# Percentage of Dark Matter

1. Dec 6, 2015

### Huej

If luminosity increases, hypothetically, for a given galaxy, would the percentage of dark matter in that galaxy be smaller? What if the colors of the stars were to turn redder?

I was thinking if the luminosity were to increase, that would mean the percentage of the normal matters increases, thus the percentage of dark matter would decrease. But when I looked at the formula: Mass of stars: (M/L) ratio * Luminosity of stars, I was thinking the mass would be constant since the luminosities in the equation would cancel out.

2. Dec 6, 2015

### rootone

Other than through gravity, 'Dark matter' only interacts very rarely with 'Normal matter', (if it does at all).
Therefore It should not significantly affect the luminosity of stars.

3. Dec 6, 2015

### Huej

I don't think it's much of a interaction of dark matter and normal matter, but more of how one indirectly affects the other. So if there is higher fraction of dark matter, there would be smaller fraction of stars/gas (assuming total gravitational mass of the galaxy is constant) and impact the total luminosity. The question that I'm trying to answer does say smaller/larger, so I'm wondering which of the two, even if the impact is small. Wondering if my reasoning makes sense though.

4. Dec 6, 2015

### Chronos

I cant think of any reason the presence of dark matter would impact luminosity. We know of galaxies with great variance in the amount of dark matter present and it apparently has no apparent effect on their luminosity or spectral features.

5. Dec 6, 2015

### rootone

Luminosity means the amount of photons which are getting emitted.
Apparently dark matter does not have any consequences at all for photons, (other than gravitational lensing)
... but that is by no means a dead certainty.

Last edited: Dec 6, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook