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"
Clyde W. Tombaugh: 1906-1997 Credit and Copyright:
4 February 1997
Astronomer Clyde Tombaugh, discoverer of Pluto, died on January 17th. Inspiring many during his long and exceptional career, he had been living in Las Cruces, New Mexico with his wife of 60 years, Patsy. Today would have been his 91st birthday. He is pictured above in 1995 in his backyard with a telescope he knew well - a 9 inch Newtonian reflector he built in 1927 with discarded farm machinery and car parts. Using this telescope under the dark night skies of Western Kansas, he made drawings of Mars and Jupiter and submitted them to Lowell Observatory in 1928. Hired to work at Lowell in 1929, Tombaugh embarked on a systematic photographic search for the long sought Planet X with a newly constructed 13 inch astrograph. In 1930 Tombaugh triumphed in his struggle to find the 9th planet, discovering faint and distant Pluto orbiting at the edge of our Solar System. Founding father of New Mexico State University's Astronomy Department, he retired as professor emeritus in 1973 but continued to tour as a lecturer and promoter until failing health prevented it. Always an active stargazer, he was asked by the Smithsonian if they could have the telescope he used to make his 1928 drawings. His response: "I told them I was still using it."