Determining the Expanse of Space
From the time of Galileo to the beginning of the 20th century, the telescope technology advanced, and the night sky with it stars, planets, gas and dust clouds (nebula), and other objects were charted in great detail. The problem was that we could see lots of stars, but had no way of knowing how far away there were because stars vary in their brightness in addition to their distance. Astronomers have developed several methods to directly or indirectly measure the distance to object is space.
It was in 1923 that Edwin Hubble found dozens of uniquely identifiable variable stars in the Andromeda nebula and then determined that Andromeda was at least 10 time more distant than the most distant stars in the Milky Way. He was first to determine that Andromeda was a separate system which he named a galaxy. The Milky Way is an obvious band of densely distributed stars and clouds of dust visible as a band in the clear night sky (Figure 1.35). Before Hubble's discovery, it was thought to be the Milky Way represented the entire Universe, and that unusual shaped spiral nebulae (galaxies) were part of the Milky Way. With Hubble's discovery, it became evident that Earth and the Sun's Solar System was within the greater Milky Way Galaxy.
The Andromeda Galaxy is a spiral galaxy (Figure 1.36). It is the closest large galaxy to our Milky Way Galaxy and is one of the few visible to the naked eye. It is the most distant object in space that can be seen without magnification.
The Andromeda Galaxy can be seen in the northern hemisphere on clear autumn nights. It is located about 2.25 million light-years away from Earth. (A light year is the astronomical distance that light can travel in a year; approximately about 9.4607 x 1012 kilometers or about 6 trillion miles.) Andromeda is estimated to contain about 1 trillion stars. Astronomers estimate that the Milky Way and Andromeda galaxies will eventually collide (merge) in about 4.5 billion years in the future.