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"
CoRoT Satellite Discovers Rocky Planet
23 September 2009
How similar is exoplanet CoRoT-7b to Earth? The newly discovered extra-solar planet is the closest physical match yet, with a mass about five Earths and a radius of about 1.7 Earths. Also, the home star to CoRoT-7b, although 500 light years distant, is very similar to our Sun. Unfortunately, the similarities likely end there, as CoRoT-7b orbits its home star well inside the orbit of Mercury, making its year last only 20 hours, and making its peak temperature much hotter than humans might find comfortable. CoRoT-7b was discovered in February by noting a predictable slight decrease in the brightness of its parent star. Pictured above, an artist's depiction shows how CoRoT-7b might appear in front of its parent star. The composition of CoRoT-7b remains unknown, but given its size and mass, it cannot be a gas giant like Jupiter, and is very likely composed predominantly of rock. Future observations will likely narrow the composition of one of the first known rocky planets discovered outside of our Solar System.