Our galaxy, the Milky Way, is typical: it has hundreds of billions of stars, enough gas and dust to make billions more stars, and at least ten times as much dark matter as all the stars and gas put together. And it’s all held together by gravity.
Like more than two-thirds of the known galaxies, the Milky Way has a spiral shape. At the centre of the spiral, a lot of energy and, occasionally, vivid flares are being generated. Based on the immense gravity that would be required explain the movement of stars and the energy expelled, the astronomers conclude that the centre of the Milky Way is a supermassive black hole.
Other galaxies have elliptical shapes, and a few have unusual shapes like toothpicks or rings. The Hubble Ultra Deep Field (HUDF) shows this diversity. Hubble observed a tiny patch of sky (one-tenth the diameter of the moon) for one million seconds (11.6 days) and found approximately 10,000 galaxies, of all sizes, shapes, and colours. From the ground, we see very little in this spot, which is in the constellation Fornax.
After the Big Bang, the Universe was composed of radiation and subatomic particles. What happened next is up for debate – did small particles slowly team up and gradually form stars, star clusters, and eventually galaxies? Or did the Universe first organize as immense clumps of matter that later subdivided into galaxies?
The shapes of galaxies are influenced by their neighbours, and, often, galaxies collide. The Milky Way is itself on a collision course with our nearest neighbour, the Andromeda galaxy. Even though it is the same age as the Milky Way, Hubble observations reveal that the stars in Andromeda’s halo are much younger than those in the Milky Way. From this and other evidence, astronomers infer that Andromeda has already smashed into at least one and maybe several other galaxies.