Galaxy

A galaxy is a vast system of stars, stellar remnants, interstellar gas, dust, and dark matter held together by gravity. The term comes from the Greek word for 'milky,' referring to the Milky Way, the galaxy that contains our Solar System. Galaxies vary greatly in size, from small dwarfs with just a few thousand stars to enormous supergiants containing up to one hundred trillion stars.

Galaxies are grouped based on their shapes, such as elliptical, spiral, or irregular. The Milky Way, for example, is a spiral galaxy. Scientists estimate that 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 up of dark matter, with only a small portion visible as stars and nebulae. At the centers of many galaxies lie enormous supermassive black holes. Galaxies can sometimes collide over millions of years, changing their shapes and structures.

Etymology

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

Early astronomers saw galaxies through telescopes and called them spiral nebulae. They weren't sure what they were, thinking they might be groups of stars. But as better telescopes showed, these were actually huge collections of stars far away from our own Milky Way galaxy. Some called them 'island universes', but the name 'galaxy' became the common term later on.

Nomenclature

Many galaxies have been identified, but only a few have well-known names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers use numbers from special lists, or catalogues, to identify galaxies. These include the Messier catalogue, the New General Catalogue, and the Catalogue of Principal Galaxies. For example, Messier 109, also called M109, is a spiral galaxy with several numbers from different catalogues, such as NGC 3992 and UGC 6937.

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. It wasn’t until the year 1610 that Galileo Galilei used a telescope and saw that the Milky Way is truly 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, 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. Recent studies suggest there may be as many as two trillion galaxies in the universe.

Types and morphology

Galaxies come in three main types: ellipticals, spirals, and irregulars. The Hubble sequence helps describe these types based on their shapes. It focuses on visual appearance, which might miss some important 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. Elliptical galaxies also have fewer heavy elements because star formation stopped after an early burst.

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 a 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 come close to each other, they can change each other's shapes. This happens because of the strong pull of gravity between them. Sometimes, galaxies pass close by without merging, which can twist their shapes and mix their gas and dust. In other cases, galaxies can crash into each other and merge together to form a new, larger galaxy. Our own 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 create stars very quickly in a process called a "starburst." During a starburst, a galaxy might use up its gas for making stars faster than it can replace it. These starbursts usually 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 emit powerful waves of energy called radio waves that stretch far beyond what we can see. These radio waves come from huge jets of energy shooting out from a very bright area at the center of the galaxy. These galaxies are studied to learn more about the powerful forces at work in space.

Main article: Active galactic nucleus

A few galaxies have very bright centers that shine much brighter than the rest of the galaxy. These bright centers are called active galactic nuclei. They are powered by supermassive black holes that pull in and heat up surrounding material, making the area very bright. Some of these galaxies also shoot out powerful jets of energy.

Main article: Luminous infrared galaxy

Some galaxies shine very brightly in a type of energy called infrared light. This happens because they are forming new stars very quickly, heating up dust that glows in infrared light. These galaxies are often formed when smaller galaxies merge together. They were more common in the early universe.

Physical diameters

Galaxies don’t have clear edges. Instead, their stars slowly thin out as you move away from the center. Because of this, measuring a galaxy’s true size is tricky. Scientists use different methods to estimate how big galaxies are.

One way is to measure how much space a galaxy takes up in the sky, called the angular diameter. Another method uses how bright the galaxy looks. By tracing where the galaxy’s light fades out, scientists can estimate 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 methods help us understand and compare the sizes of galaxies, even though each has its own strengths and weaknesses.

Larger-scale structures

Galaxies are often found in groups and clusters. Most galaxies are not alone but are part of groups where they stay close together because of gravity. These groups can merge over time to form larger clusters. In these clusters, the space between galaxies can heat up to very high temperatures.

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 their own magnetic fields that affect how they move and change. These fields help shape the spiral arms we see and move gas around, which is important for new stars to form.

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

Formation and evolution

Galaxies formed from tiny changes in matter after the Big Bang. These changes caused gas to clump together, eventually forming the first stars. These early stars were very hot and bright, and their light spread through space.

As galaxies grew, they developed different parts like clusters of old stars and huge black holes at their centers. Over time, galaxies can collide and merge, changing their shapes. The Milky Way, our galaxy, might collide with the Andromeda galaxy in the far future. Eventually, galaxies will stop making new stars and will be made up of dead or faded stars and black holes.