stars

1. The Deaths of Stars
• What happens when a star uses up all its hydrogen in
its core?
• What is the evidence that stars really evolve?
• How will the sun die?
• What happens to an evolving star in a binary system?
• How do massive stars die?

4. Q: How can the contraction
of an inert helium core
induce a hydrogen burning
shell?

7. Q: The Helium core contracts and
heats the star enough to induce a
hydrogen-burning shell… so what
stops the helium core from contracting
to zero radius (keep in mind that He
fusion has not set in yet….)?
A: Degeneracy pressure!
The core becomes very dense…
and two laws of quantum
mechanics become important:
1. Energy is quantized
2. Pauli exclusion principle
The electrons are not free to
change their energy.
All energy levels below the
“Fermi energy” are filled.
Q: What happens when we “push” on
this gas?
A: Nothing! To compress it requires
tremendous energy because we
would have to change the electron’s
energy state. It resists compression!
Q: What if we increase the temperature?
A: This temperature mostly goes into
speeding up the nuclei… not the electrons.

8. Upshot:
Degenerate matter resists compression, and changing
the temperature has little effect on the pressure.
Q: Think about what is written above…
what are the consequences…?
A: No pressure-temperature thermostat!
Q: So what?!?
=> they no longer can be main-sequence
stars (recall that the evolution of main-sequence
stars is governed by hydrostatic equilibrium and the
pressure-temperature thermostat).
Q: Why is degenerate matter so difficult to compress?

9. Q: Why does helium
fusion require a higher
temperature than
hydrogen fusion?
Stars more massive than 3
Msun begin He fusion
gradually – the stars
contract rapidly enough
that the cores do not
become degenerate.
What about less massive
stars?
Core temperature
increases… enough to start
He fusion… but the pressure
cannot compensate because
the core is degenerate.
We get a runaway effect
called the “helium flash.”
He fusion via the
“triple-alpha process”:
4He + 4He  8Be + g
8Be + 4He  12C + g

10. Helium flash last a few
minutes…
Produces more energy
than an entire galaxy!
Star becomes so hot that
it becomes no longer
degenerate and the T-P
thermostat kicks in again
for the He fusion core.
Wouldn’t see much because
the outer layers of the star
absorb the energy.
Eventually the process
repeats except with the
heavier elements.
More massive than 3 Msun
and less than 0.4 Msun => no
He flash.
Q: How does degenerate matter
trigger the helium flash?
Notice the position of
the giant stars on the
HR diagram…
Q: Why does the expansion of the star’s
envelope make it cooler and more luminous?

11. Q: If the stars at the turnoff point
have a mass of 4 Msun, how old is the
cluster?
Clusters: Cluster stars form at about the
same time… => all about the same
age.
=> If you can identify the stars that are just
leaving the main-sequence…
find the masses of them…
=> Find the age of the cluster! (and all the
stars therein!)
2.5
1
M
 
So what we get is a group of stars at
different stages of evolution!
Q: How can star clusters confirm
theories of stellar evolution?

13. Q: How long will a 0.4 solar mass
star spend on the main-sequence?
Lower main-sequence stars:
Red dwarfs: between 0.4 &
0.08 Msun
Completely convective
Live for ~ 100 billion years
Medium-mass stars:
Between 0.4 & ~3-4 Msun
“Burn” H & He but not carbon
Eventually become white dwarfs
~ size of Earth,
Teaspoon weighs over 15 tons!
“Electron degenerate”
Q: Why don’t red dwarfs become
giants?

14. Planetary nebulae:

15. Planetary nebulae:
V838 Monocerotis
Eskimo Nebula NGC 2392
Eta Carinae
Hourglass Nebula
Helix Nebula
NGC 6751 Glowing Eye
Cat's Eye Nebula NGC6543

16. Bow Tie Nebula NGC 2440
Q: What causes an aging star to
produce a planetary nebula?
Helium burning shell
He fusion reactions are
extremely temperature
sensitive: ~T40
=> Star becomes
unstable (because
of the T40)
Small increase in T =>
star expands
Star expands => cools
Star cools => star
contracts
Eventually, these
pulsations expel the
outer layers of the star
And we get a PN….

17. Ring Nebula Q: This has an angular
diameter of 76” and is at a
distance of 5,000 ly. What is
the diameter?
Q: Suppose we found that
the radius is 1pc and
Doppler shifts show that
the gas is moving at 30
km/s. How old is this
nebula (i.e., how long ago
did it start to form)?

18. White dwarfs (again):
• ~ size of Earth
• ~ 25,000 K
• very dense
(1cm3 ~ 6,600 lbs!)
• Usually 0.6 Msun
• < 1.4 Msun
• Electron degenerate
• No fusion (dead)
• ~ 9 billion years to cool
=> “black dwarfs”
• Most common star next
to red dwarfs
1.4 Msun = “Chandrasekhar limit”
Q: Why can’t a white dwarf have a
mass greater than 1.4 solar masses?
Q: Stars up to 8 solar masses can eventually
become white dwarfs… how can this be!?!
Q: If a star the size of the sun collapses to form a WD the
size of Earth, by what factor will its density increase?
3
4
3
mass
volume
M
r


 

19. Evolution of binary systems:
Matter inside a star’s Roche
surface is gravitationally bound
to the star, but…
Matter can be transferred
from one star to the other
through the inner Lagrangian
point.
Two ways in which matter can
be transferred through L1;
1. Stellar wind (slow)
2. If the star expands past its
Roche surface (rapid)
Gravitational field of the stars
combined with the rotation of
the system define the “Roche
surface.”

20. The “Algol paradox”
The less massive star became
a giant while the more
massive star remained on the
main-sequence!?!
This would
correspond to the
Algol system
2.5
1
M
 
Mass transfer explains this paradox!
Q: How can we explain the Algol paradox?

21. Accretion disks:
Because stars rotate, matter
that leaves the star has
angular momentum…
Conservation of angular
momentum creates an
accretion disk.
Tidal forces and friction cause
two things to happen;
1. Heats the disk
2. Dissipates the angular
momentum and allows the
gas to fall to the star
If the accreting star happens to
be a white dwarf…
One of two things can happen…
Nova or supernova….

22. Novae:
A star that appears for a while
and then fades away…
It’s not a new star, but an old
star flaring up.
Hydrogen is accreted from
the binary partner onto the
white dwarf.
 Very hot, dense layer
of hydrogen accumulates
on the white dwarf
surface.
This layer grows denser
and hotter until…
BAM!
Hydrogen fuses in a sudden explosion
that blows the surface off the star.
Nova Cygni 1975
~ 100,000 more luminous than the
sun.
Explosion lasts only minutes to
hours, the brightness fades in ~ 1-3
months.

23. The Fate of our Sun
and the End of Earth
• Sun will expand to a red
giant in ~ 5 billion years
• Expands to ~ Earth’s orbit
• Earth will then be
incinerated!
• Sun may form a planetary
nebula (but uncertain)
• Sun’s C,O core will
become a white dwarf

24. The Deaths of Massive Stars: Supernovae
Final stages of fusion in
high-mass stars (> 8
Msun), leading to the
formation of an iron core,
happen extremely rapidly:
H  He ~ 7 Myr,
O Si ~6 months
Si  Fe burning lasts
only for ~ 1 day.
Iron core ultimately
collapses, triggering an
explosion that destroys
the star:  Supernova
Fewer nuclei combined
with the fact that the
energy per reaction
decreases as the atomic
mass increases leads to
this rapid rate.

25. Supernovae II:
Once the iron core is
created, reactions
involving iron remove
energy in two ways:
1. Fe nuclei capture electrons
 Fe nuclei break apart
into smaller nuclei, and the
degenerate electrons that
supported the core are
removed  core contracts
2. T so high that the average
photon is a gamma ray 
nuclei absorb these gamma
rays and break apart, the
removal of these gamma
rays cools the core and
allows it to contract even
more
With these combined energy sinks, the core
collapses in less than a tenth of a second!
The core becomes either a neutron star or a
black hole while the outer layers are blown to
smithereens!
Q: How can the inward collapse of the
core produce an outward explosion?
A: As the matter falls inward, it creates a
shockwave that travels outward.
This shockwave is aided by two additional
sources of energy;
1. The disrupted nuclei in the core produce a
flood of neutrinos which cool the core and
allow it to collapse further. This collapse
heats the gas outside the core giving the
shock wave an additional boost.
2. This flow of energy also creates
turbulence which further drives the shock
wave outward.

26. Supernovae III:
All the elements in the core
are destroyed, leaving
protons, neutrons, and
electrons (and possibly
other exotic particles…)
The explosion is so violent
that heavy elements are
produced in the outer
layers during the
explosion… all elements
heavier than Fe were
created in a SN explosion!
A typical SN explosion
produces ~ 1028 Mt of TNT!
(Equivalent to 3 million solar
masses of TNT)

27. Supernova remnants:
Cassiopeia A
N 49
Veil Nebula
NGC 6960
N 63A

28. Type Ia, Ib, and II Supernovae:
Type II: Type I:
Contain hydrogen lines No hydrogen lines
Produced by the
collapse of a massive
star
Leaves behind a
neutron star or a black
hole
Type Ia Type Ib, Ic
Produced by a
collapsing massive
star which lost its
envelope to a
binary companion
Produced when a WD
accretes enough matter
to exceed the
Chandresekhar limit
WD completely blown
apart… no NS or BH. (The
WD contains usable fuel….)
Type Ib = Type II in which
the massive star lost its
atmosphere…

29. Type I and II Supernovae
Core collapse of a massive star:
type II supernova
If an accreting white dwarf exceeds the
Chandrasekhar mass limit, it collapses,
triggering a type Ia supernova.

30. The Famous Supernova of 1987:
Supernova 1987A
Before At maximum
Unusual type II supernova in the Large
Magellanic Cloud on Feb. 24, 1987
“Unusual” in that it appears
to be a type II SN (massive
core collapse) but no traces
of a neutron star have been
found…
This may be because the NS
is enshrouded in a dense dust
cloud, or matter fell back onto
the NS creating a black hole.
Believed to be the result of a
merger of two stars in a
binary system ~ 20,000
years ago which created the
blue supergiant that
exploded.
Q: What is the difference
between a nova and a
supernova?

31. Observations of Supernovae
SN 1994D in
NGC 4526

32. In 1054 AD, Chinese
astronomers recorded a
“guest star” in the
constellation Taurus.
The “new star” was
bright enough to see
during daytime!
After a month, it slowly
faded… vanishing after
~ two years.
synchrotron radiation
The Crab Pulsar is
roughly 25 km (~16 mi.)
in diameter and rotates
~ 30 times/second!
It’s slowing in its
rotation by 38
nanoseconds/day
due to energy loss
by the pulsar wind.