Hubble sequence

Stars live in big groups.

These are called galaxies. We can group them by their shapes.

Some look like smooth, round balls. These are called elliptical galaxies. Others look like spinning wheels.

These are called spiral galaxies. Some have a bar in the middle. Some galaxies look messy and odd. These are called irregular galaxies. It is fun to see their shapes!

How do we group the stars in space?

In 1926, Edwin Hubble made a system to do this. It is called the Hubble sequence. Many people call it the Hubble tuning-fork diagram. This is because the drawing looks like a tuning fork.

One group is called elliptical galaxies. These look like smooth, oval shapes. They do not have much detail. Another group is called lenticular galaxies.

These have a bright center and a flat disk. But they do not have spiral arms.

Most galaxies are spiral galaxies.

These have a bright center called a bulge. They also have arms that spin around. Some spirals have a bar in the center.

These are called barred spirals.

Some galaxies do not fit these shapes. They are called irregular galaxies.

These look messy or odd. They do not have a clear center or arms. Scientists still use this system to study the sky today.

How do we organize the vast collections of stars in our universe?

In 1926, Edwin Hubble published a system to group galaxies by their shapes. This is known as the Hubble sequence. Many people call it the Hubble tuning-fork diagram. This name comes from the way the diagram is drawn. It looks very much like a tuning fork used in music.

This system is still used by professional and amateur astronomers today.

The system works by looking at how a galaxy appears. It divides regular galaxies into three main groups. The first group is elliptical galaxies, which are labeled with the letter E. These galaxies look like smooth, glowing ovals or ellipses. They do not have much detail in their light. The second group is called lenticular galaxies, or S0. These have a bright central bulge and a flat disk. However, they do not have visible spiral arms.

The third group is the spiral galaxies. These have a central bulge and arms that wrap around. Some of these spirals also have a bar-like shape in the middle.

History shows how our understanding of these shapes has grown. Edwin Hubble originally thought lenticular galaxies were just a guess. He believed they were a middle step between ellipticals and spirals. Later, his own observations helped prove he was right. Allan Sandage later included the S0 class in a definitive version of the sequence. Other scientists like Gérard de Vaucouleurs added more detail later on. He added a fourth spiral class called Sd.

This helped make the classification even more specific.

There are many specific names and numbers in this system. Elliptical galaxies range from E0, which is nearly circular, to E7. Lenticular galaxies include examples like M85 or NGC 5866. Spiral galaxies use letters like Sa, Sb, or Sc to show how tight their arms are. For example, Sa spirals have tightly wound arms. Sc spirals have loosely wound arms with many star clusters. Our own Milky Way is a barred spiral, often classed as SBc.

Some galaxies are called irregulars because they have no regular shape at all.

This system helps us connect what we see to how galaxies work. Even though the names "early-type" and "late-type" are used, they do not mean one comes after the other. Hubble was careful to say this was just a way to name positions. We can see differences in the stars within these groups. Spiral galaxies often have many young stars in their arms. Elliptical galaxies are mostly made of older stars. This helps scientists study the many different ways galaxies can form and change.

The Hubble sequence is a morphological classification scheme used to group galaxies by their shapes.

Published by Edwin Hubble in 1926, this system is often called the Hubble tuning-fork diagram. This nickname comes from the visual shape of the diagram itself. It is a fundamental tool in extragalactic astronomy. Both professional researchers and amateur astronomers use it today to organize the vast diversity of the universe. It allows scientists to categorize galaxies based on their visual appearance, such as how they look on photographic plates.

The sequence begins with elliptical galaxies, denoted by the letter E. These galaxies have smooth, featureless light distributions. They appear as ellipses in images. Astronomers label them with an integer from 0 to 7 to show their degree of ellipticity. An E0 galaxy is nearly circular. An E7 galaxy is much more stretched out. This number is ten times the ellipticity, rounded to the nearest integer. However, how a galaxy looks depends on its orientation. A flat disk seen face-on might look like a round E0 elliptical.

At the center of the tuning fork lie the lenticular galaxies, or S0 type.

These galaxies act as an intermediate class. They possess a bright central bulge like an elliptical galaxy. They also have an extended, disk-like structure like a spiral galaxy. However, lenticular disks lack visible spiral arms. They also do not actively form many new stars. Hubble originally thought these galaxies were a necessary hypothetical stage between ellipticals and spirals. Later observations by Hubble and others proved this was correct. Allan Sandage later included the S0 class in the definitive version of the sequence.

The two parallel branches of the tuning fork contain the spiral galaxies. A spiral galaxy consists of a central bulge and a flattened disk. The disk usually features two arms in a spiral structure. About half of all spirals are barred spirals, denoted as SB. In these galaxies, a bar-like structure extends from the bulge. The spiral arms then begin at the ends of this bar.

Regular spirals are denoted simply by the letter S.

Spiral galaxies are further divided by the appearance of their arms. Astronomers use lowercase letters to show these specific details. Sa or SBa types have large, bright bulges and tightly wound, smooth arms. Sb or SBb types have somewhat fainter bulges and less tightly wound arms. Sc or SBc types have smaller bulges and loosely wound arms. These arms are clearly resolved into individual nebulae and stellar clusters. Gérard de Vaucouleurs later added a fourth class called Sd or SBd. These have very loosely wound, fragmentary arms. Most of the light in these galaxies is in the arms rather than the bulge. Our Milky Way is classified as a barred spiral, specifically type SBc.

Some galaxies do not fit into the regular sequence. These are called irregular galaxies. Hubble defined two main types of irregulars. Irr I galaxies have asymmetric profiles and lack a central bulge. They often contain many clusters of young stars. Irr II galaxies have smoother but still asymmetric appearances. They are not clearly resolved into individual stars. De Vaucouleurs called Irr I galaxies "Magellanic irregulars" after the Magellanic Clouds. He also identified a sub-class, Sm, which shows some evidence of spiral structure. The Small Magellanic Cloud is an example of an Im type, which has no obvious structure.

There is a common misunderstanding regarding the terms "early-type" and "late-type" galaxies. Elliptical and lenticular galaxies are called early-type. Spirals and irregulars are called late-type. Many people mistakenly believe this implies an evolutionary sequence. They think ellipticals turn into spirals over time. Hubble insisted this was not the case. He stated the names refer only to the position in the sequence. In reality, current evidence suggests the opposite of an evolutionary path. The early universe was dominated by spirals and irregulars. Present-day ellipticals likely formed from the mergers of these earlier galaxies. This shows how the sequence helps us study the physical properties of the cosmos.