26.2 Types of Galaxies
3 min read•june 12, 2024
come in various shapes and sizes, each with unique characteristics. From elliptical to spiral and irregular, these cosmic structures showcase the diversity of our universe. Understanding galaxy types helps us grasp the broader picture of galactic evolution and formation.
Galaxy shapes can change over time through mergers, interactions, and internal processes. These transformations affect star formation rates, gas content, and overall structure. By studying galaxy dynamics and composition, we gain insights into the complex interplay of visible matter, , and stellar populations.
Galaxy Types and Characteristics
Types of galaxy structures
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- exhibit smooth, elliptical shape with no distinct features
- Contain mostly old, red stars and little to no gas or dust for new star formation
- Classified based on their ellipticity from E0 (nearly spherical) to E7 (highly elongated)
- feature distinct spiral arms extending from a central
- Contain a mix of young, blue stars (in spiral arms) and older, red stars (in bulge)
- Rich in gas and dust, allowing for ongoing star formation (, )
- Classified as barred (SB) with central bar or unbarred (SA) without bar
- lack a distinct, organized structure
- Often smaller than elliptical and (Large and )
- May have been distorted by gravitational interactions with other galaxies
- and the provide a classification system for these structures
Galaxy shape evolution
- Galaxy mergers and interactions can cause galaxies to collide and merge
- Resulting in a change of shape and structure ()
- Mergers can trigger intense star formation and lead to the formation of larger galaxies
- Secular evolution involves gradual changes in a galaxy's structure over time due to internal processes
- Formation of a central bar or the winding up of spiral arms
- Environmental effects can strip galaxies in clusters of their gas and dust by the intracluster medium
- can transform spiral galaxies into lenticular (S0) galaxies
Components of spiral galaxies
- Bulge forms the central, spheroidal component of a spiral galaxy
- Contains mostly older, red stars and little to no gas or dust
- Can be classical (smooth, elliptical) or pseudobulges (-like with younger stars)
- Disk is the flat, rotating component of a spiral galaxy
- Contains a mix of young, blue stars and older, red stars
- Rich in gas and dust, allowing for ongoing star formation
- Divided into the thin disk (younger stars and gas) and the thick disk (older stars)
- Spiral arms are regions of enhanced star formation and density within the disk
- Contain many young, massive, blue stars and bright ()
- Thought to be caused by density waves that compress gas and trigger star formation
- Can be grand design (well-defined, symmetric) or flocculent (patchy, less distinct)
Galaxy dynamics and composition
- surrounds and extends beyond the visible galaxy
- Influences the , which shows how rotation speed varies with distance from the center
- Stellar populations within galaxies can be categorized based on age and composition
- Population I stars are younger, metal-rich stars found in the disk
- Population II stars are older, metal-poor stars found in the halo and bulge
- Some galaxies contain , extremely luminous central regions powered by supermassive black holes
Key Terms to Review (29)
Accretion disk: An accretion disk is a rotating disk of gas and dust surrounding a massive central object, such as a black hole. It forms as material spirals inward due to gravitational forces and frictional heating causes it to emit electromagnetic radiation.
Active Galactic Nuclei: Active galactic nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. These galactic nuclei emit radiation across the entire electromagnetic spectrum, from radio waves to high-energy gamma rays, making them some of the most energetic phenomena in the universe.
Andromeda: Andromeda is the nearest major galaxy to the Milky Way, located approximately 2.5 million light-years away. It is a spiral galaxy similar in structure and size to our own Milky Way galaxy, and it is a prominent feature in the northern night sky, visible to the naked eye under clear conditions.
Andromeda galaxy: The Andromeda Galaxy, also known as M31, is the closest spiral galaxy to the Milky Way and is on a collision course with it. It is approximately 2.537 million light-years from Earth and is the largest galaxy in the Local Group.
Antennae Galaxies: The Antennae Galaxies, also known as NGC 4038 and NGC 4039, are a pair of interacting spiral galaxies located approximately 45 million light-years away from Earth. They are named for the long, antenna-like tails of stars, gas, and dust that extend from the galaxies, a result of their ongoing collision and merger process.
Bulge: A bulge is a distinctive structural component found in certain types of galaxies, particularly spiral galaxies. It refers to the central, spherical, or elliptical region that protrudes above the plane of the galaxy's disk, often containing older stars and a supermassive black hole at the center.
Dark matter: Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible to current instruments. It exerts gravitational forces and is thought to constitute approximately 27% of the universe's mass-energy content.
Dark matter halo: Dark matter halo is a theoretical, spherical region surrounding galaxies, including the Milky Way, composed mostly of dark matter. It extends beyond the visible components of the galaxy and contains most of its mass.
Dark Matter Halo: A dark matter halo is a hypothetical distribution of dark matter that surrounds and extends well beyond the visible component of a galaxy. Dark matter is a mysterious substance that does not emit, reflect, or absorb light, yet it makes up a significant portion of the universe's total mass and plays a crucial role in the formation and evolution of galaxies.
Disk: A disk is a flat, circular structure that is a key component in the structure and classification of various astronomical objects, such as star clusters and galaxies. It refers to a flattened, rotating distribution of matter that is a fundamental building block of many celestial bodies.
Elliptical galaxies: Elliptical galaxies are a type of galaxy characterized by their elliptical shape and smooth, featureless appearance. They lack the distinct arms seen in spiral galaxies and typically contain older, red stars.
Elliptical Galaxies: Elliptical galaxies are a type of galaxy characterized by their smooth, elliptical shape and lack of distinct spiral arms or other prominent features. They are one of the three main classifications of galaxies, along with spiral and irregular galaxies, and are known for their old, red stellar populations and relatively simple structures.
Galactic Rotation Curve: The galactic rotation curve is a graph that depicts the orbital velocities of stars and other objects as a function of their distance from the center of a galaxy. This curve provides valuable insights into the mass distribution and overall structure of a galaxy.
Galaxies: Galaxies are vast systems composed of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. They vary greatly in size and structure and are fundamental building blocks of the universe.
Galaxy Morphology: Galaxy morphology refers to the classification and study of the physical shapes and structures of galaxies. It is a fundamental aspect of understanding the diverse types of galaxies that exist in the universe and the processes that shape their evolution.
HII Regions: HII regions are large, diffuse clouds of ionized hydrogen gas found in star-forming regions of galaxies. These regions are characterized by the presence of hot, young, and massive stars that emit intense ultraviolet radiation, which ionizes the surrounding hydrogen gas, creating a glowing, emission-line nebula.
Hubble Sequence: The Hubble Sequence is a classification scheme for galaxies based on their visual morphology, which was developed by the astronomer Edwin Hubble. It provides a systematic way to categorize and understand the diverse shapes and structures of galaxies in the universe.
Irregular Galaxies: Irregular galaxies are a type of galaxy that do not have a well-defined structure or shape, unlike the more common spiral and elliptical galaxies. They are often asymmetrical and lack the distinct features found in other galaxy types, making them appear irregular or chaotic in appearance.
Irregular galaxy: An irregular galaxy is a type of galaxy that lacks a distinct, regular shape like spiral or elliptical galaxies. They often appear chaotic and have no central bulge or spiral arms.
Large Magellanic Clouds: The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way, located approximately 163,000 light-years from Earth. It is one of the closest galaxies to our own and is visible to the naked eye in the southern hemisphere, appearing as a glowing, irregular-shaped cloud in the night sky.
Lenticular Galaxies: Lenticular galaxies are a class of galaxies that exhibit characteristics of both elliptical and spiral galaxies. They have a prominent central bulge, a flattened disk-like structure, and minimal or no spiral arms, making them appear lens-shaped or lentil-like in appearance.
Lenticular galaxies are considered intermediate between elliptical and spiral galaxies, sharing some properties of both types. They are an important part of the classification scheme for the various shapes and structures observed in galaxies throughout the universe.
Milky Way: The Milky Way is the galaxy in which our solar system is located, comprising hundreds of billions of stars and vast amounts of gas and dust. It is a spiral galaxy, with a central bulge and a rotating disk of stars, gas, and dust. The Milky Way is an essential component in understanding the structure, formation, and evolution of the universe, as it provides a window into the larger cosmic landscape.
Milky Way Galaxy: The Milky Way Galaxy is the spiral galaxy that includes our Solar System, characterized by its barred structure and multiple spiral arms. It is one of billions of galaxies in the universe and contains over 200 billion stars.
Orion Nebula: The Orion Nebula is a vast, luminous cloud of gas and dust located in the Milky Way galaxy, situated in the constellation of Orion. It is one of the most well-known and extensively studied star-forming regions in our galaxy, providing valuable insights into the processes of stellar birth and early stellar evolution.
Ram Pressure Stripping: Ram pressure stripping is a process that occurs in galaxy clusters where the hot, diffuse gas between galaxies, known as the intracluster medium (ICM), exerts a force on the interstellar medium (ISM) of a galaxy as it moves through the cluster. This force can remove the gas from the galaxy, stripping it of its fuel for star formation.
Small Magellanic Clouds: The Small Magellanic Cloud is a dwarf irregular galaxy that is a satellite of the Milky Way galaxy. It is one of the closest galaxies to the Milky Way, located approximately 200,000 light-years from Earth, and is a prominent feature in the southern night sky.
Spiral galaxies: Spiral galaxies are a type of galaxy characterized by their flat, disk-like shape with spiral arms winding outward from the center. They contain a mix of young and old stars, gas, and dust, making them sites of active star formation.
Spiral Galaxies: Spiral galaxies are a type of galaxy characterized by a central bulge surrounded by a flat, rotating disk with spiral arms. They are one of the most common and visually striking galaxy types in the observable universe.
Stellar Population: Stellar population refers to the different groups or categories of stars found in various astronomical objects, such as galaxies and star clusters. These populations are distinguished by their age, chemical composition, and kinematic properties, which provide insights into the formation and evolution of these celestial bodies.