Star
A star is a massive, luminous sphere of plasma held together by self-gravity, in which nuclear fusion reactions in the core generate energy that is radiated as light and heat. Stars are the fundamental building blocks of galaxies and are responsible for synthesising most of the elements heavier than hydrogen and helium in the universe. The life cycle of a star—from molecular cloud collapse to final remnant—depends primarily on its initial mass, with more massive stars burning hotter and dying faster.
Classification of Stars by Spectral Type (Morgan–Keenan System)
| Spectral Type | Temperature (K) | Colour | Example Star | Mass (M☉) |
|---|---|---|---|---|
| O | ≥ 30,000 | Blue | Theta Orionis C | ≥ 16 |
| B | 10,000–30,000 | Blue-white | Rigel | 2–16 |
| A | 7,500–10,000 | White | Sirius A | 1.4–2.1 |
| F | 6,000–7,500 | Yellow-white | Procyon A | 1.04–1.4 |
| G | 5,200–6,000 | Yellow | Sun | 0.8–1.04 |
| K | 3,700–5,200 | Orange | Epsilon Eridani | 0.45–0.8 |
| M | 2,400–3,700 | Red | Proxima Centauri | 0.08–0.45 |
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Related Terms
Stellar Nuclear Fusion
Stellar nuclear fusion is the thermonuclear process occurring in a star's core whereby lighter atomic nuclei are forced together under extreme temperature and pressure to form heavier nuclei, releasing enormous amounts of energy according to Einstein's mass–energy equivalence. In main-sequence stars like the Sun, the dominant process is the proton–proton (pp) chain, which converts hydrogen into helium; more massive stars rely on the CNO (carbon–nitrogen–oxygen) cycle. This energy release provides the radiation pressure that counteracts gravitational collapse, maintaining a star's long-term equilibrium known as hydrostatic balance.
Main Sequence Star
A main sequence star is a star in the longest and most stable phase of its life, during which it fuses hydrogen into helium in its core to balance gravitational contraction through radiation pressure, a state called hydrostatic equilibrium. On the Hertzsprung–Russell diagram, main sequence stars form a diagonal band called the Zero Age Main Sequence (ZAMS) running from hot, luminous blue stars (upper left) to cool, dim red dwarfs (lower right). The Sun has been on the main sequence for approximately 4.6 billion years and will remain there for another ~5 billion years before evolving into a red giant.
Hertzsprung-Russell Diagram
The Hertzsprung–Russell (H–R) diagram is a fundamental scatter plot in stellar astrophysics that plots stellar luminosity (or absolute magnitude) on the vertical axis against surface temperature (or spectral type/colour index) on the horizontal axis—with temperature increasing to the left—revealing that stars cluster into distinct evolutionary groups. The diagram was developed independently by Ejnar Hertzsprung (1905–1913) and Henry Norris Russell (1913) and remains the cornerstone tool for understanding stellar structure and evolution. The main sequence diagonal, the giant branch, the horizontal branch, the asymptotic giant branch, and the white dwarf region each represent different stages of stellar life and can be used to estimate stellar ages, distances, and populations in star clusters.
From Old English "steorra", related to Proto-Germanic "*sterron" and Latin "stella", ultimately from Proto-Indo-European root "*h₂stḗr" meaning star. The scientific study of stars as physical objects developed primarily in the 19th century with the advent of spectroscopy.