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"
Cone Nebula Infrared Close-Up
6 June 2002
After astronauts repaired NICMOS - the Near Infrared Camera and Multi-Object Spectrometer - during the latest Hubble Space Telescope servicing mission, astronomers were quick to turn the sophisticated instrument on the photogenic stellar nursery known as the Cone Nebula. This remarkable NICMOS close-up of the Cone Nebula dramatically confirms that the Hubble's infrared vision has been restored. Gas and dust clouds at the blunted tip of the cone-shaped star-forming region are seen here in false-color covering an area about half a light-year across. Toward the left hand side of the picture, the four bright stars with diffraction spikes are also present in visible light images and are in front of the Cone Nebula, itself 2,500 light-years away. But the fainter stars to their right are embedded in or behind the nebula's obscuring dust clouds and are revealed only in this penetrating infrared view.