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"
Zodiacal Light Panorama
5 April 2012
Sweeping from the eastern to western horizon, this 360 degree panorama follows the band of zodiacal light along the solar system's ecliptic plane. Dust scattering sunlight produces the faint zodiacal glow that spans this fundamental coordinate plane of the celestial sphere, corresponding to the apparent yearly path of the Sun through the sky and the plane of Earth's orbit. The fascinating panorama is a mosaic of images taken from dusk to dawn over the course of a single night at two different locations on Mauna Kea. The lights of Hilo, Hawaii are on the eastern (left) horizon, with the Subaru and twin Keck telescope structures near the western horizon. On that well chosen moonless night, Venus was shining as the morning star just above the eastern horizon, and Saturn was close to opposition. In fact, Saturn is seen immersed in a brightening of the zodiacal band known as the gegenschein. The gegenschein also lies near 180 degrees in elongation or angular distance from the Sun along the ecliptic. In the mosaic projection, the plane of our Milky Way Galaxy runs at an angle, crossing the horizontal band of zodiacal light above the two horizons. Nebulae, stars, and dust clouds of the bulging galactic center are rising in the east.