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 Orion Nebula in Infrared from HAWK-I
18 July 2016
The deepest infrared image of the Orion Nebula has uncovered a bonanza of previously unknown low-mass stars and -- quite possibly -- free floating planets. The picturesque nebula is best known in visible light where it shows a many bright stars and bright glowing gas. Catalogued as M42, the Orion Nebula at a distance of 1300 light years is the closest major star forming region to Earth. One can peer into Orion's pervasive dust in infrared light, as was done again recently with the sophisticated HAWK-I camera attached to one of the European Southern Observatory's Very Large Telescopes in the high mountains of Chile. High resolution versions of the featured infrared deep image show many points of light, many of which are surely brown dwarf stars but some of which are best fit by an unexpectedly high abundance of free-floating planets. Understanding how these low mass objects formed is important to understanding star formation generally and may even help humanity to better understand the early years of our Solar System. Remastered version of this image: Recolored and in greater detail