4U 1820−30 is one of the most unusual and well-studied X-ray binaries containing a neutron star, located approximately 20,000 light-years from Earth in the globular star cluster NGC 6624 in the direction of the constellation Sagittarius. The system consists of a neutron star and a very nearby white dwarf, which is gradually losing matter: this matter is captured by the neutron star's powerful gravity, forming an accretion disk and, as it falls toward its surface, releasing a tremendous amount of energy in the form of X-rays. The system's orbital period is only about 11.2 minutes, making it one of the closest known binary systems. The neutron star itself has a diameter of approximately 20–25 km, but contains a mass comparable to that of the Sun, giving it colossal density and gravity. Its surface gradually accumulates primarily helium, which, under enormous pressure and temperature, can suddenly begin thermonuclear combustion, causing powerful X-ray flares. During the most powerful flares, the radiation reaches the Eddington limit, and light pressure can push back the outer layers of the neutron star's atmosphere: in some observations, the photosphere expanded to more than 140 km, although the neutron star itself remains only a few dozen kilometers in size. 4U 1820−30 has also been observed to undergo so-called superbursts, lasting several hours and associated with deeper thermonuclear processes. Of particular interest is the neutron star's rotation rate: analysis of NICER observations indicates a frequency of approximately 716 revolutions per second, or approximately 43,000 revolutions per minute; if this result is definitively confirmed, 4U 1820−30 will be one of the fastest-rotating neutron stars known. With this rotation, the speed of matter at the equator reaches approximately 25% of the speed of light, or about 75,000 km/s. In other words, a point on the equator moves approximately 75 kilometers in one thousandth of a second. #neutronstar #scary #spacetok #physicstok - @holvek