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Milky Way

Nucleus Of The Milky Way

What is in the nucleus of the Milky Way? If we look with optical telescopes, we see nothing. The interstellar dust obscures the optical light. The center of the Milky Way does, however contain very strong sources of radio waves, infrared light, and x rays. One such source, called Sagittarius A*, appears to lie at the precise center of the galaxy, the point about which the entire system rotates.

The vast energy omitted by Sagittarius A* comes from a region that is less than one light day in diameter (about the size of the solar system) compared to over 120,000 light years for the entire galaxy. There is more energy produced in a very small volume of space than we can easily explain. There is certainly not enough room in this volume to contain enough stars to explain the energy production. What produces so much energy in such a small region of space? Most astronomers think that there is a supermassive black hole with the mass of a million suns, in the core of the Milky Way. Black holes are so highly compressed that a supermassive black hole capable of explaining the energy output of the Milky Way's core would still have a small volume.

Quasars and other active galaxies also emit far more energy than can easily be explained from a small region in their nuclei. An active galaxy is a galaxy with at least 100 times the energy output of the Milky Way. Quasars are among the most energetic and distant types of active galaxy. These galaxies are also thought to contain supermassive black holes in the nucleus, even more energetic than the one in the nucleus of the Milky Way. The nucleus of the Milky Way may be a quieter version of the nucleus of an active galaxy or a quasar.

There are many mysteries concerning the Milky Way, including the antimatter fountains, the nature of the energetic activity at its core, the unknown composition of the dark matter in the halo, and the uncertain process by which it formed.



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Bok, Bart J. and Priscilla F. Bok. The Milky Way. Cambridge: Harvard University Press, 1981.


Binney, James. "The Evolution of Our Galaxy." Sky & Telescope 89 (March 1995): 20-26.

Trimble, Virginia and Samantha Parker. "Meet the Milky Way." Sky & Telescope 89 (January 1995): 26-33.

Van den Berg, Sidney, and James E. Hesser. "How the Milky Way Formed." Scientific American (January 1993): 72-78.

Verschuur, Gerrit L. "In the Beginning." Astronomy (October 1993): 41-45.

Verschuur, Gerrit L. "Journey into the Galaxy." Astronomy (January 1993): 33-39.

Paul A. Heckert


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Cepheid variable star

—A type of star that varies in brightness as the star pulsates in size. Cephied variables are important distance yardsticks in establishing the distance to nearby galaxies.


—The flat disk-shaped part of the Milky Way galaxy that contains the spiral arms.

Galactic cluster

—A cluster of roughly a few hundred young stars in a loose distribution. Also called an open cluster.


—A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.

Globular cluster

—A cluster of roughly 100,000 older stars in a compact spherical distribution.


—A spherical distribution of older stars and clusters of stars surrounding the nucleus and disk of our galaxy.

Light year

—The distance light travels in one year, roughly 9.5 trillion kilometers or 6 trillion miles.

Milky Way

—The galaxy in which we are located.


—The central core of a galaxy.

Spiral arms

—The regions where stars are concentrated that spiral out from the center of a spiral galaxy.

Spiral galaxy

—A galaxy in which spiral arms wind outward from the nucleus.

Additional topics

Science EncyclopediaScience & Philosophy: Methane to Molecular clockMilky Way - History, Structure Of The Milky Way, Formation Of The Milky Way, Nucleus Of The Milky Way