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A galaxy is a gravitationally bound collection of stars and interstellar gas and dust, and usually some amount of dark matter. Galaxies take many different shapes; some of them are spherical or ellipsoidal (elliptical), some are flat and disc-like (lenticular) and may have spiraling arms of dense star-forming regions (spiral or barred spiral). Other times, they have no clearly defined shape (irregular) and resemble very large nebulae.

Galaxies range in size from just a few thousand to hundreds of thousands of lightyears across, and contain millions, billions, or trillions of stars. Our own galaxy, the Milky Way, is roughly 100,000 lightyears across and contains between 100 billion and 400 billion stars, a large fraction of which are in binary or triple systems.

A civilization that controls all of the resources available in an entire galaxy is called a Type 3.0 Civilization.

Constituents

Stars

The visible component of a galaxy is made up of stars, with galaxies containing a variable number of them, depending on the size of the galaxy. Dwarf galaxies contain a few billion stars (109), and giant galaxies can contain trillions of stars (1014).

Stellar Remnants

Galaxies also contain stellar remnants, from stars that have died during the lifespan of the galaxy.

The most significant of these is a central black hole, present in many galaxies, with a mass that can be on the order of millions of times that of the Sun. Infalling matter leads to a vastly increased radiation production the center of the galaxy, creating something called an active galactic nucleus.

Interstellar Gas

Interstellar gas is the sparse gas that is between stars, with varying density. The densest interstellar gas is cold phase (~102 K), found in molecular clouds, where new stars are formed in the spiral arms. There is also the warm phase (~104 K), found between molecular clouds, and the hot phase (~106 K), resulting from gas that has been shock heated by recent supernovae.

Cosmic Dust

Cosmic dust is made up of small solid grains that are found in the interstellar medium and released from supernovas as they fuse dense elements.

Dark Matter

The most major component of the galactic mass is dark matter, making up the majority of the mass of a galaxy. Dark matter exists in a diffuse halo around the galaxy, and does not interact with any of the fundamental forces other than gravity. This leads to an unusual rotation curve for the galaxy, with velocity being roughly constant with distance rather than falling off as would be expected.

Galactic Classes
E · S0 · SA · SB · SM · Irr
dSph · dE · dIrr

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