National Geographic : 2014 Mar
OR Hydrogen Helium Carbon Oxygen OR Neon Silicon Iron Sagittarius A* (black hole) Orbits of stars Approximate area enlarged THE MILKY WAY Our solar system THE SINGULARITY 1 1 2 2 3 3 4 4 5 5 ACCRETION DISK EVENT HORIZON STATIC LIMIT X-RAY JETS 16millionmiles 0.7 inch although their name suggests emptiness, black holes are the most densely filled objects in the universe, giving them enormous gravitational pull. Stellar black holes, formed from the collapse of giant stars, can compact the mass of ten suns to the size of new york City. Supermassive black holes at the center of galaxies can have the mass of billions of suns. their origin remains a mystery. a star’s Birth—and Death each star is a balancing act, with crushing gravity pulling inward against an interior nuclear blast furnace pushing out. When the star’s fuel is exhausted, gravity wins and the star implodes. But its fate and the elements it forges before its collapse are foretold by its original size. the more massive the star, the more violent its ending. The Power of Gravity einstein showed a century ago that the mass of stars, planets, and all other matter exerts a gravitational force, bending space like a rubber sheet. the greater the mass of an object, the more powerful the effect. the immense mass of a black hole generates a gravitational “sink” from which not even light can escape. sagittarius a* in 1974 scientists discovered a very compact source of radio waves originating from a region in the Sagit- tarius constellation, 26,000 light-years from earth. dubbed Sagittarius a* (Sgr a*), the source is now known to be a supermassive black hole at the center of our galaxy weighing more than four million suns. a whirling disk of superheated gas and dust likely spins at near light-speed around Sagit- tarius a*. the disk emits heat, radio noise, and x-ray flares but is placid compared with accre- tion disks in other galaxies. hydrogen gas clouds condense into a compact whirl. a star is born when hydrogen fuses into helium atoms. Stars greater than four to eight times the mass of our sun fuse carbon and oxygen. As heavier elements fuse at the core, lighter elements move outward. as a star grows hotter, ever heavier elements combine to fuel its fires. iron accumulates in the core. When the inert iron core becomes sufficiently massive, it collapses. the star explodes in a supernova. heavier stars collapse infinitely deep to become stellar black holes. Stars smaller than some 20 times the sun’s mass collapse into neutron stars. a light star, lacking the gravitational heft to fuse carbon and oxygen, sheds its outer layers to create a planetary nebula. its remaining mass becomes a white dwarf. according to einstein’s equations, at the center of a black hole a star’s entire mass has collapsed into an infinitely dense, dimensionless point called a singularity. Singularities likely don’t really exist but point to a mathematical hole in our understanding of gravity. the event horizon, extending some eight million miles around Sgr a*, is the boundary beyond which even light cannot escape the black hole’s gravity. though tranquil today, Sagittarius a* may have fed on a star or gas cloud a hundred times the mass of the sun as recently as 20,000 years ago. the meal produced x-ray jets blasting outward from the black hole’s poles, which are tilted 15 degrees from the plane of the galaxy. Perceiving the Invisible Black holes can’t be seen, but their loca- tion can be plotted by measuring their effect on the orbits of surrounding stars. Clouds of interstellar gas and dust are pulled in, and whole stars that venture too close are torn apart and devoured. Insatiable Force Sun White dwarf Neutron star Black hole approximate size of earth if it collapsed to a black hole; it would weigh the same as earth today. JaSon treat and alexander SteGMaier, nGM Staff art By Mark a. GarliCk SourCeS: aV ery BroderiCk, PeriMeter inStitute for theoretiCal PhySiCS, uniVerSity of Waterloo, Canada; uCla GalaCtiC Center GrouP a black hole’s spin can twist space, speeding or slowing matter orbiting nearby. the static limit is the orbit where objects traveling at light speed against the black hole’s spin seem to stand still. Venture to the event horizon and beyond on our digital editions.