motion is also not in the same direction as most other stars in the area. Those two bits of information about its motion, together with its low metallicity, lead many astronomers to believe that Arcturus may be a halo star passing through our part of the galaxy. The Milky Way galaxy is surrounded by a roughly spherical halo of low metallicity stars that move in big randomly oriented elliptical orbits. That contrasts with disk stars that generally move in orbits that are fairly similar to one another around the center of the galaxy in a large disk.  There is some debate whether Arcturus is really a halo star, but that is what most sources say about it.
So, go out, look up, and contemplate one of the nearest halo stars, and one of the nearest red giants.
 -Astroprof
Image courtesy of WikimediaÂ
Lab Lemming on June 4, 2007 at 7:29 am: 1
Even here in the Southern Hemisphere, we can currently see Arcturus low, but bright in the NE sky. Of course, with Alpha Centauri, Canopus, Sirius, Venus and Jupiter all in the sky at the same time, it doesn’t quite stick out as much…
Deborah Byrd on June 5, 2007 at 3:18 am: 2
Good morning Astroprof, sorry to contact you here, but your contact page is disconnected.
Early June - particularly this week - is great for seeing Venus and Jupiter. Here are some links from Earth & Sky:
Earth flies between the sun and Jupiter on June 5
http://www.earthsky.org/radioshows/51217/earth-flies-between-sun-and-jupiter-on-june-5
Venus’ peak evening appearance in June 2007
http://www.earthsky.org/radioshows/51277/venus-peak-evening-appearance-in-june-2007
Dazzling Venus and Jupiter balance in early summer skies
http://www.earthsky.org/article/venus-and-jupiter-balance-in-early-summer-skies
Keep up the good work!
Deborah
Donna on June 5, 2007 at 7:07 pm: 3
Arcturus was the very first star I ever identified in the night sky. I was about 10 years old and my 4th grade teacher was teaching “the stars and planets”. From that moment on I was hooked! I went to the library and “followed the Big Dipper’s” handle…and there it was! I had a name and the beginning of a lifetime of searching and enjoyment of the night sky!