BLACK DOG OBSERVATORY

CDD Images of Galaxies

Galaxies are huge gatherings of dust, gas and stars (suns) all bound together by gravity. The smallest are called dwarf galaxies that contain only a few million stars and are a few hundred light years across. Then there are 'normal galaxies' such as our own Milky Way Galaxy which contains hundreds of billions of stars. The largest are giant Elliptical Galaxies which span hundreds of thousands of light years and contain trillions of stars.

Galaxies of roughly the same size can merge with each other to become a much larger one. However what's called 'galaxy cannibalism' occurs when a small galaxy is swallowed by a much larger one. The Sagittarius Dwarf Galaxy is currently being swallowed up by the Milky Way. In either case, because the distances between individual stars are so vast, even though the galaxies appear to crash together, the stars contained in them do not collide with each other. The massive gravitational forces of the galaxies interact to form a new shape.

In the main, there are three types of galaxies:

1. Spiral Galaxies

   • These galaxies, such as the Milky Way Galaxy, have bluish spiral arms of bright young stars surrounding the central nuclear bulge. The closest of spiral galaxies that can be seen with an unaided eye away from city/suburban sky is the Andromeda Galaxy - it looks like a hazy patch of light. There are two types, barred spirals and ordinary spirals. Their diameters range from 10,000 to over 300,000 light years and their mass from about 1 billion to 500 billion times the mass of our Sun. Our solar system is located in one arm of a spiral galaxy; this is what we see as the Milky Way.

2. Elliptical Galaxies

   • These galaxies do not have any spiral features and their appearances may be like a flat cigar, a football or a spherical shapes. These types of galaxies are the result of mergers between spiral galaxies. The orbits of stars in ellipticals are elongated but the galaxies themselves barely rotate.

3. Irregular Galaxies

   • These galaxies do not have defined structures and often have a high gas content. Nonetheless, they are the most numerous of all types. They are mostly faint groups of stars much smaller than spirals. Some merging galaxies may also be called irregular. The Small Magellanic Cloud (SMC) is a visible example of an irregular galaxy. The SMC is visible to the naked eye under dark country skies (away from cities and large towns).