# Light in astronomy

1. Mar 3, 2006

### AstroCook

Hi all,
I'm currently working on a uni essay, the subject of which is how the properties of light are (and have been) used to discover and observe objects in astronomy. So far I'm looking into how the value of c is used, how doppler shift is used to determine velocities, how looking at the different parts of the EM spectrum can give a different view of the cosmos and something else that emcompasses spectroscopy/black body curves. If anyone can suggest additional relevant topics or websites that would be greatly appreciated or if you just want to discuss the above subjects that's also great.
Also, I read that the faster a galaxy rotates the greater its apparent brightness. Can anyone confirm this, if so would you be able to explain how it works or recommend a good site?
Thanks.

2. Mar 5, 2006

### SpaceTiger

Staff Emeritus
You could also discuss polarization measurements, which are important in most areas of astronomy. The topics you listed are far too broad for a specific discussion in this thread. Perhaps you can zero in on aspects of them that confuse you.

The faster a galaxy rotates, the more massive it is. The more massive it is, the brighter it is (usually). However, this would be absolute, not apparent brightness.

3. Mar 6, 2006

### Nereid

Staff Emeritus
Note that it doesn't work the other way round (as far as we know) - some of the brightest (absolute, not apparent) galaxies seem to have little rotation - the cD monsters which reside at the hearts of rich clusters.

4. Mar 6, 2006

### SpaceTiger

Staff Emeritus
Yes, thank you Nereid, that's an important distinction that I neglected to make. There is actually a very tight relationship between luminosity (or absolute brightness) and rotation velocity known as the Tully-Fisher relationship:

$$L_s \propto v^4$$

Note, however, that is only applies to spiral galaxies. Elliptical galaxies obey a similar relationship, but it instead relates luminosity and velocity dispersion:

$$L_e \propto \sigma^4$$

This is known as the Faber-Jackson relationship and is part of the fundamental plane of elliptical galaxies. The distinction is made because, as Nereid said, elliptical galaxies don't exhibit organized rotation -- the stellar velocities are more isotropic. In both cases, however, you're measuring a quantity that corresponds approximately to the mean speed of stars at large distances from the center of the galaxy.