Spiral galaxy

Spiral galaxies are a special type of galaxy that were first described by Edwin Hubble in his 1936 book The Realm of the Nebulae. They are part of what is called the Hubble sequence. Most spiral galaxies have a flat, spinning disk filled with stars, gas, dust, and a bright bunch of stars in the middle called the bulge. Around this, there is usually a faint area called the halo, which contains many stars grouped in special clusters.

An example of a spiral galaxy, Messier 77 (also known as NGC 1068)

These galaxies are called "spiral" because they have spiral shapes that stretch out from the center. These spiral arms are places where new stars are constantly being born. They glow brightly because of the young, hot stars found there. Many spiral galaxies also have a bar-like shape coming from the center, with the spiral arms starting at the ends of this bar. Over time, the number of galaxies with these bars has grown.

The Milky Way, the galaxy we live in, is a barred spiral, though it is hard to see the bar from our position inside it. Together with irregular galaxies, spiral galaxies make up about 60% of all galaxies we see today. They are usually found in areas where there are not many other galaxies, and they are rarely seen in the very busy areas where lots of galaxies cluster together.

Structure

Spiral galaxies are made up of several different parts. They usually have a flat, spinning disk with stars, gas, and dust. This disk often shows spiral patterns, which is why these galaxies are called "spiral" galaxies.

In the middle of the disk, there is a big group of older stars called the bulge. Around the galaxy, there is a faint area filled with stars, some of which are in round clusters. At the very center of the bulge, there is a supermassive black hole. The way these parts look and how much they matter can change a lot from one galaxy to another.

Tuning-fork-style diagram of the Hubble sequence

Spiral arms are the long, thin areas of stars that stretch out from the center of spiral galaxies. They look like spirals and give these galaxies their name. These arms are bright because they have many young, blue stars.

A bulge is a large, tight group of stars in the middle of most spiral galaxies. The stars in the bulge of some galaxies are older, while in others they are younger.

Many spiral galaxies also have a bar-shaped group of stars. This bar can be strong or weak and is seen in about two-thirds of all spiral galaxies.

Barred spiral galaxy UGC 12158

Most of the stars in a spiral galaxy are either close to a single flat plane, moving in circles around the center, or in a round bulge around the core. Some stars are in a spherical halo around the galaxy. These halo stars are older and have fewer heavy elements than stars in the disk. They may have come from small galaxies that merged with the spiral galaxy.

The oldest known spiral galaxy is BRI 1335-0417. Its light took 12.4 billion years to reach Earth. Another very old spiral galaxy is BX442, which is about eleven billion years old. Researchers think its shape is influenced by a small companion galaxy.

Main article: Spiral arm

Main article: Galactic bulge

Origin of the spiral structure

The study of how spiral galaxies form their spiral shapes began with Bertil Lindblad in 1925. He noticed that stars could not stay in a perfect spiral shape forever because the galaxy rotates, causing any fixed spiral arm to twist and wind tighter over time. This is known as the winding problem.

Spiral galaxy NGC 6384 taken by Hubble Space Telescope

There are two main ideas about why spiral arms look the way they do. One idea is that spiral arms are areas where there is more stuff, called density waves, which cause new stars to form. The other idea is that the arms form because of shock waves created by exploding stars and stellar winds, which trigger new star formation in a chain reaction. These ideas can both be true in different ways.

Bertil Lindblad suggested that spiral arms are areas where there is more material. When gas enters these areas, it gets squeezed and forms new stars, including bright blue stars that light up the arms.

Frank Shu and C. C. Lin later explained that these arms are like waves moving through the galaxy. Stars and gas move through these waves, and when they do, the extra material can cause new stars to form. The arms look brighter because they contain many young, bright stars. As these stars age and dim, the arms still look clear because new stars keep forming.

Distribution of stars in spirals

The similar distribution of stars in spirals

The stars in spiral galaxies are arranged in thin, flat disks. The brightness of these stars decreases as you move away from the center of the galaxy. Scientists use special formulas to describe how the stars are spread out. These formulas help us understand the size and total brightness of the galaxy’s star disk. The way the light is spread out in these disks is the same, no matter how bright the galaxy is overall.

Spiral nebula

Before we knew that spiral galaxies were separate from our own Milky Way, they were called spiral nebulae. This was because Lord Rosse used his big telescope, Leviathan, to see the spiral shape of galaxies for the first time. In 1845, he found the spiral shape of a galaxy later called the "Whirlpool Galaxy".

Drawing of the Whirlpool Galaxy by Rosse in 1845

Later, scientists wondered if these spiral shapes were their own galaxies or just parts of our Milky Way. In 1923, Edwin Hubble showed that these spiral shapes were indeed whole galaxies outside our own. After that, the term spiral nebula was no longer used.

Milky Way

The Milky Way used to be thought of as a normal spiral galaxy. In the 1960s, astronomers started to believe it might actually be a barred spiral galaxy. This was proven true in 2005 when observations from the Spitzer Space Telescope showed that the central bar in the Milky Way is bigger than people thought before.