How is distance in light years calculated?

Question

I always have wondered how is some distance between earth and other universe or distant asteriod is calculated in light year?

Say for example i recently saw a news saying a asteroid to be away from earth about 900 light years,Since one cant practically test this out because a human life time will not be enough...so how is it calculated and what is the proof of its correctness??

Answer 1

Close one of your eyes and holdup a finger in front of you. Then shut that eye and open the other and you'll see that the finger appears to move (because the place you're viewing it from has changed). That's parallax, mentioned in the answer by Vincent above. If we look at a celestial object through a telescope on two dates six months apart (so the Earth's right on the other side of the Sun the second time) its apparent movement gives us a very accurate idea of how far away it is (1 parsec is a parallax second of arc with Earth's orbit as a baseline, which is about 3.3 light years).

The further away in space a celestial body, the less it will appear to move by this method, until there is no apparent movement. However, we're lucky that we have what are called "standard candles" as the next best thing. Certain types of star will always shine with the same luminosity, so when we measure their apparent brightness from Earth we can also tell how far away they are. When these stars are relatively nearby, we can confirm our measurements using parallax, to ensure they're correct. Other standard candles exist for different distance scales. The farthest is something called a type 1a supernova. This is a massive explosion detectable billions of light years away, but always the same brightness. Hence we can tell reasonably precisely how far away (and so also how far back in time) the explosion took place.

As for the other part of your question, an asteroid 900 light years away would be too small and dim for us to detect. However, we have started finding large (Jupiter-sized) planets around other stars at that sort of distance, and when the Kepler probe is launched next year it will be able to detect Earth-sized planets at that distance.

Answer 2

Light travels about 5.8 TRILLION miles per year--- just multiply 5.8 trillion times 900 !! See the numbers are so large they become nonsense.

Answer 3

All I know is a light year is 5.8 trillion miles. Light travels at a speed of 186,000 miles a sec.

Answer 4

We'll start with intrinsic brightness.

Imagine that you are at a standard distance from the Sun.  Maybe it's a light year.  How bright is it?  Now imagine that you do the same thing for Vega.  Is Vega (from 1 light year) brighter or dimmer than the Sun (from 1 light year).  If you know the distance to a star, and you know how bright it seems to be, you can compute it's intrinsic brightness.  That allows you to compare one star to another.

You can also compare stars to each other by the study of the spectra of the light they produced.  And, you can tell if one star is very much like another.  So if you have a nearby star, whose intrinsic brightness you know, and another star, that matches it, you can work out how far away it is.  You know it's intrinsic brightness, and you know how bright it appears to be.

For nearby stars, the distance can be determined by looking at the star, then 6 months later, looking again.  It will have moved with respect to the background very distant stars.  This movement is an angle, and simple geometry can get you the distance.  It's a bit more complicated than this, but that's it in a nutshell.

For extremely distant stars, in galaxies billions of light years away, you need a star that is bright enough so you can see it.  That would be an exploding star.  Such stars can be a billion or ten billion times brighter than normal.  Their light can be separated from other stars in their host galaxy.  Now certain exploding stars, type 1a supernova, have known intrinsic brightness.  So if you measure how bright it appears to be, you can work out how far away it is.

You can also look at it's red shift - an artifact of the expansion of the Universe similar to the Doppler shift, you can find out how fast it was moving away from us at the time. So you can calibrate red shifts to the distances you've found.

I'm leaving out a few details, but that's it in a nutshell.

I highly recommend Alex Filippenko's What's new in Astronomy, 2003 video series from The Teaching Company.  Opps.  I don't see it there.  Well, he's great, so perhaps his new course: Course Image   Understanding the Universe will do it for you.

He also goes over this in an October 2006 public lecture that is available as an audio podcast.  He does refer to slides, and i've seen them in his video series, but i think you'll get the idea from the audio alone.  He's quite descriptive.  And there are some great questions he answers at the end.

Answer 5

for example, if u want to measure the distance to a star, u use the method of parallax. thats where u measure the position of a star in the sky at different points of the earths orbit around the sun, and use trigonometry to solve it

Answer 6

  We know light travels about 30 billion cm per second or about 186,000 miles.

  The sun is 8 light minutes away. It's distance and the distance to the closest stars is done by triangulation.

  Farther bodies are estimated by apparent brightness.

  The light that left the body in question started towards you 900 years ago.

Answer 7

There are a few methods to evaluate distance. For relatively near objects (but outside of the solar system) parallax is used. This involves assessing the apparent motion of the star against a background of more distance stars due to the motion of earth in its orbit around the sun. This creates a triangle with a small angle, that is nevertheless measurable. From this method the unit "parsec" was coined: it is the distance that an object would be at in order to have this apparent motion to be 1 arc second (1/3600 of a degree) wide. A parsec is about 3.26 light years. This method works up to about 300 light years before the error grows to much.

Beyond that, other methods, like intrinsic luminosity (the fact that the absolute brightness of a star is a function of its spectral type, and that its apparent brightness is thus dependent on its distance) are used. This method also applies to the intrinsic brioghtness of certain type of supernova, which can then be gauged to determine the distance to the galaxy that contains said star that went supernova.

For very distant objects (million of light years away), the Doppler redshift, using the Hubble constant, is the best method.

Answer 8

you would have to take the speed of light and times it by 900 to get its distance by taken a few angles and the color of a star is how they come up with its distance have you ever seen workers on the side of a road looking into a device and they have one on bothsides of the road they are taking readings to calicalate a distance same theory applies in space just more advance