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"
The First Image of an Extra Solar Planet
10 May 2005
It's the faint red object, not the bright white one that might be a historic find. The white object is surely a brown dwarf star. Quite possibly, however, the red object is the first direct image of a planet beyond our Solar System. The intriguing possibility was first reported last year, but many astronomers weren't then convinced that the "planet" was not just a background star. Earlier this year, the 2M1207 star system was imaged twice more in an effort to resolve the issue. To the delight of the scientific team, the objects kept the same separation, indicating that they are gravitationally bound. The faint red object 2M1207b is therefore 100 times fainter, intrinsically, than the bright white brown dwarf 2M1207a -- a characteristic well explained by a planet roughly five times the mass of Jupiter. The discovery - still subject to further confirmation - is considered a step toward the more ambitious goal of imaging Earth-like planets orbiting distant stars. The above image was taken with the high-resolution adaptive-optic NaCo camera attached to the 8-meter Very Large Telescope Yepun in Chile.