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"
NASA's X-43A Scramjet Sets Air Speed Record
29 March 2004
Using oxygen from the air itself, a NASA experimental jet propelled itself past Mach 7 in the atmosphere above the Pacific Ocean this weekend. The small automated X-43A Hyper-X craft was dropped from a huge converted B-52 bomber and then accelerated by a standard Pegasus rocket. At Mach 7, seven times the speed of sound, the X-43A separated and the novel scramjet kicked in. Atmospheric oxygen was then scooped up, combined with onboard hydrogen, and combusted in flight to propel the X-43A to record air speeds during maneuvers over the next 10 seconds. Engines of ramjet design have been suggested as a satellite launch method without heavy fuel tanks and even romanticized for interstellar space travel. The previously acknowledged air-speed record for jet-powered flight was Mach 3.3 for the decommissioned SR-71. Re-entering space rockets can start as high as Mach 36 before the atmosphere decelerates them. The X-43A, depicted in the artist's illustration above, might well propel itself past Mach 10 in future tests.