SkyCaramba weekly astronomy blog for the week ending March 17, 2012
Venus and Jupiter are pairing up this week in Aries. I’ve told you about that already. Now, I want to tell you something about the constellation they’re meeting in.
The Ancient Greeks imagined Aries to be a ram lying down with its head turned to the right. It’s hard to visualize it now, especially since most of us aren’t living that close to livestock. The small number of stars making up the constellation doesn’t help.
Aries is usually listed at the top of newspaper astrology columns. Astrologers wrongly believe that the sun is about to enter Aries at the time of the March equinox. Actually, the sun is just now getting into Pisces. It won’t be in Aries for a little over a month. The sun used to enter Pisces around March 21. But over many thousands of years, the earth wobbles like a top as it rotates. The phenomenon is called the precession of the equinoxes. The sun is also moving as it orbits the Milky Way’s galactic center. These movements cause the positions of the constellations to shift. I don’t deal with astrology, so I’ll leave it to astrologers to explain why they prefer to think the sun’s about to enter Aries now.
The bright star in Aries is Hamal. This word is an old Arabic word meaning “the lamb”. It’s not quite as bright as Jupiter and nowhere near as bright as Venus. Hamal is 66 light years away and is likely to be among the first stars whose surface features can be seen from Earth. Astronomers already can see that it appears darker around the edges than at the center, just like our sun. That’s just the way it looks though. We’re not looking into much of a star’s gases when we look at the edge of the disk, so we shouldn’t expect it to look so bright there to highly sensitive equipment that can detect small differences.
Sheratan is the second brightest star in Aries and is 60 light years away. It’s very interesting to astronomers because of a companion star orbiting it every 107 days. We can’t see it, even with the best of telescopes. But instruments that measure Doppler shifts in light beams can tell it’s there. The star shifts colors very slightly toward the blue end of the spectrum as it moves toward us and very slightly toward the red end as it moves away. Sheratan’s companion orbits closer than Venus to our sun. And it’s a highly elliptical orbit.
Mesarthim is 204 light years away. It’s also a double star. Unlike Sheratan, Mesarthim is easily resolved into two stars. Telescopes have revealed the double to astronomers since 1664. The two stars orbit each other about 12 times as far apart as Pluto and our sun. And they take 5,000 years to finish one orbit.
Below are some links to more information about the stars of Aries and the precession of the equinoxes. Enjoy the view. ¡SkyCaramba!
http://stars.astro.illinois.edu/sow/hamal.html
http://stars.astro.illinois.edu/sow/sheratan.html