Different Types of Galaxies in the Universe

Galaxies are massive star systems that contain stars, planets, interstellar gas, cosmic dust, and other components. These are some of the main components and characteristics that provide a definition of a galaxy:

• Stars: galaxies contain a large number of stars which vary in size, age, and luminosity. Some galaxies may have only a few million stars, while others may contain hundreds of billions. Just as there are stars that die, there are many others that are born. This is a long process which can take around a million Earth years. Learn more about Earth's most important star with our article on why the sun is yellow.
  
  
• Interstellar gas and cosmic dust: in addition to stars, galaxies also contain interstellar gas and cosmic dust. This material is essential for new stars and planetary systems to form within the galaxy. The gas and dust are often found in vast clouds, and their composition can vary, influencing the types of stars that form.
  
  
• Dark matter: the majority of the mass belonging to a galaxy is thought to be composed of dark matter. This is a form of matter that does not emit or interact with electromagnetic light and is, therefore, invisible. Dark matter has a notable gravitational influence on both the formation and evolution of galaxies. Fritz Zwicky proposed the concept in 1933 due to evidence of a ‘non-visible mass’ that would influence the orbital velocities of clusters in galaxies. It is also believed to play a crucial role in holding galaxies together.
  
  
• Shapes and structures: galaxies can have a variety of shapes and structures. The main types of galaxies include spirals, ellipticals, irregulars, and lenticulars. These differences are a result of the distribution and movement of the stars and also the gas in the galaxy. We will learn more about the different galaxy types in the next section.
  
  
• Clusters: galaxies tend to group together into larger structures. These formations are known as galaxy clusters. Galaxy clusters can contain anywhere from a few galaxies to thousands of them, all held together by gravitational attraction. They are the largest known gravitationally bound structures in the universe, and studying them provides insights into the large-scale structure of the cosmos.
  
  
• Evolution: galaxies evolve over time as stars are born, die, and change positions. Additionally, gravitational interactions between neighboring galaxies can lead to collisions and mergers, having an effect on their structure and shape. These events can trigger new star formation and significantly alter the appearance of galaxies.
  
  
• Observation: astronomers study galaxies through telescopes and other instruments. Observing galaxies at different wavelengths of light (such as visible light, infrared, and radio waves) provides valuable information about their composition, formation, and evolution. Advanced technology has allowed astronomers to observe galaxies billions of light-years away, offering a glimpse into the early universe.
  
  
• The large-scale universe: galaxies are the fundamental building blocks of the large-scale universe. The observable universe contains billions of galaxies. These galaxies are distributed throughout space in large structures called filaments and superclusters. These structures form a vast cosmic web that defines the architecture of the universe.

There are a wide variety of types of galaxies in the universe. Below, we show you some examples of galaxies that we know about so far.

This type of galaxy has distinctive spiral arms that extend from a central core. These spiral shapes can be seen in the picture below. Spiral galaxies are subdivided into two main categories:

• Regular spiral galaxies: where the arms are well defined and tend to be smooth and tightly wound.
• Barred spiral galaxies: which have a central bar crossing the nucleus. This bar is composed of stars and can significantly influence the motion of stars and gas within the galaxy.

The Milky Way is an example of a spiral galaxy. This is our Galaxy, which contains the planet Earth and the other elements of our solar system. Spiral galaxies are known for their ongoing star formation, particularly in their spiral arms, making them some of the most active and visually striking galaxies in the universe.

Elliptical galaxies have an elliptical shape and lack spiral arm structures. This is due to the loss of some of their interstellar matter. They usually contain a large number of old stars and do not usually have much active star formation, i.e., fewer new stars are being born. Elliptical galaxies tend to be more massive than spiral galaxies but are less luminous. They range from nearly spherical to highly elongated shapes and can vary greatly in size, from dwarf ellipticals to giant ellipticals.

As shown in the picture below, irregular galaxies do not have a well-defined shape and commonly show a rather chaotic structure. Unlike elliptical galaxies, irregular galaxies usually have a high rate of star formation. As a consequence, their irregularity may be the result of collisions or interactions between other galaxies. These interactions can lead to dramatic changes in their appearance, often resulting in peculiar features and high-energy environments.

Dwarf galaxies are much smaller and less massive than ‘normal’ galaxies. This is because dwarf galaxies are made up of only several million stars, a paltry amount when compared to most other galaxies which contain many billions. These galaxies are often found orbiting larger galaxies, such as the Milky Way. They are generally characterized by relatively few stars and resources. Despite their small size, dwarf galaxies play a crucial role in the cosmic ecosystem, often serving as building blocks for larger galaxies through mergers.

Lenticular galaxies have an intermediate shape between spiral and elliptical galaxies. They are classified as ‘S0’ in the Hubble Sequence, i.e., the classification of galaxies according to their morphology or form. They have a flat disk like spiral galaxies, but they lack spiral arms protruding around them. This is because they have lost much of their interstellar matter, similar to elliptical galaxies. Both young and old stars can be found in lenticular galaxies. These galaxies often represent a transitional stage in the evolution of galaxies, possibly formed from the exhaustion of gas in a spiral galaxy.

These types of galaxies are in a process of gravitational interaction or collision with each other. During these interactions, gravitational forces can deform the structures of galaxies. In doing so, they form interesting features such as tidal tails, star bridges, and perturbations in their shapes. Studies that investigate the nature and frequency with which interactions between galaxies occur are essential to understanding how galaxies form and how they evolve. Interacting galaxies can lead to the formation of new star systems and may eventually merge into a single, more massive galaxy. We can see a representation of such interacting galaxies in the picture below.

These are some of the most common types of galaxies. It is important to note that the universe is home to a wide variety of galaxy shapes and sizes that continue to be the subject of study and research in astronomy. Recent advancements in technology have allowed astronomers to discover and analyze even more exotic and rare types of galaxies, further enriching our understanding of the cosmos.

To learn more about our own galaxy of the Milky Way, you can take a look at our related articles which discuss the names and composition of Neptune's rings and what are the 27 moons of Uranus called?

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