# How are Cepheids used to measure distances?

• Mooky
In summary, Cepheids are stars whose brightness varies periodically. The longer their period, the brighter they are. Different Cepheids have different brightness and periods, and this is used to measure distances. Uncertainties in the calibration of the Cepheid relationship led to several re-assessments of the size of the universe.
Mooky
Hello,

I've read about this in four different textbooks and in several websites, but I still don't get it: how are Cepheids used to measure distances? I understand that Cepheids are stars whose brightness varies periodically; that the longer their period, the brighter they are; and that different Cepheids have different brightness and periods.

But, how is that used to measure distances?

Thanks,
M

Note that the longer the period, the higher their intrinsic brightness. So you measure the period, and then from the relation between period and intrinsic brightness, you now know their intrinsic brightness - that is how bright they really are. Then you measure their apparent brightness - that is how bright they appear. Because of the inverse square law, if we have two objects of the same intrinsic brightness at two different distances, the one further away will appear fainter, just like with two identical street lights - the one further away will appear fainter. So if I know the intrinsic brightness and the apparent brightness, I can calculate the distance. Mathematically, the relation between the two is given here:

http://en.wikipedia.org/wiki/Magnitude_(astronomy)

That explains nothing, mate. They would need a Cepheid close enough for parallax measurement to use as a standard first.

Cryptonic said:
That explains nothing, mate. They would need a Cepheid close enough for parallax measurement to use as a standard first.

In fact, for many years, this point was the weakest link in the astronomical distance scale, since no Cepheids were close enough to use parallax to measure their distance. So historically other methods were used to get the distance to the nearest Cepheids and calibrate the relationship. Uncertainties in the calibration of the Cepheid relationship led to several re-assessments of the size of the universe. Try reading this:

However, this is no longer an issue, since the Hubble space telescope was now able to measure the parallax of Delta Cephei, the prototypical cepheid. See this:

http://en.wikipedia.org/wiki/Delta_Cephei

Cryptonic said:
That explains nothing, mate. They would need a Cepheid close enough for parallax measurement to use as a standard first.

And there now 273 such objects. There should be many more when Gaia launches.

Thank you, phyzguy. I wish textbooks were as clear and concise as you are.

## 1. What are Cepheids and how are they related to distance measurement?

Cepheids are a type of variable star that pulsates in a regular pattern, with a period of several days to a few weeks. Scientists have found that there is a direct relationship between the period of pulsation and the absolute magnitude of the star, making them ideal for distance measurement.

## 2. How are Cepheids used to measure distances?

Cepheids are used as a standard candle, which means that their known luminosity can be used to calculate their distance from Earth. By observing the period of pulsation and comparing it to the star's apparent magnitude, scientists can determine the distance using the inverse square law.

## 3. Why are Cepheids important for measuring distances in the universe?

Cepheids are important because they are one of the few objects in the universe that have a known and consistent luminosity. This allows scientists to accurately measure distances to faraway objects, such as galaxies and supernovae, and gain a better understanding of the size and scale of the universe.

## 4. Can Cepheids be used to measure distances beyond our own galaxy?

Yes, Cepheids have been used to measure distances to galaxies beyond our own Milky Way. They have been used as standard candles to measure the distances to other galaxies in our local group, as well as in more distant galaxies up to tens of millions of light-years away.

## 5. Are there any limitations to using Cepheids for distance measurement?

While Cepheids are an important tool for distance measurement, there are some limitations. For example, they are only useful for measuring distances up to a certain point, as their pulsation period becomes too long to accurately measure beyond a certain distance. Additionally, Cepheids can be affected by factors such as interstellar dust and metallicity, which can impact their luminosity and make distance measurements less accurate.

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