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Planets (at least, some of them like Earth) aren't exactly spherical - but what about stars?

Is Sun perfectly spherical, for example? What may be the reasons if it isn't? What about other stars?

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nothing real is perfectly spherical. – Walter Feb 9 '14 at 18:01
@Walter the question is mostly dedicated to its reason in the aspect of stars. – Zoltán Schmidt Feb 9 '14 at 19:53
up vote 7 down vote accepted

No, no stars have an exactly spherical shape. The reason for this is that the centrifugal force of the star's rotation is much greater at the equator of the star than it is at the poles, for the simple reason that the rotational velocity is greater. This greater centrifugal force pushes the equator outwards, stretching the star into an oblate shape. This is called gravity darkening.

Because we have only visually resolved the surfaces of a few other stars, it's not something that is commonly directly observed (though the effect can also be observed from stellar spectra). Regulus is one star that has been observed as an oblate spheroid spectrographically. The image below shows the star Altair, directly imaged using the CHANDRA space telescope. Go here for an animation!

enter image description here

Some stars are also non-spherical due to the effects of a nearby star in a close binary orbit. Much like how the Moon causes tides on Earth, two stars can stretch each other's surfaces. If they're very close to one another, as in the picture below, there can even be mass transfer between them (ref common envelope).

enter image description here

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As well as being oblate, like most large spinning bodies, there is a high degree of turbulence in any heated gaseous body, whether that be large gas planets, like Jupiter, or stars.

Have a look at this NASA photo of the sun as an example:

enter image description here

You can see a wide range of non-spherical details.

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No (1. All stars rotate to some degree and hence are slightly oblate. 2. stars have no surface like a rocky planet, what we see is the photosphere, i.e. the surface from which light escapes without being (much) absorbed or scattered. This surface, however, is constantly moving due to convection. 3. close binary stars are affected by their partner's tide.)

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