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"
Spokes in the Helix Nebula
4 September 2008
At first glance, the Helix Nebula (aka NGC 7293), looks simple and round. But this well-studied example of a planetary nebula, produced near the end of the life of a sun-like star, is now understood to have a surprisingly complex geometry. Its extended loops and comet-shaped features have been explored in Hubble Space Telescope images. Still, a 16-inch diameter telescope and camera with broad and narrow band filters was used to create this sharp view of the Helix. The color composite also reveals the nebula's intriguing details, including light-year long, bluegreen radial stripes or spokes that give it the appearance of a cosmic bicycle wheel. The spoke features seem to indicate that the Helix Nebula is itself an old and evolved planetary nebula. The Helix is a mere seven hundred light years from Earth, in the constellation Aquarius.