November 16, 2007

Lyra

Today, I'll talk about another constellation in the summer triangle: Lyra the Harp. This instrument was crafted by Hermes from a tortoise shell and was played by the legendary Greek musician Orpheus during Jason and the Argonauts' quest for the golden fleece. It was also used by Orpheus to charm Hades, the god of the Underworld, to convince him to release his wife, the nymph Eurydice. As they left the Underworld together, he looked back to ensure she was there, breaking the deal he made with Hades, and so Eurydice was gone forever. Zeus later had an eagle (or in some legends a vulture) carry his lyre up into the heavens.

In Chinese mythology, two of the stars in the summer triangle, Vega and Altair, form the pair known as the Weaving-Girl and the Herd Boy, who were separated somehow (there are many variations on how and why). They are now permanently separated by the great sky river of the Milky Way except for once a year when a bridge of birds allows them to meet. Vega is also known as the Arc-Light of the sky thanks to its bright blue-white color and domination of the late summer sky.

Vega is the brightest star in Lyra, the 5th brightest star in the night sky. Its apparent brightness is almost exactly zero on the magnitude scale (where smaller numbers mean brighter objects), and because it gives off similar amounts of light in all parts of the visible spectrum (we say it has a flat spectrum), it is often used as a standard to make sure astronomical instruments are working properly. About 12,000 years ago, Vega was pretty close to the North Celestial Pole, but it has since moved thanks to the Earth's precession. Vega is also fairly close in the sky to the apex of the sun's motion, revealed by the proper motion of surrounding stars in the sky that seem to diverge from that point.

If you remember the excellent movie "Contact", then you may recall that Vega was the star being used for calibration by the SETI team when it suddenly started receiving signals from it. Vega is only about 27 light years away (in the film, we started receiving the signal 54 years after our first broadcasts had made it that far from Earth) and is about 60 times more luminous than our Sun. The next two brightest stars make up part of the little diamond that represents the body of the harp.

Sheliak comes from the Arabic for tortoise (after the shell from which the harp was made). This is a very interesting contact eclipsing binary star about which much has been written. The two stars in this system are so close together that they cannot be resolved separately, but it can be seen due to the fluctuations (in Doppler shift) of the system's spectral lines. These stars orbit each other about every 13 days. Because they are so close, the outer layers of these stars actually touch one another, so there is ongoing mass transfer in this system, which makes the evolution quite complicated.

Epsilon Lyrae is found very near Vega, just above and to the left if Vega is at the top of the constellation, above the diamond. This famous object is also called the Double Double. Two close binary pairs widely separated with both still visible in the same field of view, first noticed by Herschel in 1779. Another famous object in this constellation is RR Lyrae, the prototype of a certain type of variable star. Stars like this are important because we can easily determine their absolute luminosities based on easy-to-measure properties (like how fast they vary) and so such stars act as standard candles. This is critical for distance determination, a topic I'll talk about in depth another time.

The most famous deep sky object in this part of the sky is M57, the Ring Nebula, found between Sheliak and Gamma Lyrae (near the bottom of the diamond, also called Sulaphat). This is a classic example of a planetary nebula, a star in an advanced stage of evolution which has slowly blown off its outer atmosphere over thousands of years. This expanding sphere of gas and dust appears to us as a ghostly ring through an 8" or larger telescope. There is also a very nice little spiral galaxy (IC 1296) very close (often within the same field of view for a wide-angle eyepiece).

The globular cluster M56 is also located below the diamond, on a line almost in the direction the diamond points. Vega and Sheliak are about as far apart as Sheliak and M56, if you are trying to find it with a small telescope. M56 lies within the band of the Milky Way while the diamond of Lyra is just outside of it. Actually, M56 is closer to Albireo (in Cygnus) then any of the bright stars in Lyra's pattern!

Posted by Observer at November 16, 2007 03:54 PM
Comments

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

Actually, you can make out M57 in a scope as small as 6" in diameter and still make out the Ring quite easily.

BTW "Contact" is a great film indeed, one of my favorites!

Hopefully see you at TG.

Posted by: Phil on November 17, 2007 12:34 PM