1.1 Hubble sequence
6 min read•august 20, 2024
The Hubble sequence is a fundamental tool for classifying galaxies based on their visual appearance. Developed by in 1926, it organizes galaxies into categories like elliptical, spiral, and irregular, providing insights into their structure and composition.
This classification system reveals important trends in galaxy properties along the sequence. From elliptical to spiral galaxies, we observe changes in star formation rates, gas content, and stellar populations, reflecting the diverse evolutionary paths of these cosmic structures.
Hubble's galaxy classification system
- Developed by Edwin Hubble in 1926, this system categorizes galaxies based on their morphological appearance
- Provides a framework for understanding the diversity of galaxy shapes and structures observed in the universe
Tuning fork diagram
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- Visual representation of Hubble's classification scheme resembling a tuning fork
- Places elliptical galaxies on the "handle" and spiral galaxies on the two "prongs" (normal and barred)
- Arranges galaxies along a sequence from elliptical to spiral, with lenticular galaxies as a transitional class
Elliptical galaxies
- Designated as "E" galaxies followed by a number indicating their ellipticity (E0 to E7)
- Smooth, featureless appearance lacking well-defined structures like spiral arms
- Range from nearly spherical (E0) to highly elongated (E7) shapes
- Contain mostly old, red stars with minimal ongoing star formation
Spiral galaxies
- Divided into two main classes: normal spirals (S) and barred spirals (SB)
- Characterized by a central surrounded by a flat, rotating with spiral arms
- Subclassified as Sa, Sb, or Sc based on the tightness and prominence of spiral arms
- Sa galaxies have tightly wound, smooth arms and large central bulges
- Sc galaxies have loosely wound, patchy arms and small central bulges
Barred spiral galaxies
- Contain an elongated bar-like structure extending from the central bulge
- Bar composed of older stars and acts as a mechanism for funneling gas into the galaxy center
- Subclassified as SBa, SBb, or SBc based on the bar strength and spiral arm appearance
Irregular galaxies
- Lack any distinct regular structure or symmetry (neither elliptical nor spiral)
- Often appear chaotic and fragmented, with no clear central bulge or organized spiral arms
- Typically smaller in size compared to other galaxy types
- Examples include the Large and Small Magellanic Clouds, satellite galaxies of the Milky Way
Lenticular galaxies
- Designated as "S0" and considered a transitional class between elliptical and spiral galaxies
- Contain a central bulge and a disk component but lack well-defined spiral arms
- Minimal ongoing star formation and gas content compared to spiral galaxies
- May represent an evolutionary stage as spirals lose their gas and spiral structure
Properties along the Hubble sequence
Bulge vs disk dominance
- Relative prominence of the central bulge and surrounding disk varies along the Hubble sequence
- Elliptical galaxies are bulge-dominated with no significant disk component
- Spiral galaxies exhibit a range of bulge-to-disk ratios, with Sa galaxies being more bulge-dominated and Sc galaxies more disk-dominated
Gas and dust content
- Amount of interstellar gas and dust decreases from late-type spirals (Sc) to early-type spirals (Sa) and is minimal in elliptical galaxies
- Higher gas and dust content in spiral galaxies supports ongoing star formation
- Elliptical galaxies have little to no cold gas and dust, indicating a lack of star formation
Star formation rates
- Star formation activity follows the gas and dust content along the Hubble sequence
- Highest star formation rates observed in late-type spirals (Sc) and irregular galaxies
- Early-type spirals (Sa) have lower star formation rates, while elliptical galaxies show minimal to no ongoing star formation
Stellar populations
- Age and composition of stellar populations vary along the Hubble sequence
- Elliptical galaxies dominated by old, low-mass stars with minimal young, blue stars
- Spiral galaxies contain a mix of old and young stellar populations, with ongoing star formation in the disk
- Late-type spirals (Sc) have a higher proportion of young, blue stars compared to early-type spirals (Sa)
Color differences
- Galaxies exhibit a color gradient along the Hubble sequence due to differences in stellar populations
- Elliptical galaxies appear redder due to the predominance of older, low-mass stars
- Spiral galaxies show a range of colors, with late-type spirals (Sc) appearing bluer due to the presence of young, massive stars
- Color differences reflect the varying star formation histories and stellar content of galaxies
Limitations of the Hubble sequence
Observed vs intrinsic properties
- Hubble classification based on observed morphology, which can be affected by viewing angle and distance
- Galaxies with similar intrinsic properties may appear different depending on their orientation relative to the observer
- Edge-on spiral galaxies can be misclassified as lenticular or elliptical due to the obscuration of spiral arms
Impact of galaxy orientation
- Inclination of a galaxy's disk relative to the line of sight can impact its observed morphology
- Face-on spiral galaxies reveal clear spiral arm structures, while edge-on spirals appear as elongated, featureless disks
- Orientation effects can lead to misclassification or ambiguity in Hubble types
Difficulty classifying distant galaxies
- At large distances, galaxies appear smaller and fainter, making morphological classification challenging
- Lack of resolution and signal-to-noise ratio can hinder the detection of fine structures like spiral arms or bars
- Distant galaxies observed at earlier cosmic epochs may have different morphologies compared to nearby galaxies
Existence of peculiar galaxies
- Some galaxies exhibit unusual or distorted morphologies that do not fit well into the Hubble sequence
- Peculiar galaxies often result from galaxy mergers, interactions, or other disruptive events
- Examples include ring galaxies, tadpole galaxies, and galaxies with tidal tails or bridges
- Presence of peculiar galaxies highlights the limitations of a purely morphological classification scheme
Revisions to the Hubble sequence
De Vaucouleurs classification system
- Extends the Hubble sequence by adding more detailed subclasses and criteria
- Introduces intermediate classes (Sab, Sbc) and finer distinctions within elliptical and spiral categories
- Considers additional features such as inner and outer ring structures, lenses, and asymmetries
Yerkes classification system
- Developed by W. W. Morgan and based on the central concentration of light in galaxies
- Classifies galaxies into seven categories (a, af, f, fg, g, gk, k) based on the degree of central concentration
- Provides a quantitative measure of galaxy structure independent of the Hubble sequence
Importance of bar structures
- Presence of bars recognized as a significant morphological feature in galaxy classification
- Bars can influence gas dynamics, star formation, and the overall evolution of galaxies
- Inclusion of categories (SBa, SBb, SBc) in the Hubble sequence acknowledges the importance of bars
S0 galaxies as a transitional class
- Lenticular (S0) galaxies initially placed between elliptical and spiral galaxies in the Hubble sequence
- Subsequent research suggests S0 galaxies may represent an evolutionary stage rather than a strict morphological class
- S0 galaxies could result from the transformation of spiral galaxies through gas stripping or merger processes
Physical interpretation of the Hubble sequence
Evolutionary sequence vs morphological sequence
- Hubble sequence originally interpreted as an evolutionary sequence, with galaxies evolving from elliptical to spiral
- Modern understanding recognizes the Hubble sequence primarily as a morphological classification scheme
- Evolutionary connections between different galaxy types are more complex and multifaceted than a simple linear sequence
Relation to galaxy formation and evolution
- Morphology of galaxies reflects their formation and evolutionary histories
- Elliptical galaxies likely formed through mergers or rapid collapse of gas clouds, leading to a burst of star formation and subsequent quiescence
- Spiral galaxies formed through more gradual accretion of gas and angular momentum, allowing for prolonged star formation in the disk
Environmental effects on morphology
- Galaxy morphology can be influenced by the local environment, such as galaxy clusters or groups
- Tidal interactions, ram-pressure stripping, and harassment can alter the morphology and gas content of galaxies
- Elliptical and S0 galaxies more common in dense cluster environments, while spirals dominate in less dense regions
Mergers and interactions
- Galaxy mergers and interactions play a crucial role in shaping galaxy morphology and evolution
- Major mergers between galaxies of comparable mass can disrupt spiral structures and lead to the formation of elliptical galaxies
- Minor mergers and tidal interactions can trigger bar formation, enhance star formation, and cause morphological distortions
- Hubble sequence does not fully capture the diversity and complexity of galaxy morphologies resulting from mergers and interactions
Key Terms to Review (20)
Active Galactic Nucleus: An active galactic nucleus (AGN) is a region at the center of some galaxies that emits an exceptionally high amount of energy, often outshining the rest of the galaxy. This phenomenon is typically associated with supermassive black holes that accrete matter, leading to the emission of intense radiation across various wavelengths, including radio, infrared, optical, and X-rays. AGNs are crucial in understanding galaxy evolution and play a significant role in the structure and dynamics of galaxies.
Barred spiral: A barred spiral galaxy is a type of spiral galaxy that features a central bar-shaped structure made of stars, which extends from the core out towards the spiral arms. These galaxies are characterized by their unique morphology, which influences star formation and dynamics within the galaxy. The presence of a bar can significantly affect the distribution of stars and gas, leading to distinctive patterns in their rotation and evolution.
Bulge: In the context of galaxies, a bulge refers to the dense, central region found in many galaxies, typically characterized by a high concentration of stars, gas, and dust. Bulges can vary in size and composition, playing a crucial role in the overall structure and dynamics of a galaxy, influencing star formation and the evolutionary processes of the galaxy as a whole.
Disk: In the context of galaxies, a disk refers to a flattened, rotating structure that contains stars, gas, and dust. This component is typically found in spiral and some irregular galaxies, where it forms the main region of star formation and contains the galaxy's spiral arms. The disk is essential for understanding galaxy morphology and dynamics, as well as the overall structure of galaxies in the universe.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in establishing the field of extragalactic astronomy and is best known for Hubble's law, which describes the expansion of the universe. His work not only led to the classification of galaxies but also revolutionized our understanding of the cosmos, connecting various concepts like the cosmic web and the cosmological principle.
Elliptical galaxy: An elliptical galaxy is a type of galaxy characterized by its smooth, featureless light profile and an ellipsoidal shape, often containing older stars with little to no ongoing star formation. These galaxies range from nearly spherical to elongated structures and are usually found in larger galaxy clusters. Their structure and composition connect them to various cosmic phenomena and the broader structure of the universe.
Galactic Merger: A galactic merger occurs when two or more galaxies collide and combine to form a single, larger galaxy. This process can significantly alter the structure and dynamics of the galaxies involved, influencing star formation rates and leading to the creation of new galactic features. Galactic mergers are a crucial part of galaxy evolution and have implications for supermassive black hole formation, quasar activity, tidal interactions, and phenomena like galactic cannibalism.
Halo: In astronomy, a halo refers to the diffuse, extended region of stars, gas, and dark matter surrounding a galaxy, particularly prominent in spiral galaxies. This area is crucial for understanding galaxy formation and evolution, as it contains ancient stars and the materials from which galaxies formed. The halo plays an essential role in how galaxies interact with their environment and helps to inform the classification of galaxies based on their structure and morphology.
Hubble's Law: Hubble's Law states that the velocity at which a galaxy is receding from us is directly proportional to its distance from us. This fundamental observation supports the idea that the universe is expanding, linking it to various phenomena like galaxy formation and the structure of the cosmos.
Irregular galaxy: An irregular galaxy is a type of galaxy that lacks a distinct shape or structure, distinguishing it from more organized types like spiral and elliptical galaxies. These galaxies often appear chaotic and have an uneven distribution of stars, gas, and dust, leading to their non-uniform appearance. Irregular galaxies are typically smaller and can be found in various environments, often interacting with other galaxies which can influence their shape and star formation activity.
Lenticular Galaxy: A lenticular galaxy is a type of galaxy that features a central bulge surrounded by a flat, disc-like structure with a noticeable lack of spiral arms, resembling a combination of features found in both spiral and elliptical galaxies. These galaxies are often found in dense environments and can provide insight into the evolutionary processes between different galaxy types, particularly through the Hubble sequence, where they are categorized as S0 galaxies.
Photometry: Photometry is the science of measuring the intensity of light and its properties, especially as it relates to celestial objects. This measurement plays a vital role in understanding the brightness and luminosity of stars, galaxies, and other astronomical phenomena, allowing astronomers to categorize objects, analyze their composition, and understand their distances and environments.
Quasar: A quasar is a highly luminous and energetic active galactic nucleus powered by a supermassive black hole at its center, emitting immense amounts of radiation across the electromagnetic spectrum. Quasars are among the most distant and ancient objects known in the universe, making them crucial for understanding cosmic evolution and the structure of galaxies.
Redshift: Redshift is the phenomenon where light from an object is shifted towards longer wavelengths, typically observed as a shift toward the red end of the spectrum. This effect occurs when an object moves away from the observer, providing key insights into the expansion of the universe and the nature of celestial bodies.
Space Observatory: A space observatory is a scientific facility located in space that is equipped with telescopes and other instruments to observe celestial objects and phenomena beyond Earth's atmosphere. By operating outside of the distortions caused by the atmosphere, these observatories can gather clearer data across a wide range of wavelengths, including visible light, infrared, and X-rays, enabling astronomers to study galaxies, stars, and cosmic events in unprecedented detail.
Spectroscopy: Spectroscopy is the study of the interaction between light and matter, particularly focusing on how light is absorbed, emitted, or scattered by atoms and molecules. This technique allows astronomers to analyze the composition, temperature, density, and motion of celestial objects, providing crucial insights into their physical properties and behaviors.
Spiral galaxy: A spiral galaxy is a type of galaxy characterized by its distinct spiral arms that wind outward from a central bulge, typically containing a mix of young and old stars, gas, and dust. These galaxies are often rich in star formation, particularly in the arms, and are one of the most common galaxy types observed in the universe. Their structure and formation provide insights into the evolutionary processes of galaxies and their environments.
Star formation rate: Star formation rate (SFR) is a measure of the amount of mass converted into stars in a specific region of space over a given time, typically expressed in solar masses per year. This concept is crucial in understanding the evolution of galaxies, as it directly relates to their structure, characteristics, and lifecycle, affecting various classifications and types of galaxies, especially spiral galaxies where star formation is often actively occurring.
Telescope: A telescope is an optical instrument that gathers and magnifies light, allowing astronomers to observe distant celestial objects in greater detail. By collecting more light than the human eye can, telescopes enable the study of various types of astronomical phenomena, including stars, planets, and galaxies, and they are essential tools in understanding the structure and evolution of the universe.
Vesto Melvin Slipher: Vesto Melvin Slipher was an American astronomer known for his pioneering work in the study of galaxies, particularly for providing some of the first evidence for the expansion of the universe through redshift measurements. His observations of the light spectra from distant galaxies revealed a systematic shift toward longer wavelengths, indicating that these galaxies were moving away from Earth, a fundamental observation that contributed to our understanding of cosmic expansion and helped lay the groundwork for later developments in cosmology.