The Galaxies
The Milky Way Galaxy Is a spiral galaxy in which our solar system is located. The center of the galaxy lies in the Sagittarius Constellation. It is about 100,000 ly across, 2,000 ly thick, and our solar system is located 26,000 ly away from the center of the galaxy. There are hundreds of billions of stars in our galaxy. All these stars orbit around a super massive black hole millions of M ʘ at the galaxies center.
Structure of the Milky Way Galaxy At the center of our galaxy is a central bulge, which is a flattened sphere of stars of 12,000 ly in diameter. Further out we have a disk of stars that is made up of arms of stars that spiral around the galaxy. Then our galaxies disk is surrounded by a spherical shell of star and other matter called the halo.
Galactic Nucleus The Galactic nucleus or bulge at the center of the galaxy is very crowed with stars. If you lived on a planet in the galactic nucleus it would always be day time. Near the center of the galaxy there are hundreds of stars within 1 ly.
Halos Orbiting the center of our galaxy other than the disk, are two spherical shells of stars and other matter. The first shell is about 100,000 ly away from the center of the galaxy and contains many globular clusters. A globular cluster is a group of stars that are very close together. The next most outer shell has stars that are believed to contain some of the oldest stars in the universe.
The Milky Way is not the only galaxy Prior to the twentieth century astronomers though all stellar objects were in our galaxy. In 1932, Hubble found evidence that the Milky Way was not the only galaxy in the universe. Hubble did this by studying Cepheid variable stars.
Cepheid Variable Stars A Cepheid variable star is a pulsating star that varies in brightness periodically. By observing a Cepheid s period and apparent magnitude the distance to the star can be determined. Hubble studied one such star in what was thought to be a nebula, but Hubble determined that it was 2.2 million ly outside of our galaxy. He determined that this was not a nebula but in fact another Galaxy: The Andromeda Galaxy.
Galactic Cannibalism Astronomers have observed 24 small galaxies that orbit our galaxy in the halo. These smaller galaxies are losing stars because the gravity of the Milky Way. Eventually these galaxies will cease to exist, due to being ripped apart by our galaxy. This process of a larger galaxy consuming smaller galaxies is called galactic cannibalism. Most of the stars in the halos are though to be remnants of smaller galaxies.
The Galaxy is rotating Our galaxy is rotating, our sun rotates around the center of the galaxy at 500,000 mi/h. However there is a mystery to the rotation of the stars in the galaxy. The stars toward the outer parts of the of the galaxy are rotating much faster than we calculate they should be.
Dark Matter Since stars are orbiting much faster than the should be, there must be more gravitational force from the galaxy that acts on distant stars that we can t see. Therefore there must be matter beyond the visible matter that we see. Nearly 90% of our galaxies mass is yet to be discovered. This missing mass we refer to as dark matter. Whatever it is we can t see it in any wavelength of light. Scientists estimate that a spherical halo of dark matter should extend our 12 times further from the galactic nucleus than we are.
Types of Galaxies There are five main types of galaxies. Spiral, barred spiral, lenticular, elliptical, and irregular galaxies.
Spiral Galaxies Spiral galaxies: a galaxy with arms that spiral around a central bulge. Spiral galaxies are classified by the letter S or SA. A lower case letter next to the S is given depending on how tight the arms are wrapped around the center. For example: Sa, Sb, Sc,
Spiral Galaxy
Barred Spiral Galaxy Barred spiral galaxy: a spiral galaxy with what looks like a bar of stars that passes through the central bulge. Barred spiral galaxies are classified by the an SB. A lower case letter next to the SB is given depending on how tight the arms are wrapped around the center. For example: SBa, SBb, SBc,
Barred Spiral Galaxy
Lenticular Galaxy Lenticular Galaxy: in between a spiral and elliptical galaxy. They have a thin disc shape but no spiral arms. Lenticular galaxies are classified by an S0 or SB0.
Lenticular Galaxy
Elliptical Galaxy Elliptical galaxies: round to football shaped with no arms. These range in size from the biggest to the smallest galaxies in the Universe. Elliptical galaxies are classified by an E. A number from 0-7 is used next to the E depending on how eccentric the shape of the galaxy. For example: E0 E7
Elliptical Galaxy
Irregular Galaxy Irregular galaxies: are galaxies that do not have a regular or symmetrical shape. Irregular galaxies are classified by Irr
Irregular Galaxy
Clusters Galaxies are not spread out evenly throughout the Universe. Galaxies orbit each other in groups called clusters. A cluster can have from hundreds to thousands of galaxies clumped together. A clusters is about 10 million ly in size.
Superclusters Clusters of galaxies are also clumped together in superclusters. A supercluster can have dozens of clusters in it. A supercluster is about 150 million ly in size. It is estimated that there is about 10 million super clusters in the Universe.
Quasars
Quasars Quasars are the brightest objects in the Universe. They are about 100 times brighter than our galaxy that contains 100 billion stars. They are powered by accelerating material entering a super massive black hole at the center of a galaxy. They are a part of a group of objects called active galactic nuclei.
Expansion of the Universe From the red shift between superclusters of galaxies the Universe is expanding. There is an energy that is unknown called dark energy that is causing this expansion. There for all galaxies are moving away from each other. Collisions between galaxies are still possible on a small scale due to gravity of large galaxies being able to pull in smaller ones.
The Universe Astronomers estimate that are at least 100 billion galaxies in the Universe. Our galaxy has 100 billion stars. Our star the Sun has 8 planets. From this we can see just how many stars and planets there are in our Universe. But remember this is just the visible matter we can see. There is still much more we haven t discovered yet.