The Milky Way Galaxy

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The Milky Way Galaxy

A spiral galaxy, type Sbc,

centered in Sagittarius

[MW_AAT]

The Milky Way is the galaxy which homes 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 hydrogene

clouds have revealed that the Milky Way is a

spiral galaxy of Hubble type Sb or Sc. It is still

not clear if it has a bar structure (so that it would be type SB) 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.

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).

As we are situated within the outer regions of this galaxy, only about 20 light

years above the equatorial symmetry plane but about 28,000 light years from the

Galactic Center, 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 been confirmed by the data of

ESA’s astrometric satellite Hipparcos.

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:

Our image was obtained by

David Malin of the AAT, 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 Coral Cooksley of the

Anglo Australian Observatory.

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