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"
The 47 Ursae Majoris System
17 August 2001
Watching and waiting, astronomers have uncovered the presence of more than 70 planets orbiting stars other than the Sun. So far almost all these extrasolar planets have crazy elongated orbits, lie uncomfortably close to their parent stars, or are found in bizarre, inhospitable systems. Yet a reported new planet discovery indicates for the first time that a nearby sun-like star, 47 Ursae Majoris (47 UMa), has at least two planets in nearly circular orbits more reminiscent of Jupiter and Saturn in our own familiar Solar System. The planets are too distant and faint to be photographed directly. Still, 13 years of spectroscopic observations of 47 UMa have revealed the wobbling signature of a second planet intertwined with one previously known. In this artist's illustration, the worlds of 47 UMa hang over the rugged volcanic landscape of a hypothetical moon. The moon orbits the newly discovered planet, imagined here with Saturn-like rings, while the previously known planet is visible as a tiny crescent, close to the yellowish star. Closer still to 47 UMa is another tiny dot, a hypothetical Earth-like water world. About 51 light-years distant, 47 UMa can be found in planet Earth's sky near the Big Dipper.