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The Question

(Submitted March 29, 1998)

It's probably a really easy question (its just for curiosity's sake), but how far away, in light years, is the furthest star visible from the earth? (by any means possible). The name of this star is not important. Do you think there are stars in the outer regions of the universe that we can not see? What are your theories why this is so?

The Answer

The intrinsically brightest star [a supernova] is thought to be about a million times brighter than our own Sun. In astronomy, the brightness of stars is expressed in "magnitudes". This is a logarithmic scale, that works as follows. Our Sun has an intrinsic or absolute magnitude of about 5. This is the apparent magnitude our Sun would have if it were 32.6 light years away. A star 100 times brighter would have a magnitude of 0; a star 10000 times brighter would have a magnitude of -5; a star 1000000 (i.e. a million) times brighter would have a magnitude of -10.

With the Hubble telescope, using an exposure time of several hours, one can see stars to about 30th magnitude. This is about 10 billion times fainter than our Sun, if it were 32.6 light years away. The brightness of any object falls off as the square of the distance from the observer, so the Hubble telescope could just see our Sun if it were 3.26 million light years away. If you were to replace our Sun with a star a million times brighter, it could be seen about a thousand times further away, i.e., about 3 billion light years.

In answer to your last question, since this estimate is only for the very brightest stars, and since the distance I obtained is still less than the size of the visible Universe (about 15 billion light years), there are surely many faint stars at great distances which we cannot see.

J.K. Cannizzo
for Ask an Astrophysicist

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