How scientists decide that a particular star is so many (say 100 ) light years away.When light takes 100 year?

Answer 1

To correct lovingdaddy's misinformation, parallax measurements have been made out to about 1500 light years, although the uncertainty at that distance is about 10%. Within that distance, we can find a number of Cepheid variables, stars whose brightness varies with a period that is related to their brightness. Thus, we can determine the distance of other Cepheid variables, even in other galaxies. This was how it was first realized that that there WERE other galaxies - up to then, "spiral nebulae" were widely believed to be in our own galaxy.

It is difficult to measure the distance to a lone star more than 1500 LY away, because we have no way to figure its intrinsic brightness. But with large clusters, we can fit the distribution of brightness versus color to the known pattern (the Hertzsprung-Russel diagram) and calculate the distance from the amount by which distance decreases brightness.

From distance estimates of galaxies based on Cepheid variables, astronomer Edwin Hubble realized that the red shift (Doppler shift of light due to velocity away from us) increased with increasing distance. This observation led to the theory of the expanding universe, and the determination of the Hubble Constant, which relates distance to red shift.

None of these stellar distances can be measured with the precision with which we can measure nearby objects, of course. The distance to the Moon can be determined within a couple of inches using laser rangefinders.

As far as the effect of the time lag, the motion of stars within our galaxy is such that a star's position usually does not change much in 100 years, or even 1000 years. That's why the constellations have stayed the same for thousands of years. Very distant galaxies are another story. The distant universe is moving away from us so fast that astrophysicists recognize three different values of the distance to a high red-shift galaxy: the distance it was when the light we see left the galaxy, the distance the light actually traveled (longer, due to the expansion of the space the light traveled through), and the distance the galaxy is at now.

Answer 2

A star is considered to be a 100 lights years away because it takes a beam of light 100 years to travel that distance (which is just under 590,000,000,000,000 miles.) That may seem large but in astronomical terms it's relatively small. In fact the milky way galaxy we live in is about 100,000 light years wide.

So for objects only 100 light years away scientists use a method called stellar paralax. To see an example of this hold your finger out at arms length and close one eye. Then open that eye and close your other eye. Notice how your finger shifts position. Now move your finger closer to your face, changing eye's again. You'll see that the closer you hold your finger to your face the more it seems to shift.

Scientists do the same thing from earth. They measure a star position in the sky at one point on the earths orbit and then measure it again 6 months later when the earth is on the opposite side of the sun. Depending on how far the star shifts astronomers are able to calculate it's distance.

This method is usually only accurate up to 100 light years but there have been satellites that have used paralax to observe distances up to 1600 light years.

See the web site below to see how scientists determine the distance to even farther stars.

Answer 3

There is an awful lot of conjecture involved.

The orbit of the earth allows scientist to directly measure the angular distance. Think January and July and the earth will be on opposite ends of its trip around the sun. Unfortunately, even with the most sophisticated equipment, the furthest that any star could be directly measured is 30 light years. Everything beyond that is really conjecture based on brightness and relative position to other stars of known distance.

So to answer your question, anything beyond 30 light years is really just B.S. They cannot prove it.

Answer 4

They can only say where it WAS 100 years ago.

Answer 5

The Red Shift!!!!

Answer 6

(Say 100)