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"
Orion Rising
25 December 2002
Orion always comes up sideways ... and was caught in the act earlier this month by astronomer Jimmy Westlake, stargazing eastward over the Rocky Mountains north of Leadville, Colorado, USA. To make this gorgeous image, Westlake placed his camera on a tripod for two exposures. The first lasted for 18 minutes allowing the stars to trail as they rose above the mountain range. After a minute long pause, the second exposure began and lasted only 25 seconds decorating the end of each trail with a celestial point of light. The three bright stars in Orion's belt stand in a nearly vertical line above the mountain peak right of center. Hanging from his belt, the stars and nebulae of the Hunter's sword follow the slope down and to the right. A festive yellow-orange Betelgeuse is the brightest star above the peak just left of center, but brighter still, planet Saturn shines near the upper left corner. In the foreground on planet Earth, a frozen lake and snowy mountains are lit by a four day old crescent Moon. Happy Holidays and Best Wishes from APOD!