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"
Worlds of a Distant Sun: 47 Ursae Majoris b
5 October 1997
In the last few years, observational astronomy has given humanity evidence of the existence of worlds beyond the solar system. Solar-type stars are now inferred to harbor planets of approximately Jupiter mass - some residing in temperature zones which could conceivably support liquid water and therefore life! Above is a hypothetical scene near one such planet whose sun, 47 Ursae Majoris (47 UMa), is very similar to our own. In our sky, 47 UMa appears as a faint, inconspicuous star below the cup of the "the Big Dipper". (Our own sun would be equally inconspicuous when viewed from 47 UMa ...) Astronomers G. Marcy and P. Butler announced the discovery of a planet associated with this star in 1996 and reported it to have a mass of about 2.4 Jupiters or more with an orbital period of 3 years. This artist's extrasolar vision pictures the detected planet, 47 UMa b, as a gas giant surrounded by a ring of material - analogous to our own gas giant Saturn. In the foreground lies a hypothetical moon of 47 UMa b. Could such a moon support life? 47 UMa is only 44 light years distant, fairly close by astronomical standards - yet there is evidence for planetary systems which are closer still. NASA plans to explore nearby planetary systems using spaceborne observatories.