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"
Kepler's Streak
9 March 2009
Streaking skyward, a Delta II rocket carries NASA's Kepler spacecraft aloft into the clear night of March 6. The dramatic scene was recorded in a time exposure from the crowded pier in Jetty Park at the northern end of Cocoa Beach, Florida, about 3 miles from the Cape Canaveral launch site. Kepler's mission is to search for Earth-like planets orbiting in the habitable zone of other stars. A planet orbiting within a star's habitable zone would have a surface temperature capable of supporting liquid water, an essential ingredient for life as we know it. To find Earth-like planets, Kepler's telescope and large, sensitive camera will examine a rich star field near the plane of our Milky Way Galaxy. Located in the constellation Cygnus, Kepler's field of view will allow it to monitor the brightness of many stars in the solar neighborhood and detect a slight dimming as a potential Earth-like planet crosses in front of the star.