HomeScienceOuter SpaceBeyond the Milky Way: How Many Galaxies Are There in the Universe?

Beyond the Milky Way: How Many Galaxies Are There in the Universe?

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

A galaxy is a vast collection of gas, dust, and billions of stars, all bound together by gravity. Galaxies are also huge, with diameters of billions of billions of kilometers.

To properly understand what a galaxy is, we need to start by looking at our own Milky Way galaxy.

Our sun is just one star out of billions of other stars in a galaxy called the Milky Way. Just as 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 out on a really clear, dark night, you may have noticed a thin, faint band of stars and light stretching across the sky. This is our Milky Way galaxy viewed from the inside. We see a thin line because our galaxy is shaped like a thin disk, and we’re looking at the edge of the disk.

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Looking at the center of this disk, we see a brighter region called the galactic core. Stars at the core are grouped much closer together, forming a shape that resembles a ball peeking out from the top and bottom of the disk.

By mapping the positions and movements of the stars in the Milky Way, we can form 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.

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We would see the bright core, which would appear reddish yellow because the stars are cooler here. From that core would spiral a number of spiral arms, bluish in color because they contain hotter stars. The Milky Way would look a bit like a vortex.

Beyond the Milky Way

Astronomers are convinced that our Milky Way has spiral arms because we see many other similar galaxies when we look at the universe. Most other galaxies that are thin disks, similar to our Milky Way, also have twisting spiral arms. We call these spiral galaxies.

However, not every galaxy looks 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 elliptical galaxies and they are mostly made up of cooler, redder stars. There are also galaxies that have no particular shape at all. These are called irregular galaxies.

Working out how many galaxies there are in the universe is actually quite difficult. Many galaxies are too faint or small to easily observe, even with the most powerful telescopes. Despite this, astronomers came up with a clever way to work this out. Astronomers focused the Hubble Space Telescope on a small patch of the sky for 11.3 days, collecting light from galaxies both near and far.

This little patch of sky was full of galaxies, nearly 10,000, of all different sizes and shapes. By multiplying this number by the number of times this small patch of sky would fit in the entire sky, astronomers estimated between 100 and 200 billion galaxies. However, this number will almost certainly change as we learn more about our universe in the future.

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(This is a PTI story syndicated through The Conversation)

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