Introduction to the Milky Way Galaxy

The Milky Way is our galaxy, the home of our Solar System. Estimates indicate that there are 400 billion stars in the Milky Way and some scientist believe that many have planets like our Sun. Thousands of Milky Way stars are clusters and nebulae including at least almost all objects of Messier's catalog which are not galaxies on their own (the only possible exception may be M54 which may belong to SagDEG, a small galaxy which is currently in a close encounter with the Milky Way, and thus our closest known intergalactic neighbor). All the objects in the Milky Way Galaxy orbit a common center of mass, called the Galactic Center.

As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Radio astronomial investigations of the distribution of hydrogen clouds have revealed that the Milky Way is a spiral galaxy. It has a pronounced disk component exhibiting a spiral structure, and a prominent nuclear region which is part of a notable bulge/halo component. It is still not clear if it has a bar structure or not.

The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer: Many of the dwarf Local Group members are satellites or companions of the Milky Way. The closest of all is above-mentioned SagDEG at about 80,000 light years from us and some 50,000 light years from the Galactic Center, followed by the more conspicuous Large and Small Magellanic Cloud at 179,000 and 210,000 light years, respectively.

The spiral arms of our Milky Way contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from this matter. On the other hand, the bulge component consists of old stars and contains the globular star clusters; our galaxy has probably about 200 globulars, of which we know about 150. These globular clusters are strongly concentrated toward the Galactic Center: From their apparent distribution in the sky, the center of the Milky Way lies at a considerable distance in the direction of Sagittarius and not near our Solar System as had been thought previously.

Our solar system is thus situated within the outer regions of this galaxy, well within the disk and only about 20 light years above the equatorial symmetry plane but about 28,000 light years from the Galactic Center [PopView, illustration]. Therefore, the Milky Way shows up as a luminous band spanning all around the sky along this symmetry plane, which is also called the Galactic Equator. Its center lies in the direction of the constellation Sagittarius, but very close to the border of both neighbor constellations Scorpius and Ophiuchus. The distance of 28,000 light years has recently (1997) been confirmed by the data of ESA's astrometric satellite Hipparcos. Other investigations published consequently have disputed this value and propose a smaller value of some 25,000 light years, based on stellar dynamics; a recent investigation (McNamara et.al 2000, based on RR Lyrae variables) yields roughly 26,000 light years. These data, if of significance, wouldn't immediately effect values for distances of particular objects in the Milky Way or beyond.

Our Solar System is situated within a smaller spiral arm, called the Local or Orion Arm, which is merely a connection between the inner and outer next more massive arms, the Sagittarius Arm and the Perseus Arm.

Similar to other galaxies, there occur supernovae in the Milky Way at irregular intervals of time. If they are not too heavily obscurred by interstellar matter, they can be, and have been seen as spectacular events from Earth. Unfortunately, none has yet appeared since the invention of the telescope (the last well observed supernova was studied by Johannes Kepler in 1604).

More photos are available with the link at the bottom or at sites with links above. For perspective of the Milky way and other heavenly objects, digital composites, and illustrations are also available. Examples of composites and illustrations:

Relative location, illustration
Composite, all Milky Way observations
360 degree panarama composite