Galaxies

Galaxies are colossal systems held together by gravity, containing anywhere from millions to trillions of stars alongside vast clouds of gas, dust, stellar remnants, and enormous quantities of invisible dark matter. They come in a variety of shapes and sizes, shaped over billions of years by mergers, star formation, and gravitational forces.

At the centre of most large galaxies lurks a supermassive black hole — in our own Milky Way, this is Sagittarius A*, with a mass 4 million times that of our Sun.

• Spiral — Rotating disc with sweeping arms. The Milky Way and Andromeda are both spirals.
• Elliptical — Featureless, egg-shaped. Can contain trillions of stars. Often old, star-formation complete.
• Lenticular — Disc-shaped like a spiral but without defined arms. A transition type.
• Irregular — No defined shape. Often the result of collisions or gravitational distortion.
• Dwarf Galaxies — Small, faint. The Milky Way has over 50 dwarf galaxy satellites.

Our home galaxy is a barred spiral galaxy approximately 100,000 light-years in diameter. It contains an estimated 100–400 billion stars and at least as many planets. The solar system sits roughly 26,000 light-years from the galactic centre, on the Orion Arm — a minor spiral arm between two major ones.

The Milky Way and the Andromeda Galaxy (M31) are on a collision course. In approximately 4.5 billion years, the two galaxies will begin merging — eventually forming a single massive elliptical galaxy sometimes called "Milkomeda."