The Age of the Universe: Part 3

Not all stars stay the same brightness. There is a constant tug-of-war inside a star between the thermonuclear reactions trying to expand the star and gravity trying to hold it all together. In some stars, this swings back and forth pretty dramatically. We all these variable stars.

One kind of variable star is called a Cepheid variable (so named because one of the first identified was in the constellation Cepheus). One of astronomy's unsung heroes, Henrietta Leavitt, discovered that there was a relationship between the amount of time between peaks of brightness (called the period) and the range of brightnesses. So now you could tell how bright one of these stars was by simply measuring how long it took for the star to go from bright to dim to bright again.

With this new tool for measuring the absolute magnitude of a star, Edwin Hubble and his assistant Milton Humason, discovered Cepheid variables in a faint cloud called M31. They were able to determine its distance and discovered that it was over 2 million light years away from the Milky Way. It was a separate galaxy... and now the measured universe was between 500 to 2000 times older than creationists would have us believe.

But M31 (also known as the Andromeda galaxy) is part of a group of galaxies called the Local Group. We were poised to solve one of the greatest mysteries of all time.