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 Northern Milky Way
25 August 2003
Many of the stars in our home Milky Way Galaxy appear together as a dim band on the sky that passes nearly over the Earth's north and south poles. Pictured above is the part of our Galaxy that passes closest over the north pole. Placing your cursor over the image will bring up the names of several constellations and bright stars. The diffuse white Galaxy glow is created by billions of stars, while red patches are large emission nebulas, usually marking areas where bright stars have recently formed. In the north, all of the lights visible at night and all lights that created this image were emitted within the past few thousand years from within the Milky Way Galaxy -- except one. On the upper right is a small faint patch designated M31, the Andromeda Galaxy. M31 is a spiral galaxy similar to our Milky Way but so distant it emits the oldest light distinguishable by the unaided eye -- light that takes over two million years to reach us.