Stars are classified by properties such as temperature, luminosity, and evolutionary stage; galaxies are classified by shape and structure. The main types of galaxies are spiral, elliptical, and irregular, while stars can be distinguished, for example, by color and brightness.
This classification helps show how individual stars differ from the larger systems they belong to. In this article, we explore types of stars and galaxies through clear examples. For an overview of how these objects are structured and connected, see our article “Stars and Galaxies: How the Universe’s Key Objects Are Structured and Connected.”
| Object | Class | Characteristic |
|---|---|---|
| Sun | G2V star | Main sequence |
| Proxima Centauri | Red dwarf, class M | Red star |
| Sirius B | White dwarf | Compact stellar remnant |
| Milky Way | Barred spiral galaxy | Disk and spiral structure |
| M87 | Elliptical galaxy | Rounded shape without prominent spiral arms |
| Large Magellanic Cloud | Irregular galaxy | No clear symmetrical shape |
- 7 letter classes — O, B, A, F, G, K, M The main sequence of stellar spectral classification
- G2V The Sun’s spectral and luminosity class
- 4 main morphological types Spiral, elliptical, lenticular, and irregular galaxies
What characteristics are used to classify stars?
Stars are classified by spectral class, surface temperature, luminosity, and position on the Hertzsprung–Russell diagram. Spectral classes are denoted by the letters O, B, A, F, G, K, and M; this sequence reflects changes in surface temperature.
What the classification tells us
A star’s spectrum reveals its temperature and chemical properties. A letter designation alone is not enough for a complete classification: luminosity and position on the Hertzsprung–Russell diagram help distinguish stars with different properties.
The Sun is designated G2V: G2 indicates its subtype within class G, while the Roman numeral V identifies it as a main-sequence star. Blue O- and B-class stars have higher surface temperatures than yellow G-type stars; class M stars are usually red.
How do the main types of stars differ?
The main types of stars differ in temperature, color, mass, and evolutionary stage: red dwarfs are low-mass class M stars, yellow dwarfs resemble the Sun, blue O- and B-class stars are hotter and blue-white, while white dwarfs are compact stellar remnants.
- Red dwarfs: Proxima Centauri is a low-mass star of class M.
- Yellow dwarfs: This is the usual name for Sun-like main-sequence stars. The Sun itself is an example, with a spectral class of G2V.
- Blue stars: Classes O and B are distinguished by their high temperatures and blue-white color. Rigel in the constellation Orion is a blue supergiant.
- White dwarfs: These are compact stellar remnants. Sirius B belongs to this type and is part of the Sirius system.
These terms describe different properties, so they should not be treated as a single scale: classes M, O, B, and G indicate spectral type, while “supergiant” and “white dwarf” describe other characteristics of a star. For example, the Sun is classified as G2V, while Rigel is a blue supergiant; together, they illustrate the difference between color class and type of stellar object.
How are main-sequence stars, giants, and stellar remnants distinguished?
Stars are distinguished by spectral class and luminosity class: the spectrum describes the properties of their radiation and their surface temperature, while the Roman numeral indicates the star’s luminosity group. For example, V denotes a main-sequence star, III a giant, and I a supergiant.
The Sun is designated G2V: G2 indicates its spectral subclass, while V denotes the main sequence. Betelgeuse, by contrast, is a red supergiant; the color name alone does not tell us that the star belongs to class I.
Stars and stellar remnants
A white dwarf is not an ordinary main-sequence star: it is a stellar remnant with a different physical status and size. That is why classification by spectrum and luminosity is more precise than the everyday label “red star”: it distinguishes, for example, the G2V Sun from the supergiant Betelgeuse and does not confuse a white dwarf with an ordinary star.
What shapes and types of galaxies are recognized?
Galaxies are divided into four main shapes: spiral, elliptical, lenticular, and irregular. They are distinguished by the structure of their disks, the presence of spiral arms, and their degree of symmetry; familiar examples include the Milky Way, M87, and the Magellanic Clouds.
- Spiral: These galaxies have a disk and spiral arms. The Milky Way and the Andromeda Galaxy belong to this type.
- Elliptical: They appear rounded or elongated and have no prominent spiral arms. One example is M87 in the Virgo Cluster.
- Lenticular: These galaxies combine a disk with a central bulge but have no visible spiral arms. Their shape is intermediate between spiral and elliptical systems.
- Irregular: They have no clear symmetrical shape. The Large and Small Magellanic Clouds are examples of these galaxies.
These characteristics describe a galaxy’s visible structure, not its size or brightness. A system can therefore be classified by its shape: having a disk, for example, does not by itself make a galaxy spiral—a lenticular galaxy also has a disk, but no prominent arms.
What characteristics are used to compare galaxies?
Galaxies are compared by their observable shape and structure: in particular, astronomers assess the disk, arms, roundness, and presence of stable symmetry. These characteristics allow an object to be assigned a morphological type, but on their own they do not explain how a galaxy formed or changed over time.
- Spiral: Their disks and spiral arms are distinguishable. The Milky Way is a barred spiral galaxy, while Andromeda is a large spiral galaxy in the Local Group.
- Elliptical: These galaxies typically have a rounded profile. M87 belongs to this type and is known for its compact, bright core.
- Irregular: They lack the stable symmetry needed to identify them as spiral or elliptical based on their characteristic outlines.
Morphological classification describes how a galaxy looks and how its observable structure is arranged. For example, calling the Milky Way a barred spiral galaxy tells us about its shape; it does not establish how that structure originated or provide a history of the galaxy’s changes.
When can classification by color or shape be misleading?
The color and shape of a star or galaxy can be misleading because its observed appearance depends not only on the object’s properties but also on the conditions under which it is observed. Interstellar dust absorbs and scatters starlight, so a star’s apparent hue may differ from the one determined by its temperature alone.
A star’s spectral class cannot be reliably determined from a photograph: classification requires a spectrum showing how light is distributed across wavelengths. Luminosity classes also require additional data, so color or brightness in an image alone cannot provide a complete classification.
Why an image of a galaxy does not always reveal its type
A galaxy’s visible shape depends on the angle of its disk relative to the observer. Spiral arms may be easy to distinguish in a galaxy seen face-on, but harder to spot when the disk is viewed edge-on; a rounded outline in an image therefore does not prove that the galaxy is elliptical.
Limited observational quality can hide structural details: an image may not reveal the arms, bar, or other features important for classification. That is why a galaxy’s shape is assessed with its viewing angle and available detail in mind, rather than by its overall outline alone.
How are stars and galaxies connected?
Stars and galaxies are connected because stars are components of galaxies, but they belong to different levels of classification: a star is an individual object, while a galaxy is a system of stars and other components. Star classification therefore describes the properties of individual stellar objects, while galaxy classification describes the structure of the system as a whole.
The Milky Way contains stars of different spectral classes; the Sun is a G2V-type star. A star’s type is determined by its spectrum and luminosity, while a galaxy’s type is based on the overall shape of the system. In other words, G2V describes an individual star, whereas a galaxy’s classification refers to the shape of its entire collection of stars and other components.
For a broader look at how cosmic objects are connected, read our article “Stars and Galaxies: How the Universe’s Key Objects Are Structured and Connected.”
Frequently asked questions
What type of star is the Sun?
Is Proxima Centauri a red dwarf?
What are the main types of galaxies?
Can you determine a star’s class from its color in an image?
Key takeaways
- The Sun is a G2V star: G2 indicates its spectral class, and V its main-sequence status.
- Proxima Centauri is a red dwarf, while Sirius B is a white dwarf.
- The Milky Way and Andromeda are spiral galaxies; M87 is elliptical.
- A galaxy’s appearance in an image does not always reveal its type: the disk’s tilt and the quality of observations matter.
