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 Colors and Mysteries of Centaurus A
12 August 2002
Why is spiral galaxy Centaurus A in so much turmoil? The above composite image shows different clues to the unusual galaxy's past in different bands of light. In low energy radio waves, shown in red, lobes across the thick swath of dust glow brightly. In more energetic radio waves, depicted in green, a bright jet is seen emanating from the galaxy's center. In optical light, shown in white, the stars that compose much of the galaxy are visible. Recently, Centaurus A has recently been imaged in X-ray light by the Chandra X-ray Observatory. The X-rays, depicted in blue, show arcs of hot gas shooting out from the center in an explosion that likely happened about 10 million years ago. One hypothesis that would explain the turmoil would be if Centaurus A devoured a smaller galaxy about 100 million years ago.