Stellar Velocity Page 12

Applications of Velocity

We can use the Doppler method of measuring radial velocity for any luminous celestial object, be it a star, nebula, cluster of stars or an entire galaxy. In this way astronomers have build up a detailed picture of motions within our galaxy and between us and other galaxies at great distances. Below we will briefly discuss further specific applications of this method in modern astronomy.

Rotation of a star

We will never be able to see the size of a star in a telescope, no matter how close the star or how good the resolution of the telescope is. Even the nearest stars are too far away. But we may see an effect in the line spectrum of a star due to the rotation about its own axis.

Unless we look down onto one of the poles of rotation of the star, one side of the star will move towards us while the other side will move away. This means that the line spectrum will show both redshift and blueshift of the same lines, which results in a broadening of the lines.

Original image from
www.hsc.csu.edu.au (edited)

Individual lines can be broadened due to the abundance of a certain element, we discussed that in the Module "Stellar Radiation". But when several lines are equally broadened, this means that we have a measure for the rotation of the star. This effect can be enhanced by dynamic processes in the star's photosphere, which will make a decison about the actual rotation of the star more difficult.

There are many physical properties of a star that astronomers try to unravel from a line spectrum, and this rotational broadening can be a source of additional information as much as it can be a nuisance, because it obscures information about other properties. So it would be incorrect to suggest that the broadening of spectral lines will simply tell how a star rotates, but in some cases, especially for nearby giant stars, this may be the case.