What To Know
- The luminosity of elliptical galaxies is typically expressed in terms of the absolute magnitude, which is the brightness of the galaxy as seen from a distance of 10 parsecs (32.
- The classification of elliptical galaxies is a complex process that involves considering multiple factors, including shape, luminosity, and morphological features.
- The dense environment of galaxy clusters can promote the merger and stripping of galaxies, leading to the formation of elliptical galaxies.
Elliptical galaxies, characterized by their smooth, featureless appearance and lack of spiral arms, are classified based on their shape and luminosity. The two primary classification systems used are the Hubble sequence and the Revised Hubble sequence.
Hubble Sequence
The Hubble sequence, proposed by Edwin Hubble in the 1920s, classifies elliptical galaxies based on their ellipticity, or how elongated they appear. The ellipticity is denoted by the axis ratio, which is the ratio of the galaxy’s minor axis to its major axis.
- E0: Circular galaxies with an axis ratio of 1:1
- E1: Slightly elliptical galaxies with an axis ratio of 1:1.5
- E2: Moderately elliptical galaxies with an axis ratio of 1:2
- E3: Highly elliptical galaxies with an axis ratio of 1:3
- E4: Very highly elliptical galaxies with an axis ratio of 1:4
- E5: Extremely elongated galaxies with an axis ratio of 1:5
Revised Hubble Sequence
The Revised Hubble sequence, proposed in 2002, refines the Hubble sequence by introducing additional subcategories and addressing some limitations of the original system. It includes the following subclasses:
- E0-4: Circular to highly elliptical galaxies
- E5-7: Extremely elongated galaxies
- E(pec): Peculiar elliptical galaxies that do not fit into the regular sequence
Luminosity Classification
In addition to shape, elliptical galaxies can also be classified based on their luminosity. Luminosity is a measure of the total energy emitted by the galaxy. The luminosity of elliptical galaxies is typically expressed in terms of the absolute magnitude, which is the brightness of the galaxy as seen from a distance of 10 parsecs (32.6 light-years).
- Luminous elliptical galaxies: Absolute magnitude brighter than -20
- Intermediate-luminosity elliptical galaxies: Absolute magnitude between -18 and -20
- Dwarf elliptical galaxies: Absolute magnitude fainter than -18
Morphological Features
Elliptical galaxies often exhibit additional morphological features that can help in their classification:
- Bulges: Central, spheroidal regions of high luminosity
- Disks: Flattened regions surrounding the bulge
- Bars: Elongated structures that cross the galaxy’s center
- Rings: Circular or elliptical features around the galaxy’s center
Environmental Factors
The environment in which an elliptical galaxy is located can also influence its classification. Elliptical galaxies in dense galaxy clusters tend to be more luminous and rounder than those in isolated regions.
Formation and Evolution
Elliptical galaxies are believed to form through the merger of smaller galaxies. Over time, the gravitational forces of the merging galaxies cause them to lose their spiral structure and become smoother, rounder objects.
Final Note
The classification of elliptical galaxies is a complex process that involves considering multiple factors, including shape, luminosity, and morphological features. By understanding the different classification systems, astronomers can gain insights into the formation, evolution, and diversity of these massive celestial objects.
Frequently Discussed Topics
Q: What is the difference between E0 and E5 galaxies?
A: E0 galaxies are circular, while E5 galaxies are extremely elongated.
Q: Are all elliptical galaxies the same size?
A: No, elliptical galaxies can range in size from dwarf galaxies to giant elliptical galaxies.
Q: Do elliptical galaxies contain any stars?
A: Yes, elliptical galaxies contain vast numbers of stars, mostly old, low-mass stars.
Q: Why are elliptical galaxies often found in galaxy clusters?
A: The dense environment of galaxy clusters can promote the merger and stripping of galaxies, leading to the formation of elliptical galaxies.
Q: Can elliptical galaxies form new stars?
A: Yes, but at a much slower rate than spiral galaxies. Elliptical galaxies typically have low levels of gas and dust, which are necessary for star formation.
