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"
Galactic Centre Starscape
29 June 2000
Thirty thousand light-years distant, beyond the majestic dust clouds of the constellation Sagittarius, lies the centre of our Milky Way Galaxy. Hidden from optical view by the dust, the Galactic Centre region is a relatively unexplored starscape. But infrared light can more easily penetrate the dust and this recently released Infrared Space Observatory (ISO) mosaic, together with other similar images, shows about 100,000 previously unseen stars of the Milky Way's central regions. Huge obscuring dust clouds still seem to crowd the area especially in the left part of the infrared picture. Marked by the white circle, the centre itself is missing from the mosaic because it is so bright that it would saturate ISO's sensitive camera. The stars are mostly evolved red giants, intrinsically cool, large, bright stars that have swollen after exhausting their central supply of hydrogen fuel. The detailed properties of the red giant stars can be very revealing as these stars contribute to the interstellar gas and dust clouds, enriching their galactic environment with carbon and other elements. Their motions also trace the mass distribution in the Galactic Centre and may support the idea that our Galaxy grew by swallowing smaller, nearby galaxies.