Flattened Stars
Published on Jan 11, 2008 at 1:26 pm.
No Comments.
Filed under conference blogging, stars.
Yesterday I saw a poster by Ming Zhao, a graduate student at University of Michigan. He works with John Monnier. The poster was presenting CHARA interferometric data on the stars Rasalhague (Alpha Ophiuchi) and Alderamin (Alpha Cephei). This is similar to work that was released earlier this year on the star Altair. Each of these giant stars rotates so fast that they are substantially flattened, as in this reconstructed image of Altair. You can read more about Zhao and Monnier’s work on Altair on a web page set up by Monnier for that purpose.
I don’t have their images for these new stars, but they look similar to Altair, only more oblate. These two newly studied stars are rotating nearly as fast as they can be before flying apart. Rasalhague is 20% wider across its equator than from pole to pole, and it is spinning at nearly 90% of its breakup speed. Alderamin is 26% wider across its equator than from pole to pole, and it is spinning at an astonishing 94% of its breakup speed. In all three of these stars, the polar regions are substantially hotter than the equatorial regions. This is something that has become apparent in recent decades: the spectral type of a star may depend upon what direction you are looking at it. For example, Rasalhague is considered a spectral type A5, but it would appear to be spectral type A1 at its poles and spectral type A9 at its equator. Similarly, Alderamin is considered spectral type A7. However, it would look like spectral type A2 at its poles and spectral type F5 at its equator. One bit of information that would have been nice, and was not provided on the poster, was the actual rotational periods of these two stars, only the percentage that they were spinning relative to their breakup speed (which also was not provided).
Still, this is interesting work to me. We typically talk about stars as being spherical in our introductory astronomy classes. For the last few years, though, I’ve given some examples of non-spherical stars flattened by their rotation. It is interesting to me to think about how many different ways things can manifest themselves in this universe of ours!
-Astroprof
Image Credit: John Monnier (Univ. Michigan)
