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 Snows of Paranal
11 August 2011
Recorded last week, this dawn portrait of snowy mountain and starry sky captures a very rare scenario. The view does feature a pristine sky above the 2,600 meter high mountain Cerro Paranal, but clear skies over Paranal are not at all unusual. That's one reason the mountain is home to the European Southern Observatory's Very Large Telescope. Considering the number of satellites now in orbit, the near sunrise streak of a satellite glinting at the upper left isn't rare either. And the long, bright trail of a meteor can often be spotted this time of year too. The one at the far right is associated with the annual Perseid meteor shower whose peak is expected tomorrow (Friday, August 12). In fact, the rarest aspect of the picture is just the snow. Cerro Paranal rises above South America's Atacama desert, known as the driest place on planet Earth.