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"
Slightly Above Mars Pathfinder
12 September 2000
If you could have hovered above the Pathfinder mission to Mars in 1997, this is what you might have seen. Directly below you is the control tower of Sagan Memorial Station. Three dark solar arrays extend out to collect valuable energy, surrounded by light-colored deflated airbags that protected Pathfinder's instruments from directly colliding with the rocky Martian surface. The left solar panel has ramps down which Pathfinder's rolling robot Sojourner started its adventure to nearby rocks. Sojourner itself is visible inspecting a rock nicknamed Yogi at 11 o'clock. Rocks cover the Martian surface, with Twin Peaks visible on the horizon at 9 0'clock. The distant sky is mostly orange. This image is a recently released digital combination of panoramic pictures taken by Pathfinder on Mars and a picture of a Lander scale model back on Earth. The Mars Pathfinder Mission was able to collect data for three months, sending back information that has indicated a wet distant past for Mars.