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"
The Milky Way Over Tenerife
5 April 2011
Have you ever seen the band of our Milky Way Galaxy? Chances are you have never seen it like this -- nor could you. In a clear sky from a dark location at the right time, a faint band of light is visible across the sky. This band is the disk of our spiral galaxy. Since we are inside this disk, the band appears to encircle the Earth. The above spectacular picture of the Milky Way arch, however, goes where the unaided eye cannot. The image is actually a deep digital fusion of nine photos that create a panorama fully 360 across. Taken recently in Teide National Park in Tenerife, Canary Islands, Spain, the image includes the Teide volcano, visible near the image center, behind a volcanic landscape that includes many large rocks. Far behind these Earthly structures are many sky wonders that are visible to the unaided eye, such as the band of the Milky Way, the bright waxing Moon inside the arch, and the Pleiades open star cluster (can you find it?). The deep exposure also brings out many sky wonders normally beyond human perception, many of which are labelled on the annotated image version, including Barnard's Loop, visible above as the half red ring below the Milky Way band.