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"
Aurora in Colorado Skies
30 October 2003
Vivid auroral displays were triggered by a cloud of high energy particles and magnetic fields from the Sun that collided with planet Earth's magnetosphere yesterday, October 29, at about 06:30 Universal Time. The collision was anticipated, following an intense solar flare and coronal mass ejection detected on October 28, and many anxious skywatchers were rewarded with an enjoyable light show. While aurorae don't normally haunt skies in the southern United States, they were reported from locations in Missouri, Texas, New Mexico, and California in the early morning hours. Near Yampa, Colorado astronomer Jimmy Westlake also spent early yesterday morning enjoying the stormy space weather. He was impressed by this colorful apparition of the northern lights -- produced by oxygen and nitrogen atoms excited by collisions with energetic particles from the magnetosphere and returning to lower energy states, at altitudes of 100 kilometers or more. Brighter stars shine through the extreme high-altitude glow which shows much lower clouds and the distant horizon in silhouette.