Cepheid Variables in Distant Galaxies

In summary: Without knowing the actual math it gets a little confusing when you have to pieces together things from 15 different sources.In summary, the Cepheid variable in question would show time dilation effects due to the expansion of the universe.
  • #1
Ty1erC
10
0
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.
 
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  • #2
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
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
Periodicity is the only significant variable once you know the type of Cepheid being observed - which is rather easily determined by its spectrum.
 
  • #5
Chronos said:
Periodicity is the only significant variable once you know the type of Cepheid being observed - which is rather easily determined by its spectrum.

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
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
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
Chronos said:
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.

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
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
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.
 

What are Cepheid Variables?

Cepheid variables are a type of pulsating star that exhibits periodic changes in brightness. They were first discovered by astronomer Henrietta Swan Leavitt in 1908.

How are Cepheid Variables used in astronomy?

Cepheid variables are used as a standard candle for measuring distance in the universe. This is because their period of pulsation is directly related to their intrinsic brightness, allowing astronomers to calculate their distance from Earth.

Why are Cepheid Variables important in studying distant galaxies?

Cepheid variables are important in studying distant galaxies because they can be seen and measured at great distances, providing valuable information about the size and age of the universe. They also allow astronomers to create a cosmic distance ladder, which is used to measure distances to even more distant celestial objects.

How do scientists identify Cepheid Variables in distant galaxies?

Scientists use a technique called photometry to identify Cepheid Variables in distant galaxies. This involves measuring the amount of light emitted by stars in different colors, which can reveal changes in brightness over time.

Can Cepheid Variables be used to measure the expansion of the universe?

Yes, Cepheid Variables can be used to measure the expansion of the universe. By measuring the distances to galaxies at different points in time, scientists can track the rate at which the universe is expanding. This has led to the discovery of the accelerating expansion of the universe and the existence of dark energy.

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