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.
26 May 2014
It is the most expensive and complex ground-based astronomy project ever -- what will it see tonight? The Atacama Large Millimeter Array (ALMA) project consists of 66 dishes, many the size of a small house, situated in the high altitude Atacama Desert in Northern Chile. Together, ALMA observes the skies in high-frequency radio light, a band usually used only for local communication due to considerable absorption by humid air. The thin atmosphere and low humidity above ALMA, however, enable it to see deep into our universe in new and unique ways that allow, for example, explorations of the early universe for chemicals involved in star formation, and observing local star systems for signs of disks that form planets. The above time-lapse video shows the course of four ALMA antennas over one night. The Moon sets early in the video, while three dishes repoint in unison. Background stars continually rotate up, the central band of our Milky Way Galaxy pivots around and eventually exits off to the right, while halfway through the Small and Large Magellanic Clouds -- satellite galaxies near our Milky Way -- rise up from below the horizon. Car headlights momentarily illuminate the dishes, while an occasional Earth-orbiting satellite crosses overhead. Daylight ends the video but not ALMA observations -- which typically run both all night and all day. Best space images: Free APOD Lecture in Paris on June 17