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Galaxies

A galaxy is a large group of stars, dust, gases and dark matter, mutually bound by gravity and rotating about a common centre of mass. While all galaxies share these characteristics, they have them in almost endless variations.

The first variation is size. Galaxies vary from a few thousand to over quarter of a million light-years across. The smaller specimens, such as the Large and Small Magellanic Clouds, visible in the Southern Hemisphere skies, are actually Dwarf Galaxies, and are often gravitationally bound to a larger galaxy. Larger galaxies usually have clearly defined centres, surrounded by a central bulge of stars that are usually older than stars further from the centre. Though only indirectly visible through galactic haze, the centre of our own Milky Way galaxy has an active source of radiation, known as Saggitarius A-Star. The radiation output and turbulence suggest that a super-massive black hole lurks at the centre of our galaxy. These black holes seem to inhabit the centres of most large galaxies.

Galaxies also come in a bewildering variety of shapes. In 1927, Edwin Hubble formulated a classification of galaxies according to their shapes: lenticular, spiral, elliptical and irregular. The image below is known as ‘The Hubble tuning fork’ and demonstrates the various galaxy shapes.
The two most familiar shapes are the lenticular, or lens-shaped galaxy, which has a smooth appearance and a central bulge tapering to the edges. Lenticular galaxies are regarded as the oldest, because most of their stars are old and very few new stars are being created. The other is the spiral galaxy, which shows much more structure. Spiral galaxies are named for their fan-like appearance, with vane-like arms spiralling outward from their centres to their edges, apparently the result of gravitational perturbations across the galaxy. Gravitational variations in a galaxy behave rather like wind blowing over water, causing the stars to concentrate into wave patterns, while not significantly affecting their normal orbital movement around the galaxy’s centre. Even in spiral galaxies there are many variations, depending on the type of spiral. While some have their spirals starting near the centre, others have a smooth centre with the spiral structures further out. Another variation is the ‘bar’ structure often seen across the centre of some spiral galaxies. The bar structure can vary from barely detectable to very developed. These galaxies usually have their spirals coming off the ends of the central bar structure.

Where does our Milky Way fit into the picture? Curiously, we seem to know less about our galaxy than we do about galaxies millions of light-years away. This is due to the difficulties of trying to see it from the inside through layers of dust and gas. It is considered to be a barred spiral galaxy with about 200 billion stars and about 100,000 light-years across. We are 28,000 light-years from its centre, on a spur of the Orion Arm of the galaxy, with the Carina Arm on our inside toward the centre and the Perseus Arm on our outside.