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"
Eyeful of Saturn
30 April 2004
Now a bright speck of light wandering through Earth's night sky, magnificent planet Saturn lies nearly 1.5 billion kilometers from the Sun. But after an interplanetary voyage of seven years the planet's stunning rings nearly fill the field of the Cassini spacecraft's narrow angle camera in this image recorded on March 27. Tip to tip, the ring system spans about 270,000 kilometers. Named for discoverers, the large, easily visible gap in the rings is known as the Cassini division, while the narrower outer gap is the Encke division. Illuminated from below and to the right, the rings cast a shadow on Saturn's upper hemisphere, interrupted where sunlight streams through the Cassini division and creates a light blue streak. At the left, Saturn also casts a stark shadow across the planet girdling rings. On July 1, the Cassini spacecraft is scheduled to fire its main engine and enter Saturn orbit.