Tuesday, February 11, 2014

Star of the Week: Altair

Returning once more to the Summer Triangle, I'd like to introduce readers to Altair.

Altair (false color) imaged by MIRC at the CHARA Array on Mt. Wilson
Credit: John Monnier (University of Michigan)

Altair has the distinction of being one of the few stars (besides the Sun) which has been directly imaged. What you're looking at there, in false color, is a reconstructed image of Altair from observations with the CHARA (Center for High Angular Resolution Astronomy) Array, an array of 1m telescopes that uses the combined light from each instrument to collect high resolution observations of astronomical objects. In a sense it acts like a much larger telescope and I'll explain CHARA here in an upcoming post.

Altair is the brightest star in Aquila and is classified as A7V star. It is 16.7 ly from Earth, a neighbor and a close one at that on a galactic scale.  If you look at the picture above, something that should be noticeable is that Altair is not spherical. This is because it rotates at a high speed. Since stars are made of gas, a rapidly rotating star will have it's shape altered by the motion, bulging outward. You might also notice that there are light and dark spots. This indicates areas where the star is different luminosities (white being more luminous). This is because luminosity and temperature are related to surface gravity, which in turn depends on mass and radius. In Altair, the surface gravity is lower at the radius, meaning there is lower gas pressure, cooling the gas there and decreasing luminosity.