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"
NGC 300: A Cosmic Gemstone with Stars and Gas Clouds
16 July 2026
This sparkling, colorful gemstone is a spiral galaxy, NGC 300. It is one of the closest spiral galaxies to Earth, only about 6 million light-years away. But does it really look like this? Here is a more standard portrait of it. This unusual image combines the light from the stars and dust within the galaxy with the light from ionized clouds of interstellar gas shown in red (Sulphur), green (Hydrogen) and blue (Oxygen). Combining red and green light in different proportions makes yellow or orange light, most visible in the image. Light from other ionized gases is also at work in neon signs, fluorescent tubes and street lights. These massive clouds of ionized gas are typically created by young, massive stars that produce high-energy ultraviolet radiation capable of ionizing the gas. Massive stars are short-lived, compared with lighter stars like our sun, and explode as supernovas at the end of their lives. Some of the colorful clouds in the image could be hiding supernova remnants.