| Topic: |
Science > Physics |
| User: |
"ayaz" |
| Date: |
05 Nov 2007 12:00:26 PM |
| Object: |
Supernova 1987A! |
article taken from science magazine, 1988 with insights into stellar
evolution etc...
Supernova 1987A!
NO EVENT IN NATURE IS MORE VIOLENT AND POWERFUL than the death of a
massive star in the form of a type II supernova. For the star it is
the end of a comparatively brief but brilliant life, or at least the
transition to a more exotic state. For astronomers it provides not
only spectacular fireworks but a unique testing ground for theories of
stellar evoluation and explosion (1).
Throughout history the occurrence of over 620 supernovae had been
recorded prior to February of 1987 (12 more were detected by
December). Almost all have occurred at vast distances because it is
only at such great distances that astronomers can sample a sufficient
number of galaxies to compensate for the fact that each produces no
more than a few supernovae per century. For example, our own galaxy is
believed to produce a type II supernova about once every 40 years, but
most go undetected because, bright as they are, their optical
emissions are totally obscured by dust. This is especially so for type
II supernovae because they involve stars situated very close to the
plane of the Milky Way Galaxy. Thus although several dozen supernovae
have liekly occurred in our galaxy, only five have been visible to the
naked eye in the last 1000 years. The brightest of these, SN 1006, was
as bright as the quarter moon; allegedly one could read by its light.
Of these five historical supernovae perhaps two or three were of type
II. The last supernova clearly visible to the naked eye, a type, I
occurred in 1604, 4 years before the invention of the telescope, and
was observed extensively by Johannes Kepler (a much fainter one,
marginally visible to those having good vision occurred in 1885 in a
relatively nearby galaxy, Andromeda).
Imagine then the joy and amazement of the world astronomical community
when the announcement went out on 24 February 1987 that a bright type
II supernova, clearly visible to the unaided eye, was occurring in a
galxy, "just next door" (160,000 light-years), the Large Magellanic
Cloud (LMC). Ever the optimists, astronomers refer to this spectacular
event, which was discovered by Ian Shelton at the Las Campanas
Observatory in Chile, as simply "Supernova 1987A," the first supernova
discovered in 1987. Though one in a lengthy series of supernovae that
have been detected, by virtue of its proximity and brightness, this
one has proved unique in many ways.
It is the first supernova for which an ordinary stellar progenitor has
been identified. As well shall discuss, knowing the properties of the
star that exploded gives theoreticians a great advantage in
understanding the behavior of the supernova. The supernova occurred in
an iregular galaxy, the Large Magellanic Cloud. Although not unique in
this regard, it is very rare to find a type II supernova in a galaxy
of this type. The light curve, (that is, how bright the supernova is
as a function of time), was initially fainter by a factor of 10
compared to other type II's. Perhaps the type of galaxy and type of
supernova are related--irregular galaxies produce fainter supernoave,
which makes them harder to detect. Supernova 1987A was also unique in
being observed as a neutrino source. The enormous binding energy of
the neutron star that was produced at the center of the explosion came
out almost entirely in neutrinos emitted during the first 10 seconds
of the explosion. These were detected in deep underground experiments
in both the United States and Japan. The first optical detection came
only 3 hours after the neutrinos, 1 hour after the shock wave from the
exploding core broke through the surface of the star. Such an early
detection is aain unique in the annals of supernovae and severely
constrains the models. More unique properties of 1987A include a light
curve powered at peak entirely by radioactive decay (a universal
property of type I supernovae, but unique hitherto for type II's); an
accurate determination of the iron produced by explosive
nucleosynthesis, 0.07 solar masses; the detection, in mid-August, of x-
radiation, the scatteed photons liberated by the radioactive decay
of.sup.56.Co to sup.56.Fe; the detection, in late fall 1987, of the
[gamma]-rays themselves; the observation in November of an infrared
spectrum dominated by heavy elements, seemingly produced in the
explosion; the observation beginning about 150 days after the
explosion of ultraviolet emission lines from nitrogenrich
circumstellar material believed to be ejected by the supernova
progenitor thousands of years before it exploded; and the
identification of an unusual "companion" source that, at least early
on, was about 10% as bright as the supernova itself. Finally, it
should not be overlooked that 1987A was the first (and perhaps last)
supernova to appear on the cover of Time magazine and to have its owns
TV show (an episode of NOVA, which was broadcast in October 1987).
These unique results and many more that will surely follow have come
about because, for the first time, astronomers have been able to study
a bright supernova at all wavelengths on a frequent basis. In the
southern hemisphere, radio, infrared, and optical telescopes in Chile,
Australia, New Zealand, and South Africa have monitored the supernova
on an almost daily basis. From an airplane in New Zealand the Kuiper
airborne infrared telescope has obtained spectra of the supernova.
From space the International Ultraviolet Explorer (IUE) has studied
the optical brightness and ultraivolet spectrum of the supernova; the
Japanese x-ray satellite, Gina, and instruments on the Soviet space
station, Mir, have studied the x-ray emission; and the Solar MAximum
Mission (SMMe compelmented by balloon flights in Australia and in
Antarctica have observed the [gamma]-rays. Finally from deep
underground, the only detectors in the Northern Hemisphere to see the
supernova have witnessed the neutrino burst as it propagated through
Earth from the other side. As members who participated in this global
effort we offer a personal observation. Especially during the first
few weeks following the supernova, observers and observatories of all
nations and from all continents shared data, speculations, and the
shear exhilaration of the moment. Little was held back. In the process
some mistakes were made, but were quickly subjected to test and the
errors freely admitted and corrected. It was science as its best. The
data we have now will occupy theoreticians for at least a decade, but
the memory of the shared experience will last even longer.
The Star That Exploded
As soon as an accurate location for the supernova had been determined,
astronomers (we among them) rushed to their charts to see what star
had exploded. What we found initially came as a surprise. As expected
there was a very bright star, hence a very massive star, situated on
plates taken before the supernova at just the loccation (now
determined to 0.05 of a second for arc) of the supernova. The star's
name was Sanduleak (Sk) -69 [deg.]202. That it was a massive star came
as no surprise. It is believed that only stars more than about eight
times the mass of the sun (M* = 1.989 x 10.sup.33 g) can become type
II supernovae. Lighter stars do not experience the more advanced
nuclear-burning stages: carbon, neon, oxygen, and silicon burning
required to produce an unstable iron core. The surprise was the color
of the star. It was blue, not red.
By simple blackbody radiation theory the color of a star of given
distance and brightness determines its radius. According to the theory
worked out to explain hundreds of other more distant supernovae
observed prior to 1987A the radius of the star that exploded should
have been very large, several times the distance from Earth to the sun
(150 x 10.sup.6 km). Instead, the radius of Sk -69 [deg.]202 was
determined to be only 1/10 of this, or about 50 times the radius of
our own sun (solar radius, R* = 696,000 km). Indeed, the faintness of
the early light curve compared to other type II supernovae (if
something 50 million times the luminosity of the sun can be regarded
as "faint") is now understood in tems of the smaller radius of the
progenitor star. Regardless of its starting radius the supernova must
expand to about 10.sup.9 km before light can leak out and the
supernova becomes bright at visual wavelengths. Starting with the same
amount of energy from shock-wave passage, a blue supergiant, because
it is about a factor of 10 smaller in radius than a red supergiant,
must expand by a greater factor and in doing so loses more of its
internal heat to expansion. Thus less energy is available to provide
light.
But why was the presupernova star blue? Two solutions have been
advanced (and indeed were discussed as possibilities before 1987A):
(i) that the star had lost a great deal of mass prior to exploding and
was thus almost a bare helium core, or (ii) that the different
composition of LMC, in particular the smaller abundance of heavy
elements, especially carbon and oxygen, compared to our own galaxy,
changed the evolution of the star so as to make it explode with a
smaller radius. The former hypotesis finds support in the existence of
a class of stars, Wolf-Rayet stars, that are believed to be massive
stars that have lost all their hydrogen envelope. Many such stars are
seen in the 30-Doradus region near where Sk -69 [deg.]202 exploded.
These stars are not just blue, they are ultraviolet. Could it be that
slightly less extreme mass loss could leave just a trace of envelope
on the star, enough to make it a blue supergiant? The other
explanation, that of decreased metallicity (astronomers call all
elements heavier than helium "metals"), would be in accord with the
low frequency of bright type II supernovae in irregular galaxies and
would offer the possibility of making a blue supergiant out of a star
that still retained most of its envelope. Observations of the
supernova itself, as opposed to observations of the presupernova star,
are much more consistent with a star that retained 5 to 10 M* of
envelope (2, 3). Thus the low metallicity hypothesis is currently
favored by many astronomers as the chief cause of why the star was
blue though some mass loss probably occurred as well.
A large body of theory concerning the evolution of massive stars and
the production of supernovae was already in place prior to 1987A and
many aspects of the progenitor star are now generally agreed upon.
Here we follow an evolutionary scenario presented for Sk -69 [deg.]202
by Woosley and co-workers (2). Similar descriptions of the
presupernova evolution are available (3, 4).
The star known for a time as Sk -69 [deg.]202 was born about 10
million years ago having a mass within a few solar masses of 20 M*.
For 90% of its life it was powered, as are most stars in the sky,
including the sun, by the fusion of hydrogen to form helium. Its
luminosity during this period was about 60 thousand times that of the
sun and its color, an intense blue; indeed most of the radiation of
this star (classified by astronomers as an "O" star) came out in the
ultraviolet (Fig. 1). The central temperature and density during this
period were about 40 x 10.sup.6 K and 5 g cm.sup.-3., respectively,
and the star's radius, about 6 R*. After exhausting hydrogen and
producing an almost pure helium core within its inner 6 M*, the
central regions of star contracted and heated up. Energy continued to
be generated by hydrogen fusion in a thick shell surrounding this
helium core. Meanwhile the surface layers of the star expanded while
the core contracted and got hotter and denser. When the central
temperature reached about 170 million derees (density 900 g
cm.Sup.-3.e a new series of nuclear reactions was ignited in the
center of the star as helium began to fuse to form carbon and oxygen.
During the time between central hydrogen depletion and helium ignition
the surface of the star expanded to more than 10.sup.8 km, about the
distance from Earth to the sun, and its luminosity roughly doubled to
100,000 times that of the sun. The star had become a red supergiant
(right hand side of Fig. 1).
Now helium burned for another million years producing a central core
of carbon and oxygen of about 4 M*. During this period an unknown
amount of mass was lost from the surface of the star in the form of a
stellar wind so that, after helium burning, the star was left with a
helium core (having an embedded core of carbon and oxygen) of total
mass about six times that of the sun, capped by an unknown mass of low
density envelope where hydrogen had still not burned to helium. When
the helium was exhausted in the center of the star, the process of
contraction began anew, but this time the surface of the star also
participated in the shrinking. a slight decrease in the lminosity
coming out of the core made the star incapable of continued support of
its red giant envelope so that it contracted by a factor of 10 and
once again became blue (moving to the left on Fig. 1). When the
central temperature reached 700 million degrees (150,000 g
cm.sup.-3.), carbon began to burn in a new series of reactions that
produced neon, sodium, and magnesium and powered the star for about
1000 years. The evolution of the star was becoming extremely rapid by
this time. The nuclear burning of heavy fuels that have large
electrical charges, and hence great Coulomb repulsion barriers to
overcome, requires increasingly extreme values of temperature. But at
temperatures above about 500 million degrees, just when fuel is
already running short, a new and increasingly efficient process
begains to radiate away energy. Copious high energy radiation ([gamma]-
rays) present in the plasma produces electron-positron pairs. Most of
the time these pairs just annihilate to give back the [gamma]-rays
from which they were formed. But occasionally an electron and a
positron will annihilate to produce a neutrino-antineurtino pair. The
neutrinos easily escape the star and, beginning with carbon burning,
carry away more energy than does radiation from the star's surface.
The neutrino emission rate is very temperature sensitive (scaling
approximately as its ninth power), thus the burning of heavier fuels
can power the star for an ever decreasing period. Beyond carbon
ignition the evolution in the inner few solar masses of the star
proceeds so rapidly that the envelope does not have time to readjust.
The star remains as it was, a blue supergiant of about 50 R*, and that
is the configuration in which it dies.
The core continues to evolve, however. After carbon burning, it
contracts, heats up, and undergoes a brief period of nuclear
readjustment in which neon converts to (more) oxygen and magnesium.
This process, which takes place at 1.5 billion degrees and 10.sup.7 g
cm.sup.-3 over a period of several years, releases energy and is
called "neon burning." Oxygen burning, chiefly to silicon and sulfur,
follows, again lasting several years at a temperature of 2.1 billion
degrees. By now the neutrino losses from pair annihilation have become
prodigious, amounting to 10 billion times the luminosity of the sun
and 100,000 times the (photon) luminosity of the star. One final
nuclear-burning stage remains and it is a complicated one. The most
abundant nuclei in the center of the star are now isotopes of silicon
and sulfur, chiefly sup.28.Si, sup.30.Si, sup.32.S, and sup.34.S in
comparable amounts. Direct fusion of any of these isotopes to form
nuclei of the iron group is impossible. The temprature required is so
great that the radiation bath would tear apart the silicon and sulfur
first, and indeed that is what happens. At a temperature of about 3.5
billion degrees and a density near 10.sup.8 g cm.sup.-3., in a process
that lasts only a few days, a portion of the silicon "melts" into a
sea of free helium nuclei ([alpha]-particles), neutrons, and protons
that add onto residual silicon and sulfur nuclei, ultimately producing
elements of the iron group (sup.54.Fe and sup.56.Fe are most
abundant).
When this "silicon burning" has been completed in the inner 1.4 M* of
the star, no more nuclear energy can be obtained by the rearrangement
of neutrons and protons into heavier (or lighter) species. It is the
end of the star's evolution. Gravity has not diminished, indeed, it
has only become stronger with each successive stage of contraction and
burning. Having no other source of energy to support itself, the core
does again what it has done ever since the star was born. It contracts
and heats up. Two processes accelerate this contraction; both are
important. First there is the process of electron capture. Most of the
pressure supporting the star comes from the electrons. Removing
electrons thus removes pressure. Electrons are removed by merging with
protons inside heavy iron group nuclei to produce species that are
even more neutronrich. Second is the process of photodisintegration,
the tearing down of nuclei by the high energy radiation into [alpha -
particles, neutrons, and protons. This process, which begins during
silicon burning, proceeds with increasing efficiency as the
temperature gets higher. But this is essentially undoing all the
reactions that went into building heavy elements out of hydrogen and
helium in the first place; it takes a great deal of energy. Because
energy is being spent on photodisintegration rather than in providing
heat and pressure, a slight contraction will make gravity stronger
without providing a corresponding increase in the pressure. Because of
electron capture and photodisintegration, the core now collapses very
rapidly. The composition and structure of the star at this point are
summarized in Fig. 2.
The Explosion Mechanism
Once the collapse has commenced in earnest, it continues until the
central density in the star has risen by a factor of about one
million. This takes only a few tenths of a second as a configuration
initially about the size of Earth collapses to a radius of only about
50 km. The velocity during the collapse reaches about 70,000 km
s.sup.-1 (one-fourth the speed of light!) in the outer portion of the
iron core. However, because of the weaker gravity experienced by
layers further out and because the information that the core has
collapsed must propagate outwards as a sound wave of finite speed, the
neon, carbon, and helium shells as well as the hydrogenic envelope do
not participate in this collapse. Although pressure support has
essentially disappeared in the center of the star, the outer layers
hang suspended with inadequate time to respond.
The central density rises to several times that of the atomic nucleus
(2.4 X 10.sup.14 g cm.sup.-3) at which point the nuclear force,
ordinarily attractive and responsible for holding nuclei together,
changes sign and becomes repulsive. Once this occurs the resistance to
further collapse is very great. The nuclear pressure, plus that of a
(by now) highly relativistic gas of electrons, causes the inner part
of the core to halt and spring back. The inner region that rebounds as
a unit consists of about 0.7 M*, (that is, about one-half of the
collapsing iron core). Outside, matter is falling supersonically and
continues, whereas further down the collapse has halted. As it runs
abruptly into the "brick wall" of the rebounding inner core, a shock
wave forms, a surface where matter meets matter at supersonic speed.
For a time the expansion of the inner core, plus the energy that the
infalling matter gets by bouncing off of that core, pushes the shock
out. If all goes well (unfortunately it rarely does in the computer
models of 20 M* stars), the shock continues on out, finally exiting
the collapsed core with enough energy (about 10.sup.51 erg) to eject
the rest of the star into space with high velocity. This phenomenon of
a "superelastic bounce," which runs so contrary to common intuition,
can be well demonstrated by dropping two balls, one a mushy beach
ball, the other a hard tennis ball, in contact with each other along
the vertical axis. The lower (mushy) ball rebounds a short distance
and communicates much of its energy to the upper ball that continues
to a much greater altitude than the point where the ensemble was
released.
This is called a "prompt hydrodynamical explosion" (5). When it works
the shock is out of the core and the explosion is under way in only
about 20 ms. The difficulty, however, is that the expanding shoch was
loses a great deal or energy as it beats its way upstream against the
infalling outer core. Momentum is not the problem one might imagine
because the falling material simply bounces off of the shock, changing
direction but preserving speed and energy. The problem is energy
dissipation--neutrinos lost because of the high temperature interior
to the shock and, again, photodisintegration. For every 0.1 M* that
the shock disintegrates to neutrons and protons it loses 1.7 X 10.sup.
51 erg, roughly equal to the final kinetic energy of a successful
supernova explosion like 1987A. If the shock always starts at about
the same place then its success or failure will obviously depend in a
sensitive manner upon how large the iron core is. Larger iron cores
will experience more losses due to photodisintegration and are less
likely to explode by this mechanism. It has proved difficult in
practice to cause the explosion of iron cores larger than about 1.35
M* by the unaided prompt mechanism.
If the prompt mechanism fails, as it must for some critical mass of
iron core, another means must be found to explain stellar explosion.
Otherwise, for all our effort, we have just created a big black hole.
The shock would halt its outward motion; the core would grow to
several solar masses by accretion; and then, suddenly, the core would
collapse inside its event horizon to be followed within a few hours by
the rest of the star. This might be an exciting event to some, but it
would also be dim and definitely not a supernova. In recent years a
second mechanism has been called upon (6) to avoid this dismal
prospect. In some ways a return to earlier notions (7) of the 1960s,
this mechanism draws upon the enormous energy in the neutrinos
released by the collapsing core during its first one second. At least
99% of the binding energy of the neutron star that forms, roughly 2 X
10.sup.53 to 3 X 10.sup.53 erg, comes out in neutrinos. These
neutrinos, being neutral and massless (or nearly massless) have great
penetrating power. Once they escape the core, they stream freely
through the rest of the star. But the energy in these neutrinos is 100
times that needed for a shock wave to give a powerful supernova. The
problem then is channeling some small fraction of the neutrino energy
to the proper place and at the proper time to help the shock along and
get the explosion going again. The proper place is right underneath
the shock that still exists where matter is accreting onto the dense
iron core. Here electron neutrinos and antineutrinos deposit energy as
they are captured by neutrons and protons and scatter off of
electrons. This provides heat and pressure so that, after a few tenths
of a second, the shock wave starts moving out again. Some
theoreticians have referred to this as "the pause that refreshes."
Because of its longer time scale this mechanism has also come to be
known as the "delayed explosion mechanism."
It was initially hoped that observations of SN 1987A would resolve a
controversy over which mechanism dominates in the explosion of 20 M*
stars--prompt or delayed. Unfortunately, most of the observable
properties of the supernova: velocities, spectra, light curve and so
on, are not sensitive to how the star explodes, but only the fact that
about 10.sup.51 erg is somehow deposited in the central regions of the
star. As mentioned before, delayed explosions tend to be favored if
the iron core mass exceeds 1.35 times that of the sun. Model
calculations (see discussion above) and the energy of the neutrino
signal (see discussion below) suggest that the iron core in Sk
-69[deg.]202 was slightly larger than this, perhaps 1.4 M*. On the
other hand, delayed explosion tend to have less kinetic energy ([is
less than or =] 10.sup.51 erg) than prompt ones ([is greater than or
=] 10.sup.51 erg). Thus astronomers are keen to measure the energy
associated with the expansion of SN 1987A. For the time being, such
estimates are mired in uncertainty concerning the mass of the hydrogen
envelope at the time Sk -69[deg.]202 exploeed, that is, how much mass
was lost to the stellar wind during the red supergiant stage. Smaller
envelope masses slow down the expanding helium core less and, for a
given observed velocity, imply less expansion energy. Larger enveloped
on the other hand require more energy. For now the best estimate of
the explosion energy, based upon light curve and velocity, is 1 X
10.sup.51 erg, within a factor of 2. Clearly this does not resolve the
debate on mechanism. Also one must realize that the arguments relating
explosion energy and iron core mass to mechanism are based upon
theoretical estimates of arguable precision.
However it was born, it is clear that a powerful shock did propagate
through Sk -69[deg.]202, leading to its explosion. To state the
obvious, we saw a supernova. Moreover, the very high emission
temperature observed on the first day of the supernova was
characteristic of a shock wave breaking through the surface of a star.
We also know that the core collapsed to a neutron star or black hole
because, for the first time, we saw the neutrino burst. The known
radius of Sk -69[deg.]202 and the timing between the arrival of the
neutrinos and the first optical observations are consistent with the
supersonic propagation of a signal originating at the center of the
star. Finally, as we shall discuss later, the light curve of SN 1987A
has, since the end of the first month, been powered by the decay of a
radioactive nuclide, sup.56.Co. This short-lived species could only
have been synthesized and ejected from a massive star by a strong
shock.
The Neutrino Burst
When all is said and done, the most exciting and unique aspect of SN
1987A will remain the detection of the neutrino burst that signaled
the collapse of its iron core to a neutron star. The numbers are
awesome. The luminosity of the supernova in all flavors of neutrinos
was, during the first second, about 10.sup.53 erg s.sup.-1. Adopting
for the luminous matter in galaxies a universal function (8) of [is
approx.]4 X 10.sup.41 erg s.sup.-1 Mpc.sup.-3 and a radius for the
observable universe (all that matter from which we could have received
light since the Big Bang) of about 10 billion light-years, one finds
that the luminosity of the "universe" is about 5 X 10.sup.52 erg
s.sup.-1. The suprenova exceeds this and generates all its energy in a
region less than 30 miles across. Expressed another way the sun lives
for 10 billion years radiating at 3.9 X 10.sup.33 erg s.sup.-1. Thus
the total output of the sun in its life will be about 10.sup.51 erg.
The supernova radiates 100 times this in less than one second. All the
nuclear weapons in the world, on the other hand, could only power the
sun for a few millionths of a second. Supernovae are by far the most
violent events in the universe. For comparison, an energetic quasar,
3C-273, emits only 10.sup.47 erg s.sup.-1 (one-millionth of the
supernova neutrino luminosity at peak), though it does so for a much
longer period of time.
By the time the neutrino signal, currently estimated to have been 2 X
10.sup.53 to 3 X 10.sup.53 erg, has traveled the 160,000 light-years
to Earth it has been reduced by geometry to a flux (proportional to 1/
r2) of a modest 50 billion neutrinos per square centimeter. Because
they interact with matter so weakly most of these neutrinos stream
right through Earth with no interaction. But because there are so many
of them, a sufficiently large detector might hope to snag a few.
Indeed, we estimate that one neutrino of approximately 10 million
electron volts (MeV) coming from SN 1987A was stopped in the bodies of
each of roughly one million people worldwide on 23 February 1987. This
is small, however, compared to the neutrino flux received over a
lifetime from the sun and neither is of any biological consequence
whatsoever.
More to the point, since no once felt these neutrinos, large detectors
had been set up at various points around the world, in Japan, in the
United States, in the Soviet Union, and in Italy. These detectors were
originally constructed to search for proton decay (the revised
Kamiokande II detector was to search for solar neutrinos) but were
also well instrumented for detecting neutrinos from supernovae.
Indeed, the search for neutrinos from supernovae had been one of the
secondary goals of the experiments, but, of course, no one was certain
when, or even if, a supernova would happen so nearby in our lifetimes.
But neutrinos were indeed detected on 23 February. The Kamiokande II
detector in Japan observed 11 events (9) on February 23.316 (universal
time), and the IMB (Irvine-Michigan-Brookhaven) detector, located in
Cleveland, Ohio, saw eight at the same time (10). All the neutrino
detectors are located in the Northern Hemisphere and the supernova was
in the Southern. The neutrinos detected had come through Earth and
were on their way back out into space. The 11 neutrinos detected by
Kamiokande had a mean energy of 15.4 MeV and arrived over a period of
12 seconds (Fig. 3). The eight events observed by IMB, which has a
higher energy threshold for detection (20 MeV versus 7 MeV), had a
mean energy of 32.5 MeV and arrived over an interval of 6 seconds.
Both sets of detections individually have very high statistical
significance (see inset, Fig. 3) and, taken together, it is certain
that a cosmic neutrino event was observed. Given the arrival time,
within 3 hours of the first optical record of SN 1987A, and the good
accord of the signal in neutrino energy, number of neutrinos, and
duration with that predicted beforehand for a type II supernova (11),
it is also certain that this signal came from the supernova.
Serendipitously, the near simultaneous arrival of neutrinos of quite
dissimilar energy after traveling for 160,000 years places limits on
the mass of the electron natineutrino that are better than previous
laboratory limits. In particular, the mass of the particle v.sub.e can
be no greater than 14 eV/c.sup.2 at the 90% confidence level (12).
Offsetting somewhat the triumph of theory in predicting the properties
of the neutrino burst observed by the Kamiokande and IMB detectors is
the puzzle of the signal acquired on the same day by another neutrino
detector experiment situated in Europe beneath Mt. Blanc (13). Five
events in the energy range 7 to 11 MeV were recorded in the space of 7
seconds on February 23.12, that is 4.7 hours before the Kamiokande/IMB
detection. To have such an occurrence within hours of the known onset
of the brightest supernova in four centuries is indeed suggestive of
an association. The signals from Kamiokande and IMB are so significant
and mutually confirming that there is no doubt that they saw the
supernova. Could there have been two signals that day?
Many theorist have attempted to find a way of answering this question
in the affirmative. Their models are imaginative, but generally
incredible. There are two principal difficulties. First the early
light curve, especially the observations of McNaught and the upper
limit set by Jones (see discussion below), are consistent with a shock
wave starting at the center of a blue supergiant at the time
Kamiokande and IMB saw the burst, but not with a shock wave starting
at the Mt. Blanc time (2, 3). A second neutrino signal could
conceivably have been generated later, for example by a phase
transition in the neutron star, but then the second signal would
follow, not precede Kamiokande/IMB. Second, no strong signal was
reported at IMB or Kamiokande at the time of the Mt. Blanc detection
(actually one count was seen just above threshold in the Kamiokande
detector, but the experiments themselves claim that this was a
background event; there was no increase in the subthreshold counting
rate). These other detectors are more sensitive and should have seen
the signal if Mt. Blanc did. It is our evaluation that either the
detection at Mt. Blanc was a statistical fluke, or something very
unusual happened on 23 February (minus 160,000 years) that will take
us a long time to understand.
Observations of the Light Curve
When Ian Shelton announced his discovery of SN 1987A on the night of
24 February, astronomers in the Southern Hemisphere immediately began
searching recent photographs of the Large Magellanic Cloud to
determine when exactly Sk -69[deg.]202 had begun to brighten. By good
fortune, Shelton had taken a plate of the same field the night before
his discovery, which showed that light from the supernova had first
arrived within the last 24 hours. Soon word was received that R. H.
McNaught, observing from Siding Spring, Australia, had serendipitously
recorded the supernova at a visual magnitude of 6.4 (that is, over 200
times brighter than Sk -69[deg.]202 had been when it was a blue
supergiant) only 8 hours later than Shelton's 23 February plate, which
had shown nothing unusual. As it turned out, McNaught's photograph had
been taken a mere 3 hours after the detection of the neutrino burst by
Kamiokande and IMB. Although McNaught was justifiably disappointed
that he had failed to examine his photograph in time to have been the
official "discoverer" of SN 1987A, his observation remains of
fundamental importance in documenting the extremely rapid rise of this
supernova. A second crucial observation (actually a nondetection!) of
SN 1987A during the first few hours of 23 February was made by the New
Zealand amateur astronomer A. Jones, who independently discovered of
SN 1987A the next night only a few hours after Shelton. On the night
of the 23rd, however, Jones did not notice the supernova while
scanning the appropriate region of the sky with a small telescope,
suggesting that SN 1987A was at least three times fainter than when
McNaught photographed it a scant 78 minutes later. These two early
data points in the light curve had proved invaluable in constraining
hydrodynamical models of SN 1987A, and serve to dramatically emphasize
the important role that amateurs still play in modern astronomy.
Once informed of the discovery of SN 1987A, professional astronomers
at observatories in Chile, New Zealand, Australia, and South Africa
began to intensively monitor its brightness at optical and infrared
wavelengths. After the rapid rise displayed during the first few
hours, the visual brightness of SN 1987A leveled off at a value that
was roughly a factor of 10 times less than would have been expected
for a "normal" type II supernova. Approximately 24 hours after core
collapse, the first acurate photometric measurements showed that the
temperature of the supernova had already dropped to 15,000 K (compared
with a theoretical value of over 300,000 K at the moment the shock
broke through the surface of Sk -69[deg.]202). This dramatic decrease
in temperature continued over the next week as the outer layers of Sk
-69[deg.]202 underwent rapid adiabatic expansion. Soon, SN 1987A was
one of the reddest objects in the sky visible to the naked eye. By the
20th day of observation, the temperature had dropped to a value of
approximately 5500 K, where it stayed for the next 70 days (until late
May).
As SN 1987A evolved, one of the major challenges for observers was to
measure the "bolometric" light curve, which is simply the energy
radiated over all wavelengths as a function of time. In practice, this
required that frequent brightness measurements be obtained at optical
and infrared wavelengths with ground-based telescopes, and (for the
first few days) in the ultraviolet with IUE. The resulting light curve
as calculated by groups at Cerro Tololo Inter-American Observatory in
Chile and the South African Astronomical Observatory (14) is shown in
Fig. 4. The slight difference between the two curves is not due to
observational error, but arises instead from different computational
methods and different assumptions of the amount of interstellar dust
extinction.
Figure 4 shows that over the first 7 days after core collapse, the
bolometric luminosity of SN 1987A decreased sharply (as did the
temperature) in response to the rapid expansion of the shock-heated
surface of the star. Soon thereafter the hydrogen in the envelope,
which had been ionized by the initial shock wave, began to slowly
recombine in an inward propagating wave. Since the opacity increases
steeply between the neutral and ionized zones owing to the scattering
off of free electrons, the photosphere (that is, the surface at which
the bulk of the observed radiation is emitted) closely tracks the
hydrogen recombination wave during this phase. Likewise, the observed
temperature levels off at 5000 to 7000 K, which is the value at which
hydrogen recombines at the low densities ([is approx.]10.sup.-13 g
cm.sup.-3) prevalent in the envelope. This so-called "plateau" phase
lasts until the recombination wave encounters the helium mantle, which
the model calculations indicate occurred around the 40th day after
core collapse for SN 1987A. The bolometric luminosity during this
phase slowly increased, reflecting the fact that the photosphere,
although receding in mass coordinates, was still growing in physical
size as a result of the continuing expansion of the envelope.
Were there no other source of energy besides the initial shock wave,
the bolometric luminosity of SN 1987A would have begun to suddnely
drop after about one month as the recombination wave passed through
the base of the hydrogen envelope. In fact, as shown in Fig. 4, the
luminosity continued to climb at a steady rate and did not reach a
maximum until approximately 85 days after core collapse. The question
on every astronomer's mind during this period was "What is now
powering the light curve?" Two possibilities were suggested early on:
(i) radioactivity from sup.56.Co produced in the initial explosion, or
(ii) reprocessed energy from a rapidly spinning pulsar buried at the
center. Fortunately, the radioactivity hypothesis could be easily
checked observationally since shortly after maximum, when the helium
mantle began to grow transparent, the bolometric luminosity should
start to decline exponentially at a rate dictated by the 77.1 day half-
life of sup.56.Co. Thus, in early June when the light curve of SN
1987A finally began to turn down, the theorists anxiously awaited word
from the observers. Within a few weeks, a steady decline rate in the
bolometric luminosity of 0.010 mag day.sup.-1 set in, precisely as
predicted from the decay of sup.56.Co. From this "radioactive tail,"
which has continued at the same slope through late November, it
follows that 0.07 M* of sup.56.Ni were produced in the initial
explosion (see Fig. 4). The error in this determination (20 to 30%) is
set entirely by our knowledge of the distance to the Large Magellanic
Cloud and the amount of dust extinction in the line-of-sight. (Note
the radioactive decay sequence: sup.56.Ni decays to sup.56.Co with a
half-life of 6.1 days; sup.56.Co decays to stable sup.56.Fe.)
SN 1987A was no longer underluminous by the time the bolometric light
curve had made the long, slow climb to maximum, and then settled onto
its radioactive powered tail. This observation tells us that the basic
mechanics of the explosion of Sk -69[deg.]202 (that is, the collapse
of the iron core, and the resulting outward propagation of the shock
wave) were essentially identical to those that occur in other type II
supernovae. The key difference was the fact that Sk -69[deg.]202 was a
blue instead of a red supergiant, and hence was considerably more
compact initially. This means that everything we learn from the
remaining evolution of SN 1987A, as the products of explosive
nucleosynthesis and, perhaps, the neutron star are revealed, should
apply equally well to other type II supernovae with similar mass
progenitors.
Information from the Spectrum
The first optical spectra of SN 1987A (see Fig. 5, top) revealed broad
"P Cygni" emission lines of hydrogen and helium atop a strong blue
continuum (15). The term "P Cygni" is used by observers to refer to an
emission line that is accompanied by blueshifted absorption. Such line
profiles are an unmistakable signature of gas in outflow, and are
observed not only in supernovae but also in hot stars that are
experiencing substantial mass loss (P Cygni is just such a star). The
blueshifted absorption is produced by gas in the line of sight, and
hence provides a direct measure of the outflow velocity. In the first
spectra of SN 1987A, velocities as great as 30,000 km s.sup.-1 were
deduced from the hydrogen line profiles, offering dramatic testimony
of the huge kinetic energy imparted by the shock wave.
As the photospheric temperature of SN 1987A dropped, the appearance of
the optical spectrum changed rapidly (see Fig. 5, center). The helium
lines, which arise from highly excited energy levels, disappeared
within 5 days of outburst. At the same time, absorption lines of lower
ionization species such as neutral sodium and doubly-ionized calcium,
iron, and scandium began to strengthen. Rather unexpectedly, strong
lines of singly ionized barium and strontium were also identified
(16). It is important to realize that the spectral lines at this stage
were still being formed in what had been the hydrogen envelope of the
progenitor, Sk -69[deg.]202. In the atmospheres of most stars (the
sun, for example), barium and strontium are trace elements that show
up only very weakly in the spectrum. Hence, the unusual strength of
these lines in SN 1987A suggests that the barium and strontium
abundances in the hydrogen envelope of Sk -69[deg.202 were anomalously
high. Barium and strontium are produced primarily in the helium
burning zones of massive stars by the so-called "s-process" whereby
iron nuclei are converted to heavier elements through the slow
addition of neutrons. Certainly the s-process must have operated in Sk
-69[deg.]202 before it exploded, but for barium and strontium to have
such high abundances in the hydrogen envelope strongly suggests that
material from the helium burning zone was mixed close to the surface
at some stage either by convective mixing as a read supergiant prior
to the explosion or during the supernova outburst itself.
Further evidence that Sk -69[deg.]202 was once a red supergiant has
come from ultraviolet spectra obtained with IUE (17). SN 1987A was a
strong source in the ultraviolet for only the first few days following
core collapse. However, the persistent IUE observers continued to
obtain daily spectra on the chance that the supernova might
unexpectedly brighten again. Their diligence was rewarded in mid-July
(approximately 150 days after outburst) when emission lines of
nitrogen began to be detected. The narrowness of the line profiles
showed that the emission was not coming from the supernova itself.
Instead, the origin was apparently a pre-existing shell of low
velocity nitrogen-rich material at a radius of approximately one-half
light-year that was ionized by the initial burst of ultraviolet
radiation that accompanied shock outbreak from the surface of Sk
-69[deg.]202. The high nitrogen abundance, about 30 times more
abundant compared to carbon as in the sun, implies that this gas had
initially been processed in the hydrogen burning zone, was mixed to
the surface during the red supergiant phase, and then lost as a
stellar wind.
Figure 5, bottom, shows the optical spectrum as it appeared in
September, nearly seven months after outburst. Note the striking
increase in the strengths of the emission lines relative to the
continuum as the spectrum slowly evolves from that of a star to a
nebula. As the outer layers of SN 1987A continue to expand, the heavy
elements synthesized in the explosion become visible. By November the
infrared spectrum taken by two groups using the Kuiper Airborne
Telescope (18) showed that this had happened. An emission line
spectrum, qualitatively similar in appearance to Fig. 5, bottom,
showed prominent features due not only to hydrogen, but singly ionized
iron, cobalt, and nickel. Other features were also identified and
attributed to silicon, sulfur, and the molecule carbon monoxide. The
strengths of the lines were such that they could not have been
produced by just the small abundances of these elements present in the
star since birth, but require large quantities of the heavy elements
to have been synthesized by the star, either before it exploded
(probable in the case of carbon and oxygen) or during the supernova
itself (silicon and heavier elements). Of particular interest are
features attributed to singly ionized cobalt, the strength being such
that they must reflect the large abundance of radioactive .sup.56.Co
known to have been produced in the explosion as .sup.56.Ni.
As time passes and more spectra are obtained at all wavelengths and as
more accurate calculations of the radiation transport are carried out,
it should prove possible to obtain reliable abundance estimates from
the relative emission line strengths, which can then be compared with
the model predictions. In addition, the line shapes and precise
energies can be used to map the compositional structure of the ejected
material as a function of its ejected velocity. An important issue--
has there been extensive mixing during the first year of the
explosion? Are some heavier elements moving faster than lighter ones
in contrast to the simple spherically symmetric model (Fig. 2)?
Observations at X-Ray and [gamma]-Ray Wavelengths
From the light curve it is clear that radioactivity has been produced
in the supernova; to be precise, 0.07 M.sub.* of .sup.56.Ni was
synthesized as the shock wave went through the innermost layers to be
ejected. Within a few weeks this .sup.56.Ni had all decayed to .sup.
56.Co which, owing to its longer half-life, is still present in
appreciable amounts. Each time a .sup.56.Co nucleus decays to a stable
nucleus of .sup.56.Fe it emits a number of [gamma]-rays of discrete
energy. This is because the cobalt decay leaves iron in a highly
excited (nuclear) state and, just like an atom in an excited state,
the relaxation to the ground state emits one or more photons of
specific energy. If the supernova had become rapidly transparent to
these [gamma]-rays owing to expansion, clumping, or jets, they would
have been detected early on by SMM. During the first 150 days, at
least, they were not detected because the [gamma]-rays produced deep
inside the supernova were all trapped. As they diffuse out, collisions
of the [gamma]-rays with electrons reduce their energy so that they
become first x-rays and finally optical emission. Even now, one year
later, 90% of the energy from radioactive decay is still coming out at
optical, ultraviolet, and infrared wavelengths. This is why the
bolometric light curve tracks the half-life of .sup.56.Co so well.
As the supernova expands however, a fraction, and ultimately all of
the [gamma]-rays escape unimpeded. First one sees hard x-rays, and
then [gamma]-lines of specific energy. Based upon models that fit the
optical light curve and were calculated beforehand, it was expected
(2, 19) that the hard x-radiation would reach a level detectable to
Ginga around the end of 1987. In fact x-radiation having the predicted
properties appeared several months earlier, which suggests either that
there is less matter between us and the center of the supernova than
most people thought or that the radioactive .sup.56.Co has somehow
been mixed out into the overlying ejecta (20-22). The alleged mixing
could have been due to the hydrodynamics of the shock wave that
exploded the star or might have been caused by the expansion of
the .sup.56.Ni and .sup.56.Co region owing to the energy from
radioactive decay.
Beginning in early August x-rays were detected by two experiments on
board the Russian space station Mir and by the Japanese X-ray
satellite Ginga (23). The very hard spectrum observed in both cases,
peaking around 20 keV with detectable emission extending, for the
instruments on Mir, above 100 keV (Fig. 6), is consistent with what
was expected from [gamma]-rays from .sup.56.Co decay that have been
degraded by scattering and that is almost certainly its origin (19-22,
24). Since its detection the x-ray signal has increased only about a
factor of two and may already have reached its peak. Observations in
October and November showed that the hard x-ray emission ([is greater
than or =]40 keV) was essentially unchanged since early September.
Theory predicts a roughly constant flux of hard x-rays for the first
two hundred days after detection (20). Surprisingly there is a second,
time variable component of x-ray emission in the Japanese measurements
though not yet reported at comparable sensitivity by instruments on
Mir. This component is soft (4 to 10 keV), but turned on at about the
same time as the hard signal. It does not seem a likely consequence of
radioactive decay and has been attributed instead to a shock wave
interacting with matter around the supernova (24). Why two components
attributed to distinctly different mechanisms should turn on at the
same time is a mystery as is the rapid time variation of the soft
component.
Once hard x-ray emission had been detected, the [gamma]-rays
themselves could not be far behind (19-22, 26). Beginning in fall
1987, a series of experiments, both satellite and balloon borne, began
to detect the characteristic 847 keV and 1238 keV lines that
accompany .sup.56.Co decay to .sup.56.Fe. First came SMM, which
measured (27) during the period August through October a flux for the
847 and 1238 keV lines of about 1 X 10.sup.-3 and 0.6 X 10.sup.-3
[gamma]-rays cm.sup.-2 s.sup.-3., respectively. During October through
January four balloon flights carrying [gamma]-ray detectors, three out
of Alice Springs, Australia and one in Antarctica, also detected the
supernova at about the same flux level. So far the quality of the
data, although convincing in showing that the supernova is indeed
emitting [gamma]-rays at the anticipated energies, has not been
adequate to provide detailed information on the velocity distribution
of the ejected radioactivity. Thus, scientists are eagerly looking
forward to the next round of balloon flights occurring March and April
1988 in Australia. At least a half-dozen detectors will be flown,
again out of Alice Springs, some of them having considerably greater
sensitivity than any of the previous instruments used to study the
supernova.
Such missions are often quite adventurous. As this article goes to
press, one experimental group headed by Jim Matteson and Bob Lin of
the University of California has just flown their balloon-borne
detector from Australia over Africa to a rough landing on a rooftop
just outside Rio de Janeiro, Brazil. Data analysis will commence as
soon as the instrument is retrieved from the local fire station.
The Mystery Spot
In late March, just 1 month after Sk -69[deg.]202 exploded, a team of
astronomers from the Harvard-Smithsonian Center for Astrophysics
traveled to Cerro Tololo Inter-American Observatory in Chile to obtain
high-resolution pictures. Using a complicated image reconstruction
technique called "speckle" interferometry, the team hoped to directly
observe the expansion of the supernova over the next year or so. At
this early date, it seemed rather unlikely that they would be able to
resolve anything yet, even with the speckle technique. Nevertheless,
the team ignored their prejudices and ended up making one of the most
surprising discoveries of all (28).
As expected, the images obtained of SN 1987A did not show any
measurable extension. Amazingly, however, a second object was clearly
visible to the south of the supernova at a projected separation of 18
light days. Even more astounding was the fact that this "mystery spot"
was only a factor of 12 times fainter than the supernova, or roughly
150 times brighter than Sk -69[deg.]202 had been before it exploded.
In fact, pre-outburst photographs of the region of the supernova
showed clearly that the Sk -69[deg.]202 had been the brightest star in
the field. Thus, the mystery spot was obviously something new
associated with the supernova.
The properties of the mystery spot were so unexpected that most
theorists were (for once!) at a loss to explain its appearance. If it
were gas participating in the explosion, it would have to have been
ejected at a velocity of at least 0.6 times the speed of light. On the
other hand, it seemed unlikely that the spot was a cloud of gas or
dust at rest being illuminated or ionized by radiation since it would
have been able to intercept only a tiny fraction of the total
luminosity of the supernova. Two of the more interesting suggestions
were that the spot was a fragment of the neutron star that had been
catapulted away, or that it was material being lit up by a
relativistic jet. An obvious test of some of these ideas was to see if
the spot was stationary or moving. Hence, results from the Harvard-
Smithsonian team's next attempt at speckle imaging in late May and
early June were eagerly anticipated. But as fate would have it, the
spot was no longer visible, implying that it was at least 40 times
fainter than the supernova. Further observations made in July were
equally unsuccessful. If the light is the result of a jet it would be
difficult to understand why no strong radio or x-ray emission was
observed in early April. Where too are the [gamma]-lines (see above)
that should be very bright if material were directly ejected from near
the core?
So what was the mystery spot? Unfortunately, in the absence of further
data, it is very difficult to say. Our best bet at the moment is to
look for other observed phenomena that might somehow be related to the
existence of the spot. For example, from late March to mid April 1987,
two symmetric emission features appeared in the blue and red wings of
the hydrogen lines. Was this simply a coincidence? Still more
intriguing are the optical polarization observations reported by
astronomers at the Anglo-Australian Observatory (29). These data show
that the intrinsic polarization of the continuum and emission lines of
SN 1987A was significant and variable, which necessarily implies that
the geometry of the explosion was not perfectly spherical. As it turns
out, the axis of symmetry deduced from the polarization observations
is identical to within the errors to the angle between the supernova
and the mystery spot. Perhaps this is yet another coincidence--or
perhaps it is not. In the months and years to come there may be other
clues. But for now, at least, the nature of the mystery spot remains
just that, a mystery.
The Future
In the immediate future the attention of observers will be focused
upon obtaining further information on the composition and distribution
of matter in the exploding debris. What elements were made in the
explosion and how much of each? Is the material homogeneous or has it
begun to clump? Is the supernova spherically symmetric or deformed?
Are there jets? Has there been extensive mixing of material created in
the deep interior with material further out or has the spherical
distribution of Fig. 2 been approximately retained? Further study of
the [gamma]-rays from .sup.56.Co decay will aid in answering some of
these questions. The intensity of the [gamma]-rays as a function of
time will tell how much material lies between us and the decaying
atoms. How the bolometric light curve deviates from the strict
exponential decay of its radioactive power source will give similar
information. The shapes of the [gamma]-ray lines may give a handle on
the extent to which the supernova has mixed. So too will the shapes of
emission lines observed in the infrared and optical from heavy
elements like iron, oxygen, silicon, sulfur, and calcium.
But what of the collapsed object that lies at the center? Theory and
observation both tell us that either a neutron star or black hole has
been born. There was a neutrino signal and the energy can only be
explained by gravitational collapse of the stellar core to one of
these two objects. Theory predicts that the mass of the collapsed
remnant is 1.4 M.sub.*., a value consistent with the properties of the
neutrino signal, in which case it is very likely a neutron star. Is it
a pulsar? That depends upon the magnetic field strength and rotation
rate of the neutron star. It also depends critically upon the density
of material surrounding the neutron star. Even a little bit of matter
falling back from the expanding debris could choke the pulsar
mechanism. The very good agreement of the present rate of decline of
the optical light curve with that expected from the decay of .sup.
56.Co shows that if there is any source of energy other than
radioactivity, it must have a small effect on the light curve. This
implies that if the neutron star is a pulsar with a magnetic field
similar to the one in the Crab Nebula (4 trillion gauss), it cannot be
rotating very rapidly. Otherwise it would contribute light at an
unacceptable level. Current numbers imply that a pulsar, if present,
is rotating slower than about once every 20 ms. This is still fast,
but not a millisecond pulsar as some might have expected.
To actually see pulsed emission from a central source the expanding
supernova must become transparent. For example, so long as a typical
light ray coming from the neutron star scatters along the way we must
observe at Earth a hodge podge of signals that have come along paths
of varying length. This washes out any regular pulsation that might
exist at the source. The optical depth to electron scattering remains
large for about two years following the explosion. After that, seeing
a possible pulsar depends upon the wavelength at which one observes
and the orientation of the system. If there is a pulsar it may make
its presence known by its contribution to the bolometric luminosity of
the remnant (30) long before pulsations are seen, if indeed they are
ever seen. On the other hand the radio pulsar mechanism may be shorted
out by accretion if just a trace of matter is falling back onto the
core from the ejecta. If the neutron star is an accreting x-ray
emitter it might make its presence known when the supernova has
declined in luminosity to that of the brightest of these sources,
about 10.sup.39 erg s.sup.-1 1 year from now.
Farther out, still moving at about 1/10 the speed of light, the shock
wave is bound for interstellar space. By now it has gone about one-
half trillion miles. In other supernovae in the past this shock wave,
by interacting with gas around the supernova, has generated intense
radio and x-ray emission. The present supernova is somewhat anomalous
in having a lower density in its vicinity, a property attributed to
the fact that it originated from a blue supergiant rather than a red
one. Blue supergiants have weaker stellar winds. But observations and
at least some of the theoretical models suggest that the progenitor
star, Sk -69[deg.]202, was at some point in its life, perhaps as
recently as 20,000 years ago, a red supergiant. Observations from IUE,
for example, show spectroscopic evidence for low velocity, nitrogen-
rich material surrounding the supernova. As time passes, from one year
to several decades, the blast wave should impact this circumstellar
shell giving rise to strong radio and x-ray emission.
Whatever occurs from this point on will be new and exciting. The great
beauty of this supernova is that, again owing to its proximity, we
will be able to observe it at all wavelengths for a long time to come.
Direct measurements of radioactive decay in freshly synthesized
elements, the birth of a pulsar, the evolution of a young supernova
remnant, all are likely spectacles over the next few years. But the
most important and exciting events will come unforetold as Supernova
1987A continues to be the answer to an astronomer's prayer--"Surprise
me!"
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| User: "Uncle Al" |
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| Title: Re: Supernova 1987A! |
05 Nov 2007 02:04:41 PM |
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ayaz wrote:
article taken from science magazine, 1988 with insights into stellar
evolution etc...
This is 2007, 20 years latyer. Are there any scholarly updates, git?
Supernova 1987A!
NO EVENT IN NATURE IS MORE VIOLENT AND POWERFUL than the death of a
massive star in the form of a type II supernova.
[snip]
Hypernova, gamma burstar, orbital merge of white dwarfs or neutron
stars... Learn something before you spew.
The star's
name was Sanduleak (Sk) -69 [deg.]202.
[snip]
Sonofagun, a fact!
Hey stooopid - where is the supernova 1987a remnant?
978 lines posted, and for what?
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2
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