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"
Coronal Inflow
29 November 2001
The active Sun has thrown a lot our way lately, including storms of particles streaming outward in the solar wind and clouds of plasma which triggered awesome auroral displays. Still, a growing body of intriguing observations from the LASCO instrument on board the space-based SOlar and Heliospheric Observatory (SOHO) indicates material also flows back toward the Sun, starting from over 2 million kilometers above its visible surface. Relatively hard to detect against the outflowing solar corona, a dark inflowing cloud's relative motion is tracked above in two highly processed images recorded an hour apart. The solar surface, graphically shown by the yellow quarter circle at the lower right, is blocked from view by a smooth occulting disk. Fighting against a solar wind outflow of about 120 kilometers per second the cloud seems to be moving inward at 50-100 kilometers per second. Occasionally appearing as often as once per hour, the clouds, seem to be dragged in by collapsing magnetic field loops rather than gravity alone. Researchers are now working to relate this surprising inflow to the solar wind and magnetic environment of the Sun.