Galaxies
A galaxy is a large group of stars, gas, and dust bound together by gravity. Our solar system resides in the Milky Way galaxy, a spiral galaxy that is part of a group of galaxies called the Local Group.
There are billions of galaxies in the Universe, but only three outside our Milky Way Galaxy can be seen without a telescope - the Large and Small Magellanic Clouds and the Andromeda galaxy. The Large and Small Magellanic Clouds are about 160,000 light years away and are satellites of the Milky Way. They can be seen from the southern hemisphere. The Andromeda Galaxy is a larger galaxy that is about 2.5 million light years away and can be seen from the northern hemisphere with good eyesight and a very dark sky. The other galaxies are even further away from us and can only be seen through telescopes.
The smallest galaxies may contain only a few hundred thousand stars and be several thousand light years across, while the largest galaxies have trillions of stars and may be hundreds of thousands of light years across. Galaxies can be found by themselves, in small groups and in large clusters. It is very rare to find stars in the space in between galaxies.
Galaxies sometimes collide with each other, with interesting results. These collisions can trigger bursts of star-formation in addition to changing the shapes of the galaxies that collide. However, when galaxy collisions occur, individual stars do not collide, due to the vast distances between them.
Interacting galaxies imaged by the Hubble Space Telescope. (Credit: NGC 4676: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA; Antennae: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration UGC 8334: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University))
Galaxy classification
Galaxies are classified by shape. There are three general types: elliptical, spiral, and irregular.
Perhaps the most familiar kind of galaxy are spiral galaxies. They have a distinctive shape with spiral arms in a relatively flat disk and a central "bulge". The bulge has a large concentration of stars. The arms and bulge are surrounded by a faint halo of stars. The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. Our Milky Way Galaxy is a spiral galaxy.
Some spiral galaxies are what we call "barred spirals" because the central bulge looks elongated – like a bar. In barred spirals, the spiral arms of the galaxy appear to spring out of the ends of the bar.
Examples of spiral galaxies imaged by the Hubble Space Telescope. (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)
As their name suggests, elliptical galaxies are round or oval, with stars distributed fairly uniformly throughout. They have a bulge and halo, like spiral galaxies, but don't have the flat disk of stars. The stars in ellipticals tend to be older.
Irregular galaxies have no identifiable shape or structure to them. They are often chaotic in appearance, without a bulge or any trace of spiral arms. The different shapes and orientation of galaxies are a result of their history, which may have included interactions with other galaxies.
Examples of an elliptical galaxy (NGC 1132) and an irregular galaxy (NGC 1472A). (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)
Updated: February 2016