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"
Venus and the Pleiades
15 April 2004
Venus still rules the western skies after sunset as the brilliant evening star. While wandering the ecliptic with its fellow naked-eye planets earlier this month, it passed near the Pleiades star cluster, providing a striking photo opportunity for earthbound skygazers. Cataloged as M45, the Pleiades stars make for a lovely sight on their own, often shown in long exposure images immersed in hazy blue reflection nebulae. In this picture though, recorded on the evening of April 3rd, brilliant Venus closes with the Seven Sisters and overwhelms the light from the delicate cosmic clouds. The view offers a study in contrasts as Venus appears about 700 times brighter than Alcyone, the Pleiades brightest star. With Venus just over 5 light-minutes from Earth, Alcyone and the other Pleiades cluster stars are about 400 light-years distant. Formed out of the contracting nebula which gave birth to the Sun, Venus is also roughly 4.5 billion years old. The stars of the Pleiades are likely aged a mere hundred million years.