How do they measure distance in light years?

Question

Astronomers tell us that Star X or Galaxy Y is 10 billion light years away. How do they know? They have never been there. They don't have a ruler telling them that the star is that far away. What are they using to help them to tell us about these vast distances?
And how can they measure the Milky Way Galaxy? They tell us it is 100 million light years across. What is the standard they are measuring it with?
Sorry, I never took physics or astronomy, so put it in layman's terms if possible.

Thank you darkphoenix262 for your correction on the size of the Milky Way Galaxy.

Answer 1

All these answers mentioning parallax, but in essence, measuring a star's distance using the Earth's orbit as a baseline is basic trigonometry.

Imagine an iscoceles triangle, where the base is the "diameter" of the Earth's orbit, and the sides are the line from the Earth to star at, say Jan1, and the line to the star at July 1.

It's such a long thin triangle because although the Earth's orbit is 300 million km across, the distance to the star dwarfs it to such an extent that the angle at the top of the triangle (mathematically speaking, the angle that subtends the Earth's orbit at the star), is less than one second of arc for the NEAREST star.

Though the angle is tiny, it can be measured for the nearest stars.

An easy way to work it out is that a radian, which is the angle subtended by the length of a circle's radius projected onto the arc of the circles, is about 57 degrees. Therefore, if the parallax angle to the star is one second of arc, ( a scond is one 3600th of a degree), then the distance to the star is 57 times 3600 times the width of the Earth's orbit.

This distance is termed a Parsec, and is also another measure astronomers use, and is about 3.26 light years. No star is that near, as I said above, the nearest star has a parallax angle of less than one second of arc.

Answer 2

The distances of the stars are so great that astronomers have found it too inconvenient to speak of them even in millions of miles. They measure the distance to a star by the time it takes the light from the star to reach the earth. The unit is a light year: the distance that light travels in a year. On method for measuring the distances makes use of trigonometry as follows: If we know the length of the base of the triangle and the size of the two angles at the base, we can find the distance to the apex of this triangle. In order to have a triangle with a base long enough, we sight a star in the summer, and then again in winter, when the earth is on the opposite side of the sun. Since we know the base length, twice the distance to the sun, and the observation angles of the star, we easily find the distance to the star.

Answer 3

There are several different techniques.

For close stars, the method of paralax is used. In this, the star is observed six months apart (so the earth is on opposite sides of its orbit). The different placement of the earth causes an apparent shift in the position of the star (called paralax). The size of this shift is related to how far away the star is with smaller shifts corresponding to larger distances.

For stars of intermediate distances and in clusters of stars, the cluster is observed over time and the 'railroad' effect is measured. This effect is caused by perspective and is related to the vanishing point of the motion in the sky (like railroad lines appear to come together at the horizon). By relating the rate at which the cluster approaches this vanishing point, it is possible to tell how far away the cluster is.

Once these two methods are done for enough stars, it is possible to catalog the various types of star and how their light, temperature, and size are related. Then, for any new star, the temperature and type of star can be found through the light it emits. This tells the brightness of the star which can be used to tell how far away it is (when compared to how bright it appears).

At the level of close galaxies, Cepheid variables are used. These are stars that vary in a regular pattern with the length of the variation related to how bright the star is. These are used to find the distances to the galaxies the stars are in. It is also possible to use certain types of supernovae (which have a standard brightness) to find the distances to galaxies. Once this is done for enough glaxies, it is possible to related the red shift of the galaxy and the distance to that galaxy.

Answer 4

Through parallax. We know how far we are from the sun and how fast we travel through space (since we're going in a big ellipse and it takes 365.24 days to orbit the sun, we can calculate our speed) So given that data, we observe a star at one point in our orbit and then observe it again later (or several times over several weeks, so we get a good spread of data) Then it's just a matter of trigonometry: you measure the angle between the star and the horizon where you've observed it and determine how much it's shifted over time due to the motion of the earth. If you take the distance the earth has travelled as the base of a triangle (and just assume it's close enough to a straight line for it not to matter much, which over the distance of millions of light years, it isn't), you can calculate the distance the star was from the earth at either of the points at which you measured it. You do this several times to make sure your measurements were accurate enough and that you didn't make any heinous math errors, and come up with a decent calculation.

This only works for relatively close stars (within about 100 parsecs or so) Anything further out and you have to use either spectroscopic parallax (a misnomer, since it actually has nothing to do with parallax) or for pulsars, stellar luminosity. In spectroscopic parallax, you look at the color of the star and spectrally analyze it; since we know what elements create different colors when heated, we can tell the composition and relative temperature of a star, and thus how bright it really is. Pulsars, on the other hand, actually vary in how bright they appear to be, but the period (how often) the star pulses is proportional to its actual luminosity, so we know how bright it really is. Either way, since we know that observed brightness is proportional to actual brightness or luminosity and inversely proportional to the square of the distance from the light source (the Inverse Square rule, as it's called in physics circles), we can calculate the distance from that.

As for measuring the Milky Way Galaxy, they basically do it by figuring out the distance to all the stars we can see, and where there appears to be a gap, that's where the galaxy ends and the rest of the universe begins. Harlow Shapley did this in 1917 by observing globular clusters, groups of stars that appear to ring the edges of the galaxy, and came up with a diameter of 100,000 light years, with us somewhere around 30,000 light years from the center. Later corrections and calculations by other astronomers have refined this number, but at 144,000 light years, with us around 26,000 light years from the center, he wasn't too far off.

Answer 5

I believe there are several independent methods, which broadly agree with each other. One method is this:

There is a particular kind of star which astronomers believe they understand fairly well (I think they're pulsars), and for which they can always calculate the real brightness. So, if they look at one of these stars in our galaxy, they can figure out how bright it *really* is, observe how bright it actually *appears*, and hence work out how far away it must be, in order to appear that bright in the sky.

Hopefully other people will explain other methods... red shift etc. :-)

Your best option is to look at a good website which explains this kind of thing.

NB Always take notice of what 'mathematician' has to say - His answers tend to be excellent.

Answer 6

A light year is the distance light travels in 365 days; based on the speed of light being 186,000 miles per second. That is the accepted unit of measure for distances of items in space.

Answer 7

LIGHT YEARS is a unit of distance . it is the distance travelled by light in a year .
1 light year = 60*60*24*365*3*10^8 meters

Answer 8

i learned this in science class, they mark the spot of a star in the sky and then half a year later they mark the spot it move 2 and then they use a formula to find out where it is at, don't ask me the formula, i'm only starting high school so i don't know, look it up, this is a tested formula to, they used it in small scale experiments to test it, it also called the doppler effect and there is blue shift, coming towards u and red shift, moving away from u

Answer 9

That is the measurement, its 10 light years away. A lightyear is the measurement

Answer 10

its called parallax, which is the observable relative motion of two objects at different distances, like when your sitting in a moving car looking out the window and the hydro poles are passing by you at rapid pace but the far off trees appear to be moving by slowly.