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"
Satellites Collide in Low Earth Orbit
18 February 2009
How often do satellites collide? Although minuscule space debris may strike any satellite on occasion, the first known collision between time two full satellites occurred only last week. Even though thousands of satellites have been launched, the low collision rate is caused by the great vastness of space. Last week, however, a defunct Russian communications satellite named Cosmos 2251 smashed right into an operational US communications satellite named Iridium 33 over Siberia, Russia. Both satellites were destroyed. The sheer number of massive particles in a dispersing debris cloud, depicted in an inset image above, increases the risk that other operating satellites might be struck by a harmful fast-moving projectile. The collision occurred in low Earth orbit only 750 kilometers up, a height shared by many satellites but significantly higher than the 350-km high human-occupied International Space Station. Since satellites may disintegrate when struck by fast-moving space junk, the crash focuses concern that a future dramatic satellite collision may one day start an ablation cascade of increasingly more collisions. The result could then render future human space flights increasingly risky and expensive satellite lifetimes increasingly short.