How many galaxies are there outside the Milky Way? – Rosella, 15, Hong Kong
A galaxy is a huge collection of gas, dust and billions of stars held together by gravitational forces. Galaxies are also very large, billions of kilometers in diameter.
To properly understand what a galaxy is, we need to start by looking at our own Milky Way.
Our sun is just one star among billions of others in a galaxy called the Milky Way. In the same way that the Earth revolves around the Sun, the Sun also revolves around the center of the Milky Way.
When we look at the night sky, the stars we see with our eyes are all part of the Milky Way. If you’re outside on a very clear, dark night, you’ve probably seen a thin string of stars and lights stretching across the sky. This is our galaxy, the Milky Way, seen from the inside. We see a thin line because our galaxy is a thin disk and we are looking at the edge of the disk.
If we look towards the center of this disk, we see a much brighter region called the galactic nucleus. The stars in its core are clustered closer together, forming a spherical shape that rises from the top and bottom of the disk.
By tracking the positions and movements of the stars in the Milky Way, we can begin to form a picture of what our galaxy would look like if we could look down on the disk. The general shape will be a circle.
We will see a bright core, which will appear reddish yellow, because the star is much cooler here. Numerous spiral arms erupt from this core, which is blue because it contains much hotter stars. The Milky Way will look like a vortex.
Beyond the Milky Way
Astronomers believe our galaxy, the Milky Way, has spiral arms because we see so many other similar galaxies when we look out into the universe. Most other thin-disk galaxies similar to the Milky Way also have undulating spiral arms. We call them spiral galaxies.
However, not all galaxies look like this. Some of the other galaxies we see in the universe appear as soft, hazy ellipses of light, between the shapes of a basketball and a rugby ball. We call them elliptical galaxies, and they’re mostly made up of cooler, redder stars. There are also galaxies that have no definite shape. These are called irregular galaxies.
It is very difficult to know how many galaxies there are in the universe. Many galaxies are too faint or too small to be easily observed, even with the most powerful telescopes. However, astronomers have found a clever way to solve this problem. Astronomers pointed the Hubble Space Telescope at a small patch of sky for 11.3 days and collected light from near and distant galaxies.
This little patch of sky is filled with galaxies, around 10,000, of all sizes and shapes. By multiplying that number by the number of times a small patch of sky fits into the whole sky, astronomers estimate between 100 and 200 billion galaxies. This number will almost certainly change, as we learn more about our universe in the future.
Written by Nicholas Bone, University of Portsmouth (Conversation)
