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"
Fermi's First Light
28 August 2008
Launched on June 11 to explore the universe at extreme energies, the Gamma-ray Large Area Space Telescope has been officially renamed the Fermi Gamma-ray Space Telescope, in honor of Nobel Laureate Enrico Fermi (1901-1954), pioneer in high-energy physics. After testing, Fermi's two instruments, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT), are now regularly returning data. Fermi's first map of the gamma-ray sky from the LAT is shown in this false-color image, an all-sky view that looks toward the center of our Milky Way Galaxy with the galactic plane projected across the middle. What shines in the gamma-ray sky? Along the galactic plane, energetic cosmic rays collide with gas and dust to produce the diffuse gamma-ray glow. Strong emission from spinning neutron stars or pulsars, and distant active galaxies known as blazars, can be identified by placing your cursor over the map. A prelude to future discoveries, the remarkable result combines only 4 days of observations, equivalent to a year of observations with the Compton Gamma-ray Observatory mission of the 1990s. In addition to the ability to monitor gamma-ray bursts, the greatly improved sensitivity will allow Fermi to look deeper into the high-energy Universe.