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"
NGC 4603 and the Expanding Universe
27 May 1999
NGC 4603, a galaxy with majestic spiral arms and intricate dust lanes, is 108 million light-years away. Its distance has been accurately measured by astronomers using one of the fundamental yardsticks of the extragalactic distance scale - pulsating variable stars known as Cepheids. Though intrinsically very bright, Cepheids are faint and difficult to find at such large distances (the bright "spiky" stars seen above are foreground objects). Thanks to the Hubble Space Telescope's sharp vision, more than 36 beckoning Cepheids have been identified in NGC 4603, now the most distant galaxy in which these stars have been located. In fact, using the Space Telescope to pick out Cepheids in galaxies closer than NGC 4603, the Hubble Key Project Team has recently announced the completion of their 8 year effort to precisely measure galaxy distances and the expansion rate of the Universe - the Hubble Constant. Based on their comparison of galaxy distances and recession speeds, they report that the Hubble Constant is 70 kilometers per second per megaparsec to an accuracy of 10 percent. This means a galaxy should appear to recede 160,000 miles per hour faster for every 3.3 million light-year increase in distance away. Accurately measuring the Hubble Constant was one of the major goals for the Hubble Space Telescope when it was launched in 1990.