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"
Spitzer's Trifid
13 February 2020
The Trifid Nebula, also known as Messier 20, is easy to find with a small telescope. About 30 light-years across and 5,500 light-years distant it's a popular stop for cosmic tourists in the nebula rich constellation Sagittarius. As its name suggests, visible light pictures show the nebula divided into three parts by dark, obscuring dust lanes. But this penetrating infrared image reveals the Trifid's filaments of glowing dust clouds and newborn stars. The spectacular false-color view is courtesy of the Spitzer Space Telescope. Astronomers have used the infrared image data to count newborn and embryonic stars which otherwise can lie hidden in the natal dust and gas clouds of this intriguing stellar nursery. Launched in 2003, Spitzer explored the infrared Universe from an Earth-trailing solar orbit until its science operations were brought to a close earlier this year, on January 30.