Roma

I am curious as to how astronomers measure the distance of stars. I read that the triangulation method is good only for stars that are up to about 100 ly away, and for stars further than that, something called "cepheid variable" is used. I kind of understand how it works, but does anybody have any information or links as to how accurate this method is? Is this the most accurate way of measuring the distance to these stars? Thanks.

edit to add: Are there any legit criticisms of the "cepheid variable" method?

-Roma

EvilMudge

Cepheid variables work on the basis that the luminosity (source brightness) of the star varies over a period of time proportional to the mass of the star. So if you know how bright the star is, then by measuring it's apparent brightness you can work out how far away it is.

The only problems with it are the following:
Is the star being obscured by dust clouds (you have to do spectral analysis to be sure)?
Is this star really a cepheid variable? Is it an eclipsing binary that appears to be a cepheid?
In the first case you must estimate the absoprtion of the dust. In the second they are primarily used to measure the distance to a cluster or galaxy containing numerous cepheids, so that the errors will average out.

There are also candidates for a similar method for galaxies, whereby distances to clusters can be evaluated using similar means.

Asha'man

Here is a nice link that talks about measurements using Cepheid stars: Finding Distance To Nearby Galaxies Using Cepheid Variable Stars

The idea is simple: if we know the absolute brightness of an object, and we know it's apparent brightness, then we can compute the distance.

There is a direct relationship between the period of a Cepheid variable star and it's absolute brightness, so we can use this to measure the distance to nearby galaxies.

For galaxies that are further away, the easiest measurement is to look at the redshift of the galaxy. Since the universe is expanding uniformly, the speed at which a galaxy is receding from us should be a good indicator of it's distance. There is some error here, because the galaxy may have some motion of it's own.

More recently, another form of "standard candle" has been found, the type Ia supernova. Due to understanding the mechanism involved, we know that a certain type of supernova always happens to a star with the same mass. Knowing the amount of mass, we therefore know the amount of energy produced, and therefore the absolute brightness. Just like the Cepheids, knowing the absolute brightness and the apparent brightness easily yields a distance measurement. This technique is used for the furthest measurements, out to about 12 billion light years.

As far as accuracy goes, these techniques are all pretty accurate, there is no controversy about these methods that I am aware of. The galaxy redshift technique is probably the least accurate, but I think it easily produces measurements within 5% of the other techniques. That is probably an important thing to note: these different techniques have been tested against each other, and all produce essentially the same results.

Friar Bellows

There are dozens of different ways to measure astronomical distances. This page summarises some of them:

http://www.astro.ucla.edu/~wright/distance.htm