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"
M57: The Ring Nebula
4 May 1998
It looked like a ring on the sky. Hundreds of years ago astronomers noticed a nebula with a most unusual shape. Now known as M57 or NGC 6720, the gas cloud became popularly known as the Ring Nebula. It is now know to be a planetary nebula, a gas cloud emitted at the end of a Sun-like star's existence. As one of the brightest planetary nebula on the sky, the Ring Nebula can be seen with a small telescope in the constellation of Lyra. The Ring Nebula lies about 4000 light years away, and is roughly 500 times the diameter of our Solar System. In this recent picture by the Hubble Space Telescope, dust filaments and globules are visible far from the central star. This helps indicate that the Ring Nebula is not spherical, but cylindrical. Perhaps the Ring Nebula would appear differently if viewed sideways.