What is a Galaxy?

Galaxy - A component of the Universe made up of gas and a large number (usually more than a million) of stars held together by gravity.  The two most common types of galaxies are spiral and elliptical.

Spiral Galaxies


Andromeda

Spiral galaxies consist of a nucleus, a disk, a halo, and spiral arms.  Interstellar material such as gas and dust are found in and near the spiral arm structure in the disk of the galaxy.  This is a picture of the nearest large spiral galaxy, Andromeda.


M 83

The spiral arms are sites of star formation, which typically contain young, bright stars.  An example is Galaxy
M 83.


M 100

Our own Galaxy is a spiral. Spirals that are face-on towards us appear like giant pinwheels, like Galaxy M 100.


NGC 4565

With other galaxies, we can only see the outer edge.  This image shows Galaxy NGC 4565 edge-on.  

Spiral galaxies, like our own Milky Way, contain hundreds of billions of stars (many like our Sun), gas, and dust.  They stretch across a hundred thousand light years of space.  Each star or gas cloud revolves around the center of a spiral galaxy in its own unique orbit, which takes typically a hundred million years or more to complete.   Most of the stars and gas clouds in a given spiral galaxy are revolving in nearly the same direction.  This gives the galaxy the appearance of a flattened, spiral disk, which contains most of the stars and gas clouds.  Most spiral galaxies are actively forming new stars within the coldest, densest, and dustiest clouds.  They appear to be converting their gas supply into new stars at rates they can sustain for many billions of years.  However, not all galaxies are forming stars. 

 

Elliptical Galaxies


M32

As their name suggests, elliptical galaxies have elliptical or spheroidal like shapes.


M87

The majority of their stars are old, and they have much less dust and gas than the spirals.  Most have not formed stars for billions of years.

Elliptical galaxies often appear as spherical or football-shaped collections of stars that are nearly devoid of gas and dust (compared to spiral galaxies) and most have not formed stars for billions of years.

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Last Updated: 12/2/98