How do astronomers measure distances? How do they know a galaxy is 10 million light years from Earth?

sequoianoir

Nearby stars appear to move with respect to more distant background stars due to the motion of the Earth around the Sun. This apparent motion (it is not "true" motion) is called Stellar Parallax. This motion can be seen by just looking at an object a few feet in front of you and alternate your view between each of your eyes. With respect to the background it will "move" from left to right to left as you have first left eye closed and right eye open, and then reverse them. This only works with "close" stars and the movement gets less and less the further the star is from the Earth. Jan 09 04, 1:37 PM

sequoianoir

Yes it is triangulation, but this is only possible for "close" stars. More distance stars and galaxies are calculated by a few methods. A candle viewed from 10 feet is a lot brighter than when viewed from 50 feet, 100 feet or 1000 feet, if it is visible at all. Knowing how bright a candle is at source, how much light it gives out, and then measuring its apparent brightness from an unknown distance allows its distance to be calculated. Unfortunately different candles, and similarly different stars, burn hotter and/or brighter and there is no standard from which to make the calculation. However collecting the light from a star and analysing its chemical composition and makeup using a spectrometer gives much of the information required. This is easily demonstrated just looking at different flames found on Earth, such as candle flames, bunsen burners and acetylene torches. Yellow flames are relatively cool, blue flames much hotter and white very hot. Other calculations can give its mass and hence size. Another parameter used is its "Red shift". Light, just like sound, the Doppler effect, changes frequency if the source is moving. With sound it is why a police siren changes note as it passes you by. Coming towards you the speed of the car is added to the speed of sound, having passed it is subtracted. With light there is a color shift and the light appears to have more red in it. The faster it is moving away then the redder it will appear. It is known that stars and galaxies that are moving away at greater speeds are also more distant. Jan 09 04, 5:23 PM