Black Holes The mass of a neutron star cannot exceed about 3 solar masses. If a core remnant is more massive than that, nothing will stop its collapse, and it will become smaller and smaller and denser and denser. Eventually, the gravitational force is so intense that even light cannot escape. The remnant has become a black hole.

Space Travel Near Black Holes The gravitational effects of a black hole are unnoticeable outside of a few Schwarzschild radii—black holes do not “suck in” material any more than an extended mass would.

Space Travel Near Black Holes (cont’d.) Matter encountering a black hole will experience enormous tidal forces that will both heat it enough to radiate, and tear it apart… Figure 22-20. Black-Hole Heating Any matter falling into the clutches of a black hole will become severely distorted and heated. This sketch shows an imaginary planet being pulled apart by a black hole’s gravitational tides. “Spaghettification”!

+ (Perhaps) “Primordial” Mini- or Micro-Black Holes !

“Black Hole Comparison”: Link to Video, “Black Hole Comparison”: https://www.youtube.com/watch?v=QgNDao7m41M

Observational Evidence for Black Holes: “Gravitational Lensing” (from the Gravitational Deflection of Light)

Gravitational Lensing (cont’d.) Figure 22-22. Gravitational Light Deflection The gravitational bending of light around the edges of a small, massive black hole makes it impossible to observe the hole as a black dot superimposed against the bright background of its stellar companion.

Observational Evidence for Black Holes This bright star has an unseen companion that is a strong X-ray emitter called Cygnus X-1, which is thought to be a black hole! Cygnus X-1 is a very strong black-hole candidate: Its visible partner is about 25 solar masses. The system’s total mass is about 35 solar masses, so the X-ray source must be about 10 solar masses. Hot gas appears to be flowing from the visible star to an unseen companion. Short time-scale variations indicate that the source must be very small. Figure 22-23. Cygnus X-1 (a) The brightest star in this photograph is a member of a binary system whose unseen companion, called Cygnus X-1, is a leading black hole candidate. (b) An X-ray image of field of view outlined by the rectangle in part (a). Of course, X ray cannot be seen directly; X ray emitted by the Cygnus X-1 source were captured in space by an X-ray detector aboard a satellite, changed into radio signals for transmission to the ground, and changed again into electronic signals that were then viewed on a video screen, from which this picture was taken. (Harvard-Smithsonian Center for Astrophysics)

Observational Evidence for Black Holes The existence of black-hole binary partners for ordinary stars can be inferred by the effect the holes have on the star’s orbit, or by radiation from infalling matter. Figure 22-24. Black-Hole Binary Artist’s conception of a binary system containing a large, bright, visible star and an invisible, X-ray-emitting black hole. This painting is based on data obtained from detailed observations of Cygnus X-1. (L. Chaisson)

Observational Evidence for Black Holes There are several other black-hole candidates as well, with characteristics similar to those of Cygnus X-1. The centers of many galaxies contain supermassive black hole—about 1 million solar masses. Figure 22-25. Active Galaxy Many galaxies are thought to harbor massive black holes at their centers. Shown here in false color is the galaxy 3C296. Blue color shows the distribution of stars in the central elliptical galaxy; red shows huge jets of radio emission extending 500,000 light-years across. (See also the opening image of Chapter 24.) (NRAO)

Observational Evidence for Black Holes Recently, evidence for intermediate-mass black holes has been found; these are about 100 to 1000 solar masses. Their origin is not well understood. Figure 22-26. Intermediate-Mass Black Holes? X-ray observations (inset below) of the interior of the starburst galaxy M82 (at top, about 100,000 light-years across and 12 million light-years away) reveal a collection of bright sources thought to be the result of matter accreting onto intermediate-mass black holes. The black holes are probably young, have masses between 100 and 1000 times the mass of the Sun, and lie relatively far (about 600 light-years) from the center of M82. The brightest (and possibly most massive) intermediate-mass black hole candidate is marked by an arrow. (Subaru; NASA)

Space Travel Near Black Holes A probe nearing the event horizon of a black hole will be seen by observers as experiencing a dramatic redshift as it gets closer, so that time appears to be going more and more slowly as it approaches the event horizon. This is called a gravitational redshift—it is not due to motion, but to the large gravitational fields present. The probe, however, does not experience any such shifts; time would appear normal to anyone inside. Similarly, a photon escaping from the vicinity of a black hole will use up a lot of energy doing so; it cannot slow down, but its wavelength gets longer and longer…

Time Dilation near the “Event Horizon” of a Black Hole: