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Week #9 Notes:
The Death of Stars:
Recycling
Introduction
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The more massive a star is, the
shorter its stay on the main
sequence.
The most massive stars may be there
for only a few million years.
A star like the Sun, on the other
hand, is not especially massive and
will live on the main sequence for
about ten billion years.
Since it has taken over four billion
years for humans to evolve, it is a
good thing that some stars can be
stable for such long times.
Introduction
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This week, we will first discuss what will
happen when the Sun dies: It will follow the
same path as other single lightweight stars,
stars born with up to about 10 (but possibly as
low as 8) times the mass of the Sun.
They will go through planetary nebula (see
figure) and white-dwarf stages.
Then we will discuss the death of more
massive stars, greater than about 8 or 10
times the Sun’s mass, which we can call
heavyweight stars.
They go through spectacular stages.
Some wind up in such a strange final state—a
black hole—that we devote the entire next
chapter to it.
The Death of the Sun:
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Most solitary (that is, single) stars containing less than
about 8 –10 times the Sun’s mass will have the same fate.
(Later we will see that stars in tightly bound binary
systems can end their lives in a different manner.)
Since the Sun is a typical star in this mass range, we focus
our attention on stars of one solar mass when describing
the various evolutionary stages of stars.
Red Giants
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As fusion exhausts the hydrogen
in the center of a star (after
about 10 billion years on the
main sequence, for the Sun), its
core’s internal pressure
diminishes because
temperatures are not yet
sufficiently high to fuse helium
into heavier elements.
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Gravity pulls the core in,
heating it up again.
Hydrogen begins to “burn” more
vigorously in the now hotter
shell around the core. (The
process is once again nuclear
fusion, not the chemical burning
we have on Earth.)
Red Giants:
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The new energy causes the outer layers of the star to swell by a
factor of 10 or more.
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The solar surface will be relatively cool for a star, only about 3000
K, so it will appear reddish.
Such a star is called a red giant.
Red giants appear at the upper right of temperature-luminosity
(Hertzsprung-Russell) diagrams.
The Sun will be in this stage, or on the way to it after the main
sequence, for about a billion years, only 10 per cent of its
lifetime on the main sequence.
Red giants are so luminous that we can see them at quite a
distance, and a few are among the brightest stars in the sky.
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Arcturus in Boötes and Aldebaran in Taurus are both red giants.
Red Giants:
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The contracting core eventually
becomes so hot that helium will
start fusing into carbon and oxygen
nuclei, but this stage will last only a
brief time (for a star), perhaps 100
million years.
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During this time, the star
becomes slightly cooler and
fainter.
Not being hot enough to fuse
carbon nuclei, the core once again
contracts and heats up, generating
energy and causing the surrounding
shells of helium and hydrogen to
fuse even faster than before.
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This input of energy makes the
outer layers expand again, and
the star becomes an even
larger red giant.
Planetary Nebulae:
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As the star grows still larger during the second red-giant phase, the
loosely bound outer layers can continue to drift outward until they leave
the star.
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Perhaps the outer layers escape as a shell of gas, in a relatively gentle
ejection that we can think of as a “cosmic burp.”
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Or perhaps they drift off gradually (in the form
of a “wind”), and a second round of gas
sometimes comes off at a more rapid pace.
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This second round of gas plows into the first
round, creating a visible shell (see figure).
Planetary Nebulae
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We know of about a thousand such glowing shells of gas in our
Milky Way Galaxy.
Each shell contains roughly 20 per cent of the Sun’s mass.
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They are exceptionally beautiful.
In the small telescopes of a hundred years ago, though, they
appeared as faint greenish objects, similar to the planet Uranus.
These objects were thus named planetary nebulae.
The remaining part of the star in the center is the star’s exposed
hot core, which reaches temperatures of 100,000 K and so
appears bluish.
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It is known as the “central star of a planetary nebula.”
It is on its way to becoming a white dwarf (see next section).
Planetary Nebulae
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We now know that planetary
nebulae generally look
greenish because the gas in
them emits mainly a few
strong spectral emission lines
that include greenish ones,
specifically lines of doubly
ionized oxygen (see figure).
Planetary Nebulae
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The best-known planetary nebula is the Ring Nebula
in the constellation Lyra (see figure, top).
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It is visible in even a medium-sized telescope as a tiny
apparent smoke ring in the sky.
Only photographs reveal the vivid colors.
The Dumbbell Nebula is another famous example.
The Helix Nebula (see figure, bottom) is so close to
us that it covers about half the apparent diameter in
the sky as the full moon, though it is much fainter.
Each planetary-nebula stage in the life of a Sun-like
star lasts only about 50,000 years; after that time,
the nebula spreads out and fades too much to be
seen at a distance.
Planetary Nebulae
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The Hubble Space Telescope has viewed planetary nebulae with a
resolution about 10 times better than most images from the ground, and
has revealed new glories in them.
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Its infrared camera provided views of different aspects of some of the
planetary nebulae (see figure).
To our surprise, planetary nebulae turn out to be less round than
previously thought.
The stars aren’t losing their mass symmetrically in all directions.
White Dwarfs
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Through a series of winds and planetary-nebula
ejections, all stars that are initially up to 8 (or perhaps
even 10) times the Sun’s mass manage to lose most of
their mass.
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The remaining stellar core is less than 1.4 times the Sun’s
mass. (The Sun itself will have only 0.6 of its current mass
at that time, in about 6 billion years.)
This results in a type of star called a white dwarf (see
figure).
White Dwarfs
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When its remaining mass (about 0.6 solar masses) is
compressed into a volume 100 times smaller across, which is a
million times smaller in volume, the density of matter goes up
incredibly.
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A single teaspoonful of a white dwarf would weigh about 5 tons!
A white dwarf ’s mass cannot exceed 1.4 times the Sun’s mass;
it would become unstable and either collapse or explode.
This theoretical maximum was worked out by an Indian
university student, S. Chandrasekhar (usually pronounced “chan
dra sek´ har” in the United States), en route to England in
1930. It is called the “Chandrasekhar limit.”
A major NASA spacecraft, the Chandra X-ray Observatory, is
named after him.
White Dwarfs
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Because they are so small, white dwarfs are very faint and
therefore hard to detect.
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Only a few single ones are known.
We find most of them as members of binary systems.
Even the brightest star, Sirius (the Dog Star), has a white-dwarf
companion, which is named Sirius B and sometimes called “The
Pup” (see figure).
White Dwarfs
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A very odd and interesting system of white dwarfs was discovered
with the Chandra X-ray Observatory. From the periodicity of the xray observations (see figure, left), the system is thought to
contain two white dwarfs orbiting each other so closely thatthe
orbit has a period of only five minutes (see figure, right)!
Summary of the Sun’s Evolution
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The entire “post-main-sequence” evolution
of the Sun, a representative solitary lowmass star, can be tracked in a temperatureluminosity diagram (see figure), or
Hertzsprung-Russell diagram
The Sun “moves” through the diagram, but
of course we really mean that the
combination of luminosity and surface
temperature changes with time, and that
this changing set of values is reflected by a
“trajectory” in the diagram.
Binary Stars and Novae
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Single stars evolve in a simple manner.
In particular, their main-sequence lifetime
depends primarily on their mass.
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Most stars, however, are in binary systems,
and the stars can exchange matter.
Surrounding each star is a region known as
the Roche lobe, in which its gravity
dominates over that of the other star (see
figure).
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The Roche lobes of the two stars join at a
point between them, forming a “figure-8”
shape. (Édouard Roche was a 19thcentury
French mathematician.)
Binary Stars and Novae
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Consider two main-sequence stars.
As the more massive star evolves to
the red giant phase, it fills its Roche
lobe, and gas can flow from this
“donor” star toward the lower-mass
companion (see figure, top).
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The recipient star can gain
considerable mass, and it subsequently
evolves faster than it would have as a
single star.
Note that the flowing matter forms an
accretion disk around the recipient
star because of the rotation of the
system (see figure, bottom).
Binary Stars and Novae
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If one star is already a white dwarf and the companion
(donor) fills its Roche lobe (for example, on its way to the red
giant phase), a nova can result (see figure).
For millennia, apparently new stars (novae, pronounced
“no´vee” or “no´vay,” the plural of nova) have occasionally
become visible in the sky.
Binary Stars and Novae
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Actually, however, a nova is an old star that brightens by a
factor of a hundred to a million (corresponding to 5 to 15
magnitudes) in a few days or weeks.
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It then fades over the course of weeks, months, or years.
The ejected gas may eventually become visible as an expanding
shell.
Core-Collapse Supernovae
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Stars that are more than 8 –10 times as massive as the Sun whip
through their main-sequence lifetimes at a rapid pace.
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A star containing 15 times as much mass as the Sun may take only 10
million years from the time it reaches the main sequence until it fully
uses up the hydrogen in its core.
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These prodigal stars use up their store of hydrogen very quickly.
This timescale is 1000 times faster than that of the Sun.
For these massive stars, the outer layers expand as the helium core
contracts.
The star has become so
large that we call it a
red supergiant.
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Betelgeuse, the star
that marks the shoulder
of Orion, is the bestknown example (see
figure).
Core-Collapse Supernovae
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Eventually, the core temperature reaches 100 million degrees,
and the triple-alpha process begins to transform helium into
carbon.
Some of the carbon nuclei then fuse with a helium nucleus
(alpha particle) to form oxygen.
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Each stage of fusion gives off energy.
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Finally, even iron builds up.
Layers of elements of progressively lower mass surround the
iron core, somewhat resembling the shells of an onion.
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The carbon-oxygen core of a supergiant contracts, heats up, and
begins fusing into still heavier elements.
The ashes of one set of nuclear reactions become the fuel for the
next set.
But when iron fuses into heavier elements, it takes up energy
instead of giving it off.
No new energy is released to make enough pressure to hold up
the star against the force of gravity pulling in; thus, the iron
doesn’t fuse.
Core-Collapse Supernovae
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Instead, the mass of the iron core increases as nuclear fusion of
lighter elements takes place, and its temperature increases.
Eventually the temperature becomes so high that the iron
begins to break down (disintegrate) into smaller units like
helium nuclei.
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The core can no longer counterbalance gravity, and it collapses.
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This breakdown soaks up energy and reduces the pressure.
The core’s density becomes so high that electrons are squeezed
into the nuclei.
They react with the protons there to produce neutrons and
neutrinos.
Additional neutrinos are emitted spontaneously at the
exceedingly high temperature (10 to 100 billion kelvins) of the
collapsing core.
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All of these neutrinos escape within a few seconds, carrying large
amounts of energy.
Core-Collapse Supernovae
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The collapsing core of neutrons overshoots its equilibrium size
and rebounds outward, like someone jumping on a trampoline.
The rebounding core collides with the inward-falling surrounding
layers and propels them outward, greatly assisted by the
plentiful neutrinos (only a very tiny fraction of which actually
interact with the gas).
The star explodes, achieving within one day a stupendous
optical luminosity rivaling the brightness of a billion normal
stars.
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It has become a supernova (see figure on next slide), and it will
continue to shine for several years, gradually fading away.
So much energy is available that very heavy elements, including
those heavier than iron, form in the ejected layers.
The core remains as a compact sphere of neutrons called a
neutron star.
There is even some evidence that occasionally, the neutron star
further collapses to form a black hole.
Core-Collapse Supernovae
Core-Collapse Supernovae
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Such supernovae (the
plural of supernova), known
as Type II (they show
hydrogen lines in their
spectra, unlike Type I
supernovae), mark the
violent death of heavyweight
stars that have retained at
least part of their outer layer
of hydrogen.
Since the fundamental
physical mechanism is the
collapse of the iron core,
they are also one type of
core-collapse supernova.
Observing Supernovae
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Only in the 1920s was it realized that
some of the “novae”—apparently
new stars—that had been seen in
other galaxies (see figure) were
really much more luminous than
ordinary novae seen in our own Milky
Way Galaxy.
These supernovae are very different
kinds of objects.
Whereas novae are small eruptions
involving only a tiny fraction of a
star’s mass, supernovae involve
entire stars.
A supernova may appear about as
bright as the entire galaxy it is in.
Observing Supernovae
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Unfortunately, we have seen very few supernovae in our
own Galaxy, and none since the invention of the
telescope.
The most recent ones definitely noticed were observed by
Kepler in 1604 and Tycho in 1572.
A relatively nearby supernova might appear as bright as
the full moon, and be visible night and day.
Since studies in other large galaxies show that supernovae
erupt every 30 to 50 years on the average, we appear to
be due, although a few supernovae have probably
occurred in distant, obscured parts of our Galaxy.
Observing Supernovae
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Photography of the sky has revealed some two dozen regions of
gas in our Galaxy that are supernova remnants, the gas spread
out by the explosion of a supernova (see figure, left).
The most studied supernova remnant is the Crab Nebula in the
constellation Taurus (see figures, right).
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The explosion was noticed widely in
China, Japan, and Korea in a.d. 1054;
there is still debate as to why
Europeans did not see it.
Observing Supernovae
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If we compare photographs of
the Crab taken decades apart,
we can measure the speed at
which its filaments are
expanding.
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Tracing them back shows
that they were together at
one point, at about the time
the bright “guest star” was
seen in the sky by the
observers in Asia,
confirming the
identification.
The rapid speed of expansion—
thousands of kilometers per
second—also confirms that the
Crab Nebula comes from an
explosive event.
13.2c Observing Supernovae
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The Chandra X-ray Observatory is giving us highresolution x-ray images of supernova remnants (see
figures).
The x-rays reveal exceptionally hot gas produced by the
collision of the supernova with gas surrounding it.
13.2d Supernovae and Us
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The heavy elements that are formed and thrown out by both corecollapse supernovae and white-dwarf supernovae are necessary for life
as we know it.
Directly or indirectly, supernovae are the only known source of most
heavy elements, especially those past iron (Fe) on the Periodic Table of
the Elements.
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So we humans, who depend on heavy elements for our existence, are
here because of supernovae and this process of recycling material.
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They are spread through space and are incorporated in stars and planets
that form later on.
Specifically, the Sun and our Solar System were made from the debris of
many previous generations of stars.
Think about it: The carbon in your cells, the oxygen that you breathe, the
calcium in your bones, and the iron in your blood are just a few examples of
the elements produced long ago by stars and their explosions. (Other
examples are the silver, gold, and platinum in jewelry—but these are not
vital for the existence of life!)
Thus, as the late Carl Sagan was fond of saying, “We are made of star
stuff [or stardust].”
Supernova 1987A!
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An astronomer’s delight, a supernova quite
bright but at a safe distance, appeared in 1987.
On February 24 of that year, Ian Shelton, then
of the University of Toronto, was photographing
the Large Magellanic Cloud, a small galaxy
170,000 light-years away, with a telescope in
Chile.
Fortunately, he chose to develop his
photograph that night.
Supernova 1987A!
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When he looked at the photograph still in
the darkroom, he saw a bright star where
no such star belonged (see figures).
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He had discovered the nearest supernova
to Earth seen since Kepler saw one in
1604.
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He went outside, looked up, and again saw
the star in the Large Magellanic Cloud, this
time with his naked eye.
By the next night, the news was all over
the world, and all the telescopes that could
see Supernova 1987A (the first supernova
found in 1987) were focused on it.
Some of these telescopes, as well as the
Hubble Space Telescope, continue to
observe the supernova on a regular basis
to this day.
Supernova 1987A!
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Hubble’s high resolution shows clear views of an
inner ring of material produced prior to the
supernova, slowly expanding around the supernova
(see figure, left).
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The supernova debris is in the process of meeting up
with the ring, and the collision should cause the
supernova debris and the ring to brighten substantially
over the next few years.
Already, we see many individual “hot spots”
brightening in the ring of material (see figure, below).
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Two outer rings are also visible.
Supernova 1987A!
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One exciting thing about such
a close supernova is that we
even know which star had
erupted!
Pre-explosion photographs
showed that a blue supergiant
star had been where the
supernova now is (see figure).
Supernova 1987A!
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The Chandra X-ray Observatory image (see figure) shows
hot gas, millions of kelvins, matching the optical bright
spots.
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The optical and x-ray spots result from a collision of the
shock waves with the fingers of cool gas.
Scientists expect the ring to brighten still more.
Neutron Stars
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The collapsed core in a core-collapse
supernova is a very compact object,
generally a neutron star consisting
mainly of neutrons, as already
mentioned in Section 13.2a.
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They are like a single, giant atomic
nucleus, without the protons (see
figure).
Neutron stars have measured masses of
about 1.4 Suns, but some might exist
with up to 2 or 3 solar masses;
astronomers don’t understand the limit
for neutron stars as accurately as they
know that 1.4 solar masses is the limit
for white dwarfs.
Neutron Stars
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Neutron stars are only about 20 or 30 km across (see figure); a
teaspoonful would weigh a billion tons.
At such high densities, the neutrons resist being further compressed; they
become “degenerate.”
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When an object contracts, its magnetic field is compressed.
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A pressure (“neutron-degeneracy pressure”) is created, which
counterbalances the inward force of gravity.
As the magnetic-field lines come together, the field gets stronger.
A neutron star is so much smaller than the Sun that
its magnetic field should be about a trillion times
stronger.
When neutron stars were first discussed theoretically
in the 1930s, the chances of observing one seemed
hopeless.
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But we currently can detect signs of them in several
independent and surprising ways, as we now discuss.
The Discovery of Pulsars
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Recall that the light from stars twinkles in the sky because
the stars are point-like objects, with the Earth’s
atmospheric turbulence bending the light rays.
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Similarly, point-like radio sources (radio sources that are so
small or so far away that they have no apparent length or
breadth) fluctuate in brightness on timescales of a second
because of variations in the density of electrons in
interplanetary space.
In 1967, a special radio telescope was built to study this
radio twinkling; previously, radio astronomers had mostly
ignored and blurred out the effect to study the objects
themselves.
The Discovery of Pulsars
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In 1967 Jocelyn Bell (now Jocelyn Bell Burnell)
was a graduate student working with Professor
Antony Hewish’s special radio telescope (see
figure).
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As the sky swept over the telescope, which
pointed in a fixed direction, she noticed that the
signal occasionally wavered a lot in the middle
of the night, when radio twinkling was usually
low.
Her observations eventually showed that the
position of the source of the signals remained
fixed with respect to the stars rather than
constant in terrestrial time (for example,
always occurring at exactly midnight).
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This timing implied that the phenomenon was
celestial rather than terrestrial or solar.
13.3b The Discovery of Pulsars
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Bell and Hewish found that the signal, when spread out, was a set of
regularly spaced pulses, with one pulse every 1.3373011 seconds (see
figure).
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But soon Bell located three other sources, pulsing with regular periods of
0.253065, 1.187911, and 1.2737635 seconds, respectively.
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The source was briefly called LGM, for “Little Green Men,” because such a
signal might come from an extraterrestrial civilization!
Though they could be LGM2, LGM3, and LGM4, it seemed unlikely that
extraterrestrials would have put out four such beacons at widely spaced
locations in our Galaxy.
The objects were named pulsars—to indicate that they gave out pulses
of radio waves—and announced to an astonished world.
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It was immediately
apparent that they were an
important discovery, but
what were they?
What Are Pulsars?
The Crab, Pulsars, and Supernovae
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Several months after the first pulsars had been discovered,
strong bursts of radio energy were found to be coming from the
direction of the Crab Nebula.
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The discovery of a pulsar in the Crab Nebula made the theory
that pulsars were neutron stars look more plausible, since
neutron stars should exist in supernova remnants like this one.
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Observers detected that the Crab pulsed 30 times per second,
almost ten times more rapidly than the fastest other pulsar then
known.
This very rapid pulsation definitively excluded white dwarfs from
the list of possible explanations.
And the case was clinched when it was discovered that the clock in
the Crab pulsar was not precise—it was slowing down slightly.
The energy given off as the pulsar slowed down was precisely the
amount of energy needed to keep the Crab Nebula shining.
The source of the Crab Nebula’s energy had been discovered!
The Crab, Pulsars, and Supernovae
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Astronomers soon found, to their surprise,
that an optically visible star in the center of
the Crab Nebula could be seen apparently
to turn on and off 30 times per second.
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Actually the star only appears “on” when its
beamed light is pointing toward us as it
sweeps around.
Long photographic exposures had always
hidden this fact, though the star had been
thought to be the remaining core because
of its spectrum, which oddly doesn’t show
any emission or absorption lines.
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Later, similar observations of the star’s
blinking on and off in x-rays were also found
(see figure).
The Crab, Pulsars, and Supernovae
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Even more recently, the high-resolution observations by
the Hubble Space Telescope and Chandra X-ray
Observatory of the Crab Nebula revealed interesting
structure near its core (see figure).
A Pulsar with a Planet
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Earlier in this book, in Chapters 2 and 9, we described the
discovery of planets around other stars.
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But these extra-solar planets were not the first to be discovered.
In 1991, the first ones were discovered by observing a pulsar.
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The detections were from observations of a pulsar that pulses very
rapidly—162 times each second.
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radio pulses varied slightly
(see figure), indicating that
something is orbiting the
pulsar and pulling it slightly
back and forth.
A Pulsar with a Planet
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Alex Wolszczan, now at Penn
State, has concluded that the
variations in the pulse-arrival
time are caused by three
planets in orbit around the
pulsar.
These planets are 0.19, 0.36,
and 0.47 A.U. from the pulsar,
within about the same distance
that Mercury is from the Sun.
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They revolve in 25.3-, 66.5-,
and 98.2-day periods,
respectively.
The system is 2000 light-years
from us, too faint for us to
detect optically.
A Pulsar with a Planet
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The presence of the planets was conclusively verified when they
interacted gravitationally as they passed by each other.
The two most massive planets are calculated from the
observations to be somewhat larger than Earth, each containing
about 4 times its mass.
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The existence of a fourth planet farther out in the system is
possible but uncertain.
Astronomers think that neutron stars are formed in supernova
explosions, so any original planets almost certainly didn’t
survive the explosion.
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The innermost planet is much less massive than Earth.
Most likely, the planets formed after the supernova explosion, from
a disk of material in orbit around the neutron star remnant.
These pulsar planets are not the ones on which we expect life
will have arisen!
Review of Stars: