Star
A star is a massive, self-luminous sphere of plasma held together by its own gravity. It shines by converting hydrogen into helium through nuclear fusion in its core, releasing energy across the electromagnetic spectrum.
Stars form in large clouds of gas and dust—called nebulae—where regions collapse under gravity, creating protostars. When core temperatures reach millions of degrees, fusion ignites, marking the birth of a star. Observatories like Hubble and missions such as NASA’s Infrared telescopes have imaged this process in action.
The majority (~90%) of stars are main-sequence stars, fusing hydrogen into helium. These include a broad range of masses—from red dwarfs (small, long-lived, faint) to blue giants and supergiants (massive, hot, and short-lived). Our Sun is a middle-aged G-type main sequence star.
As stars exhaust their hydrogen fuel, their evolution depends on mass. Lower-mass stars become red giants then white dwarfs. More massive stars undergo successive fusion stages, end in supernova explosions, and leave behind neutron stars or black holes.
Stars vary in brightness, size, and color. They are classified using spectral types (O, B, A, F, G, K, M) based on surface temperature and absorption lines. For example, O- and B-type stars are hot and blue; M-type are cool and red.
Stellar remnants include white dwarfs (Earth-sized cores of former stars), neutron stars (city-sized remnants of supernovae), and black holes (extreme-density objects from the most massive stars).
Stars are not static—many rotate, exhibit magnetic activity (like sunspots and flares), and broadcast stellar winds. Their lifecycle enriches the interstellar medium with heavier elements, seeding future generations of stars and planets.
Stars often exist in groups—binary or systems within star clusters and galaxies. Their properties are studied via brightness, spectra, parallax, variability, and statistical surveys by missions like Gaia and Kepler.
Sources
APODs including "Star"
Andromeda Nebula: Var!
6 April 1996
In the 1920s, using photographic plates made with the Mt. Wilson Observatory's 100 inch telescope, Edwin Hubble determined the distance to the Andromeda Nebula - decisively demonstrating the existence of other galaxies far beyond the Milky Way. His notations are evident on the plate shown above (the image is a negative with stars appearing as black dots against the white background of space). By intercomparing plates, Hubble searched for "novae", stars which underwent a sudden increase in brightness. He found several on this plate and marked them with an "N". Later he discovered that one was actually a type of variable star known as a cepheid - crossing out the "N" he wrote "Var!" (upper right). Thanks to the work of Harvard astronomer Henrietta Leavitt, cepheids, regularly varying, pulsating stars, could be used as "standard candle" distance indicators. Identifying such a star allowed Hubble to show that Andromeda was not a small cluster of stars and gas within our own galaxy, but a large galaxy in its own right at a substantial distance from the Milky Way. Hubble's discovery is responsible for our modern concept of a Universe filled with galaxies. Information: The Scale of the Universe Debate in April 1996