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"
Galaxy Wars: M81 versus M82
7 May 2025
In the upper left corner, surrounded by blue arms and dotted with red nebulas, is spiral galaxy M81. In the lower right corner, marked by a light central line and surrounded by red glowing gas, is irregular galaxy M82. This stunning vista shows these two mammoth galaxies locked in gravitational combat, as they have been for the past billion years. The gravity from each galaxy dramatically affects the other during each hundred-million-year pass. Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. But M81 left M82 with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays. This big battle is seen from Earth through the faint glow of an Integrated Flux Nebula, a little studied complex of diffuse gas and dust clouds in our Milky Way Galaxy. In a few billion years, only one galaxy will remain.