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"
Stars in the Infrared Sky
18 September 1996
What if you could see infrared light? Because this light is less absorbed by dust than visible light, you could peer into the center of our Milky Way Galaxy. The stars there are normally hidden from direct view by the interstellar dust clouds which line the Galactic plane. Above is a false color image of the entire sky made by the DIRBE instrument onboard NASA's COBE Satellite using infrared light with a wavelength of 3.5 microns - about 7 times longer than the wavelength of visible light. The galactic plane runs horizontally along the middle of the image. At this wavelength, the cool stars in our galaxy shine brightly and can be seen to define the plane of the Milky Way and the central bulge. Interplanetary dust, which tends to lie along the plane of our own solar system, scatters sunlight and emits radiation at these wavelengths too. The faint glow it produces results in the "S" shape apparent in this infrared all-sky view.