What Are Galaxies? Learn Everything about Galaxy Formation and Evolution

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Updated on: Educator Review By: Michelle Connolly

Have you ever looked up at the night sky and wondered what those twinkling lights are? They’re not just stars; they’re part of something much bigger and even more amazing called galaxies!

Galaxies are giant groups of stars, gas, and dust that all stick together in the massive universe. They hold billions of stars, just like our Sun, swirling together in different shapes and sizes. Our very own planet, Earth, lives inside a galaxy called the Milky Way. 

So, are you ready to travel beyond the stars and learn more about the incredible galaxies? Let’s learn and have fun!

What is a Galaxy?

A galaxy is a giant group of stars, dust, and gas, all held together by an invisible force called gravity. Just like the Earth’s gravity keeps you from floating away, gravity pulls all the stars in a galaxy closer to each other.

What’s Inside a Galaxy?

Galaxies contain more than just stars. Let’s learn about the different components that make up a galaxy.

1. Sparkling Stars

The main ingredient in a galaxy is stars. They are giant balls of burning gas that light up the universe, just like the Sun lights up our planet Earth. Stars come in all sizes and colours—some are even hundreds of times bigger than the Sun! Galaxies can hold billions, even trillions of stars, making them truly enormous and dazzling.

How do all these stars stay together? Stars are very massive, which means they have a lot of gravity. This strong gravity pulls on other stars and gas nearby, keeping them all trapped within the galaxy. In other words, stars pull on each other and stay together.

2. Dust

Space dust is made up of tiny particles like sand, ice, and even space rocks floating around in the space between the stars.

While it might seem like dust wouldn’t be very important, it actually plays a crucial role in galaxies. Sometimes, this dust gets clumped together in big clouds. When light from nearby stars shines through it, it can create beautiful, colourful formations called nebulae. These colourful clouds come in all shapes and sizes and look like beautiful wisps of smoke in space. 

3. Interstellar Gas

Just like air fills a balloon, gas fills the space between stars in a galaxy. Gas is made up of tiny particles, even smaller than dust particles, and it’s spread out between the stars and dust in a galaxy.

This gas is like the fuel for a car because it’s what helps to create new stars. When the gas gets squeezed together by gravity, it gets very hot and dense, and eventually, it can spark and ignite, forming a new star.

4. Dark Matter

In a galaxy, a mysterious substance called dark matter makes up most of its mass, even though it’s invisible to our telescopes. While we can’t see it directly, we know it exists because it has gravitational effects on visible matter like stars and gas.

Scientists are still trying to understand what dark matter is made of and how it formed. Studying its impact on galaxies helps them understand how they evolve and behave.

Types of Galaxies

Galaxies come in different shapes and sizes, just like houses in a city! While there are many types, astronomers have identified three main categories based on their appearance: spiral, elliptical, and irregular.

1. Spiral Galaxies

A spiral galaxy looks like a giant pinwheel made of stars, dust, and gas. These magnificent galaxies have beautiful, flattened disks with long, winding arms stretching out from the centre. The Milky Way, our own galaxy, is a spiral galaxy. 

The spiral arms of a galaxy are filled with young, hot stars, gas, and dust. These arms are constantly creating new stars, making them the most vibrant and colourful parts of the galaxy.

At the centre of a spiral galaxy lies a bright, dense region called the central bulge. This bulge contains mostly older stars packed tightly together.

2. Elliptical Galaxies

Unlike the swirling arms of spiral galaxies, elliptical galaxies are smooth and round. They come in various shapes, ranging from round to oval, and unlike spiral galaxies, they lack distinct arms.

Elliptical galaxies are filled with mostly older stars, which explains why they appear less colourful than spiral galaxies. Since they have less gas and dust, they don’t form many new stars.

They are relatively calm and stable compared to the bustling activity in spiral arms. They haven’t been involved in many collisions with other galaxies, which can trigger star formation.

3. Irregular Galaxies

Unlike the organised structure of spiral and elliptical galaxies, irregular galaxies don’t have a specific shape. They can be lumpy, clumpy, or even elongated. They are quite rare compared to the other two types.

Irregular galaxies often contain a mix of young and old stars, gas, and dust. This is because they are often formed by the collision of two galaxies, which disrupts their structure and triggers episodes of star formation.

Scientists are still learning a lot about irregular galaxies, and their chaotic appearance makes them an exciting area of ongoing research.

Galaxy Formation and Evolution

Galaxies have a story of their formation that stretches back billions of years. Let’s explore everything about galaxy formation and evolution.

1. The Big Bang

Billions of years ago, the universe was incredibly hot and dense. Then, the universe began to expand and cool down in a massive explosion called the Big Bang

During this expansion, tiny fluctuations in the density of the universe occurred, making some areas slightly denser than others. These denser regions became the seeds for future galaxies. Over time, gravity pulled the matter in these areas closer together, causing them to collapse and condense.

2. Gas Clouds and Star Formation

As these dense regions collapsed, they formed giant clouds of mostly hydrogen gas, the same kind of gas that makes up a large portion of interstellar gas today. Within these clouds, gravity continued to pull pockets of gas together, becoming denser and hotter.

Eventually, the pressure and temperature within these dense pockets became high enough to ignite nuclear fusion, the process that powers stars. This marked the birth of the first stars in the galaxy, shining brightly and marking the beginning of a new era.

3. Growth and Evolution

Galaxies are not static structures; they are constantly evolving over billions of years. New stars are constantly being born in the dusty arms of spiral galaxies, while older stars in elliptical galaxies continue their long journey of burning and fading.

The ongoing formation of stars, the influence of dark matter, and even potential future collisions with other galaxies all contribute to the dynamic and ever-changing nature of our universe.

Exploring Our Own Galaxy: The Milky Way

Our Milky Way is classified as a spiral galaxy. It has a flattened disk filled with stars, gas, and dust and two spiral arms winding out from its centre. Our solar system, with our planet Earth, is located within one of these arms, called the Orion Arm.

The Milky Way is estimated to contain hundreds of billions of stars, making it a truly massive collection. While we can’t see the entire Milky Way due to the dust within it, on a clear night, you can sometimes see a faint band of light stretching across the sky. This is the combined light of billions of stars that form the Milky Way’s disk.

What’s Inside the Milky Way Galaxy?

The Milky Way isn’t just filled with stars; it also contains a variety of other celestial objects. 

  • Black Hole: At the centre of the Milky Way lies a massive black hole called Sagittarius A*. It’s a region of extremely strong gravity with such a powerful pull that not even light can escape its grasp. Though black holes are invisible due to their immense gravity, scientists can detect their presence by observing the behaviour of stars and gas around them.
  • Nebulae: These are colourful clouds of gas and dust, sometimes forming beautiful shapes like the Eagle Nebula or the Lagoon Nebula. They are where new stars are born.
  • Star Clusters: These are groups of stars held together by gravity, often containing hundreds or even thousands of stars. They can be young and vibrant or older and more loosely bound.
  • Planets and Planetary Systems: While we haven’t yet discovered other planets like Earth within the Milky Way beyond our own solar system, scientists believe they exist and are actively searching for them.

Other Galaxies Beyond the Milky Way

Our Milky Way galaxy is one of countless galaxies in the vast universe, each with its own unique characteristics. Here are a few remarkable examples to spark your curiosity

1. Andromeda Galaxy (M31)

Located only about 2.5 million light-years away, the Andromeda Galaxy is our closest giant galactic neighbour. It’s also a spiral galaxy, similar to the Milky Way, but slightly larger.

Interestingly, astronomers predict that the Milky Way and the Andromeda Galaxy will eventually collide in billions of years, forming a single, even larger galaxy.

2. Whirlpool Galaxy (M51)

Located about 31 million light-years away, the Whirlpool Galaxy is a spiral galaxy that exhibits beautiful, well-defined spiral arms. Unlike our edge-on view of the Milky Way, the Whirlpool Galaxy presents itself face-on, offering a stunning view of its beautiful spiral arms.

The Whirlpool galaxy is known for its active star formation, with large, bright regions of gas and dust where new stars are born. It is also considered one of the most visually stunning galaxies observed from our vantage point.

3. Triangulum Galaxy (M33)

Located about 3 million light-years away, the Triangulum Galaxy is the third member of the Local Group, a small cluster of galaxies that includes the Milky Way and Andromeda.

Compared to the Milky Way and Andromeda, the Triangulum Galaxy is a dwarf galaxy containing significantly fewer stars. 

4. Sombrero Galaxy (M104)

This iconic galaxy gets its nickname from its distinctive appearance, resembling a wide-brimmed hat with a dusty central bulge and a faint disk surrounding it.

The Sombrero Galaxy was initially thought to be an elliptical galaxy. However, recent observations suggest it might be a lenticular galaxy, a type of spiral galaxy with a prominent central bulge and faint or no spiral arms.

5. Messier 87 (M87)

Unlike the spiral galaxies mentioned earlier, Messier 87 is a giant elliptical galaxy located about 54 million light-years away. It has a smooth, rounded shape and contains hundreds of billions of stars.

This galaxy is also home to one of the most massive black holes ever discovered, with a mass billions of times that of our Sun.

Interesting Facts about Galaxies

  1. Light travels so fast that it takes a year to travel one light-year, making other galaxies incredibly far away.
  2. The most distant galaxy ever discovered, the galaxy GN-z11, discovered in 2016, is estimated to be over 13.4 billion light-years away.
  3. Scientists estimate that there are billions, maybe even trillions, of galaxies in the observable universe, each containing billions of stars and planets.
  4. Galaxies can collide and merge over time, creating even larger galaxies. Our Milky Way is predicted to collide with Andromeda Galaxy in several billion years.
  5. Galaxies aren’t alone in space. They are often accompanied by smaller galaxies called dwarf galaxies, which orbit them like tiny satellites.
  6. Almost every large galaxy is believed to have a supermassive black hole at its centre, with masses millions or billions of times the Sun’s mass.
  7. Galaxies constantly recycle their material. Stars eventually die, returning their elements back into the galaxy’s gas and dust, which can then form new stars and planets.
  8. Galaxies can experience massive outflows of gas and dust called galactic fountains. This material can be ejected from the galaxy because of the combined effects of stellar winds, supernova explosions, and the pull of the central black hole.
  9. The Milky Way is part of a small group of galaxies called the Local Group, but there are vast clusters and superclusters of galaxies throughout the universe.
  10. While we can’t travel to distant galaxies, scientists use powerful telescopes and instruments to study their properties, helping us understand the formation and evolution of these magnificent cosmic structures.

Ultimately, galaxies hold countless mysteries, and the universe is full of countless other wonders waiting to be discovered. If you’re curious to learn more about space, Learning Mole has many interesting lessons for you! Learn about other objects in the universe, like the dazzling streaks of meteors or the icy tails of comets, and explore the amazing space stations that scientists use to study the universe.

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