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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.
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.
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.
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 3.2 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
Theory: The Emission of Radiation Pulses
1. Today, more than 1000 pulsars have been detected, most with pulses between 0.1 and 4 seconds.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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 X-rays. 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 founded the neutron stars that theory predicted
70 years ago.
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
 Tools of Astronomy: The Distance / Dispersion Relationship
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 1.3874 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.
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 space-time
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 X-ray
emissions that result from accretion.
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
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, 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.
5. In the second 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.
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.