in a galaxy not so far away only 25
million light years
astronomers have found what looks like
the remnants of strange
celestial explosions called hypernova
hypernova are possibly the most powerful
explosions in our universe
since the big bang said q daniel wang an
astrophysicist
at northwestern university a hypernova
is a highly energetic supernova thought
to result from an extreme
core collapse scenario in which a
massive star of more than 30 solar
masses
collapses to form a rotating black hole
emitting twin
energetic jets and surrounded by an
accretion disk
this collapse happened so quickly that
the outer parts of the star
are unaware of what has taken place and
the star is subsequently exploded
by vigorous winds of newly formed nickel
56
blowing off the accretion disk and shock
waves produced
as the jets plow through the stellar
material
the hypernova whose luminosity is
powered by the radioactive decay of
nickel 56
is the result of the explosion of the
star
curious to learn more about hypernova
and their formation
keep watching
[Applause]
this model for the formation of a
hypernova
also predicts that these objects should
be accompanied
by a gamma-ray burst or grb although the
mechanism to form the gamma rays is
still a matter of debate
it is thought that they are produced
through interstellar collisions
within the jet itself whatever the
actual mechanism
the gamma rays are beamed into a narrow
cone along the direction of motion of
the jet
and are visible to us only if the jet is
pointed
in our direction astronomers estimate
that for every grb we observe
there are several hundred more we don't
see those which are oriented in
directions away from us
conclusive evidence for this hypernova
grb connection
was obtained only recently although
there were many cases where the light
curve of the prototype hypernova
sn 1998 bw could be fitted to the light
curve decay of gamma-ray burst
optical transients associated with grbs
it was not until astronomers clearly
observed the spectrum
of a hypernova within the spectrum of an
optical transient
that the connection was firmly
established in 1998 one explosion
occurred that revolutionized our
understanding of the stellar calamity
that gives rise to a grb in a nearby
spiral galaxy sn 1998 bw appeared
it bore many unusual features for
membership
of the type ic fraternity although
clearly belonging to the class of
supernova deficient in hydrogen and
helium
this explosion showed very broad
emission features and unusually strong
radio emission both characteristics of
material
moving at a sizeable fraction of the
speed of light
essentially simultaneously grb 980425
irradiated bepo sax triggering the hunt
for an afterglow
measurements of the emission from the
grb suggested that
at most 10 to the power -5 solar masses
of material was ejected in the jets
additionally the peak gamma-ray emission
was four orders of magnitude less than
many of the previously observed
cosmological bursts overall this was a
fairly
wimpy gamma-ray burst meanwhile sn
bw displayed a number of peculiarities
that led some to quickly suggest
that this explosion might be more than
just spatially coincident
with the grb they might be causally
related
in terms of its peak luminosity sn1998bw
would be regarded as rather
underwhelming by today's standards
clocking in with a peak absolute
magnitude of minus 19.5
but at the time this was many times more
luminous
than the majority of observed type ic
events
however given that this modestly high
luminosity was driven solely by matter
produced and
ejected in the explosion itself the
implication was that the explosion
was unusually energetic two pieces of
evidence were important here
the first was the high ejective
velocities while the second
was the above average peak luminosity
together these implied
that 0.5 to 0.7
solar masses of radioactive nickel had
been synthesized
10 times that seen in other core
collapse supernova
and more in line with thermonuclear type
ia events
finally the strong radio emission and
broad emission lines in the spectra
implied particularly high velocities in
at least
a sizable fraction of the ejected
material
in short sn 1998 bw was unusually
energetic
but why on their own these features in
the supernova would have sent minds
scurrying in a search of answers however
it was nearly the coincident location of
the supernova
to a gamma-ray burst seen days earlier
that caught the most attention
sn 1998 bw lay within the error box of
grb
980425 suggesting that the two events
were related
a year earlier sn 1997 cy
also shared a spatial location with
another grb
however this supernova was poorly
constrained temporarily by observations
nor was the spatial coincidence of the
gamma-ray bursts
particularly good indeed there were two
possible bursts associated with this
supernova
at different locations in the sky
without clear
observations sn 1997 cy
lost out to sn 1998 bw
as the first supernova candidate to be
linked to a long gamma-ray burst
although there certainly wasn't a cast
iron case linking the two events
the peculiar kinematics of the supernova
would readily be explained
if the grb had occurred in the star
which then somehow triggered the
supernova
various papers documenting the grb and
supernova
nervously linked the two events but
perhaps with understandable reticence
they avoided claiming that one event
directly led to the other
however careful scrutiny of the
explosion revealed that the architecture
of the supernova
matched predictions of stan wolseley's
collapse our model
although not directly observed the
progenitor was clearly a compact wr
star as the spectrum displayed neither
hydrogen
nor helium so had the elusive supernova
grb connection been bagged
confirming wool's least conjecture at
the time a somewhat oxymoronic
definition
perhaps would have been the response
nowadays most researchers would stake
their reputations on it
however it would fall to later grb
discoveries to make astrophysicists
confident to claim the link was
confirmed
at the time sn 1998 bw was documented
the term hypernova
which was coined by kochi iwamoto to
describe the apparent
additional oomph in the explosion
initial estimates place the energy in
the expanding shock wave at 10 to the
power
45 joules which is an order of magnitude
above a typical core collapse event
and far in excess of most of the type ic
supernova
that had previously been observed
although such feats
might just be feasible in the
environment of a supermassive black hole
there was a significant question as to
whether
an individual star could create so much
energy
in such a short interval of time thus
even at this stage the idea of a
hypernova
was questioned even at this point the
unprecedented and difficult to explain
energies
could best be accounted for by beaming
of energy across
our line of sight however what remains
spectacular
even in this more bashful scenario are
the velocities of a proportion
of the debris launching matter and up to
99
of light speed the term hypernova stuck
in many astronomers minds
and soundly resonated within the mass
media
however names are like ghosts and have a
nasty habit of coming back to haunt you
as more data has streamed in and the grb
supernova have
lost their top spot for sheer unbridled
energy
the term hypernova has seemed well
overstated
moreover the term hypernova has been
previously applied
to all sorts of unusually bright
supernova from hypothetical
population 3 blasts to gamma-ray bursts
pear instability explosions and any
particularly bright supernova
thus although superficially appealing
the term hypernova
is so vaguely and widely applied as to
make it effectively meaningless
so why were scientists so reticent about
making a formal link between grb
980425 and sn 1998 bw
in part this is the scientific process
evidence first
and claim second secondly with most
x-ray data coming in with low resolution
there was sufficient breadth in the
error box of the grb
to make an unambiguous link with the
supernova
thirdly the grb was unusual in that it
appeared to be very weak
so where a link was proposed between the
two explosions
such as in the paper of kenichi nomado's
group
special circumstances were invoked to
account for the possible association
for a grb with a supernova before moving
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any of our weekly videos grb 98045
was unusual and perhaps was not
functionally equivalent
to the explosions seen at cosmological
distances
giving a flavor of this resistance in
aomoto's nature article
he says if this supernova and grb-98042
are indeed associated the energy
radiated in gamma rays
is at least four orders of magnitude
less than in other gamma-ray bursts
although its appearance was otherwise
unremarkable
this indicates that very different
mechanisms
can give rise to gamma-ray bursts
looking back at this it seems a peculiar
stance given our current knowledge of
these bursts
yet the scientific view is to describe
then to explain
the claim to have found a possible
supernova grb connection
rested on a single event the concept of
reproductibility came to the fore as did
carl sagan's adage
extraordinary claims require
extraordinary evidence
thus it was perhaps understandable that
the final clincher leaking
grb's to rare core collapse supernova
would come a few years later yet despite
the reluctance
to formally link some supernova to grbs
iwamoto and co-workers produced a rather
flawless model of the supernova
involving a progenitor star composed of
carbon and oxygen co and just the sort
of star
wolsely had proposed five years earlier
as the source of grbs
at its death the model star weighed in
at 13.8 times that of the sun
and was derived from a more massive
progenitor
of 40 solar masses that had undergone
extensive mass loss
in line with wolseley's model this
stripped stellar core
was probably rotating very rapidly and
it was this high angular momentum
that helped the corpse generate a final
blaze of activity
aka the grb according to this model
throughout its life stellar winds had
ripped and stripped away at this massive
star
until it was reduced to a bare cl core
a wolf race its star at its ruin the
core of this
stripped object collapsed generating a
black hole
of 2.9 solar masses jets erupted from
the mail storm swirling around the
gaping maw
of this rapidly rotating black cauldron
these jets then
tore through the star generating the
observed grb
flanking the jets the expanded blast
wave ripped viscera from the stellar
interior
that was rich in radioactive nickel 56
this
generated a luminous supernova whose
output peaked close to that of
sn 1998 bw furthermore the
record-breaking photospheric velocity of
40 000 kilometers per second
was reproduced by the core collapse
model following these parameters
it must be emphasized that forty
thousand kilometers per second
is more than ten percent of the speed of
light and these kinds of speeds were
utterly
unprecedented in supernova observed at
the time
even at late stages of the spectra of sn
1998 bw
revealed outward velocities exceeding 10
000 kilometers per second these high
velocities
could not be explained with standard
core collapse scenarios
an altogether more energetic route
underlay them
this was the formation of a stellar mass
black hole
accompanied by jets of matter moving at
a sizeable fraction
of light speed iwamoto and colleagues
proposed that these
relativistic jets erupted from the
stellar surface
generating both these high photospheric
velocities
and the subsequent gamma rays clearly
diverging somewhat from the model of
wolseley they proposed that the
gamma-ray emission was generated
as the jets slammed into the surrounding
circumstellar medium
resulting in a powerful synchrotron
emission woolsey has suggested that the
gamma-ray emission
was generated close to the black hole in
a hot bubble of matter
and anti-matter iowa moto's model had
gamma rays produced much further afield
further evidence would subsequently
accumulate casting down on both these
explanations
but that aside the basic collapse our
models seemed to work
like wolseley some years before awamoto
suggested that the progenitor of
sn 1998 bw and grb
980425 achieved this remarkable
death as a result of high angular
momentum
or strong magnetic fields the
presumption was that this was the result
of the merger of the progenitor with a
companion star
this is something we have already seen
therefore the implication was
there in the data sn 1998 bw
was associated with grb 980425
in an interesting twist to the collapser
model
scientists proposed that the properties
of grb 980425
and the accompanying supernova could be
best explained
if the grb was observed just off the
axis
of the jet thus the somewhat
underwhelming explosion energies of grb
980425 were a consequence of beaming
and the associated relativistic events
rather than the grb being intrinsically
faint
grb 980425 might not have been so wimpy
after all
with time more grbs were captured that
displayed the distinct signature
of an underlying supernova the presence
of high levels of metals
synthesized in the explosion and then
ejected from the collapsed core in the
burst
however the picture was not uniformly
clear
nor was the chronology of the observed
events
thanks for watching everyone can you
guess the connection between the
formation of black holes and gamma-ray
bursts
do you agree that hypernova is the most
extreme
cosmological explosion that can happen
after the big
bang have you learned anything new from
this video
let me know in the comments below be
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and i'll see you next time on the
channel