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"
The Lynx Arc
5 November 2003
While chasing the spectrum of a mysterious arc in a cluster of galaxies within the obscure northerly constellation Lynx, astronomers have stumbled upon the most massive and distant star-forming region ever discovered. The notably red "Lynx arc" lies right of center in this color image of the galaxy cluster, a composite of Hubble Space Telescope and ground-based data. While the galaxy cluster lies about 5 billion light-years distant, spectroscopic studies show that the arc itself is actually a distorted image of an even more distant but enormous star-forming region. The image is formed as the closer galaxy cluster's gravity bends light like a magnifying lens, an effect explained by Einstein's theory of gravity. In fact, the monster star-forming region is nearly 12 billion light-years away and about a million times brighter than the more familiar stellar nursery, the Orion Nebula. Estimates are that the star-forming region seen as the Lynx arc contains about a million massive, hot stars, compared to the four stars which power the Orion Nebula's glow. Stars within the Lynx arc are more than twice as hot as the Orion Nebula's central stars and were formed when the Universe was a mere 2 billion years old. Still, astronomers believe that the first stars were formed at even earlier times.