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"
Hubble 25th Anniversary: Pillars of Creation
7 January 2015
To celebrate 25 years (1990-2015) of exploring the Universe from low Earth orbit, the Hubble Space Telescope's cameras were used to revisit its most iconic image. The result is this sharper, wider view of the region dubbed the Pillars of Creation, first imaged by Hubble in 1995. Stars are forming deep inside the towering structures. The light-years long columns of cold gas and dust are some 6,500 light-years distant in M16, the Eagle Nebula, toward the constellation Serpens. Sculpted and eroded by the energetic ultraviolet light and powerful winds from M16's cluster of young, massive stars, the cosmic pillars themselves are destined for destruction. But the turbulent environment of star formation within M16, whose spectacular details are captured in this Hubble visible-light snapshot, is likely similar to the environment that formed our own Sun. APOD Talk: Friday, January 9 in New York City