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"
Rocket Streak and Star Trails
30 January 2014
Fixed to a tripod and looking east across the Kennedy Space Center's Turn Basin, a camera captured these star trails as a series of short exposures over a three hour period on the evening of January 23rd. Positioned just a few miles from Space Launch Complex 41 at Cape Canaveral Air Force Station, it also captured a spectacular night launch of an Atlas V rocket carrying NASA's Tracking and Data Relay Satellite TDRS-L. Creating the trails, the apparent motion of the stars through the sky is just a reflection of the daily rotation of planet Earth on its axis. But that rotation is also the reason the rocket streak follows a path arcing east across the Atlantic. Launching toward the east, in the direction of Earth's rotation, adds the rotation velocity to the rocket and reduces the fuel needed to reach orbit. A little ironically, TDRS-L is destined for a geostationary orbit. From there, 36,000 kilometers or so above the equator, its orbital period will match Earth's rotation and the satellite will hang motionless in planet Earth's sky.