MadSci Network: Astronomy

Re: When I look at a star what do I see.

Date: Mon Aug 6 12:30:15 2001
Posted By: Chris Lintott, Undergraduate, Astrophysics, Magdalene College
Area of science: Astronomy
ID: 995577530.As

First of all, sorry for taking a while to answer.

Good question! Light is the fastest thing in the universe, but it does 
travel at a definite speed - it takes time to get places. In fact, the 
speed of light is about 300,000 kilometers every single second. At that 
speed, light from the Sun takes 8 minutes to reach us. This is a 
problem when we try and control spaceprobes - any signal sent to Jupiter, 
for example, would take several hours to reach the probe - not much good if 
you want to send an urgent signal.

The same applies to light from the stars - the light from Alpha Centuri 
left the star about 4 years ago so you are seeing the star as it was then. 
Any aliens on Alpha Centuri who happen to be looking at the Earth will see 
it as it was in 1997! If we look at the furthest galaxies, we can be 
looking many billions of years back in time!
Light travels much faster than sound. For example if you stood beneath Big 
Ben in London with a radio, you would actually hear the bongs from the 
radio first, before you heard them directly. This is because the radio 
waves travel at the speed of light, and so reach you before the sound does, 
even though they have to travel much further (to a studio and back)

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