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"
Atlantis Approaches Mir
2 April 1996
Imagine flying though space and approaching the Mir space station. The crew of the Space Shuttle Atlantis did just this in a mission that ended only two days ago. Mir, now 10 years old, is equipped for scientific experiments in astronomy, physics, materials, biology and chemistry. The top most module on Mir is an unmanned supply ship used to send food and supplies. The next module with the long boom carries telescopes and essential flight equipment and connects to the core module with living quarters and solar panels. To the left is the Spektr module carrying solar arrays and scientific equipment while on the right is a scientific module that also carries an airlock. The docking module seen at the bottom is the ultimate destination of Atlantis. The STS-76 mission left astronaut Shannon Lucid for a planned five month stay. Four more shuttle flights are currently planned to Mir, keeping a NASA astronaut continuously in space until late 1997. In late 1997, building on this jointly developed understanding and experience, the US and Russia will launch the first modules of the International Space Station. Latest Comet Hyakutake images: APOD Hyakutake Archive, JPL, Fayetteville Observer-Times, NASA's Night of the Comet, ICSTARS, Jerry Lodriguss, ScienceWeb, Crni Vrh Obs., Cent. Mich. U.