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"
Orion in Depth
19 September 2020
Orion is a familiar constellation. The apparent positions of its stars in two dimensions create a well-known pattern on the bowl of planet Earth's night sky. Orion may not look quite so familiar in this 3D view though. The illustration reconstructs the relative positions of Orion's bright stars, including data from the Hipparcus catalog of parallax distances. The most distant star shown is Alnilam. The middle one in the projected line of three that make up Orion's belt when viewed from planet Earth, Alnilam is nearly 2,000 light-years away, almost 3 times as far as fellow belt stars Alnitak and Mintaka. Though Rigel and Betelgeuse apparently shine brighter in planet Earth's sky, that makes more distant Alnilam intrinsically (in absolute magnitude) the brightest of the familiar stars in Orion. In the Hipparcus catalog, errors in measured parallaxes for Orion's stars can translate in to distance errors of a 100 light-years or so.