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"
Earth Nears Asteroid Toutatis
20 January 1997
On November 29, 1996 the Earth came within 3.3 million miles of the asteroid Toutatis. Above is a computer simulated picture of this spectacle from the surface of Toutatis (a 2.5 degree field of view looking toward Earth). In Earth's sky, Toutatis appeared only as a faint object moving against a background of stars. Also known as Earth-crossing asteroid 4179, Toutatis is in an eccentric 4 year orbit which moves it from the asteroid belt between Mars and Jupiter to just inside Earth's orbit. When the Earth passed near it in 1992 Toutatis was imaged by radar and seen to be two irregularly shaped lumps, perhaps joined by a narrow neck. This bizarre object is up to 1.5 miles wide, 2.9 miles long, and is tumbling through space. In the year 2004, on September 29, the Earth will pass very near Toutatis, closing to within a million miles (4 times the Earth-Moon distance) - the closest approach predicted for any asteroid or comet between now and 2060. Studies of Toutatis and other Earth-crossing asteroids help reveal connections between the Solar System's meteorites,main-belt asteroids and comets. These wayward asteroids also offer tantalizing targets for robotic exploration and, over time, represent potential collision hazards for planet Earth!