April 14, 2008

Corvus

Like Crater, the constellation Corvus the crow rides on the back of Hydra low in the Southern sky during the Spring, and its mythology is related to the other two constellations as I described previously. During the height of the Greek civilization, this constellation held a lot more significance than it does now because at that time it was one of about 10-15 constellations that straddled the Celestial Equator. Now, thanks to precession, it is at a declination about 20 degrees South of the Celestial Equator (thanks to Allen for that info).

Alpha Corvi is Alchiba, though like many Alphas, it is far from the brightest star in the constellation. The name translates to "tent" (from Al Hiba), which is what the four-star asterism vaguely resembles in Corvus. Another name for it historically is Al Minhar al Ghurab, which translates to the beak of the raven. Beta Corvi is Kraz, a yellow giant star about 10-15 times larger than our Sun and 160 times more luminous. Gamma is Gienah Corvi or in some cases Gienah Ghurab, depending upon whether the constellation is a crow (corvi) or a raven (ghurab).

This is the wing of the raven and the brightest star in the constellation, a blue giant fairly close to the end of its main sequence lifetime. Delta Corvi is Algorab, a double star consisting of a blue main sequence star and a cooler orange main sequence star that has just recently broken out of its early stage and has a lot of surrounding dust. Perhaps this is an early analogue to our own solar system? Gamma and Delta are the two northernmost stars in this four star asterism, and if you follow a line through them toward the East about 20 degrees, you will run into Alpha Virginis (Spica). Spica is also easy to find via the Big Dipper (follow the arc to Arcturus, speed on to Spica) and so can be used to locate Corvus.

Eta Corvi also holds some interest for Astronomers thanks to the presence of an extended disk of debris surrounding it, analagous to the Kuiper Belt in our own solar system. There even seems to be evidence of a fairly large gas giant well separated from the star, a Neptune analogue perhaps.

Now for the deep sky objects in Corvus. The most famous is certainly the pair NGC 4038 and 4039, two colliding galaxies known as the Antennae Galaxies. The titanic collision between these galaxies has probably been going on for a billion years or more. When galaxies like this collide, the stars within them usually don't affect one another. That's simply because of the spacing between stars is so large relative to the sizes of stars. It is for this same reason that stars remain orbiting in our galactic halo despite passing through the disk at high relative speeds over and over.

The gas and dust clouds, however, are extended objects and do collide, and the result is a massive burst of star formation. With many galaxies that have undergone collisions in the past, we can reconstruct what happened through simulations and also by looking at the stellar populations. How many stars of each type are present? The answer to this question can give us an idea of the star formation history of a galaxy. For some galaxies, we can identify one or more groups of stars that must have been formed during a burst of activity at one or more times in the past, likely triggered by a merger.

Where you follow a line from Gamma to Delta to lead your eye to Spica, following that same line in the opposite direction a distance roughly equal to the spacing between the stars leads you to this pair in the sky. Using these same two stars as finders, make an equilateral triangle, and the bottom vertex will be the planetary nebula NGC 4361, which vaguely resembles a spiral galaxy though this is coincidental. At magnitude 10.9, this should be visible with a good 8" or 10" scope.

There are several other galaxies and a small galaxy cluster in this region of the sky as well, but they are all much too faint for small telescopes, and no good images of them are available outside of grainy images culled from all-sky surveys. Next time, I'll complete this threesome of constellations by talking about the constellation upon which Corvus and Crater are "resting" in the heavens: Hydra the Serpent.

Posted by Observer at April 14, 2008 08:02 PM
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