LIFE AND DEATH OF MASSIVE STARS
GOALS To study the evolution of massive stars
1. ON THE EVOLUTION OF ANY STAR
1) Different mass stars evolve into different stages and at different rates
2) Mass controls evolution (gravity vs pressure force)
3) Interstellar cloud, protostar, main sequence, giant, variable and depending on mass, ends as white dwarf or supernova
2. STEPS THAT LEAD TO SUPERNOVA EXPLOSIONS
Show supernova animation.
1) Onion-like structures of giant stars (hydrogen, helium, nitrogen, carbon, oxygen, neon, sodium, sulfur, silicon, iron)
2) Review fusion of H:
1. fusion mostly occurs in the center
2. when H is used up the core collapses increasing T
3. He fusion stops core collapse and pushes the star further out creating the giant stage
4. two layer onion-like structure
3) Fusion up to iron:
1. Collapsing core makes gravity stronger
2. But the increased released energy from fusion due to progressively increased binding energy of the elements up to iron, produces an increased pressure force that matches gravity. Thus the star is in equilibrium.
3. The end of iron fusion (producing Cobalt) results in the lack of hydrostatic equilibrium.
4. Mr. H analogy. Elements heavier than iron correspond to weaker persons than Mr. Iron.
4) Therefore, Pressure force + force generated by the exclusion principle < gravity
5) electrons + protons -> neutrons + neutrinos
6) Neutrons obeying Pauli's exclusion principle stop further core collapse but falling material rebounds
7) Neutrinos blow off all onion-like layers of star = supernova explosion
3. FACTS ABOUT SUPERNOVA
1) The entire star of size Pluto's orbit is blown off leaving behind intact either a neutron star or a black hole.
2) Light like a galaxy
3) Material reaches LY in a few years. Ready for star recycling
4) Visible for centuries (1054AD)
5) The further out you look the more back in time you look
6)Production of the heavy elements
7) You are wearing a star
9) Supernova explosions are rather rare
4. NEUTRON STARS
1) Neutron stars = e- + p+ -> n + n
2) Town-size objects, dense like a nucleus (squeeze all people into a raindrop), strong gravitation, fast spinning
3) Pauli's exclusion principle (overcrowding is not allowed) applied to neutrons produces equilibrium
Recall from lesson 8, Pauli's exclusion principle = two similar spin-1/2 particles cannot have both the same position and the same velocity. If particles have nearly the same position, they must have different velocities, thus they will not stay in nearly the same position for long. For dense stars such as white dwarfs and neutron stars this means that the exclusion principle generates a repulsion force that can balance gravity.
5. BLACK HOLES
1) A Black hole is a dense object, so dense that from within a certain distance around it, nothing, not even light, can escape.
2) Event horizon = a spherical boundary around a black hole from within which nothing can escape, not even light
3) Why does nothing escape? Because the escape speed from within the event horizon is greater than the speed of light. It is not suggested that light is attracted by the mass of the black hole and slows down. No, rather the mass of the black hole distorts space and light moves in a warped space within the event horizon.
4) Light bending predicted by the theory of general relativity and experimentally confirmed during a solar eclipse. According to General Relativity, space is distorted by a black hole (or any mass). So, the Earth goes around the Sun, not because there is a force of gravity pulling it, as Newton thought, instead, according to Relativity, it is the distorted space around the Sun, (caused by the mass of the Sun), which pushes the Earth to move around the Sun.
5) How is a black hole created?
Due to the cosmic speed limit of light, there is a limit in the outward force arising from the exclusion principle. This is so because the maximum difference in the velocities of the matter particles in a massive star is limited to the speed of light. So for very massive stars the exclusion principle cannot generate enough repulsive force to balance their immense gravity and the star collapses into a point, having infinite density and forming a black hole. Black holes can also be created from dense regions in the early universe (primordial black holes).
6) Mass of a black hole and size of its event horizon
7) Mass, rotation, charge
8) Virtual particle and antiparticle pairs, exploding black holes and gamma radiation
9) Stephen Hawking combined general relativity and quantum mechanics and found that particles and radiation can escape Black holes (see Hawking essays page 80 or his "Brief History of time"). The basic idea is this:
The sum over histories = particles can take any path in space-time each associated with a certain probability. This quantum idea is so opposite than the more common but incorrect concept of an object having a single history (page 45, his essays). Thus it is possible for a particles to travel, at least briefly, with speed faster than light (page 121, using the uncertainty principle), thus they can escape the black hole.
10) Detecting a black hole
11) Trip into a black hole by twin brothers X and Y
A) First, X watches his twin brother Y take a trip towards the black hole.
1. Y is gravitationally attracted by the black hole and begins to accelerate as expected by Newtonian physics.
2. As Y's speed approaches c, instead of going ever faster, he appears to X to start slowing down.
3.Also, as seen by X, Y's clock slows down.
4. Y appears to stopfalling just above the event horizon and his clock seems to stop.
5. From X's point of view, Y hovers there forever and time seems to stop.
6. If Y decides to turn back before he crosses the event horizon, he discovers that he is younger than his brother X.
7. Time runs normally for both X and Y but time is not absolute. This means that, different observers, depending on their relative motion and the gravitation in their neighborhood, observe different time intervals for a particular event. That is why Y is younger than X.
8. In fact, time flows normally for each brother's own clock, i.e. one second every second.
9. Clocks in regions of strong gravity slow down relatively to clocks in regions of weak gravity. The word "relatively" is a key word: It is not suggested that in regions with strong gravity activities occur with a slower rhythm. Rather all activities are happening the way we are used to. It is only when we compare them relatively to different observers that time intervals do not last the same.
B) X decides to take a trip towards the black hole.
1. As X is approaching the black hole's event horizon, he accelerates to speeds close to c but time runs normally for him.
2.As the event horizon nears, the adventurer is ripped apart by the powerful gravitational force.
3. If he is not ripped apart, then while he crosses the event horizon, he perceives no sudden change to mark the crossing.
4. Except that all stars in the universe appear in his forward field of vision.
5. Pathways to other realities?
6. Einstein and Global Positioning System (GPS)
Clocks in satellites run faster than identical clocks on Earth because of the weaker gravity up there. But they do not run as fast because their orbital velocity slows them down a bit. These relativistic effects are large enough that if not considered that navigational errors of kilometers would result within hours due to the difference in time of the Earth and satellite clocks.
"According to Einstein’s theory, space and time are not the immutable, rigid structures of Newton’s universe, but are united as spacetime, and together they are malleable, almost rubbery. A massive body warps spacetime, the way a bowling ball warps the surface of a trampoline. A rotating body drags spacetime a tiny bit around with it, the way a mixer blade drags a thick batter around." (Will Clifford http://physics.aps.org/articles/v4/43)
All stars go through evolution. However, depending on their mass, they evolve at different rates and have different final form. Whether stars are very massive (having mass greater than ten solar masses) or not, all of them begin their life as interstellar clouds, then continue as protostars, main sequence and giant stars. But the very massive stars do not end up as white dwarfs, but as neutron stars or black holes. The time interval spent in each of the stages is shorter for the very massive stars. Furthermore, the giant stage of the very massive stars is not the same as the giant stage of one-solar-mass stars. As will be discussed below, the very massive stars, while in the giant stage, develop an onion-like structure with different atoms in each layer. Then, as the fusion supplies in the various layers near its end, the massive star will experience the most catastrophic explosions in the universe, called a supernova, resulting from an interplay between the force of gravity and the pressure force.
Due to mass loss during the evolution process, stars having up to 10 solar masses probably end up as white dwarfs. But what happens to more massive stars? We know that there are stars as massive as 150 times the Suns mass.
Nuclear Fusion of Heavy Elements; Onion-like Structures of Massive Giant Stars
In a massive star the inward pull of gravity creates the necessary conditions in its cores for the beginning of fusion of elements heavier than helium. As discussed earlier, the first element that fuses to produce energy is hydrogen. As a result helium is produced, which in turn fuses to create carbon. When most of hydrogen fused to helium, around the newly created helium core, a left-over spherical shell of hydrogen remained. This hydrogen does not fuse as rapidly as the inner-core hydrogen since the conditions of temperature are not as extreme in the outer part of the core. Due to the large mass of the star, further gravitational collapse creates the proper conditions of temperature for helium fusion. Remember, as we saw in lecture 7, despite the electric repulsive force two hydrogen nuclei experience, the high temperature in the core of a star is the necessary condition to bring them close enough in order to fuse. Now, in order that two helium nuclei fuse, because the electric repulsive force they experience from one another is greater than that between two hydrogen nuclei, the temperature which is necessary to bring them close enough for fusion, must be greater than the temperature required for hydrogen fusion. In general, this is the reason that increasingly higher temperatures are required for heavier elements to fuse, as the heavier elements contain more protons in their nuclei.
As was the case with hydrogen fusion, at the end of helium fusion, the newly created inner core of carbon, is surrounded by a left-over helium spherical shell which could not fuse at the same fast rate as the inner-core helium. The process of more core collapse, increased temperature, which in turn leads to the beginning of a new round of nuclear fusion reactions, resulting in the production of still heavier elements, and creating an onion-like structure, continues relatively smoothly until the core of the star contains iron (fig. below).
The different layers of an onion represent the different layers of elements. At the end of hydrogen fusion and as the massive star develops the onion-like structure, the star is a giant or supergiant star in the H-R diagrams. However, the layering of elements has an end when the inner core is composed of iron. As we get further from the center, we find shells of decreasing temperature in which fusion reactions involving elements of progressively lower mass are taking place. Starting from the outermost layer, the basic layers are: hydrogen, helium, nitrogen, carbon, oxygen, neon, sodium, silicon, sulfur and finally iron.
But what happens once the iron core is created? Up to the point where silicon and sulfur were fusing, a common result of each fusion has been to release tremendous amounts of energy. This released energy was serving as a balancing force of the stars enormous inward pull of gravity. So even though the stars core was collapsing, it was doing so in a relatively stable fashion, i.e., no violent catastrophic explosion resulted from the collapse. However, the element iron is the denser and most stable of all elements. From all fusion reactions, the one that generates the most energy is the fusion that leads to the formation of iron. Iron does not fuse as easily. But when it does to create heavier elements, and in turn, when these heavier elements fuse to form even heavier ones, the energy generated is not large enough in order to create the necessary outward pressure force for counterbalancing the increasing inward force of gravity. This has catastrophic implications on the very existence of the star. What happens when the fusion energy in the star is not enough to produce the necessary outwardly directed radiation pressure to balance the force of gravity? In other words, what happens the instant that there is no hydrostatic equilibrium in a star?
Collapse into a Ball of Neutrons
Let us try to answer the last question. Massive stars having 12 solar masses or more, have the most catastrophic fates through violent explosions called supernova. The main reason for such violent finishes is the unbalanced of forces created between the ever increasing inward pull of gravity, and the outward pressure force due to the fusion of the star's iron core as well as the fusion of the other elements in the various shells of its onion-like structure. Even those massive stars that their mass is not large enough to create an iron core, and their onion-like structure may consists of lighter than iron elements, may end up in a catastrophic supernova explosion, of lesser power however. Let us understand further the mechanism of a supernova explosion and consider its aftermath.
As the different layers of elements are created in massive stars, the star is in relative equilibrium because the inward pull of gravity is balanced by 1) the outward radiation pressure force from the fusion of elements in the various shells and 2) by the outward force of the degenerate electrons which we first encounter in discussing the evolution of one-solar-mass stars. But there is an important difference between massive stars and one-solar-mass stars; their mass! For one-solar-mass stars, the force from the degenerate electrons can match and therefore balance gravity. On the other hand, the tremendous pull of gravity in massive stars, in no way can be matched by the force from the degenerate electrons. Inevitably, this leads to further collapse of the stars core, confining electrons into even smaller regions of space and forcing them to enter the nuclei of the various elements and interact with them. Even though according to the Bohr model of the atom electrons can not be inside a nucleus, according to better models we have today, electrons may be found passing from the nucleus. In order to avoid violating the exclusion principle, electrons are forced inside the nuclei, and while there, they interact with the protons found there to form neutrons and neutrinos. The electrons inside the nuclei interact with the protons via the nuclear weak force, first encounter in discussing fusion.
Paulis exclusion principle that lead to the notion of degenerate electrons applies for neutrons as well. But the force from degenerate neutrons is much greater than that from the degenerate electrons and therefore further core compression is resisted. The collapsed core is now about 20 Kilometers in diameter and took only a few seconds to be created from the instant that electrons started interacting with protons to create neutrons. This neutron core is mainly composed of neutrons and is as dense as a nucleus (as dense as all the people in the world squeezed in a raindrop!). Surrounding the neutron core we still have the various shells of the onion-like giant star which is destined to be blown away in a violent supernova explosion about to occur during the first few moments (fractions of a second) of the creation of the neutron core. Because of neutron degeneracy, this dense core resists further compression suddenly halting the collapse. But as the material of the star is falling towards its center with speeds as high as one fourth of the speed of light, it rebounds violently from the relatively denser core in the center, much like a tennis ball rebounding from a relatively denser wall. On its way outwardly, this material blows off the outer layers of the entire star during the most catastrophic explosion in the universe, called supernova. The central temperature of a 20 solar-mass star as is exploding can reach higher than 200 billion degrees K. The aftermath of a supernova is a very dense and small neutron core known as a neutron star. But this is not the only possible end. If the star is very massive, with mass greater than 40 solar masses, the aftermath of the supernova will be a black hole, discussed below.
Let us understand a little further the main reason that star is been blown off. The neutrinos produced by electrons interacting with protons are in huge quantities. Even though neutrinos do not interact with ordinary mass so easily, the material inside a massive collapsing star is, one the one hand, denser than ordinary matter, and on the other hand, the neutrinos are produced in immense quantities, increasing their probability of interaction with the material of the collapsing star. In fact, it is believed that up to 90% of the supernovas energy comes from neutrinos and that it is the neutrinos on their way out of the star that blow off its different layers even while gravity is pulling everything inward. The blown off material will become an interstellar cloud of gas and dust (nebula) a few light years in size, that can remain visible for thousands of years. Still visible today is the radiant swirl of gas and dust called the Crab nebula that surrounds the remains of its progenitor, a star that was observed to have gone supernova in 1054 (fig. 1054). From this and other similar nebulae a new star or star system with planets will be created in billions of years.
One of the greatest most recent supernova explosions was the one in 1987 (fig. 1987) occurring in a neighboring galaxy to our own Milky Way called the Large Magellanic Cloud. The star that went supernova was about 160,000 light years away from us. Table 22.1 has the ultimate fate of stars with different masses.
Supernova and its relation to biological evolution
On average one supernova explosion occurs in the Milky Way galaxy every 25 to 100 years. Our Milky Way Galaxy is a spiral galaxy (figs. below) 100,000 LY in diameter and 1000 LY thick.
One light year (LY) is the distance traveled by light in one year. The Milky Way galaxy contains more than 400 billion stars. The most luminous and cataclysmic supernova explosions have as much luminosity as 10 billion Suns. That is, during the first few months to a year after a supernova explosion, the dying star may be more luminous than an entire galaxy of stars! At the time of the outburst, a supernova can eject material with velocities as high as 44 million miles per hour. This material which contains all known atoms that were built up by fusion are "recycled" (are available) to form new stars and planets. In fact the flood of fast moving protons during the explosion combine with iron and other nuclei to form all heavier than iron elements known to us. That includes gold, silver, uranium etc. A supernova explosion is the only mechanism known in nature that can produce these heavy elements. In addition, high-energy cosmic ray particles ejected during a supernova explosion, trapped by a galaxy's magnetic field and finally colliding with the atoms of the various life forms, could be part of the reason life forms experience steady mutations over the billion of years. Mutations are subtle changes in the genetic code that could be responsible for the evolution of life forms. On the other hand, if a supernova occurs relatively close (up to 100 LY) to a planet with life, these energetic particles can easily destroy any life form, as these particles enter the atoms is composed of and interact with the genetic materials. The good news is that in our own Milky Way galaxy we do not have that many massive stars very close. There is some evidence from the meteorites in our solar system that the formation of the solar system was triggered by a neighboring supernova explosion about 5 billion years ago. In other words, the original solar nebula was assisted in its initial collapse towards its center by the moving particles of a neighboring supernova explosion.
2. FACTS ABOUT NEUTRON STARS
After black holes which we study below, neutron stars are the densest objects in the universe. Their gravitational attraction at their surface is 1011 times greater than that on the Earths surface. They are town-size objects, spinning very fast. As fast as one rotation per 0.001 sec. It is the collapse from an earlier giant star that makes these tiny neutron stars to spin so fast. Even if the original star was not spinning as fast. This is due to the principle of conservation of angular momentum. The principle that makes a figure skater spin faster by "collapsing" her hands closer to her body. The degeneracy of neutrons keeps a neutron star in equilibrium by creating an outward force that balances the inward pull of gravity.
95% of a neutron's star composition is neutrons, with the rest protons and electrons. The relatively few protons and electrons still present on a neutron star turn out to be the reason for an interesting property neutron stars have. They are perfect cosmic lighthouses emitting regular pulses of electromagnetic radiation. Due to the stars fast spinning, the pulse periods range from as little as 0.001 sec up to 10 sec. How do we explain these pulses of electromagnetic radiation? Protons and electrons cannot remain on the stars surface so easily due to the strong magnetic field of the neutron star. As a result these charged particles are being launched into space with speeds close to that of light. The main launching points are the North and South magnetic poles of the neutron star. As these particles are moving away from the neutron star, are emitting various forms of electromagnetic radiation which also travel out from each magnetic pole. Those neutron stars that happen to have their rotation axis not aligned with their magnetic axis seem that they emit the electromagnetic radiation in pulses (fig. below). Much like a lighthouse. If the beam of electromagnetic radiation sweeps the Earth, we see a pulsating neutron star which is then called a pulsar. Since the charged particles emit electromagnetic radiation they loose energy and fall back to the surface of the neutron star, ready to be re-launched into space and to produce new electromagnetic energy in an endless cycle.
3. BLACK HOLES
The above is a simulation.
Below spinning black holes
The aftermath of a supernova explosion is a neutron star or a black hole. A black hole is a region of spacetime from which nothing, not even light, can escape, because gravity caused by the mass in it is so strong. Spacetime refers to the three coordinates from the three dimensions of space, and the one coordinate from the one dimension of time. Knowing the three coordinates of space, is like knowing a location in the universe. Knowing the coordinate of time, is like knowing the instant of an event in the universe. Therefore, knowing the four coordinates of spacetime, is like knowing the precise location of an event in the universe. So in the definition of a black hole, as a region of spacetime from which nothing can escape, it means that nothing can escape from within a region of space and at all times.
Even degenerate neutrons cannot balance the growing inward pull of gravity of very massive stars. In that case gravity will confine mass into such a small region of spacetime, that if we imagine launching an object from its surface, the escape velocity must be greater than the speed of light. But what does this mean? Let us elaborate on that.
Imagine a massive star with mass ten times that of the Sun. During most of its lifetime of about a billion years, the star will produce light by fusing hydrogen into helium. The energy released will create sufficient pressure to support the star against its own gravity, and also at some point at the end of a billion years, it will evolve to become a giant star having size about five times bigger than the Sun. The escape velocity from the surface of such a star would be 1000 km / sec. Firing an object vertically up with a smaller velocity would be dragged back by gravity. On the other hand, if the launching velocity is equal or greater than the escape velocity, then the object would escape far from the star, to infinity.
When the star exhausted its nuclear fuel, there would be nothing to maintain the outward pressure, and the star would begin to collapse under its own gravity, much like an old suspension bridge. As the star shrank the force of gravity at its surface becomes stronger and therefore the escape velocity must increase. When the radius of the star is about 30 km, the escape velocity will increase to 300,000 km / sec, the velocity of light. For a smaller-radius star, the escape velocity would be even higher than the speed of light. Therefore any light emitted from the star would not be able to escape to infinity because light can travel as fast as the speed of light, and not faster. Thus, light would be dragged back by gravity. According to Einsteins theory of special relativity, nothing can travel faster than light, so if light cannot escape, nothing else can either. Also, it seems that a beam of light emitted from the surface of such star cannot move in a straight line, but instead is bent in order to avoid escaping (fig. below).
The property of light bending near strong gravity regions has been predicted by Einsteins theory of general relativity, and has been experimentally observed during a solar eclipse. For example, a distant star hidden behind the Sun not directly visible by an Earth observer is made visible because its light is gravitationally bent as it passes near the Sun. Therefore it changes its path of motion and it is projected in a different position in the sky. This makes the star visible during a solar eclipse during which the Suns bright light is blocked by the Moon and a clearer view of some stars is possible (see fig. below).
The boundary of a black hole from within which nothing can ever escape, not even light, is called the event horizon. It is a spherical-shell-like boundary. Since from within the event horizon of the collapsed star, nothing can ever escape, not even light, and the object as well as all region of space up to its event horizon cannot be seen by an observer, then the term black hole seems appropriate. The radius of the event horizon is influenced by the mass of the black hole, radius = 2 G M / c1/2 where G is Newtons constant of gravity, M is the mass of the black hole and c is the speed of light. Smaller-mass black holes have smaller event horizons than larger-mass ones. Depending on the mass of the black hole, its event horizon could be light years in size. Also, smaller-mass black holes have greater temperature than larger-mass ones, as long it is assumed that the mass is distributed up to the event horizon. Smaller-mass black holes may exist scattered in the universe, formed not by the collapse of stars, but by the collapse of highly compressed regions in the hot, dense medium that is believed to have existed shortly after the Big Bang (lecture 10) in which the universe originated. Such "primordial" black holes having the mass of a mountain would have an event horizon of radius 10-15 m, the size of a single proton or neutron.
When a black hole is created by a gravitationally collapsed object (fig. 23.10) it can be characterized as a system with only three important parameters: its mass, its angular momentum (rotation) and its electric charge. No other detail of the collapsed object is preserved. For example a black hole does not care if the collapsed object is composed of matter or antimatter, of one type of atom or another, and whether it was spherically or irregular in shape. The properties of a black hole's mass are unknown.
No object is denser than a black hole. The density of a black hole may be infinite. This is so because all the mass of a black hole could sometimes be concentrated in a point! This point is the center of a black hole and is called a singularity.
Even though initially, as stated above, was believed that black holes do not seem to allow for anything to escape from within their event horizon, using laws of Quantum Physics, Stephen Hawking suggested a way that a black hole may indeed radiate electromagnetic radiation. How would this happen? Quantum Physics implies that space is filled with pairs of "virtual" particles and antiparticles that are constantly materializing in pairs, separating, and then recombining to annihilate each other. These particles are called virtual because unlike "real" particles, they cannot be observed directly with a particle detector. Their indirect effect can nonetheless be measured ("Lamp Shift"). Such pairs of particle-antiparticle are continuously created everywhere, for example, they are created just outside the event horizon. The energy they are created from, is from the black hole's gravitational energy. One of the particles may fall into the black hole but the other one may escape to infinity, carrying with it some of the black holes gravitational energy. Therefore, the result is that the black holes mass is reduced, since mass and energy are related through E = m c2. But through the years, a constantly-being-reduced-mass black hole will have an ever-decreasing event horizon, until it will correspond to a size smaller than the actual size of the surface of the black hole, up to which its mass is distributed. At the time, the black hole will be visible and in fact will make its presence in the universe by exploding and producing high energy gamma rays. In the long run every black hole in the universe will explode. However, a black hole having as much mass as the Sun will take 1066 years to do so, but a primordial black hole will explode in about 10 billion years.
Detecting a black hole may be difficult but it can be done. Usually in the universe stars come in pairs, called binaries. The stars are gravitationally bound to one another and are "dancing" around each other. If one of them evolves to become a supernova and end up as a black hole, and the other one becomes a giant, there is hope that the black hole will be detected. The detection will be possible while the giant star looses mass through stellar winds which streams toward the black hole and swirls around it before finally falling in. In the inner portions of the accretion disk, the matter is revolving so fast that internal friction heats it to very high temperatures and x-rays are emitted. The detection of a high energy x-rays from a region of space that appears dark is a sure indication of the existence of a black hole (fig. below).
TRIP INTO A BLACK HOLE
The View From an Outside Observer
Perhaps the most unsettling characteristic of a black hole is the effect it has on matter and time, at least to an observer safely distant from the black hole's event horizon. The laws of Newtonian physics dictate that an object, say an astronaut in a space shuttle, should accelerate toward the black hole until it disappears inside the event horizon. However, according to Einstein's theory of general relativity, this is not what will happen.
At first an observer safely distant from the black hole, sees the astronaut to begin to accelerate as expected by Newtonian physics. But as the astronaut's speed approaches the speed of light, he behaves not as dictated by Newtonian physics but by Einstein's theory of general relativity. That is, instead of going ever faster, the astronaut appears to the observer to start slowing down. The effect is so dramatic that the astronaut actually seems to almost stop falling just above the event horizon. From the observer's point of view, the astronaut hovers there forever and time seems to stop. In addition light signals sent by the falling astronaut back to the observer are red shifted due to gravitational effects (not Doppler effects), and as the astronaut sends his last signal while just above the event horizon, it is infinitely red shifted which means that even though the signal is moving with the speed of light it will have almost zero energy when it reaches the observer. Say the observer that stayed back was the twin brother of the astronaut that took the trip towards the black hole. If the astronaut returned back to his brother, and of course he could do that only if he never crossed the event horizon, then he will discover that he has aged much less than his brother that stayed back. This is a prediction from the theory of general relativity that requires that clocks slow down while being in regions of strong gravity, in relation to clocks that are in regions of weaker gravity.
The Observer Takes a Trip to a Black Hole
Since to the outside observer the falling astronaut never reaches its destination, the observer decides to become an adventurer and therefore takes a trip towards a black hole himself. As he is approaching the black hole's event horizon, he accelerates but time runs normally. But as the event horizon nears, spacetime is so powerfully distorted that the adventurer is ripped apart by the tidal forces created by the fact that his side closer to the black hole is experiencing much greater force than his side further from it. For the sake of the argument, let us say that the adventurer is some sort of a super human that the tidal forces would not rip him apart. Then, as the adventurer crosses the event horizon, he would perceive no sudden change to mark the crossing, except that all stars in the universe appear in his forward field of vision, a consequence of the fact that he is traveling with a speed close to the speed of light.