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"
Four Planet Sunset
31 July 2010
This mesmerizing sunset photo was taken from the summit of volcanic Mount Lawu, 3,265 meters above sea level, on July 21. The view looks west, toward the city lights of Surakarta (aka Solo), Central Java, Indonesia. Two other volcanic peaks, sharp Merapi (left) and Merbabu lie along the colorful horizon. Four planets shine in the twilight sky above them. Spread out near the plane of the ecliptic are Mercury, Venus, Mars, and Saturn, along with bright Regulus, alpha star of the constellation Leo. For help finding them, just put your cursor over the picture. In fact, these four planets still shine in western skies at sunset, with Venus, Mars, and Saturn grouped much more tightly this weekend and in early August. By August 12, a young crescent Moon will join the four planet sunset.