Download Chapter 15: The Deaths of Massive Stars

CHAPTER 15
The Deaths of Massive Stars
CHAPTER OUTLINE
15-1 Moderately Massive and Very Massive Stars (> 4 M)
1. Moderately massive stars have masses between 4 and 8 solar masses. Very massive
stars are over 8 solar masses. For stars with mass more than 4 solar masses, energy
transport occurs by convection in the inner part and by radiation in the outer part of the
star.
2. A star 15 times as massive as the Sun burns up its hydrogen in only 10 million years.
3. Such massive stars expand to become supergiants, with luminosities a million times
that of the Sun and absolute magnitudes of –10.
4. The most prominent red supergiant is Betelgeuse in Orion.
5. Because of greater core temperatures and pressures, supergiants produce heavier elements, such as neon, silicon, and iron.
15-2 Type II Supernovae
1. Type II supernovae, which result from massive stars, reveal prominent hydrogen lines.
They are powered by gravitational energy that is released as gravity continuously collapses the core.
2. The process by which Type II supernovae occur is not well known but is thought to
begin with the conversion of silicon to iron. The fusing of silicon to iron in a supergiant
star will take only a few days, which is a remarkably short period of time.
3. Because the iron fusion reaction absorbs more energy than it releases, the core shrinks,
heats up, but produces no new more massive elements. Once its mass reaches the Chandrasekhar limit, the core collapses violently. After reaching its minimum size (at about
nuclear density), the core rebounds, colliding violently with infalling material.
4. This collision between the infalling material and the rebounding core produces two effects:
(a) Enough energy is produced to fuse iron into heavier elements.
(b) Shock waves are sent outward that throw off the outer layers of the supergiant. These
shock waves may be further heated by neutrinos escaping the collapsed core.
5. Elements heavier than iron cannot be formed without some source of energy. Heavy
elements found here on Earth can be produced in two different processes: supernovae explosions and during the late stages of stellar evolution just before an old star expels its
outer layers and dies as a white dwarf.
Detecting Supernovae
1. The three naked eye supernovae that have been seen in our Galaxy in recent times occurred in 1054, 1572, and 1604.
2. The supernova of 1054 was the most spectacular and was seen by Chinese astronomers. The supernova of 1572 was seen by Tycho Brahe, and the one of 1604 was observed by Kepler. Unfortunately, both of these supernovae occurred just before the invention of the telescope.
Advancing the Model: Nucleosynthesis
1. Only hydrogen, helium, and lithium, the very lightest elements, were present in the
very early universe. Heavier elements up through iron were produced inside stars through
nuclear fusion reactions
2. Elements heavier than iron are created in a process called neutron capture where neutrons are forced into atomic nuclei before the nuclei can decay. This process occurs in
two environments:
a. The rapid process: inside a massive star during a supernovae.
b. The slow process: during a particular stage of stellar evolution just before an old star
expels its outer layers and dies as a white dwarf.
15-3 SN1987A
1. SN1987A was discovered in the Large Magellanic Cloud in 1987 by Ian Shelton.
2. It appears that the ejected shell of material was shed by a red supergiant with a mass of
about 20 times that of the Sun. The star ejected the material about 20,000 years ago, leaving a star with a hotter, bluer surface.
3. An increase in neutrinos was observed some three hours before the supernova was
seen, confirming an important part of the theory of supernovae. The neutrinos could
leave the star immediately while it took another three hours for the shock wave to reach
its surface and produce the luminosity increase.
4. Supernovae events are singular in the lives of stars. In some cases the entire star is
blown apart, including the core. In other cases, the core is left behind; the nature of this
core depends on the mass of the original star.
Advancing the Model: Supernovae from Moderately Los Mass Stars?
1. Some models indicate that a red giant of 4 solar masses could become a supernova.
2. At the point where it is fusing helium into carbon in a shell around the core and dropping that carbon onto the core, the core mass could reach the Chandrasekhar limit. At
that point the core undergoes violent carbon fusion.
15-4 Neutron Stars
Theory: Collapse of a Massive Star
1. A hypothesis worked out in the 1930s predicted that after the mass of a star’s core increased beyond the Chandrasekhar limit, the star will collapse further and its electrons
and protons will combine to form neutrons.
2. A neutron star is a star that has collapsed to the point at which it is supported by neutron degeneracy.
3. The diameter of a typical neutron star is only 0.2% of the diameter of a white dwarf
and the neutron star is a billion times denser.
4. Neutron stars have masses between 1.4 and about 3 solar masses. The mass of a typical
neutron star is 1.5 solar masses, its diameter is 20 km (width of small city), its density is
1015 g/cm3, and its temperature is 10,000,000 K.
Observation: The Discovery of Pulsars
1. In 1967 Jocelyn Bell discovered an unknown source of rapidly pulsating radio waves.
Subsequent discoveries of similar sources gave rise to the name pulsar.
2. A pulsar is a pulsating radio source with a regular period (between a millisecond and a
few seconds) believed to be associated with a rapidly rotating neutron star.
3. The first few pulsars discovered had pulse duration of about 0.001 second. Objects that
emit pulsing signals with such duration cannot have a diameter any greater than 0.001
light-seconds, which is 300 kilometers.
4. Such a small size ruled out white dwarfs (Earth-sized objects), leaving the hypothesized neutron star as the explanation for pulsars.
15-5 The Lighthouse Model of Neutron Stars/Pulsars
1. Today, nearly 2000 pulsars have been detected, most with pulses between 0.1 and 4
seconds.
Theory: The Emission of Radiation Pulses
1. The short duration of the pulses implied that vibrations (either of white dwarfs or neutron stars) are eliminated as a possible source. Similarly, the idea of eclipsing binaries
was eliminated due to the very short periods.
2. The likely mechanism for producing these pulses is by radiation that comes from a
small part of the surface of a rotating object. According to the lighthouse model, pulsar
behavior is explained as being due to a spinning neutron star whose beam of radiation we
see as it sweeps by.
3. A pulsar’s extremely strong magnetic field causes two opposite directed beams of radiation to be emitted along the magnetic axis of the pulsar. If the magnetic axis were not
aligned with the rotational axis, then the pulsar’s rapid rotation causes the beams to
sweep through space in a very short period.
4. If the Earth were located in the path of a beam, we would see a pulse of radio waves
each time the beam sweeps by us.
5. The observed small pulse duration implies that the angular size of the beams is small
and that we would see only a small percentage of the pulsars that exist.
Observation: The Crab Pulsar and Others
1. A few months after the discovery of the first pulsar, a pulsar was discovered in the
Crab Nebula with a period of 0.033 second (i.e., it blinks 30 times a second).
2. The Crab pulsar emits radiation in all parts of the spectrum, from radio waves to Xrays. Its total energy output is more than 100,000 times that of the Sun.
3. The source of the Crab pulsar’s energy and that of the surrounding nebula is the rotational energy of the pulsar. This was implied by the observation that the Crab pulsar is
slowing down. As the magnetic field of the pulsar propels electrons out into the nebula,
the electrons slow down the pulsar.
4. This hypothesis was confirmed quantitatively by comparing the amount of rotational
energy lost as the object slows down with the amount of energy emitted by the nebula.
5. The Crab pulsar spins so rapidly because it formed relatively recently. Over time it will
lose rotational energy, slow down, and emit less energy.
6. The final fate of a pulsar depends on its neighborhood. A pulsar in a binary system
may be spun up by transfer of matter from its companion.
7. Neutron stars in binary systems give rise to two other exotic phenomena.
(a) Infalling material may be funneled by the neutron star’s strong magnetic field onto the
magnetic poles creating hot X-ray regions, which are observed as pulses of X-rays due to
the star’s rotation.
(b) If the infalling material gets distributed evenly, fusion reactions may start that result
in a layer of helium under a layer of infalling hydrogen. When the conditions are such
that helium fusion reactions begin, the surface gets very hot and a sudden burst of X-rays
is emitted that lasts for a few seconds. This is an explosive thermonuclear reaction similar
to a nova.
8. Astronomers are now confident that they have found the neutron stars that theory predicted 70 years ago.
Advancing the Model: The Pulsar in SN1987A?
1. Astronomers were not surprised that a pulsar had not been visible at the center of the
supernova because debris from the explosion would block our view of it.
2. Two years after the supernova, astronomers in Chile detected pulses in visible and infrared light from the center, indicating a pulsar.
3. Later observations by the same astronomers as well as others could not find the pulsar
again to confirm the discovery.
4. The explanation for the pulses was found. An old video camera at the observatory was
left on during the observation, and would sometimes cause and electrical pulse with the
same frequency as the astronomers observed.
Tools of Astronomy: The Distance/Dispersion Relationship
1. Pulsars provide another method to measure distances to stellar objects.
2. Space is not a perfect vacuum, and longer wavelengths travel at slightly lower group
speeds than shorter wavelengths. The difference in speed is called dispersion.
3. For example, the visible-light portion of a pulse reaches us before the radio portion,
although at any wavelength the pulse length is the same.
4. The amount of dispersion depends on the distance the pulse travels and the properties
of the interstellar medium. If any two quantities can be measured; dispersion, distance,
and the medium properties, the third can be determined.
15-6 Moderately Massive Stars — Conclusion
1. We know that neutron stars have masses more than 1.4 solar masses but we can only
estimate that the upper limit is between 2 and 4 solar masses.
2. The evolution of a moderately massive star takes it through the following steps: protostar, main sequence star, red giant, supernova, remnant nebula/pulsar.
15-7 General Relativity
1. The special theory of relativity predicts the observed behavior of matter due to its
speed relative to the person who makes the observation.
2. The general theory of relativity expands special relativity to accelerated systems and
presents an alternative way of explaining the phenomenon of gravitation.
3. The principle of equivalence holds that the effects of acceleration are indistinguishable from gravitational effects.
4. If the principle of equivalence is valid, light should bend in the presence of a massive
object. This bending of light was confirmed during the 1919 total eclipse of the Sun.
5. General relativity has survived every test to which it has been put.
A Binary Pulsar
1. A binary pulsar was discovered in 1974. Because of the short period of revolution
(7.75 hours) of the system, it could be determined that the pulsar’s orbit precesses at the
rate of 4.2 degrees per year.
2. Observations suggest that both objects are neutron stars. The pulsar has a mass of
1.441 solar masses and its companion’s mass is 1.387 solar masses.
3. The binary pulsar system has been perfect for studying predictions of general relativity; it was the first instance of the use of general relativity to calculate a stellar property,
and the first precise determination of the mass of a neutron star.
4. This system indirectly confirms general relativity’s prediction that the two stars will
spiral closer together. The observations are in excellent agreement with the prediction
making it hard to doubt the existence of gravitational waves (i.e., the ripples in the curvature of space produced by changes in the distribution of matter).
15-8 The Fate of Very Massive Stars
1. Very massive stars differ from moderately massive stars primarily in what happens to
them when their core is compressed to a density greater than electron degeneracy can
support. In a moderately massive star, the resulting supernova leaves a neutron star. In a
very massive star, the core becomes a black hole.
Black Holes
1. Neutron degeneracy cannot support a neutron star whose mass is greater than about 3
solar masses. Such a star will collapse and become a black hole.
2. The Schwarzschild radius (RS) is the radius of a sphere around a black hole (of mass
M) from within which no light can escape.
3. The size of the Schwarzschild radius depends only on the mass of the star: RS = 3M
(where RS is in kilometers and M in solar masses).
4. A black hole is an object whose escape velocity exceeds the speed of light and therefore whose radius is less than the Schwarzschild radius.
5. An event horizon is the surface of the sphere around a black hole from which nothing
can escape. Its radius is the Schwarzschild radius.
6. As far as we know, once a star collapses inside its event horizon, nothing can stop the
star from collapsing to a single point of infinite density. This point, at the center of the
black hole, is called the singularity.
7. At the singularity, time and space do not exist as separate entities. Random things can
happen here but do not affect the world outside the event horizon.
Properties of Black Holes
1. A black hole can be described by three numbers: mass, electric charge, angular momentum. Whatever the properties of the material that formed the black hole, that information is forever removed from the universe.
2. The mass of a black hole can be measured using Kepler’s third law. The electric charge
of a black hole can also be measured, but is not considered in discussing black holes
since they quickly become neutral through accretion.
3. Black holes are thought to spin very rapidly. A spinning black hole would drag spacetime around with it. As a result, light passing near a rotating black hole on one side behaves differently than light passing by the other side. This allows us to measure the black
hole’s angular momentum.
Detecting Black Holes
1. In a binary system of a black hole and a red giant (or supergiant), material will be
pulled from the giant and will swirl around the black hole, causing X-rays to be released
from the heated material in the disk. This is one way to detect a black hole.
2. If an X-ray source is associated with a massive star, then we can hope it is a black hole.
Cygnus X-1 was the first candidate for a black hole.
3. Black hole candidates have been observed to have masses that span the entire spectrum
of allowed masses, from just above the minimum 3 solar masses or so to billions of solar
masses at the cores of galaxies.
4. We can distinguish between black hole candidates and neutron stars by studying the Xray emissions that result from accretion.
Advancing the Model: Black Holes in Science, Science Fiction, and Nonsense
Nonsense
1. There is a common misconception that a black hole is a giant vacuum cleaner. In fact,
the gravitational force of a black hole is unusually great only near the black hole.
2. Newton’s law of gravity states that the force of gravitational attraction between two objects depends only on the masses of the two objects and the distance between them, and not
on the size of the object.
3. The difference between the Sun and solar mass black hole is that you can get closer to
the center of the star while remaining outside its surface.
Predictions from Science
1. Stephen Hawking predicted that mini black holes may have formed at the beginning of
the universe. If those ever existed they would have long since evaporated by radiation.
2. An object falling into a black hole would be pulled apart by tidal forces.
3. Einstein’s theories of relativity tell us that if we watched an object fall into a black
hole, it would get redder and redder (Doppler shift-like) and the distortion of time would
cause its fall to take forever. As time is reckoned on the object, however, it would fall
into the hole very quickly.
Science Fiction
1. Writers have speculated that matter falling into a black hole may appear at another
place or another time. If it indeed occurs, we should see “white holes” where matter and
energy are appearing out of nowhere. No such phenomenon has been observed.
2. Even more speculative is the idea that the matter may come out in a parallel universe.
Because by definition we have no contact with such a universe, we have no way of verifying such speculation.
15-9 Our Relatives—The Stars
1. Astronomers divide stars into three classes: Population I, Population II, and Population
III stars. The three populations are distinguished by the amount of heavy elements they
contain.
2. The Population III class includes the very first generation of stars in the universe; they
formed using only hydrogen and helium.
3. Population II stars contain very little material in their atmospheres other than hydrogen
and helium. They are old stars. Heavy elements do exist in their cores as byproducts of
fusion reactions.
4. Population I stars contain heavier elements in their atmospheres. They are young stars.
5. The Sun is a Population I star. We know this from its spectrum but also from the fact
that the Earth contains heavier elements, and the Earth formed from the same material
that made the Sun.
Advancing the Model: Gamma-Ray Bursts (GRBs)
1. Gamma-Ray bursts (GRBs) were discovered by accident in the late 1960s. In 1991, the
Compton Gamma Ray Observatory showed that the GRBs are distributed isotropically.
2. GRBs last from 0.01 second to tens of minutes, and a few GRBs are observed each
day. For a short time period, some GRBs become the brightest objects in the gamma-ray
sky.
3. The first observation that confirmed that GRBs are very distant and therefore very energetic was made by Beppo-SAX in 1997.
4. In one scenario, the core of a very massive star (> 30 solar masses) collapses to form a
black hole surrounded by a disk; a short time later the system produces two jets of high-
speed particles that shoot out. The interaction of the jets with the expanding surrounding
supernova material results in gamma and X-rays.
5. In the second scenario, GRBs are thought to be produced during the merger of a neutron star and a black hole, or a pair of neutron stars or black holes.
6. In either scenario, a burst of gamma rays signals the birth of a new black hole.
7. Observations so far support the idea that long bursts originate from explosions of very
massive stars, while short bursts result from mergers.