How do we know how far away a star is?

Question

If scientists say a star is 10 light years away, how do they know that its not 5 or 20 instead? It's not like you can use a tape measure?

I love mantle two's theory that all stars are between 1 and 12 inches away. Classic!

Answer 1

There are generally three ways of estimating stellar distances. 1) Have you noticed when driving down the road that trees and houses near the road appear to move in the opposite direction faster than those further away? If a house is a mile from the road you might be going 70 mph and the house seems almost to be standing still. This is called parallax. For the nearer stars scientists photograph two stars which appear close together in the sky during winter, say, then take another photograph of the same stars during summer after the earth has moved over 180 million miles. They then compare the two photographs to see how much the stars appear to have moved. 2) Light varies in brightness according to the inverse square law. If you move a light source twice as far away it will appear only 1/4 as bright. Three times as far away only 1/9 as bright. Some stars are what are called variable stars. Their light varies in brightness over time. Scientists have discovered that for some of these variable stars the rate at which they vary in brightness depends upon how bright they really are, not how bright they appear to us. So by comparing how bright they appear to us versus how bright they really are and by doing a little calculation using the inverse square law scientists can get a pretty good idea of how far away they really are. 3) For stars really far away, scientists use the theory that the universe is expanding and therefore everything in it is moving away from everything else. The farther away an object is the faster it is moving. Think of blowing up a polka dot balloon. As you blow it up the dots move away from each other at an increasing rate. All forms of energy which move in waves are subject to the Doppler effect. Have you noticed how a train horn sounds higher in pitch when it is coming toward you than when it is moving away? Same with race car engines. This is the Doppler effect. It also occurs with light. Light from objects moving away from us is shifted towards the red end of the spectrum. The faster the star is moving away the more red shift. If we know what color light the star is actually producing we can get a good idea of how far away the star actually is.

Answer 2

Up to about 150 light years astronomers use parallax. Here is how they do it.

They measure carefully the position of a star against the position of other stars nearby (nearby, in the sky that is). Six months later they do the same. If the position of the target star changes with respect to the others, and they can determine which of the others have not changed with respect to each other and are much further away, then the angular change is put into the formula tangent (a/2) = distance/r, where a is the angle they measure and r is the radius of the orbit of the earth.
(They measure the angle using the diameter of the orbit of the earth, so for the equation they must take one-half the diameter).

For stars farther than that they must estimate the luminosity of the star, using its stellar type (from the Hertzsprung-Russell diagram) from it's spectrum, and estimate distance using it's apparent brightness using the inverse-square rule. If there is a lot of dust between us and the target star then this method is subject to large error.

Galactic and intergalactic distances are estimated in two ways. The first is by use of the redshift, in which distance is (said to be) proportional to velocity of recession. The second is to watch for supernovae or novae, which have a tendency to exhibit approximately the same luminosity for each type, again invoking the inverse square rule for luminosity decrease in a given distance. Obviously the parallax method is more accurate for nearby objects, but the others are so far away that small errors do not mean much.

Answer 3

You know that the fence by the side of the road moves past faster than the tee on the horizon as you drive past? This effect is called parralax. It causes nearby stars to move compared to the most distant ones as the earth orbits the sun. They wiggle back and forth.

This wiggle over the year can measure stars up to 300Light years (LY) away.

There are so many stas within 300LY away that we can fit all stars into certain categories according to their colour.

By checking the colour of any star, regardless of it's distance, we can put it into one of those categories. From this, we can work out how far it is.

Finding the distance of stars from their wiggle in the sky (trigonometric parallax) is less accurate the further away it is. 300LY is when it become so inaccurate that the colour method (specroscopic parallax) is considered more accurate.

Trig Parallax is 10-50% inaccurate. Spectroscopic parralax is >50% innacurate.

Hey ho. What can you do? Like you said, it's not like you can get a tape measure.

Answer 4

For nearby stars, within a hundred or even a few hundred light years they measure the parallax. The parallax is the slight shift in position of a nearby star as seen against the background of more distant stars that is caused by the Earth's orbital motion around the Sun. But the angle is EXTREMELY small even for nearby stars, and gets smaller and smaller for more distant stars. For more distant stars there are a number of indirect ways to measure the distance. Most of them have to do with figuring out how much light the star actually produces and then calculating its distance from how dim it looks to us.

Answer 5

They use trigonometry to determine these distances. There are properties of light and of triangles which you can use to determine really big distances. For a smaller example, you can find the height of a building by measuring your distance from it and the angle you need to tilt your head to see it. A similar method can be used, except with two people very far apart. If you know the distance between the people and the angle each person needs to look up at, you can find the height of the star with a little math. Of course, your measurements have to be extremely accurate.

Answer 6

they don't use redshift and blueshift with stars that difference is too small, they only use that with galaxies.

with nearby stars they can use parallax. like when you close one eye then close the other and open the one that you first closed (see the camera one, camera two scene in wayne's world). they take a picture of the stars at one side of earth's orbit, and then again at the other side of the orbit. (the orbit around the sun not the orbit around the earth) then they can calculate how far away the star is with trigonometry.

there was a satellite, its name escapes me right now but it measured the parallax to hundreds o stars with a lot of accuracy while it was in space.

other stars they can figure out what type of star it is and then find its apparent brightness (magnitude) then calculate how far away it is based on that.

Answer 7

There are shifts in the spectrum of light we receive from a star which vary by distance. Astronomers have developed very complex equations to translate the specific observed spectrum shifts into a close estimate of distance.

Answer 8

They know how bright a star should be by the spectrum of the light. They tell the distance by the difference in how bright it is and how bright it appears. For example, our sun, the further away someone observes it the dimmer it would appear. They could tell the distance by how bright it appears as to how bright it should be.

Answer 9

Take a yardstick out to your patio at night,stand it on a table or something, and point the other end on the star,line it up and count the inches.
I've found that the stars closest to the horizon are closer than the ones in the middle of the sky.

Answer 10

you can research this using the key words 'cepheid variable' and 'henrietta leavitt'. henrietta leavitt was this female scientist who discovered how to measure distances between the earth and stars and other planets by using cepheid variables. cepheid variables are stars whose lumonisities change periodically. i'm not sure how but she worked out a way to measure the distance between the earth and the host clusters of cephied variables and then extended her research to distances between the earth and planets and other constellations.