Download The light curves for a nova look like the following.

Nova (means new): is a star that suddenly increases greatly in
End of high mass stars
Nova
Supernova
• Type I
• Type II
Stellar nucleo-synthesis
Stellar Recycling
Credit: http://csep10.phys.utk.edu/astr162/lect/binaries/accreting.html
The light curves for a nova look like the
following.
or
brightness
The change is brightness is typical a factor of 106 (whereas
a supernova is 108, a different object all together).
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec14.html
brightness, then slowly fades back to its normal appearance over
a period of months or few weeks.
Once a decade, on average, we observe a `new' star in the
heavens.
Comparing before and after images of that region of the sky demonstrates that
novae are old stars that dramatically increase in brightness, such as Nova Herculis
shown above.
There are many reasons why a star might increase in
brightness in a sudden and explosive-like manner; the
collision of two stars, core changes, unstable pulsations.
However, novae are often recurrent, meaning that after 50
to 100 years the nova will go off again. This means that
whatever causes the brightness changes must be cyclic
(i.e. it doesn't destroy the star).
The best explanation for novae is surface fusion on a white
dwarf. By definition, white dwarfs no longer have any
hydrogen to burn in a fusion reaction. They have used all
there hydrogen at earlier phases of their life cycle.
However, a white dwarf in a binary system can `steal' extra
hydrogen from its companion by tidal stripping.
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec14.html
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A Binary system with a normal main sequence
star and an old white dwarf will look like the
following:
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec14.html
As the red giant star continues to expand it will exceed its
Roche limit and hydrogen gas will stream across to the
white dwarf, spiraling inward to form an accretion disk.
Eventually the main sequence star will evolve to
become a red giant star.
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec14.html
Hydrogen gas will build up on the surface of the white dwarf
where the surface gravity is extremely high. After a few decades,
the pressure and density of the hydrogen outer shell will reach the
point where fusion can begin and the shell explodes in a burst of
energy.
accretion disk
( So hot that
emits in
visible as well
as in
ultraviolet and
x-ray)
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec14.html
After the shell is fused, the process starts over again, thus
explaining why we see recurrent novae.
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec14.html
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O & B Stars
Form Fast
Live Fast, Burn Hot
Leave Main-Sequence Fast
Die Young, Die Explosively
q massive stars have the same types of internal changes
as low mass stars
• same types of compositional changes occur
But
q massive stars evolve more rapidly
• extra mass causes extra gravity
Credit: http://csep10.phys.utk.edu/astr162/lect/binaries/accreting.html
Dividing line between high and low mass occurs at
This dividing line really refers to
8 Msun.
the mass of the star at the time
the carbon core forms.
Star Mass
Time on MS
1 Msun
5 Msun(B-type)
10 Msun(O-type)
10 billion yrs
100 million yrs
20 million yrs
For all stars > 2.5 Msun
Helium burning starts smoothly
without any helium flash.
• extra gravity causes extra core heating
• extra core heating causes faster nuclear burning
• more luminous, so they use fuel more rapidly
• leave main sequence fast
In Brief:
§ Stars with Mass < 0.08 Msun
• failed stars
brown dwarf (nuclear burning never starts)
§ Stars with 0.08 < Mass < 0.25 Msun
• burn through Helium
• blow off their envelope and core becomes ‘He’ white dwarf
§ Stars with 0.25 < Mass < 4 Msun
• burn through Carbon
• blow off their envelope and core becomes a C-O white dwarf
§ Stars with 4 < Mass < 8 Msun (intermediate mass stars)
• burn through Carbon
• blow off their envelope with massive stellar winds and core becomes a
C-O or possibly an O-Ne-Mg white Dwarf
§ Stars with Mass > 8 Msun (high-mass stars)
• burn Carbon, Neon, Oxygen & Silicon
• build up a heavy Iron core & burning shells.
• Final stage occurs when the Iron core begins to catastrophically collapse
• supernova (boooom….)
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In Brief:
§ Stars with Mass < 0.08 Msun
• failed stars brown dwarf (nuclear burning never starts)
§ Stars with 0.08 < Mass < 0.25 Msun
• burn through Helium
• blow off their envelope and core becomes ‘He’ white dwarf
§ Stars with 0.25 < Mass < 4 Msun
• burn through Carbon
• blow off their envelope and core becomes a C-O white dwarf
High Mass Stars: M > 8 Msun
They burn through a succession of nuclear fusion fuels
and form heavier elements.
• All elements other than H and He are produced from stars.
The material in you was formed by a
star!
The process of building up heavy elements from light ones is
called nucleosynthesis.
§ Stars with 4 < Mass < 8 Msun (intermediate mass stars)
• burn through Carbon
• blow off their envelope with massive stellar winds and core becomes a
C-O or possibly an O-Ne-Mg white Dwarf
§ Stars with Mass > 8 Msun (high-mass stars)
• burn Carbon, Neon, Oxygen & Silicon
• build up a heavy Iron core & burning shells.
• Final stage occurs when the Iron core begins to catastrophically collapse
• supernova (boooom….)
is
ion
lut st
o
Ev y fa
r
ve
*
*
*
*
*
*
e.g. for star: M > 20 Msun
Hydrogen burning: 10 Myr
Helium burning:
1 Myr
Carbon burning: 1000 years
Neon burning:
~10 years
Oxygen burning: ~1 year
Silicon burning: ~1 day
Composition of high Mass Stars: M > 8
Msun
Credit: prof. W. Pogge (OSU)
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Energy is released when elements lighter than iron are fused
together. Likewise energy is released when element heavier than
iron are split apart (fission). Conversely, to fuse heavier elements
or split light elements requires extra energy.
The most stable element is Iron (26Fe56).
Need energy to split up Fe or to add to Fe.
For elements lighter than iron:
• Fusion releases energy
For elements heavier than iron:
• Fission releases energy
The universe is slowly turning to iron!
Iron is the most tightly
bound nucleus. This
means that moving
towards iron releases
energy. It is like a ball
rolling to the lowest
point.
Credit:Terry Herter, Cornell University
In stars iron plays a role of fire extinguisher
• central fire ceases
• equilibrium goes away forever
• T = several billions K
Gravity overpowers and star implodes
• core starts collapsing
• temperature reaches T >10 Billion K
• & density ~1012 kg/m3
Two Energy consuming processes kick in:
• Iron nuclei breaks into lighter nuclei e.g. He, p & n. This
process is called
• p + e n + neutrinos. (called
)
• The neutrinos escape & carry away energy.
Both processes rob the energy of core
• cooling the star
• reducing the pressure
• accelerating its collapse.
Core is collapses fast and reaches a point where
•
•
Now the opponent of gravity is there
•
•
Before the halt, gravity overshoots
takes only one sec and then ……
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At the start of Iron Core collapse, the core properties
are:
• Radius ~
km (~Rearth)
• Density ~
kg/m3
A second later, the properties are:
• Radius ~
km
• Density ~
kg/m3
• Collapse Speed ~
c!
Energetic shock waves sweep through the star
•
At shock breakout:
• Star brightens to ~10 Billion Lsun in minutes.
• Can outshine an entire galaxy of stars!
q Outer envelope is blasted off:
• accelerated to a few x 10,000 km/sec
• gas expands & cools off
q Only the core remains behind.
q After its initial brilliance, the Supernova fades out after a
few months.
q This is called .
q
Nova and supernova both represent sudden enhancement
in the brightness of a star for few weeks or months and
eventually the star becomes dim again. In the sky they
may look alike but both are quite different processes.
Differences:
v A supernova is more than a million times brighter than a
nova.
v In a matter of few months it radiates away as much energy
as Sun will radiate in its entire 1010 yrs of life period.
v A star can become supernova only once but it may become
nova many times.
v According to observations supernovae are divided into two
classes.
Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec15.html
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Type I
Type II
q
q
q
q
(rise in luminosity followed by steady,
gradual decline)
(luminosity remains at same level for a
few months after peak, before decline)
Type II: Core-collapse supernova is the same as the death of the
high mass star as described in previous slides.
Type I: (come back to the white dwarfs, descendent of low mass stars!)
A) > In binary system when white dwarf goes through nova cycles
• some material it burns out or some expels away
• but some material it keeps collecting, hence mass of WD increases slowly
If the mass of the WD exceeds the the limit of 1.4 solar mass
(called Chandrasekhar mass limit)
• electron degeneracy pressure cannot any more work against gravity
WD immediately starts collapsing/contracting
• Temperature increases rapidly
Carbon fusion begins everywhere throughout WD simultaneously
Star explodes as carbon-detonation supernova.
B) Two white dwarf in a binary system may collide and merge to
form a massive, unstable star. End result is again carbon-detonation
supernova.
v 1054 AD: "Guest Star" in Taurus
• Observed by Chinese astronomers
• Visible in daylight for 23 days
• Visible at night for ~6 months
• Left behind the Crab Nebula
v 1572: Tycho Brahe's Supernova
v 1604: Johannes Kepler's Supernova
• Important supernovae that were influential at the
beginnings of modern astronomy.
v 6000-9000 BC: Vela supernova
• Observed by the Sumerians; appears in legends about
the God Ea.
Credit: Prof. W. Pogge (OSU)
Crab Supernova Remnant: remnant of the supernova observed by
Chinese astronomers in 1054 A.D. This is type II supernova expanding into
space at several 1000 Km/sec.
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Nearest naked-eye visible supernova seen since 1604.
Explosion occurred on February 23, 1987:
15 Msun Blue Supergiant Star named SK-69o202 exploded in the Large
Magellanic Cloud. (satellite galaxy of the Milky Way ~50,000 pc away).
Particle experiments on Earth recorded a pulse of neutrinos arriving just
before the burst of light from shock breakout.
Astronomers have continued to follow its development over the last 15
years.
Vela Supernova Remnant: expansion velocities imply that its central star
exploded around 9000 B.C.
SN1987a has provided us with a great wealth of information about
supernova physics, and help to largely experimentally confirm the
basic predictions of the core-bounce picture (although with good
data, many details still remain murky).
The material in you was formed by the stars!
After
Before
There are about 115 naturally occurring elements in the
Universe. Stellar evolution theory successfully explanations
the origin of all these elements.
The process of building up heavy elements from light ones is called
nucleosynthesis:
§ Fusion in the very early Universe, immediately after the
Big Bang produced hydrogen, helium, lithium, beryllium
and boron, the first 5 elements in the Periodic Table.
§ Other elements, from carbon to iron, were formed by
fusion reactions in the cores of stars.
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He Capture:
Elements heavier than iron are not produced
in stars, so what is their origin?
Another example: Cadmium decays to form Indium
The construction of elements heavier than iron involves neutron
capture.
a nuclei can capture or fuse with a neutron because the neutron
is electrically neutral and, therefore, not repulsed like the proton.
there exist numerous free neutrons in the stars as the byproduct of many reactions.
each neutron capture by heavy nuclei produces an isotope, some
are stable, some are unstable.
unstable isotopes will decay by emitting a positron and a
neutrino to make a new element. E.g.
Fe56 + n
Fe57
(stable isotope)
Fe57 + n
Fe58
(stable isotope)
Fe58 + n
Fe59
(unstable isotope)
In about a month Iron-59 radioactively decays into cobalt-59.
Cobalt-59 captures a neutron to form Cobalt-60 and decays to
nickel-60 and so on.
Neutron capture can happen by two methods:
1) s-process (s means slow)
The s-process works as long as the decay time for unstable
isotopes is longer than the capture time. Up to the element
bismuth (atomic number 83), the s-process works, but
above this point the more massive nuclei that can be built
from bismuth are unstable.
2) r-process (r means rapid)
In this process the capture of neutrons happens in such a
dense environment that the unstable isotopes do not have
time to decay. The high density of neutrons needed is only
found during a supernova explosion and, thus, all the heavy
elements in the Universe (radium, uranium and plutonium)
are produced this way. The supernova explosion also has
the side benefit of propelling the new created elements into
space to seed molecular clouds which will form new stars
and solar systems.
Ref: http://zebu.uoregon.edu/~js/ast122/lectures/
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