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"
The Road and the Milky Way
11 June 2022
At night you can follow this road as it passes through the Dark Sky Alqueva reserve not too far from Alentejo, Portugal. Or you could stop, look up, and follow the Milky Way through the sky. Both stretch from horizon to horizon in this 180 degree panorama recorded on June 3. Our galaxy's name, the Milky Way, does refer to its appearance as a milky path in the sky. The word galaxy itself derives from the Greek for milk. From our fair planet the arc of the Milky Way is most easily visible on moonless nights from dark sky areas, though not quite so bright or colorful as in this image. The glowing celestial band is due to the collective light of myriad stars along the galactic plane too faint to be distinguished individually. The diffuse starlight is cut by dark swaths of the galaxy's obscuring interstellar dust clouds. Standing above the Milky Way arc near the top of this panoramic nightscape is bright star Vega, with the galaxy's central bulge near the horizon at the right.