 Visible to the naked eye, and studied by Persian astronomers around 900 A.D., the Andromeda Nebula was thought to be a part of the Milky Way. In fact, it was thought that all the stars in the universe were in our Milky Way galaxy. That changed in the early 1900s. In 1923, Edwin Hubble found a Cepheid variable in the Nebula. The star altered the course of modern astronomy. The star goes by the name V1. Here's Edwin Hubble's image of Andromeda, which was made on a 4x5 inch glass plate, and dated October 6, 1923. He originally identified three stars and marked each of them with an N for Nova, a class of exploding star. Later, Hubble realized that the Nova at the top right was actually a Cepheid variable. He crossed out the N and wrote VAR for variable. He added an exclamation point, because he knew that this variable would allow him to calculate the distance. You may recall from our segment on Distant Stars that we covered how Harriet Levitt discovered the relationship between Cepheid variable frequency and their intrinsic luminosity. And once we know the intrinsic luminosity of a star, we can use the apparent luminosity and the inverse square law to determine how far away the star is. And indeed, once the period was measured at 31.4 days, he knew he had another galaxy. For V1, distances to stars were measured in thousands of light years. After V1, the universe became a much bigger place. V1 was over two and a half million light years away. Andromeda is a beautiful, barred spiral galaxy with two spiral arms that glow with a massive number of new stars. This is very much like our Milky Way, but Andromeda's disc is 220,000 light years wide. That's more than twice the width of the Milky Way. But recent studies of star escape velocities indicate that both galaxies have the mass of about 800 billion suns, counting stars, gas, dust, and dark matter. Light from this magnificent galaxy left its stars just over two and a half million years ago. When the light that entered Hubble's telescope left Andromeda, there were no humans on Earth. While the light traveled towards Earth, we came into being. We created and lost great civilizations, and we built the telescope that caught the light when it finally reached our planet. In 2015, the Hubble Space Telescope captured the sharpest and most detailed image ever taken of the galaxy. It shows over 100 million stars and thousands of star clusters embedded in a section of the galaxy's disc, stretching across over 48,000 light years. It traces the galaxy from its central bulge on the left, where stars are densely packed together across lanes of stars and dust to the sparser outskirts of its outer disc on the right. Zooming into the boxed field, we see some foreground Milky Way stars in the line of sight to Andromeda, and a couple of distant spiral galaxies shining through Andromeda's disc. A large number of star clusters can be seen in this analysis of Andromeda, the large group of blue stars in the galaxy indicates star-forming regions in the spiral arms, while the dark silhouettes of obscured regions trace out complex dust structures. Underlying the entire galaxy is a smooth distribution of cooler red stars that trace Andromeda's evolution over billions of years. Careful analysis of Andromeda's proper and radial motion indicates that it is on a collision course with our Milky Way. We'll conclude our coverage of Andromeda with a look at its closing velocity. It's closing at 14.4 million kilometers per day. That's almost 9 million miles per day. But given the 2.5 million light years between us, it won't get here for around 4 to 4.5 billion years. We'll cover this in depth in a subsequent segment on colliding galaxies.