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"
A Particle Beam Jet forms HH 24
4 February 2014
If you visit HH 24, don't go near the particle beam jet. This potential future travel advisory might be issued because the powerful jet likely contains electrons and protons moving hundreds of kilometers per second. The above image was taken by the Hubble Space Telescope in infrared light in order to better understand turbulent star forming regions known as Young Stellar Objects (YSOs). Frequently when a star forms, a disk of dust and gas circles the YSO causing a powerful central jets to appear. In this case, the energetic jets are creating, at each end, Herbig-Haro object 24 (HH 24), as they slam into the surrounding interstellar gas. The entire star forming region lies about 1,500 light years distant in the Orion B molecular cloud complex. Due to their rarity, jets like that forming HH 24 are estimated to last only a few thousand years.