Galaxy types are directly linked to star formation activity levels: spiral galaxies typically exhibit high star-forming activity, whereas ellipticals are significantly less active. This connection stems from differences in the structure and distribution of gas and dust within galaxies.
The variety of galaxy types, from majestic spirals to compact ellipticals and irregular forms, reflects different stages of their evolution and internal conditions. Understanding how a galaxy’s structure affects the birth of new stars helps reveal the mechanisms behind the formation and development of the Universe. This article explores the relationship between galaxy structure and their star-forming capability, which plays a key role in astrophysical research.
Studying this relationship allows us to better understand why some galaxies continue to actively form stars while others gradually «fade away.» This is important not only for galaxy classification but also for predicting their future fate and assessing the influence of external factors, such as interactions and mergers, on their evolution.
| Galaxy Type | Shape | Star Formation Rate | Cold Gas Content |
|---|---|---|---|
| Spiral (Sa, Sb, Sc) | Spiral with arms | Up to 5 solar masses per year | High |
| Elliptical (E0–E7) | Ellipsoidal | Close to zero | Low |
| Irregular | Asymmetric | Variable, up to bursts | Medium |
- 5 solar masses per year Maximum star formation rate in spiral galaxies
- 1926 Year Edwin Hubble created the galaxy classification
- 0.05 arcseconds Resolution of the Hubble Space Telescope
- up to 10 km/s Accuracy of velocity measurements for gas and stars with the MUSE spectrograph
How Are Galaxies Classified by Shape and Structure?
Main Galaxy Types
Galaxies are classified by shape and structure into three primary types: spiral, elliptical, and irregular. This system, developed by Edwin Hubble in 1926, remains fundamental in modern astronomy. Spiral galaxies feature a flat disk with arms and a central bulge, ellipticals have an ellipsoidal shape with varying elongation, and irregulars lack clear symmetry.
For example, the elongation of ellipticals is denoted as En, where n ranges from 0 (nearly spherical E0) to 7 (highly elongated E7). Irregular galaxies, such as the Large Magellanic Cloud, do not fit into this scheme and are characterized by chaotic structure.
Subtypes and Designations
Spiral galaxies are subdivided into three subtypes: Sa, Sb, and Sc, differing in the density of spiral arms and the size of the central bulge. In Sa types, the arms are tightly wound and the bulge relatively large, whereas Sc galaxies have looser arms and smaller bulges.
- Sa: dense arms, large bulge; for example, galaxy M104 with a diameter around 50,000 light-years.
- Sb: intermediate arm density and bulge size, like galaxy M81.
- Sc: loose arms and small bulge; example — galaxy M33 in Triangulum.
Thus, Hubble’s classification reflects structural features of galaxies, enabling a connection between their shape and star formation processes and evolution. For instance, spiral galaxies with loose arms generally have higher star formation rates than compact ellipticals.
How Does Galaxy Shape Affect Star Formation Rate?
Star Formation in Spiral Galaxies
Galaxy shape is directly related to star formation rate: spiral galaxies with abundant cold gas supply form new stars much more actively, reaching up to 5 solar masses per year. In such systems, gas disks are the main sites of star formation, and the presence of large amounts of hydrogen sustains an intense process. For example, the Milky Way exhibits this level of activity on average over the past several million years.
Characteristics of Elliptical and Irregular Galaxies
Elliptical galaxies mainly consist of old stars and contain almost no cold gas, resulting in star formation rates close to zero. In contrast, irregular galaxies, including dwarfs, show variable activity: occasional star formation bursts linked to gas accretion or interactions. Star formation rates in these types are assessed through ultraviolet observations and measurements of the Hα emission line intensity, which indicates the presence of young hot stars.
- Spiral galaxies: star formation rate up to 5 solar masses per year
- Elliptical galaxies: star formation rate near 0 M☉/yr
- Irregular galaxies: variable activity with burst periods
- Measurement methods: ultraviolet emission and Hα line
What Physical Processes Connect Galaxy Shape and Star Formation?
The Role of Spiral Arms
Galaxy shape and star formation activity are closely linked to the dynamics of gas and stars within the structure; in spiral galaxies, dense spiral arms create gas compression zones where new stars form with efficiency up to a few percent of gas mass per year. In these arms, gas pressure increases, causing molecular clouds to collapse and stars to be born, as observed in the Milky Way, where star formation rates are about 1–3 solar masses per year.
Impact of External Factors
External processes, such as galaxy collisions and mergers, greatly alter shape and trigger star formation bursts, sometimes boosting rates by tens of times, typical for Ultra-Luminous Infrared Galaxies (ULIRGs). In contrast, elliptical galaxies that have lost much of their gas due to stellar winds and supermassive black hole activity show low star formation—below 0.1 solar masses per year. Irregular galaxies with dispersed gas feature irregular and localized star-forming regions, often associated with turbulence and uneven gas distribution.
- Spiral arms: gas pressure 2–3 times higher than in interarm regions.
- ULIRGs: star formation bursts up to 100 solar masses per year.
- Elliptical galaxies: star formation under 0.1 solar masses per year due to gas deficiency.
- Irregular galaxies: irregular star formation zones caused by turbulence.
When Is the Link Between Galaxy Shape and Star Formation Less Clear?
The connection between galaxy shape and star formation activity becomes less obvious when elliptical galaxies show moderate star formation or when active galactic nuclei (AGN) influence star formation patterns regardless of morphology.
Atypical Cases
Some elliptical galaxies, traditionally considered «dead,» can show weak but noticeable star formation in their cores. For example, in 2026 studies with the Hubble Space Telescope, star formation bursts with intensities around 0.1–1 solar mass per year were observed in the cores of several ellipticals. These temporary bursts may be tied to the accretion of gas clouds, enabling star formation to resume after long dormancy. At the same time, active galactic nuclei, such as systems with powerful relativistic jets, can both suppress and stimulate star formation, complicating a straightforward correlation between shape and star formation.
Methodological Limitations
Assessing star formation activity in distant galaxies faces technical challenges. Observations in the far-infrared, where thermal emission from cold gas and dust is prominent, are hindered by the high cost and limited availability of specialized instruments like the MIRI spectrometer on the James Webb Space Telescope. This complicates precise measurement of star formation rates, especially below 0.5 solar masses per year. Sensitivity and resolution limits lead to underestimation of activity in some galaxy types, weakening the observed link between morphology and current star formation activity.
- Star formation intensity in elliptical galaxy cores: about 0.1–1 solar mass per year
- Infrared observation sensitivity threshold: approximately 0.5 solar masses per year
- Key far-IR instrument: MIRI spectrometer on James Webb Telescope (2026)
What Methods and Instruments Are Used to Study Galaxy Structure and Star Formation?
Optical and Infrared Methods
Optical and infrared telescopes are widely used to study galaxy structure and star formation, providing high spatial resolution and sensitivity to different galaxy components. The Hubble Space Telescope can capture images with resolution up to 0.05 arcseconds, enabling detailed examination of star distribution and structural features. Infrared observations from the James Webb Telescope reveal young star clusters hidden by dust, thanks to its coverage from 0.6 to 28 micrometers, greatly expanding the ability to investigate active star-forming regions.
Radio and Spectroscopy
The ALMA radio observatory detects molecular gas emission with resolution down to 0.1 arcseconds, allowing study of star formation raw material and molecular cloud dynamics in galaxies. The MUSE spectrograph on the Very Large Telescope (VLT) enables velocity measurements of gas and stars with accuracy up to 10 km/s, which is crucial for analyzing kinematics and interactions within galactic structures.
- Hubble resolution: up to 0.05 arcseconds
- ALMA resolution: up to 0.1 arcseconds
- MUSE spectroscopic accuracy: up to 10 km/s
- James Webb infrared range: 0.6–28 μm
Frequently Asked Questions
What are spiral galaxies and why is star formation active in them?
Why do elliptical galaxies have almost no new stars forming?
How do galaxy mergers affect star formation?
Which instruments help study star formation in distant galaxies?
Key Takeaways
- Hubble’s classification remains fundamental for understanding galaxy types
- Galaxy shape is directly linked to its ability to form new stars
- Spiral galaxies exhibit the highest star formation rates among main types
- Galaxy mergers are a key factor in boosting star formation activity
- Modern telescopes provide the necessary resolution to study galaxy structure and processes