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"
Cepheus 1: Nearby Galaxy Hiding
1 December 1998
Some galaxies are hard to find. Besides being hidden behind the dust and bright foreground stars of our Milky Way Galaxy, recently discovered Cepheus 1 was missed previously because it is so uniformly dim. In fact, the first indication that any galaxy was there at all came in the radio band from the Dwingeloo radio telescope in the Netherlands. Close inspections of optical photographs like the one above then revealed Cepheus 1 as the low-surface brightness galaxy in the center. Cepheus 1 turns out to be only about 20 million light years distant, and so is one of the few spiral galaxies that live close by. Low-surface brightness (LSB) galaxies like Cepheus 1 are still being studied, but are known to have relatively large separations between bright stars, and to be more commonly found away from other galaxies.