And no. They’re not flat either.
Just ask Vega!
Vega is 6th brightest star in the sky (visible magnitude ≈0.03), and has been used by astronomers for distance calculations for centuries. But it is not the most spherical.
The radius of Vega at the equator is about 19% larger than the radius from the poles, and has a rotational velocity almost fast enough to fly apart.
Altair is the 13th brightest star, and is part of the summer triangle along with Vega. It rotates in around 9 hours, significantly more than the sun’s 26-36 day rotation!
Altair is one of the few stars that we have a direct image of. The equatorial radius is 25% larger than the polar radius, which is an increase of 0.4 solar radii!
Let’s leave the best to last. Achernar is 10th brightest in the sky (you can see all these stars with the naked eye!). Achernar is the primary star in a binary system, the other being a white dwarf.
Achernar is the least spherical star in the milky way. The radius at the diameter is an amazing 56% larger than the radius at the poles! It spins so fast that it is at the verge of spinning apart!
Here’s an artist’s impression of them, featuring Regulus as well. The sun is for comparison:
Why are they oblate?
Fast rotating stars have a very high centrifugal force. It’s not exactly a force, but we can treat it as such. The effects of the centrifugal force oppose gravity, so in the case of Achernar and others, their high centrifugal force diminishes some of the effects of gravity.
Stars rotates around the poles, so they don’t experience much centrifugal force in those areas. Interestingly, these flattened stars tend to be less luminous at the equator.
I used to think stars had to be spherical, but yet again I’m shown just how amazing space is! I hope you enjoyed reading this post, and I shall see you next week!