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"
Multicolor Venus
1 March 2012
Brilliant Venus now shines in western skies at twilight. Seen as the prominent evening star, the planet is a tantalizing celestial beacon even for casual skygazers. Venus can offer less than satisfying telescopic views though. The planet is shrouded in reflective clouds that appear bright but featureless at the eyepiece. Still, careful imaging with a series of color filters, as used in these composite images, can reveal subtle cloud patterns. Captured early last month from a backyard observatory in Manchester, New Hampshire, USA, the images are based on video camera frames. The data was recorded through near-ultraviolet, green, and near-infrared filters (left), and red, green, and blue filters while Venus stood high above the western horizon just before sunset. This season's evening apparition of Venus is the best one for northern hemisphere observers in 7 years. It will ultimately end with a solar transit of the planet, the last one to occur in your lifetime, on June 5/6.