How do astrophysicists determine how long ago starlight left its star and reached our planet?

How do they know if star light took a billion years to reach us?How do astrophysicists determine how long ago starlight left its star and reached our planet?If is star is close enough, simple geometry can be used to determine distance. The angle through which a star apparently moves as the earth moves from one side of its orbit to the other can be measured and the distance determined.



Most stars, however, are not near enough for that. So other techniques are used. Some stars vary in brightness, they expand and contract in size, in a rhythm directly proportional to their actual, or absolute, brightness. From a star's absolute brightness, or luminosity, the distance to the star can be calculated.



Yet another way, is to look at the spectrum of a star. Many stars with the same spectrum turn out to have the same luminosity. So, a really dim star with a certain spectrum will have a determinate luminosity, and so its distance can be calculated.



This last method, while widely used, has its problems. If the light of the star goes through dust and gas, it may appear dimmer than it actually is, and so its distance appears greater than it actually is. Consequently, mapping regions of the galaxy with lots of dust and gas is important to astronomers who try to estimate the distance to stars.



HTH



CharlesHow do astrophysicists determine how long ago starlight left its star and reached our planet?They can compare the readings they get from x-rays, light and other sources from that start to determine its distance, and then it is just a simple v*t = d problem... for the most part.How do astrophysicists determine how long ago starlight left its star and reached our planet?the speed of light is known from laboratory measurements.



the distance to the star is determined by various measurements.How do astrophysicists determine how long ago starlight left its star and reached our planet?
Depending on how far away it is, you can use several different methods.



For stars relatively close to us, we can use parallax. That's where you take measurements 6 months apart, so the earth is on opposite sides of the sun. And you calculate a massive triangle, knowing the angles and the distance between the earths two points in its orbit.



When they're further away, you can use Cepheid variable stars, and further out again, you can use certain types of supernovae.