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"
The Hubble SWEEPS Field
13 October 2006
This crowded star field towards the center of our Milky Way Galaxy turns out to be a great place to search for planets beyond our solar system. In fact, repeatedly imaging about 180,000 stars in the field over a one week period, the Hubble Space Telescope enabled astronomers to conduct the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). Their search looked for brief, periodic dips in brightness caused as a large planet eclipses or transits its parent star. Since chances of seeing such an eclipse are slim, it was a definite advantage to examine as many stars as possible. In the end, SWEEPS astronomers found 16 candidate stars (green circles identify 11 in this cropped picture) that are likely closely orbited by large Jupiter-sized planets with periods of a few days or less. Large planets orbiting so close to their stars are termed hot Jupiters. Kepler, a future NASA mission, is intended to extend the transit technique to search for Earth-sized planets.