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"
Periodic Comet Swift-Tuttle
19 February 1996
Comet Swift-Tuttle, shown above in false color, is the largest object known to make repeated passes near the Earth. It is also one of the oldest known periodic comets with sightings spanning two millennia. Last seen in 1862, its reappearance in 1992 was not spectacular, but the comet did become bright enough to see from many locations with binoculars. To create this composite telescopic image, four separate exposures have been combined, compensating for the motion of the comet. As a result, the stars appear slightly trailed. The inset shows details of the central coma. The unseen nucleus itself is essentially a chunk of dirty ice about ten kilometers in diameter. Comets usually originate in the Oort cloud in the distant Solar System - well past Pluto, most never venturing into the inner Solar System. When perturbed - perhaps by the gravity of a nearby star - a comet may fall toward the Sun. As a comet approaches the Sun, rocks, ice-chunks, gas, and dust boil away, sometimes creating impressive looking tails. In fact, debris from Comet Swift-Tuttle is responsible for the Perseids meteor shower visible every July and August. Comet Swift-Tuttle is expected to make an impressive pass near the earth in the year 2126, possibly similar to Comet Hyakutake this year or Comet Hale-Bopp next year.