Astronomers, such as Harvard’s Robert Kirshner, believe that understanding supernova remnants like the Crab Nebula is important because they can be used to measure the distance to other galaxies, which is currently an area of intense research. As we have seen, disagreements over the distances to other galaxies have recently touched off a lively debate over the big bang model of the creation of the universe.

How Do They Know How Far It Is?

When astronomers tell us that the Andromeda galaxy is two million light-years away, they are really giving us an educated guess. No one has come up with a way to measure such mind-boggling distances directly. Distances to the very closest stars, those within 200 light-years or so, can be measured directly by stellar parallax, which involves simple trigonometry. But this only works for stars so close to the earth that they appear to move slightly as the earth goes around the sun. Most stars, and all galaxies, are much farther away. At that point the guesswork begins. Even stars in our own backyard, such as the famous red supergiant Betelgeuse in Orion, are subject to guesswork, with estimated distances for it ranging from 300 light-years to over 1,000. It should not surprise us, therefore, to find disagreement among astronomers regarding galactic distances, which are a million times greater.