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Galaxies

Galaxies

Image of the band of the Milky Way from the ground

A spiral galaxy in the Hubble Ultra Deep Field image. (Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team)

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.

*Tell me more about the Milky Way

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.

Image of the band of the Milky Way from the ground

Mosaic of the Large Magellanic Cloud in ultraviolet light from Swift. (Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State))

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.

Image of the band of the Milky Way from the ground  Image of the band of the Milky Way from the ground  Image of the band of the Milky Way from the ground

Interacting galaxies imaged by the Hubble Space Telescope - NGC 4676 (right), Antennae Galaxies (middle), and UGC 8334 (left). (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.

Image of the band of the Milky Way from the ground  Image of the band of the Milky Way from the ground

Examples of spiral galaxies imaged by the Hubble Space Telescope: spiral galaxy M74 (left) and barred spiral galaxy NGC 1672 ( right). (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.

Image of the band of the Milky Way from the ground  Image of the band of the Milky Way from the ground

Examples of an elliptical galaxy (NGC 1132, left) and an irregular galaxy (NGC 1472A, right). (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)

Updated: February 2016

 

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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