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0. 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?.

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The Deaths of Stars

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the deaths of stars


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?


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:
  • Energy is quantized
  • Pauli exclusion principle

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!

All energy levels below the “Fermi energy” are filled.

Q: What if we increase the temperature?

A: This temperature mostly goes into speeding up the nuclei… not the electrons.

The electrons are not free to change their energy.



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?


He fusion via the

“triple-alpha process”:

4He + 4He 8Be + g

8Be + 4He 12C + g

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.”


Q: How does degenerate matter trigger the helium flash?

Helium flash last a few minutes…

Notice the position of the giant stars on the HR diagram…

Produces more energy than an entire galaxy!

Q: Why does the expansion of the star’s envelope make it cooler and more luminous?

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.



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!)

Q: If the stars at the turnoff point have a mass of 4 Msun, how old is the cluster?

So what we get is a group of stars at different stages of evolution!

Q: How can star clusters confirm theories of stellar evolution?


Lower main-sequence stars:

Red dwarfs: between 0.4 & 0.08 Msun

Completely convective

Live for ~ 100 billion years

Q: Why don’t red dwarfs become giants?

Q: How long will a 0.4 solar mass star spend on the main-sequence?

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”


Eskimo Nebula NGC 2392

NGC 6751 Glowing Eye

Planetary nebulae:

Helix Nebula

Eta Carinae

Cat's Eye Nebula NGC6543

Hourglass Nebula

V838 Monocerotis


Q: What causes an aging star to produce a planetary nebula?

Helium burning shell

He fusion reactions are extremely temperature sensitive: ~T40

Small increase in T => star expands

Star expands => cools

Star cools => star contracts

=> Star becomes unstable (because of the T40)

Eventually, these pulsations expel the outer layers of the star

And we get a PN….

Bow Tie Nebula NGC 2440


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)?


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?


Evolution of binary systems:

Gravitational field of the stars combined with the rotation of the system define the “Roche surface.”

Matter inside a star’s Roche surface is gravitationally bound to the star, but…

  • Two ways in which matter can be transferred through L1;
  • Stellar wind (slow)
  • If the star expands past its Roche surface (rapid)

Matter can be transferred from one star to the other through the inner Lagrangian point.


The “Algol paradox”

This would correspond to the Algol system

Q: How can we explain the Algol paradox?

Mass transfer explains this paradox!

The less massive star became a giant while the more massive star remained on the main-sequence!?!


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;
  • Heats the disk
  • 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….




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.

Nova Cygni 1975

  • Very hot, dense layer of hydrogen accumulates on the white dwarf surface.

This layer grows denser and hotter until…

~ 100,000 more luminous than the sun.

Explosion lasts only minutes to hours, the brightness fades in ~ 1-3 months.


Hydrogen fuses in a sudden explosion that blows the surface off the star.

the fate of our sun and the end of earth


The Fate of our Sunand 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
the deaths of massive stars supernovae


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.

Fewer nuclei combined with the fact that the energy per reaction decreases as the atomic mass increases leads to this rapid rate.

Iron core ultimately collapses, triggering an explosion that destroys the star:  Supernova


Supernovae II:

With these combined energy sinks, the core collapses in less than a tenth of a second!

Once the iron core is created, reactions involving iron remove energy in two ways:

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?

  • Fe nuclei capture electrons  Fe nuclei break apart into smaller nuclei, and the degenerate electrons that supported the core are removed  core contracts
  • 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
  • A: As the matter falls inward, it creates a shockwave that travels outward.
  • This shockwave is aided by two additional sources of energy;
  • 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.
  • This flow of energy also creates turbulence which further drives the shock wave outward.

Supernovae III:

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!

All the elements in the core are destroyed, leaving protons, neutrons, and electrons (and possibly other exotic particles…)

A typical SN explosion produces ~ 1028 Mt of TNT!

(Equivalent to 3 million solar masses of TNT)


Supernova remnants:

N 49

Cassiopeia A

Veil Nebula NGC 6960

N 63A


Type Ia, Ib, and II Supernovae:

Type Ib = Type II in which the massive star lost its atmosphere…

Type II:

Type I:

Contain hydrogen lines

No hydrogen lines

Type IaType Ib, Ic

Produced by the collapse of a massive star

Produced when a WD accretes enough matter to exceed the Chandresekhar limit

Produced by a collapsing massive star which lost its envelope to a binary companion

WD completely blown apart… no NS or BH. (The WD contains usable fuel….)

Leaves behind a neutron star or a black hole

type i and ii supernovae


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.

the famous supernova of 1987 supernova 1987a

Unusual type II supernova in the Large Magellanic Cloud on Feb. 24, 1987


The Famous Supernova of 1987: Supernova 1987A

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.


At maximum

“Unusual” in that it appears to be a type II SN (massive core collapse) but no traces of a neutron star have been found…

Q: What is the difference between a nova and a supernova?

This may be because the NS is enshrouded in a dense dust cloud, or matter fell back onto the NS creating a black hole.

observations of supernovae


Observations of Supernovae

SN 1994D in NGC 4526


In 1054 AD, Chinese astronomers recorded a “guest star” in the constellation Taurus.

The “new star” was bright enough to see during daytime!

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.

After a month, it slowly faded… vanishing after ~ two years.