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"
Southern Sky from 38,000 Feet
30 January 2021
Celestial sights of the southern sky shine above a cloudy planet Earth in this gorgeous night sky view. The scene was captured from an airliner's flight deck at 38,000 feet on a steady westbound ride to Lima, Peru. To produce the sharp airborne astrophotograph, the best of a series of short exposures were selected and digitally stacked. The broad band of the southern Milky Way begins at top left with the dark Coalsack Nebula and Southern Cross. Its expanse of diffuse starlight encompasses the the Carina Nebula and large Gum Nebula toward the right. Canopus, alpha star of Carina and second brightest star in Earth's night is easy to spot below the Milky Way, as is the dwarf galaxy known as the Large Magellanic Cloud. The Small Magellanic cloud just peeks above the cloudy horizon. Of course, the South Celestial Pole also lies within the starry southern frame.