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"
A Spring Sky Over Hirsau Abbey
6 May 2009
What's in the sky tonight? When strolling outside just after sunset, even if just going out to your car, a casual glance upwards can reveal a beautiful night sky also seen by many people across the Earth. To see your local version of the above image, start by facing south, and then tilt your head back. Visible nearly above you, during springtime at sunset in much of the northern hemisphere, will be the Big Dipper, part of the constellation of the Big Bear. The cup end of the Big Dipper will point to the North Star Polaris, the star around which the whole sky would seem to spin, if you could watch for hours. Polaris is at the tip of the Little Dipper, otherwise known as the constellation of the Little Bear. Depending on the time of night, other visible constellations would include Bootes, Leo, Gemini, and Auriga. The above fisheye image was taken from Germany last week. Visible around the entire image edge is the courtyard of Hirsau Abbey, once a Benedictine Monastery founded in the year 830. Moving your cursor over the image will bring up an annotated version of the above image, including the location of the planet Saturn.