Galaxy Facts
andromeda galaxy
The spiral galaxy Andromeda is thought to look very much like our own Milky Way.
  • Galaxy is derived from the Greek word meaning milk.
  • Our solar system is in a galaxy called the Milky Way.
  • Away from light pollution a spiral arm of the Milky Way can be seen in the night sky with the naked eye.
  • The Milky Way is 100,000 light years across and is thought to have formed 10 to 12 billion years ago.
  • Earth is located 26,000 light years from the center of the Milky Way.
  • The Milky Way rotates once every 250 million years.
  • There are around 200 billion stars in our galaxy.
  • There are approximately 100 billion galaxies in the visible Universe.
  • The nearest spiral galaxy to the Milky Way is Andromeda, it is 2.5 million light years from Earth.
  • The Andromeda galaxy is the furthest object that can be seen with the naked eye.
  • At the center of most galaxies there is a supermassive black hole which can be millions of times more massive than our sun.

The Formation of Galaxies

sombrero galaxy
One of the Universe's most eye catching galaxies, aptly named the Sombrero
One of the liveliest debates amongst astrophysicists is to how the galaxies actually formed. At the heart of the most accepted theory of galaxy formation is dark matter, an invisible material which is much more abundant in the Universe than normal matter. It is thought that in the colder, denser parts of the early Universe dark matter and gas began to clump together, growing larger and larger, forming the seeds of galaxies called protogalaxies.
In these protogalactic clouds star forming regions began to emerge, creating huge high mass stars. These stars were short lived which gave birth to more and more stars from their supernova explosions. Due to the effects of gravity these protogalaxies eventually collided and merged forming the large galaxies that populate the universe today.

Types of Galaxies
Spiral Galaxies
spiral galaxy
Spiral galaxies are shaped like a flattened disk with a bulge in the center from which arms spiral out. The dense center of a sprial galaxy contains older stars along with a supermassive black hole, the less populated spiral arms tend to contain younger stars. Barred spirals are spiral galaxies which have a bar shaped core, it is thought that the Milky Way is a Barred Spiral.
Elliptical Galaxies
elliptical galaxy
Elliptical galaxies have a more spherical like shape, ranging from almost round to a flattened out cigar. They have no spiral arms and are thought to contain mostly older stars. Ellipticals are thought to be the most common type of galaxy in the Universe and could be the result of smaller spiral galaxies colliding.
Irregular Galaxies
irregular galaxy
Irregular galaxies mostly have no discernable shape with no central bulge or spiral arms. It is thought they were once spiral or elliptical galaxies that have been distorted due to collisions. There are two main classifications of irregular galaxies, type Irr-I have some structure, type Irr-II have no structure.

When Galaxies Collide

antennae galaxies
The Antennae galaxies, two separate galaxies in the process of merging.
Galaxies colliding with each other is a very common event in our Universe. In fact our very own Milky Way is a product of smaller dwarf galaxies merging to form a larger one. Due to the effects of gravity the Milky Way and our nearest galactic neighbor Andromeda are hurtling towards one another at the rate of 270,000 miles (435,000 km) per hour and are due to collide in around 3 billion years.

Although the galaxies will collide the stars inside will not, the distances between stars makes that highly unlikely. Instead star systems will either be flung out into interstellar space or sent towards the newly forming core. As the galaxies interact the heat generated will create a burst of new star formation with possibly millions of new stars being born. The supermassive black holes that exist at the centers of these two galaxies will find their way to the core, joining to form a black hole that would be tens of millions of times more massive than our sun.

The hope for our solar system would be that it wouldn’t be anywhere near the center of the newly forming galaxy, that would be a very dangerous place to be. With so much material flying around Earth could be knocked off orbit or blasted by a nearby supernova explosion. On the other hand if our solar system was thrown far from the center into the tidal tails created by the collision we could watch the new galaxy take shape from a safe distance, if humans are still around in 3 billion years that is. The two galaxies will eventually merge and take shape over a period of millions of years, forming into a larger spiral or possibly elliptical galaxy.


quasar galaxy
Quasars emit enormous amounts of energy
The Milky Way is classed as a normal galaxy meaning that the supermassive black hole at its center is in a quiet stage of its evolution. At the moment it has very little material to feed on and so is in a sense fasting, as a result it generates very little energy. The total amount of energy emitted by the Milky Way is due to the amount of stars that are present in the galaxy.

On the other hand active galaxies also known as Quasars emit far more energy than the amount of stars present can account for. This is because the supermassive black holes at the center of these galaxies are not fasting, they are in fact feeding ferociously on material accreting around it, generating huge amounts of energy. As a result Quasars emit a hundred times more energy than a normal galaxy and are 10 trillion times brighter than the Sun.

The word Quaser is short for quasi-stellar radio source and were first detected in the 1960’s. We now know that the energy emitted from Quasars is not just in the form of radio waves but also in visible light, infrared, x-rays, ultra-violet and gamma rays. In addition some Quasars eject a stream of charged particles that travel near to the speed of light, active galaxies which have this jet of energy pointing towards Earth are known as Blazars.