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"
X-Rays from the Galactic Plane
14 August 2001
In February 2000, the orbiting Chandra X-ray Observatory spent 27 hours staring into the plane of our Milky Way galaxy. Its target was a spot in the small constellation Scutum, within the Milky Way's zone of avoidance where galactic gas and dust clouds block visible light, making a poor window for optical telescopes. However the penetrating x-ray observations looked through the obscurations revealing the Milky Way and the Universe beyond. The x-ray view is reconstructed above in false color. Distant active galaxies emitting high energy x-rays appear as blue dots, while reddish dots are sources of lower energy x-rays, likely stars within the Milky Way itself. Intriguing is the diffuse blue glow of high energy x-rays, distinct from the individual sources in the picture. Astronomers have long debated whether our galactic plane's apparently extended x-ray emission was due to discrete sources or diffuse hot gas. As these results suggest diffuse interstellar gas with a temperature of tens of millions of degrees Celsius is indeed the answer, other questions arise. What heats the gas to these incredible temperatures? Why does this energetic gas linger in the galactic plane?