Galaxy

A galaxy is a huge system of stars, stellar remnants, interstellar gas, dust, and dark matter held together by gravity. The word comes from the Greek word for 'milky,' because of the Milky Way, the galaxy where our Solar System lives. Galaxies come in many sizes, from tiny dwarfs with just a few thousand stars to very large supergiants with up to one hundred trillion stars.

Galaxies are grouped by their shapes, such as elliptical, spiral, or irregular. The Milky Way, for example, is a spiral galaxy. Scientists think there are between 200 billion and 2 trillion galaxies in the observable universe. These galaxies are often found in groups, clusters, and even larger structures called superclusters.

Most of the mass in a galaxy is made of dark matter, with only a little visible as stars and nebulae. Many galaxies have huge black holes at their centers. Galaxies can sometimes collide over millions of years, changing their shapes and looks.

Etymology

The word galaxy comes from an old Greek word for the Milky Way. It means 'milky'. This name describes the milky band of light we see in the night sky.

Early astronomers used telescopes to look at galaxies. They called them spiral nebulae. They were not sure what these objects were. They thought they might be groups of stars. With better telescopes, we learned these were huge collections of stars far from our own Milky Way galaxy. Some people called them 'island universes', but the name 'galaxy' became the common term.

Nomenclature

Many galaxies have names, like the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers also use numbers from special lists to name galaxies. These lists include the Messier catalogue, the New General Catalogue, and the Catalogue of Principal Galaxies. For example, Messier 109, or M109, is a spiral galaxy with numbers like NGC 3992 and UGC 6937 from different lists.

Observation history

Milky Way

Main article: Milky Way

Long ago, people wondered what the Milky Way was. The Greek thinker Democritus thought it might be made of faraway stars. Others had different ideas. In 1610, Galileo Galilei used a telescope and saw that the Milky Way is made of many, many stars.

Later, astronomers learned more about our galaxy. They discovered that the Milky Way is a huge spinning collection of stars, held together by gravity, much like our Solar System but much bigger. Scientists have studied its shape and where our Sun sits within it.

Distinction from other nebulae

Some galaxies can be seen without a telescope on a dark night. One example is the Andromeda Galaxy, which looks like a fuzzy patch in the sky. People have noticed these objects for a very long time. In the 1700s, a man named Charles Messier made a list of fuzzy things in the sky, including some galaxies. Later, others studied these objects and realized some were faraway galaxies, not part of our own Milky Way.

Multi-wavelength observation

New tools have helped scientists study galaxies in new ways. We now use more than just visible light. We can see heat light to look through dusty parts of galaxies, radio waves to study faraway galaxies, and even X-rays to see energetic events. These tools help us learn about the structure and activity in galaxies.

Modern research

In the 1950s, scientists discovered a special kind of radio signal from hydrogen gas in galaxies. This helped them map how galaxies move. They found that galaxies spin faster than their visible matter should allow, suggesting there is invisible “dark matter” helping to hold them together. The Hubble Space Telescope has provided amazing pictures and data, showing there are far more galaxies in the universe than we can see.

Types and morphology

Galaxies come in three main types: ellipticals, spirals, and irregulars. The Hubble sequence helps describe these types by their shapes. It looks at how they appear, which might miss some features like star formation in starburst galaxies or activity in the centers of active galaxies.

Many galaxies are thought to have a supermassive black hole at their center, including the Milky Way, whose core region is called the Galactic Center.

Ellipticals

Elliptical galaxies are rated based on how stretched out they are, from nearly round (E0) to highly elongated (E7). They look like smooth, round blobs and usually have very little interstellar matter. Because of this, they have fewer new stars and are made mostly of older stars. These stars orbit the center in random directions.

Spirals

Spiral galaxies look like spinning pinwheels. They have a flat disk of stars and a central bunch of older stars. Spiral arms stretch out from the center and are made of bright, young stars. In the Hubble system, they are called S followed by a letter (a, b, or c) that shows how tightly wound the arms are and how big the central bunch is. The Milky Way is a barred spiral galaxy, meaning it has a bar-shaped band of stars crossing its center.

Other morphologies

There are other special kinds of galaxies. Peculiar galaxies have unusual shapes because of gravitational pulls from other galaxies. A ring galaxy has a ring of stars around an empty center. A lenticular galaxy is a mix between elliptical and spiral galaxies. Irregular galaxies don’t fit into the usual categories.

Dwarfs

Most galaxies are actually dwarf galaxies, which are much smaller than big galaxies like the Milky Way. They have only a few billion stars. Many of these small galaxies orbit larger ones; the Milky Way has many such companions. These dwarf galaxies help scientists understand how galaxies form.

Variants

Main article: Interacting galaxy

When galaxies get close to each other, they can change shape because of gravity. Sometimes they just pass by and twist each other's shapes. Other times, they crash into each other and become one big galaxy. Our Milky Way is slowly merging with smaller galaxies nearby.

Main article: Starburst galaxy

Inside galaxies, new stars are born from clouds of gas and dust. Some galaxies make stars very fast in a process called a "starburst." During a starburst, a galaxy may use up its gas quickly. These starbursts last only a few million years. They are more common in the early universe but still happen today. Starbursts often happen when galaxies interact or merge.

Main article: Radio galaxy

Some galaxies send out strong waves of energy called radio waves. These waves come from huge jets of energy shooting out from the center of the galaxy. Scientists study these galaxies to learn more about space.

Main article: Active galactic nucleus

A few galaxies have very bright centers called active galactic nuclei. These bright centers are powered by supermassive black holes that pull in and heat up material around them. Some of these galaxies also shoot out powerful jets of energy.

Main article: Luminous infrared galaxy

Some galaxies shine very brightly in infrared light because they are making new stars quickly. This heats up dust that glows in infrared light. These galaxies often form when smaller galaxies merge together. They were more common in the early universe.

Physical diameters

Galaxies don’t have clear edges. Their stars get thinner as you move away from the center. Because of this, measuring a galaxy’s true size is tricky. Scientists use different ways to guess how big galaxies are.

One way is to measure how much space a galaxy takes up in the sky. This is called the angular diameter. Another way uses how bright the galaxy looks. By tracing where the galaxy’s light fades out, scientists can guess its size. There are also special formulas, like the half-light radius, which measures how far you have to go from the center before you’ve seen half of the galaxy’s light. These different ways help us learn and compare the sizes of galaxies.

Larger-scale structures

Galaxies are often found in groups and clusters. Most galaxies are not alone. They stay close together because of gravity. These groups can merge over time to form larger clusters.

On the largest scales, galaxies are arranged in sheets and filaments with big empty spaces in between. The Milky Way, our home galaxy, is part of a group called the Local Group, which is itself part of an even larger structure called the Virgo Supercluster.

Magnetic fields

Galaxies have magnetic fields that change how they move and look. These fields help shape the spiral arms and move gas, which helps new stars form.

The strength of these magnetic fields in spiral galaxies is about 10 microgauss, much weaker than Earth’s magnetic field. Some galaxies, like the Milky Way’s neighbors, have even weaker fields, while others that make stars quickly can have stronger fields.

Formation and evolution

Galaxies started from small changes in matter after the Big Bang. These changes made gas come together, forming the first stars. These early stars were very hot and bright, and their light shone across space.

As galaxies grew, they developed parts like groups of old stars and big black holes in the middle. Over time, galaxies can crash into each other and join together, changing their shapes. The Milky Way, our galaxy, might crash into the Andromeda galaxy far in the future. Finally, galaxies will stop making new stars and will be filled with old or faded stars and black holes.