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"
SN1987A's Cosmic Pearls
20 February 2004
In February 1987, light from the brightest stellar explosion seen in modern times reached Earth -- supernova SN1987A. This Hubble Space Telescope image from the sharp Advanced Camera for Surveys taken in November 2003 shows the explosion site over 16 years later. The snap shot indicates that the supernova blast wave continues to impact a pre-existing, one light-year wide ring of material, and the nascent central supernova remnant continues to expand. Like pearls on a cosmic necklace, bright hot spots produced as the blast wave heats material up to millions of degrees began to appear on the ring in the mid 1990s and have been followed across the spectrum by astronomers ever since. Supernova SN1987A lies in the Large Magellanic Cloud, a neighboring galaxy some 170,000 light-years away. That really does mean that the explosive event - the core collapse and detonation of a star about 20 times as massive as the Sun - occurred 170,000 years before February 1987.