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"
Star Trails and the Bracewell Radio Sundial
13 July 2018
Sundials use the location of a shadow to measure the Earth's rotation and indicate the time of day. So it's fitting that this sundial, at the Very Large Array Radio Telescope Observatory in New Mexico, commemorates the history of radio astronomy and radio astronomy pioneer Ronald Bracewell. The radio sundial was constructed using pieces of a solar mapping radio telescope array that Bracewell orginaly built near the Stanford University campus. Bracewell's array was used to contribute data to plan the first Moon landing, its pillars signed by visiting scientists and radio astronomers, including two Nobel prize winners. As for most sundials the shadow cast by the central gnomon follows markers that show the solar time of day, along with solstices and equinoxes. But markers on the radio sundial are also laid out according to local sidereal time. They show the position of the invisible radio shadows of three bright radio sources in Earth's sky, supernova remnant Cassiopeia A, active galaxy Cygnus A, and active galaxy Centaurus A. Sidereal time is just star time, the Earth's rotation as measured with the stars and distant galaxies. That rotation is reflected in this composited hour-long exposure. Above the Bracewell Radio Sundial, the stars trace concentric trails around the north celestial pole.