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"
Our Galaxy in Stars, Gas, and Dust
27 September 1999
The disk of our Milky Way Galaxy is home to hot nebulae, cold dust, and billions of stars. The red nebulae visible in the above contrast-enhanced picture are primarily emission nebulae, glowing clouds of hydrogen gas heated by nearby, bright, young stars. The blue nebulae are primarily reflection nebulae, clouds of gas and fine dust reflecting the light of nearby bright stars. Perhaps the most striking, though, are the areas of darkness, including the Pipe Nebula visible on the image top left. These are lanes of thick dust, many times containing relatively cold molecular clouds of gas. Dust is so plentiful that it obscures the Galactic Center in visible light, hiding its true direction until discovered early this century. The diffuse glow comes from billions of older, fainter stars like our Sun, which are typically much older than any of the nebulae. Most of the mass of our Galaxy remains in a form currently unknown.