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"
Caught in the Afterglow
24 November 2011
In this artist's illustration, two distant galaxies formed about 2 billion years after the big bang are caught in the afterglow of GRB090323, a gamma-ray burst seen across the Universe. Shining through its own host galaxy and another nearby galaxy, the alignment of gamma-ray burst and galaxies was inferred from the afterglow spectrum following the burst's initial detection by the Fermi Gamma Ray Space Telescope in March of 2009. As seen by one of the European Southern Observatory's very large telescope units, the spectrum of the burst's fading afterglow also offered a surprising result - the distant galaxies are richer in heavy elements than the Sun, with the highest abundances ever seen in the early Universe. Heavy elements that enrich mature galaxies in the local Universe were made in past generations of stars. So these young galaxies have experienced a prodigious rate of star formation and chemical evolution compared to our own Milky Way. In the illustration, the light from the burst site at the left passes successively through the galaxies to the right. Spectra illustrating dark absorption lines of the galaxies' elements imprinted on the afterglow light are shown as insets. Of course, astronomers on planet Earth would be about 12 billion light-years off the right edge of the frame.