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 over New Zealand
26 February 2014
Sometimes the more you look at an image, the more you see. Such may be the case for this beautiful nighttime panorama taken last week in New Zealand. Visible right off, on the far left, are common clouds, slightly altered by the digital fusion of combining 11 separate 20-second exposures. More striking, perhaps, is the broad pink aurora that dominates the right part of the image, a less common auroral color that is likely tinted by excited oxygen atoms high in Earth's atmosphere. Keep looking and you might notice a bright light just beyond the mountain on the left. That is the rising Moon -- and an even closer look will reveal faint crepuscular rays emanating from it. Musing over the image center may cause you to notice the central band of the Milky Way Galaxy which here appears to divide, almost vertically, the left clouds from the right aurora. Inspecting the upper right of the image reveals a fuzzy patch, high in the sky, that is the Small Magellanic Cloud. Numerous stars discretely populate the distant background. Back on Earth, the image foreground features two domes of the Mt. John University Observatory and a camera tripod looking to capture much of this scene over a serene Lake Tekapo.