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"
At the Limit of Diffraction
7 May 2015
Did you ever want to just look through the eyepiece of a large telescope in space? If you could, you would see a sharp view that was diffraction limited. Unaffected by atmospheric blurring that ultimately plagues earthbound observers, the angular resolution of your diffraction limited view would be determined only by the wavelength of light and diameter of the telescope lens or mirror; the larger the diameter, the sharper the image. Still, in this working earth-based snapshot a new active adaptive optics system (MagAO) is being used to cancel out the atmospheric blurring in a visual observation of famous double star system Alpha Centauri. Testing the system at the eyepiece of the 6.5 meter diameter Magellan Clay Telescope at Las Campanas Observatory, astronomer Laird Close is enjoying a historic diffraction limited view (inset) and the wide apparent separation of the close binary star system ... without traveling to low earth orbit.