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"
Southern Cross over Chilean Volcano
25 January 2021
Have you ever seen the Southern Cross? This famous four-star icon is best seen from Earth's Southern Hemisphere. The featured image was taken last month in Chile and captures the Southern Cross just to the left of erupting Villarrica, one of the most active volcanos in our Solar System. Connecting the reddest Southern Cross star Gacrux through the brightest star Acrux points near the most southern location in the sky: the South Celestial Pole (SCP), around which all southern stars appear to spin as the Earth turns. In modern times, no bright star resides near the SCP, unlike in the north where bright Polaris now appears near the NCP. Extending the Gacrux - Acrux line still further (from about four to about seven times their angular separation) leads near the Small Magellanic Cloud, a bright satellite galaxy of our Milky Way Galaxy. The Southern Cross asterism dominates the Crux constellation, a deeper array of stars that includes four Cepheid variable stars visible to the unaided eye. Just above the volcano in the image, and looking like a dark plume, is the Coalsack Nebula, while the large red star-forming Carina Nebula is visible on the upper left. Portal Universe: Random APOD Generator