The Milky Way

Our Galaxy, the Milky Way, contains about 100 billion stars, and is about 100,000 light-years in diameter (1 light-year = 9.46 x 1017 centimeters). All of the stars visible to the naked eye and through moderately large telescopes are in our Galaxy. The Milky Way consists of three main parts: the bulge, disk, and halo.


Artist's conception of the Milky Way


Bulge: The bulge is a spherical structure of stars at the center of the Galaxy with a radius of about 5000 light-years. At the very center of the bulge is the nucleus of the Galaxy which contains a supermassive black hole. This black hole may be 1-10 million times the mass of the Sun, where the mass of the Sun is M = 2 x 1030 kilograms. The densest part of the Galaxy is located in the direction of the constellation Sagittarius, a summer constellation. So, when you look at Sagittarius (which has a "teapot" configuration), you are looking in the direction of the center of the Milky Way.


This 360 degree image of the Milky Way was made from a huge infrared survey of the sky led by UVa astronomer Mike Skrutskie. Imagine the picture wrapping around you. The light areas are from starlight, the darker areas show very dark dust clouds. The bulge of the Milky Way has a slight "peanut-shape" in this view.

Disk: Most of the stars in the Milky Way are in the disk. The diameter is about 100,000 light-years while the thickness is just 1,000 light-years. The Sun is located in the disk about 28,000 light-years from the center of the Galaxy, and rotates around the center of the Galaxy with a velocity of 220 km/s. The disk contains a mixture of old and young stars, dust, and gas. The youngest stars in the Galaxy are located in the disk. These stars form in large clouds of gas that collapse in the spiral arms of the Galaxy.

Halo: The halo, which has a radius of about 300,000 light-years, is a much more extended spherical distribution of older stars and globular clusters. Globular clusters are large, spherical condensations of millions of relatively old stars. The halo also contains about a dozen dwarf galaxies that orbit the center of the Milky Way.


The globular cluster M13 in Hercules, courtesy of NOAO/AURA/NSF

The dwarf spheroidal galaxy Leo I, courtesy of the Anglo-Australian Telescope

Dwarf galaxies are very small galaxies that have masses from 1/10 to 1/100 that of the Milky Way. The three main types of dwarf galaxies are dwarf irregular (dIrr), dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies, which are all much smaller and fainter than normal irregular, spiral, and elliptical galaxies. The different types of galaxies (not including dwarfs) are summarized in the famous "Hubble Tuning Fork" diagram below.

There are several dwarf spheroidal galaxies orbiting the Milky Way. There is evidence that these satellite galaxies can be disrupted by the gravitational pull of the Milky Way. Theoretical work has shown that stars can be torn from galaxies orbiting the Milky Way, resulting in thin streams of debris trailing or leading the satellite around its orbit.

Movie: Interaction of a small dwarf galaxy with a Milky Way-like parent galaxy, made by Key Project team member Kathryn Johnston.

In 1995, a new satellite galaxy of the Milky Way was discovered in the constellation Sagittarius, which is clearly showing signs of having been disrupted. Although the Sagittarius dwarf galaxy (Sgr) is the closest satellite galaxy to the Milky Way, it was the last to be discovered because of its position behind the bulge of the Milky Way.


Artist's conception of the Milky Way-Sagittarius configuration, from Scientific American

The Majewski group and others have now mapped this satellite's tidal arms to be wrapping 360 degrees around the Milky Way.

Movie: A new model of the Milky Way, made by recent UVa B.A. student David Law, incorporates the newly mapped tidal tails of the disrupting Sagittarius dwarf galaxy, as mapped with the 2MASS catalogue by UVa department members Steve Majewski, Mike Skrutskie, David Law and Jamie Ostheimer, and Martin Weinberg (UMass). The yellow dot marks the position of the Sun and the red dots represent stars stripped from the Sagittarius dwarf galaxy by the Milky Way.

Thus, our Galaxy is a cannibal!


From The Economist


Image Credits:

Artist's conception of Milky Way and Hubble tuning fork images from www.physics.hku.hk/~nature/CD/regular_e/lectures/chap18.html.

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