Page 36 - Curriculum Visions Dynamic Book

 If you were on a planet near a black hole, this is what you might see: a small central source of light, perhaps a neutron star or quasar. Surrounding it there would be a disk—called an accretion (buildup) disk—that contains material sucked in from the surrounding space. Coming out at right angles to the disk like a searchlight is a stream of electrons.
binary star A pair of stars that are gravitationally attracted, and that revolve around one another.
black hole An object that has a gravitational pull so strong that nothing can escape from it.
gravitational field The region surrounding a body in which that body’s gravitational force can be felt.
photon A particle (quantum) of electromagnetic radiation.
We cannot detect the feature because it has gravitational fields so massive it sucks in everything, and even light (photons of radiation) cannot escape it. It is known as a black hole (and also as an active galactic nucleus).
Although the term black hole suggests a tunnel rather than a physical object, a black hole is not an absence of matter. It
is the most dense form of matter imaginable, with the most extreme gravitational field in the Universe. If the Earth were to become a black hole, it would measure no more than a few millimetres across.
The gravitational field is what sucks dust, gas, and other stars toward it, and what stops light from leaving it, which is why we cannot see it.
A black hole might be 2 light-hours across (the distance from the Sun to the Earth is 8 light-minutes). It might have a mass of a hundred million or a billion Suns.
To keep such a phenomenal gravitational field intact, the black hole has to add continuously to its mass. A black hole does so by devouring gas, dust, and even stars that happen to come too close to it. The mass each black hole needs is the equivalent of several Suns every year.
Why we can “see” black holes
Although nothing flows out of a black hole, as material flows toward it, the material heats up, and it gives out light rays, X- rays, and radio waves. Black holes can also be observed by the effects of their enormous gravitational fields on nearby objects in space.
For example, if a black hole develops from a massive star that was part of a binary star (twin) system, it will begin to devour its companion star. As this happens, it will cause the companion star to heat up and send out X-rays. The binary system Cygnus X-1 is made of a blue supergiant and an invisible companion star—the black hole.
The black hole at the center of the M87 galaxy has a mass equal to two to three billion Suns, and it affects the gases near to it by causing them to swirl around, like water draining from a bath. So “radio stars” (see page 45) may be the signature of material flowing toward black holes.
36