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"
McNeil's Nebula
19 February 2004
It was a clear, cold western Kentucky night on January 23rd as seasoned amateur astronomer Jay McNeil tried out his recently acquired 3-inch refracting telescope by imaging the area around a familiar object, the M78 reflection nebula in Orion. Days later while processing the images, he noted a substantial but totally unfamiliar nebulosity in the region! With a little help from his friends, his amazing discovery is now recognized as a newly visible reflection nebula surrounding a newborn star -- McNeil's Nebula. Pictured here at the center of this close-up, McNeil's Nebula with its illuminating young star at the tip, do not appear in images of the area before September 2003. The emergence of McNeil's Nebula is a rare event to witness and astronomers are eagerly following its development, but Orion will soon lie too close to the Sun in the sky, interrupting further observations for several months. The Orion nebula complex itself is around 1,500 light-years away. At that distance, the above image spans less than 10 light-years. Update (Feb. 19): While McNeil's Nebula was not seen on previous images, some dating back to 1951, it is reportedly apparent in an image of the region recorded in the mid-1960s and available on the SEDs web site. This would indicate that the intriguing reflection nebula and illuminating star are variable, rather than "newly" emerging.