The Milky Way Galaxy

A spiral galaxy, type Sbc, centered in Sagittarius

The Milky Way is the galaxy which is the home of our Solar System together with at least 200 billion other stars and their planets, and thousands of 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 their common center of mass, called the Galactic Center (see below).

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 of Hubble type Sb or Sc. Therefore, out galaxy has both a pronounced disk component exhibiting a spiral structure, and a prominent nuclear reagion which is part of a notable bulge/halo component. It is still not clear if it has a bar structure (so that it would be type SB) or not, but an increasing number of investigations has given some evidence for this, so that the Milky Way may look like M61 or M83, and is perhaps best classified as SABbc.

More on the structure of the Milky Way

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, Harlow Shapley has concluded that this center of the Milky Way lies at a considerable distance (which he overestimated) in the direction of Sagittarius and not rather close to us, 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. Therefore, the Milky Way shows up as 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.

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).

Milky Way pictures are wide-field exposures. Besides being attractive and often colorful, they are often suited to view the Milky Way objects (including nebulae and star clusters) in their celestial surroundings of field stars. Some fields include lots of Messier objects and thus included here:

Milky Way central region including constellations Sagittarius, Scorpius, Ophiuchus and Scutum, and map of the Milky Way Central Region, by Bill Keel of the University of Alabama
Milky Way in Sagittarius, including portions of Scorpius and Ophiuchus
Milky Way around M17, M18, and M24

Our image was obtained by David Malin of the Anglo-Australian Observatory, and shows the many Messier objects around the direction of the Galactic Center. It is copyrighted and may be used for private purpose only. For any other kind of use, including internet mirroring and storing on CD-ROM, please contact the Photo Permissions Department of the Anglo Australian Observatory.

More information on this image by David Malin

Old style AAT image

In order to obtain a picture of the whole Milky Way as it appears from Earth, one must either compose a mosaic of many photographs (optionally computer-processed), or create a drawing; fine examples may be accessed below:

In the infrared light, the structure of the Milky Way can be better investigated, as the obscurring dust clouds are of better transparency for long wavelength IR than for the visible light. The Cobe satellite has provided an infrared image of the Milky Way's central region.

The central region of the Milky Way, as those of many other galaxies, is more densely crouded with stars than the outer region, and contains a massive central object, Sagittarius A*.

Below we give some data for the Galactic Center (this and all following positions for epoch 2000.0):

Right ascension	17 : 45.6 (h : m)
Declination	-28 : 56 (deg : m)
Distance	28 (kly)

Our Sun, together with the whole Solar System, is orbiting the Galactic Center at the distance given, on a nearly circular orbit. We are moving at about 250 km/sec, and need about 220 million years to complete one orbit (so the Solar System has orbited the Galactic Center about 20 to 21 times since its formation about 4.6 billion years ago).

The Galactic North Pole is at

Right ascension	12 : 51.4 (h : m)
Declination	+27 : 07 (deg : m)

Considering the sense of rotation, the Galaxy, at the Sun's position, is rotating toward the direction of Right Ascension 21:12.0, Declination +48:19. This shows that it rotates "backward" in the Galactic coordinate system, i.e. the Galactic North Pole is actually a physical South Pole with respect to galactic rotation.

The coordinate data given here were extracted from the online coordinate calculator at Nasa's Extragalactical Database (NED) (also available by telnet).