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"
Comet Hartley 2 Flyby
5 November 2010
Follow these 5 frames clockwise starting from the top left to track the view from the EPOXI mission spacecraft as it approached, passed under, and then looked back at the nucleus of comet Hartley 2 on November 4. Its closest approach distance was about 700 kilometers. In fact, this encounter was the fifth time a spacecraft from planet Earth has imaged a comet close-up. But Hartley 2's nucleus is definitely the smallest one so far, its long axis spanning only about 2 kilometers (1.2 miles). Though Hartley 2 is small, these stunning images showing jets of dust and gas indicate an impressively active surface. The jets are seen originating from the rough surface areas, with sunlight illuminating the nucleus from the right. Remarkably, rough areas at both ends of the elongated nucleus are joined by a narrower, smooth waist. The EPOXI mission reuses the Deep Impact spacecraft that launched a probe impacting the nucleus of comet Tempel 1 in 2005.