# Cepheid Variables in Distant Galaxies

1. Feb 15, 2012

### Ty1erC

I'm trying to fully understand how Cepheid variables are able to give us accurate distance approximations to galaxies. If these galaxies are receding away from us at ever-increasing speeds, wouldn't we notice the variable stars' periods getting longer and longer over time? How can we accurately know these periods and thereby know the stars' luminosities?

Some aspect of this has to be going over my head.

2. Feb 15, 2012

### mathman

There are two problems with your assertions.

Cepheids can't be seen in far distant galaxies. Standard candles for distant galaxies are supernovae Ia.

Although the universe is expanding, the rate is too slow, compared to ordinary earth time, to see any change in the periods.

3. Feb 15, 2012

### Staff: Mentor

Well, let's assume we have a telescope big enough to see an individual cepheid variable about 5 billion light years away. Would it's period be noticeably longer than it should be compared to a cepheid variable in our own galaxy? Enough to throw off our distance measurements significantly I mean, since we can't really tell just from observing the star whether it's period is longer or shorter than it "should be" since we don't know the exact mass and composition of the star.

4. Feb 15, 2012

### Chronos

Periodicity is the only significant variable once you know the type of Cepheid being observed - which is rather easily determined by its spectrum.

5. Feb 15, 2012

### Staff: Mentor

Knowing its spectrum, would said cepheid variable at 4 billion LY from us show a slower periodicity than a similar one in our galaxy?

6. Feb 15, 2012

### Ty1erC

it was my assumption that cepheids could be used 'across the board' as a standard candle that was causing my confusion. thanks.

but if they could be used, i would presume that the periodicity would be seriously affected by the expansion of space. the further away a galaxy is, the faster it is racing away from us. again, i could be wrong.

7. Feb 16, 2012

### Chronos

The periodicity of a cepheid 4 billion light years distant would definitely show time dilation effects. Of course that is far beyond our present detection capabilities. Cepheids can only be detected out to about 30 mpc at present. That is too close to yield any meaningful measurement of time dilation due to expansion.

8. Feb 16, 2012

### Staff: Mentor

Outstanding! I always wondered if your normal time dilation rules applied at the cosmological scale. Without knowing the actual math it gets a little confusing when you have to piece together things from 15 difference sources.

9. Feb 16, 2012

### Ty1erC

would it be correct to call the effect time dilation? the galaxy in question isn't flying away from us through space, but space it self is expanding. wouldn't the longer period just be a result of the 'light source' emitting its photons from an increasingly greater distance?

10. Feb 16, 2012

### Chronos

Time dilation of light curves is definitely a factor in high z supernova studies. The basic rule is 1+z gives you the expected amount of time dilation. This was corroborated by, among others, Goldhaber, Riess and Perlmutter in their papers of the late 90's.