How is the distance of stars measured??

Question

We know Alpha Centauri is 4x10^x kms away from earth. How do tehy measure the distance???

Answer 1

Nearby stars are measured by parallax. Astronomers measure the relative positions of the stars, then wait six months (for the Earth to reach the opposite point of its orbit around the Sun) and measure again. The closest stars will have shifted their relative positions slightly, and their distance can be calculated by how much they moved. This can only be used for stars up to a few hundred light years away - the angles become too small to measure after this.

For stars further away than that, astronomers have to measure the apparent brightness of the star along with its colour spectrum - the colour spectrum indicates how big the star is and how much light it emits. Because the apparent brightness of a light source drops off predictably with distance, once astronomers know how large/bright the star should be and how bright it appears to us from our vantage point, its distance can be calculated.

Other techniques are also used too - some types of star behave in very predictable ways. These are referred to as "standard candles" (very big candles at that!) Because their true brightness is known, this can be compared with how bright they appear to us and their distance calculated. Red shift is also used to measure very distant objects - since the universe is expanding, light sources moving away from us have their spectra shifted towards red (this is known as the doppler effect - it's the same effect as when you hear a siren change pitch as the vehicle passes you, but with light instead of sound). The further something is away, the further it is shifted towards the red end of the spectrum.

Answer 2

There are several techniques, and parallax (first response) is one of them.  Another has to do with stars called Cepheid variables, which change their brightness in a regular manner depending on their size -- and maxium brigthness.  By measuring the period and the observed brightness, you can tell how far away it is.  Yet another is by red shift: at great distances, individual variations average out and the red shift of a star's spectrum gives its velocity -- and hence its distance.

Answer 3

In the case of nearby stars it is indeed parallax.

For star more than a few hundred light years away - this doesn't work

There are various method that can be used.

Check out this site for an overview:

http://www.uwgb.edu/dutchs/CosmosNotes/d...

and

http://en.wikipedia.org/wiki/Parallax#St...

Most of the other methods use some way of determining how bright stars really are, and we can measure how bright they look and thus calculate how far away they must be

Answer 4

By what's called parallax; how much its position in the sky seems to change as we go round the sun.

Answer 5

There is a great book actually called “Parallax” that explains how scientists through the ages developed the ability to measure stars’ distances.

The amount the nearest star seems to be displaced in the sky by the Earth moving from one side of its orbit to the other is about 1/4000th of a degree.  That is such a small displacement that if you haven’t got instruments accurate enough and stable enough you are going to miss it.

It took til the 19th century before the instrumentation became good enough to take a stab at it, but the 20th century to really get accurate.  

It’s an amazing story because back in the 17th century they were still trying to quell the religious belief that God created the stars fixed in the heavens.  When they couldn’t find any parallax displacement in the stars, even viewed from nearly 200 million miles apart (across the width of the Earth’s orbit), the astronomers of the times could not prove the religious aspect wrong, and had to concede that the church may have been correct.  

What they didn’t know was just how massive were the distances involved, and of course they had to take pot luck in choosing stars to test, having no idea which ones were the closest.  

How lucky we are to live in an age where we know how impossibly far away are the stars, and that they are all suns.  Imagine not knowing that?