G8 – Solar Systems, Galaxies and the Universe Approaching -03

Galaxies

Compared to Earth, the solar system is a big place. But compared to a galaxy, a solar system is just a tiny point of light to mark its central star. Our Sun is just one of the billions of stars that make up the Milky Way Galaxy. As you look up into the night sky, all the stars you see are part of the Milky Way Galaxy.

A galaxy is a very large group of stars. It can contain billions of stars. Some galaxies contain up to a trillion stars! Our galaxy, the Milky Way, contains billions of stars. Our Sun is just one of those billions of stars. If our solar system was the size of your fist, the Milky Way Galaxy would be bigger than the US! The Milky Way, where our solar system is, is only one of many billions of galaxies in the universe.

The distance between galaxies is vast. For example, the closest galaxy to the Milky Way is the Andromeda Galaxy. It is more than 2,500,000 light years away. There are many types of galaxies. Galaxies are classified according to their shape. There are spiral galaxies like our Milky Way and the Andromeda Galaxy. There are also elliptical galaxies and irregular galaxies. Different types of galaxies have different characteristics. One thing they do have in common is how they were formed.

Formation of our Solar System

Scientists now have a hypothesis of how our solar system was formed. This may also hold true for other solar systems. This hypothesis is known as the solar nebular hypothesis. Everything we now see inside our solar system, from the Sun to the smallest comet, all started from a huge cloud of gas and dust. These large clouds of gas and dust are called nebula. It is believed that the nebula was made up of mostly hydrogen and helium. It was gravity that caused the nebula to contract over time.

As the nebula contracted, it started to rotate. As it got smaller and smaller, it spun faster and faster. It is similar to when ice skaters pull their arms in as they spin. This decrease of rotational inertia caused the cloud of gas and dust to spin even faster. The high rate of spin caused the nebula to form into a disk shape. If correct, this hypothesis would explain why all the planets are found in the flat disk-shaped region. It also explains why all the planets revolve in the same direction. There is evidence to suggest our solar system formed from a nebula about 4.6 billion years ago. Currently, the nebular hypothesis is well-regarded by scientists. This means it is the most likely cause of our solar system formation. There is less agreement on how galaxies form.