Re: The rotation of the Sun

Okay, here's the lowdown.

When astronomers can't measure the rotation speeds of stars exactly. This
is because for any given star, the inclination is uncertain. Is the star's
rotation axis pointing straight towards the observer? Is it perpendicular
to the direction that the observer is looking in? Difficult to tell without
additional information. Often, it's impossible to tell.

This means that instead of listing the rotational velocities of stars,
astronomers have to list a quantity called v sin(i), where i is the
inclination of the star's axis of rotation and is the rotational velocity. 
v sin(i) is the component of the velocity that's along the observer's line
of sight. If you know i for a given star, you can determine d. Sometimes,
it's possible to find i, for example, by measuring the star's magnetic
field. 

Since sin(i) is always between 0 and 1, v can't be any bigger than v
sin(i). So v sin(i) is an upper limit. 

You can compare the v sin(i) values for various kinds of stars, to
determine which ones are fast rotators.

The brightest stars we know of are class O, which have surface temperatures
in excess of 20 000 K and v sin(i) of about 160 km/s. For stars like the
sun (class G, surface temperature 6000 K), this value is more like 4 km/s
and for stars that are fainter than the sun, this may be as low as 1 or 2
km/s. This might cause you to think that the Sun rotates pretty slowly, on
average. 

However, about 90% of stars in our galaxy belong to classes K and M, which
are fainter than the Sun. This means that the Sun actually rotates faster
than most of the stars in the Milky Way. It's only the rare, bright star
that's a rapid rotator.