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"
TYC 8998-760-1: Multiple Planets around a Sun Like Star
18 August 2020
Do other stars have planets like our Sun? Previous evidence shows that they do, coming mostly from slight shifts in the star's light created by the orbiting planets. Recently, however, and for the first time, a pair of planets has been directly imaged around a Sun-like star. These exoplanets orbit the star designated TYC 8998-760-1 and are identified by arrows in the featured infrared image. At 17 million years old, the parent star is much younger than the 5-billion-year age of our Sun. Also, the exoplanets are both more massive and orbit further out than their Solar System analogues: Jupiter and Saturn. The exoplanets were found by the ESO's Very Large Telescope in Chile by their infrared glow – after the light from their parent star was artificially blocked. As telescope and technology improve over the next decade, it is hoped that planets more closely resembling our Earth will be directly imaged. Experts Debate: How will humanity first discover extraterrestrial life?