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"
Planet Aurora Borealis
28 January 2012
Illuminated by an eerie greenish light, this remarkable little planet is covered with ice and snow and ringed by tall pine trees. Of course, this little planet is actually planet Earth, and the surrounding stars are above the horizon near Östersund, Sweden. The pale greenish illumination is from a curtain of shimmering Aurora Borealis also known as the Northern Lights. The display was triggered when a giant solar coronal mass ejection (CME) rocked planet Earth's magnetosphere on January 24th and produced a strong geomagnetic storm. Northern hemisphere skygazers will also recognize the familiar orientation of stars at the left, including the Pleiades and Hyades star clusters and the stars of Orion. Increasing solar activity has caused recent auroral displays to be wide spread, including Aurora Australis, the Southern Lights, at high southern latitudes.