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"
Lunar Occultation of Venus
2 June 2022
On May 27 Venus rose as the morning star, near the waning crescent Moon in a predawn sky already full of planets. It was close on the sky to the Moon's crescent and a conjunction of the second and third brightest celestial beacons was enjoyed by skygazers around the world. But seen from locations along a track through southeast Asia and the Indian Ocean the Moon actually passed in front of Venus in a lunar occultation. In this animated gif the 75 percent illuminated disk of Venus approaches and just begins to disappear behind the sunlit southwestern lunar limb. The telescopic frames used to construct it were captured from Reunion Island in the Indian Ocean around 4:50am local time, with the Moon and Venus very close to the eastern horizon. At the time Venus was over 180 million kilometers from Reunion Island, compared to a lunar distance of a mere 400 thousand kilometers or so. About 50 minutes later Venus emerged from behind the Moon.