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.
24 April 2021
What's happening in the sky? The pre-dawn sky first seemed relatively serene yesterday morning over Indian Harbor Beach in Florida, USA. But then it lit up with a rocket launch. Just to the north, NASA's SpaceX Crew-2 Mission blasted into space aboard a powerful Falcon 9 rocket. The featured time-lapse video -- compressing 12-minutes into 8-seconds -- shows the bright launch plume starting on the far left. The rocket rises into an increasingly thin atmosphere, causing its plume to spread out just as it is lit by the rising Sun. As the Crew-2 capsule disappears over the horizon, the landing plume of the returning first stage of the Falcon 9 descending toward the SpaceX barge in the Atlantic Ocean can be seen. Up in space, the Endeavour crew capsule is expected to dock with the International Space Station (ISS) this morning, delivering four astronauts. The Crew-2 astronauts join Expedition 65 to help conduct, among other tasks, drug tests using tissue chips -- small microfluidic chips that simulate human organs -- that run rapidly in ISS's microgravity.