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"
Fomalhaut Dust Disk Indicates Planets
11 October 2002
One of the brightest stars on the sky likely has planets. Fomalhaut, actually the 17th brightest star in the night sky, is a mere 22 light-years away but only a fraction of the age of our Sun. Recent observations in far infrared light with a detector cooled to near zero kelvins indicate a dust disk surrounding Fomalhaut that has both a hole in the center and a warped edge. Now the hole in the center indicates that dust has fallen onto interior planets -- possibly like the Earth -- while the warp at the edge indicates the gravitational pull of a planet like Jupiter or Saturn. The discovery image was taken with the SCUBA instrument through the James Clerk Maxwell Telescope in Hawaii, USA. The above illustration shows what the Fomalhaut dusty planetary system might look like from near the large planet.