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"
Carina Cliffs from the Webb Space Telescope
5 September 2022
Stars created these cliffs. Specifically, the destructive winds and energetic light from the stars in the open cluster NGC 3324 eroded away part of a mountain of dark interstellar dust in the northern part of the Carina Nebula. Several of these stars are visible toward the top of this highly detailed image taken recently by the James Webb Space Telescope, the largest astronomical telescope ever launched. Webb's large mirror and ability to see dust-piercing infrared light has enabled it to capture fascinating details in the dust, hundreds of previously hidden stars, and even some galaxies far in the distance. The featured jagged cliffs occur in part of Carina known as the Gabriela Mistral Nebula -- because when viewed in another orientation, they appear similar to the facial profile of the famous Chilean poet. These nebular cliffs occur about 7,600 light years away toward the southern constellation of Carina. Astrophysicists: Browse 2,800+ codes in the Astrophysics Source Code Library