Curious Kids: How many galaxies are there in the universe?

How many galaxies are there beyond the Milky Way? – Rosella, 15, Hong Kong

A galaxy is a massive collection of gas, dust, and billions of stars, all bound together by the force of gravity. Galaxies are also enormous, measuring billions of billions of kilometers in diameter.

To fully understand what a galaxy is, we should start by looking at our own galaxy, the Milky Way.

Our Sun is just one star among billions of other stars contained 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 can see with our eyes are all part of the Milky Way. If you’ve been outside on a really clear, dark night, you might have noticed a thin, fuzzy band of stars and light stretching across the sky. This is our galaxy, the Milky Way, seen from the inside. We see a thin line because our galaxy is shaped like a thin disk, and we are looking at the edge of the disk.

Curious Kids is a series from The Conversation that gives kids the chance to have their questions about the world answered by experts. If you have a question you’d like an expert answered, send it to curiouskids@theconversation.com and be sure to include the asker’s first name, age, and city. We cannot answer all questions, but we will do our best.

If we look towards the center of this disk, we see a brighter region called the galactic core. The core stars are clustered much closer together and form a ball-like shape that sticks out from the top and bottom of the disc.

By mapping the positions and movements of stars in the Milky Way, we can begin to build a picture of what our galaxy might look like if we could look at the disk from above. The general shape would be a circle.

We would see the bright core, which would look reddish-yellow, because the stars here are cooler. A number of spiral arms, bluish in color because they contain hotter stars, would spiral out from this core. The Milky Way would look a bit like a whirlpool.

Beyond the Milky Way

Astronomers are convinced that our Milky Way has spiral arms because we see many other galaxies like it when we look out into the universe. Most other galaxies that are thin disks similar to our Milky Way also have winding spiral arms. We call these spiral galaxies.

However, not all galaxies look like this. Some of the other galaxies we see in the universe look like smooth, fuzzy ovals of light, something between the shape of a basketball and a rugby ball. We call these galaxies elliptical, and they are mostly made up of cooler, redder stars. There are also galaxies that have no particular shape at all. They are called irregular galaxies.

Determining how many galaxies there are in the universe is actually quite difficult. Many galaxies are too faint or too small for us to observe easily, even with the most powerful telescopes. Despite this, 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 galaxies near and far away.

This tiny patch of sky was full of galaxies, nearly 10,000, of all different sizes and shapes. Multiplying that number by the number of times that tiny patch of sky would fit into the entire sky, astronomers came up with an estimate of between about 100 and 200 billion galaxies. This number will almost certainly change, however, as we learn more about our universe in the future.