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"
Hale-Bopp: A Continuing Tail
20 February 1998
Where is Hale-Bopp now? The Great Comet of 1997, one of the largest and most active comets ever, is outbound about 400 million miles from the sun. Too faint for viewing without telescopes or binoculars, Hale-Bopp is presently positioned in the very southerly constellation of Pictor. This "negative" image (black stars against white sky) of Hale-Bopp is the result of a 1 hour time exposure using the 1-metre European Southern Observatory Schmidt telescope on January 5. Clearly the comet still has a substantial tail, blown by the solar wind, that points generally away from the sunward direction. But look closely. A spiky "anti-tail" is also visible pointing toward the sun! It is likely that this anti-tail is composed of dust grains released from the comet nucleus which are too large to be easily pushed by the solar wind. As Hale-Bopp recedes from the sun its activity will subside but astronomers are still uncertain as to how long its tails will last. After a swing through the outer solar system and the Oort cloud, Hale-Bopp will pass through the inner solar system again ... around the year 5400.