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 Antennae Galaxies
22 October 1997
A ground-based telescopic view (left) of the collision between the galaxies NGC4038 and NGC4039 reveals long arcing insect-like "antennae" of luminous matter flung from the scene of the accident. Investigators using the Hubble Space Telescope to sift through the cosmic wreckage near the two galaxy cores have recently announced the discovery of over a thousand bright young clusters of stars - the result of a burst of star formation triggered by the collision. The green outline shows the area covered by the higher resolution Hubble image (right). At the distance of the Antennae galaxies (about 63 million light-years), a pixel in this image corresponds to about 15 light-years. Dust clouds around the two galactic nuclei give them a dimmed and reddened appearance while the massive, hot, young stars of the newly formed clusters are blue. How do colliding galaxies evolve with time? Determining the ages of star clusters formed in galaxy collisions can provide significant clues. The Antennae galaxies are seen in the southerly constellation Corvus.