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"
Quadrantid Meteors through Orion
20 January 2020
Why are these meteor trails nearly parallel? Because they were all shed by the same space rock and so can be traced back to the same direction on the sky: the radiant of the Quadrantid Meteor Shower. This direction used to be toward the old constellation of Quadrans Muralis, hence the name Quadrantids, but when the International Astronomical Union formulated its list of modern constellations in 1922, this constellation did not make the list. Even though the meteors are now considered to originate from the recognized constellation of Bootes, the old name stuck. Regardless of the designation, every January the Earth moves through a dust stream and bits of this dust glow as meteors as they heat up in Earth's atmosphere. The featured image composite was taken on January 4 with a picturesque snowy Slovakian landscape in the foreground, and a deep-exposure sky prominently featuring the constellation Orion in the background. The red star Betelgeuse appears unusually dim -- its fading over the past few months is being tracked by astronomers. Teachers: APOD in the Classroom