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"
Black Holes and Neutron Stars: 218 Mergers and Counting
26 March 2026
What is the sound of two black holes merging in deep space? Sound waves don't propagate in vacuum, but gravitational waves do. In 2015 we were able to "hear" them for the first time and confirm one of Albert Einstein's theoretical predictions. Each square on the grid of the featured image represents one of the gravitational wave detections announced so far by the LIGO-VIRGO-KAGRA Collaboration. These plots show how the binary pair accelerates in their orbit around each other towards merger: the rising frequency effect is called a "chirp". Although there are significantly more neutron stars than black holes, most of the detections are binary black hole mergers. That happens because black holes are heavier and their signals are louder and can be seen farther away, resulting in more detections. These events are rare, and we don't expect to see one close by in our Galaxy any time soon. But they are happening continuously throughout the cosmos.