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"
That Night over Half Dome
2 May 2014
Captured one night last May this eight frame mosaic starts on the left, down Northside Drive through Yosemite National Park. It ends thousands of light-years away though, as the arc of the Milky Way tracks toward the center of our galaxy on the right, far beyond the park's rugged skyline. That night was still moonless when the storm clouds retreated, so the rocky faces of the surrounding mountains are lit by campfires and artificial lights. Yosemite Falls is at the left. The granite face of Half Dome juts above the far horizon, near the center of the view. The remarkable flash above it is a bright meteor. Part of the annual Eta Aquarid meteor shower the colorful streak is moving up, its trail pointing directly back to the shower's radiant, low in Aquarius. This year's Eta Aquarids should peak in the moonless early morning hours of May 6 as the Earth sweeps through dust from the tail of Comet Halley.