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"
At the Core of NGC 6752
10 February 2012
This sharp Hubble Space Telescope view looks deep into NGC 6752. Some 13,000 light-years away toward the southern constellation Pavo, the globular star cluster roams the halo of our Milky Way galaxy. Over 10 billion years old, NGC 6752 holds over 100 thousand stars in a sphere about 100 light-years in diameter, but the Hubble image frame spans the central 10 or so light-years and resolves stars near the dense cluster core. In fact the frame includes some of the cluster's blue straggler stars, stars which appear to be too young and massive to exist in a cluster whose stars are all expected to be at least twice as old as the Sun. Explorations of the NGC 6752 have also indicated that a remarkable fraction of the stars near the cluster's core, are multiple star systems, supporting arguments that star mergers and collisions in the dense stellar environment can create the cluster's blue straggler stars.