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"
Comet McNaught Over Chile
27 January 2013
Comet McNaught of 2007 has been, so far, the most photogenic comet of our time. After making quite a show in the northern hemisphere in early 2007 January, the comet moved south and developed a long and unusual dust tail that dazzled southern hemisphere observers. In this image, Comet McNaught was captured above Santiago, Chile. The bright comet dominates on the left while part of its magnificent tail spreads across the entire frame. From this vantage point in the Andes Mountains, one looks up toward Comet McNaught and a magnificent sky, across at a crescent moon, and down on clouds, atmospheric haze, and the city lights. The current year -- 2013 -- holds promise to be even better for comets than 2007. In early March, Comet PANSTARRS is on track to become visible to the unaided eye, while at the end of the year Comet ISON shows possibilities that include casting a tail that spreads across the sky, breaking up, and even becoming one of the brightest comets in recorded history.