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"
Volcano of Fire Erupts Under the Stars
11 March 2015
First, there was an unusual smell. Then there was a loud bang. But what appeared to the eye was the most amazing of all. While waiting near midnight to see a possible eruption of Volc�n de Fuego (Volcano of Fire) in Guatemala last month, a ready camera captured this extraordinary image. Lava is seen running down the side of the volcano, while ash rises up, and glowing magma bubbles explode out of the caldera. Lights near the town of Escuintla can be seen in the background, one of several nearby towns that have witnessed several spectacular eruptions previously. High above, seemingly tranquil by comparison, are familiar stars from the night sky. Although the Volc�n de Fuego usually undergoes low-level activity, when the next spectacular eruption will occur is unknown. Click Through the Universe: Random APOD Generator