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Veil
ngc 6960 Western Veil
ngc 6992 - Eastern Veil
Abell 85
IC 0443 Jellyfish
  Galaxies.jpg -  Galaxies range in size from dwarfs containing tens of millions of stars to giants with a hundred trillion stars. They contain stars, gas, dust, and dark matter orbiting around their center of mass. 90% of the mass is in the form of dark matter. Dark matter was originally discovered in 1933 when Fritz Zwicky calculated that most of the mass in the Coma galaxy cluster was not visible. He claimed that without this extra mass the cluster couldn't hold together. He was ignored until Vera Rubin discovered in 1975 that the high rotational speed of galaxies required some type of non-visible mass. We now call this dark matter, something that we know nothing about but can infer must exist due to its gravitational effects on the matter that we can see. It's estimated that the Milky Way contains about 200 billion stars with at least 100 billion galaxies in the observable universe. Some astronomers believe the Milky Way is one of the older galaxies in the observable universe. We don't know exactly how galaxies form. We have observed that all galaxies with a bright central bulge appear to have a supermassive black hole at their center. The black hole seems to consistently have a mass of about 0.5% of the mass of the galaxy. One theory is that the black hole formed through gravitational attraction of gas in the early universe. Heating of the gas in an accretion disk around the black hole triggered the formation of stars around it and thus starting birth of the galaxy. This feeding resulted in a type of active galaxy known as a quasar, generating extremely intense energy over the entire electromagnetic spectrum. Eventually the solar winds associated with this activity pushed gas and stars away from the center causing the feeding to stop ending the quasar phase and resulting in a quieter inactive galaxy such the current Milky Way. Galaxies form in groups or clusters. Groups normally contain less than about 50 individual galaxies while clusters are larger, although the distinction is weak. They are the largest gravitationally bound objects in the universe. The Milky Way is part of the Local Group containing about 50 members. Its two largest galaxies, Andromeda and the Milky Way, have several satellite galaxies that revolve around them. Galaxy groups or clusters gather in even larger superclusters that don't have sufficient mutual gravitational attraction to overcome cosmic expansion. The Local Group is part of the Virgo Supercluster.  
M31 Andromeda
M33 Triangulum
M51 Whirlpool
Quasar 3C273
Quasar Q1634 706

Galaxies range in size from dwarfs containing tens of millions of stars to giants with a hundred trillion stars. They contain stars, gas, dust, and dark matter orbiting around their center of mass. 90% of the mass is in the form of dark matter. Dark matter was originally discovered in 1933 when Fritz Zwicky calculated that most of the mass in the Coma galaxy cluster was not visible. He claimed that without this extra mass the cluster couldn't hold together. He was ignored until Vera Rubin discovered in 1975 that the high rotational speed of galaxies required some type of non-visible mass. We now call this dark matter, something that we know nothing about but can infer must exist due to its gravitational effects on the matter that we can see.

It's estimated that the Milky Way contains about 200 billion stars with at least 100 billion galaxies in the observable universe. Some astronomers believe the Milky Way is one of the older galaxies in the observable universe.

We don't know exactly how galaxies form. We have observed that all galaxies with a bright central bulge appear to have a supermassive black hole at their center. The black hole seems to consistently have a mass of about 0.5% of the mass of the galaxy. One theory is that the black hole formed through gravitational attraction of gas in the early universe. Heating of the gas in an accretion disk around the black hole triggered the formation of stars around it and thus starting birth of the galaxy. This feeding resulted in a type of active galaxy known as a quasar, generating extremely intense energy over the entire electromagnetic spectrum. Eventually the solar winds associated with this activity pushed gas and stars away from the center causing the feeding to stop ending the quasar phase and resulting in a quieter inactive galaxy such the current Milky Way.

Galaxies form in groups or clusters. Groups normally contain less than about 50 individual galaxies while clusters are larger, although the distinction is weak. They are the largest gravitationally bound objects in the universe. The Milky Way is part of the Local Group containing about 50 members. Its two largest galaxies, Andromeda and the Milky Way, have several satellite galaxies that revolve around them. Galaxy groups or clusters gather in even larger superclusters that don't have sufficient mutual gravitational attraction to overcome cosmic expansion. The Local Group is part of the Virgo Supercluster.

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