Light year?

[paynesmanor]

I just want to understand this, A light year is the distance light travels in a year, So to say 300 million light years away, means 300 million years away, at the speed of light! Now for my real question how do they know that its that far away? And exactly how many miles is a light year?

[craterchains]

186 million miles an hour I think?!?

So from there do the math for hours in a year.

How do they know it is that far away?
There are about three ways now I think to determin the distance to nearby stars and galaxies. Could be more.

[BMAONE23]

light travels 669,600,000mph X 24hrs = 16,070,400,000 miles per day X 365 = 5,865,696,000,000 miles per year
5 trillion 865 billion 696 million miles = 1 light year

[iamlucky13]

Now for my real question how do they know that its that far away? And exactly how many miles is a light year?

That part of the question is complicated. Craterchains listed said there were three methods or more, and I think he's right, but I'm not sure I'm thinking of the same three.

The first is called parallax. Basically it's like triangulation. You measure the angle you view an object when the earth is in one position around the sun, then again six months later. This gives you two angles and a side length for the triangle (9th grade geometry: angle-side-angle)

According the Hubble and the Big Bang Theory, the universe is expanding, and you can use the doppler effect to calculate the distance. The further an object is, the greater the red shift. However, the movement of a particular object may upset this, especially close up where the red shift is small. For example, the Andromeda Galaxy is actually moving towards us, so this method does not work there.

The third method works only if you know how big an object is. Then you can calculate the distance by comparing the known size to the apparent size. However, often we determine how big something is by how far away it is.

Oh, I just thought of another. Based on the color spectrum of a star, you can figure out pretty well how bright it should be, and calculate the distance based on actual brightness.

[cosmo_uk]

you can also use "standard candles" - this is a class of object that has a known luminosity therefore any differences in said luminosity between 2 such objects will be down to distance.

(its a little more complicated than this but you get the idea)

eg

large cluster elliptical galaxies were used to increase the range of the hubble diagram.

more recently type 1a supernova (white dwarf detonation nearly always has same mass ) this was the observation that discovered dark energy

potentially gamma ray bursts could be used for very high red shift studies in the future

[NoelC]

To expand a bit, certain types of variable stars with a given period are predicted to be of uniform brightness. Detecting these stars at an unknown distance, based on the variable brightness period, then measurement of the absolute amplitudes of the brightness (and knowing how fast brightness falls off over distance), can give a measurement of the variable star's distance from us. This is useful for measuring the distance to nearby galaxies, for example.

-Noel