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"
Spacecrafts Streak Over Colorado
14 March 2011
Have you ever seen a "star" drifting slowly overhead, but not known what it was? Sometimes even pointing it out to friends or family will only lead to a shrug. What you are seeing, most likely, is a spacecraft in low Earth orbit reflecting back sunlight as it circles the Earth once every 90 minutes or so. Two of the brighter spacecraft in the present day sky are the International Space Station (ISS), and, when it is up, a NASA space shuttle. As relative orientations change, the brightness of reflections may also change, sometimes suddenly. Another source of bright drifting objects, Iridium communication satellites, may even appear to flare up to become brighter than any other sky object for a few seconds. Pictured above, two bright points of light separated by only a few degrees drifted together across the sky above Lory State Park, Colorado, USA last week, just after sunset. These lights were were the ISS and the space shuttle Discovery, which had undocked from the ISS a few hours earlier. Given a digital fusion of many separate camera exposures and a wide angle perspective, the pair appears above as streaks in front of point-like stars. Web sites now exist that can help you identify unknown "drifters" and even predict the time of the next pass of the ISS visible from your location.