What are Galaxies

Pre Question

What are Galaxies?

Who discovered Galaxies?

How are Galaxies Classified?

Vocabulary

Galaxy:    Galaxy is a system of Billions of stars

Galactic Clusters:     A grouping of galaxies

Nebula: Large luminous clouds of gas and dust in outer space, composed mostly of hydrogen gas

Constellation:   A group of stars visible within a particular region of the night sky

The Milky Way Galaxy

On some summer nights, you may see a bright ribbon of stars overhead.  You are looking toward the center of Earth’s galaxy. 

A galaxy is a group of stars, gas, and dust.  Many galaxies rotate around a core.  Stars are part of galaxies, and the universe contains about a hundred billion galaxies, with each galaxy containing billions of stars.

Earth’s galaxy is called the Milky Way Galaxy.  It includes more than 100 billion stars, and it is one of the largest galaxies in the universe.  It is so large that the light of a star on one side of the galaxy takes more than 100,000 light-years to reach the other side.  As the Milky Way Galaxy rotates, the sun makes one complete turn around the center every 200-250 million Earth years.

Who Discovered Galaxies

Until the 1920’s people thought the Milky Way was the only galaxy of stars in otherwise empty space.  But in 1923 the astronomer Edwin Hubble found evidence that other galaxies existed. 

Using the large Mount Wilson telescope in California, Hubble found that points of light in the constellation Andromeda were too far away to be in our galaxy.  Andromeda is a totally separate galaxy.  

Today we know that there are billions of galaxies!  The largest are much bigger than the Milky Way.  But even the smallest galaxy contains about 100,000 stars.  We also know that galaxies occur in groups called galactic clusters

Classifying Galaxies

Galaxies are classified by shape.

There are four basic types:

spiral, barred spiral, elliptical, and irregular.

Spiral Galaxies

The Milky Way Galaxy is a barred spiral galaxy.  A spiral galaxy has a bright bulge of stars in the center and rotating arms.  Earth’s solar system is in one of the Milky Way Galaxy’s spiral arms, about 30,000 light-years from the center of the galaxy.  A light-year is the distance light travels in one Earth year, about 5.6 trillion mi.

From the side a spiral galaxy looks like a thin rod with a center bulge. Astronomers.  Recently Astronomers discovered that in the center of most galaxies is a black hole.

A spiral galaxy, seen from the “top,” looks like a giant pinwheel spinning through space.  The arms wind around the center as it turns, giving the galaxy a spiral appearance.

A spiral galaxy’s arms contain young stars, protostars, dust, and gas.  The thick bulge at the center contains older stars.

Spiral Galaxies are classified by there age ranging form the youngest sc spiral galaxies to the oldest so spiral galaxies.

         Sc                Sb                    Sa                So

The age of the spiral galaxies is determined by the spread of their spiral arms and the appearance of younger large blue stars.      

The milky way galaxy is an sb spiral galaxy.

Barred Spiral Galaxies

A barred spiral galaxy is similar to a spiral galaxy.  The difference is that spirals extend from a bar of stars that stretches from the center.

Astronomers believe that the bar is caused by strong

magnetic energy in the galaxy.

Elliptical Galaxies

About half of all galaxies are elliptical.  The shapes of elliptical galaxies ranges from almost spherical like a basketball, to a shape like a flattened football.  Unlike spiral galaxies, elliptical galaxies don’t seem to rotate.

Elliptical galaxies are comprised of older red stars, and much of the light is defused by the presence of stellar dust.

No central hub is visible in Elliptical Galaxies.

Elliptical galaxies range from the younger football shaped galaxies.

To the older basketball shaped galaxies.

Irregular Galaxies

Irregular Galaxies are galaxies that do not fit the classifications of the

Galactic Clusters

Have you ever seen a faint smudge or a misty patch of stars in the night sky?  You may have been looking at a galactic cluster

  A galactic cluster is a group of galaxies.  The Milky Way Galaxy is one of about 30 galaxies in a cluster called the Local Group.  The Milky Way Galaxy is one of the lager galaxies in this cluster. 

Most of the galaxies in the Local Group are small and elliptical or irregular.

Beyond the Local Group are other galactic clusters.  Some of these are huge, with thousands of galaxies.  One of the clusters closest to the Local Group is the Virgo Cluster, which is about 50 million light-years way.