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"
Closer To Beta Pic
22 January 1998
What did our Solar System look like as the planets were forming? Since the 1980s, astronomers have been pointing toward Beta Pictoris, a young, sun-like star a mere 50 light-years distant, as a likely example. Beta Pic is surrounded by a disk of dust which we view nearly edge-on. The dust disk shines by reflected starlight and has been examined with ever increasing detail to search for signs of planetary formation. The trick is to follow the disk as close in to the star as possible, without being overwhelmed by the direct starlight. To make this Hubble Space Telescope image, a coronagraph was used to block the direct starlight and achieve the closest view yet. The false color picture shows the inner section of the dusty disk to within nearly 1.5 billion miles of the star itself, about the scale of the orbit of Uranus. The obvious warp is indirect evidence that a planet now orbits this young sun, slightly inclined to the disk. The planet's gravitational pull would produce the visible distortion.