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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Q: How can the contraction of an inert helium core induce a hydrogen burning shell?
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!
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?
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.”
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?
Red dwarfs: between 0.4 & 0.08 Msun
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?
Between 0.4 & ~3-4 Msun
“Burn” H & He but not carbon
Eventually become white dwarfs ~ size of Earth,
Teaspoon weighs over 15 tons!
NGC 6751 Glowing Eye
Cat's Eye Nebula NGC6543
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
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)?
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?
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…
Matter can be transferred from one star to the other through the inner Lagrangian point.
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!?!
Because stars rotate, matter that leaves the star has angular momentum…
Conservation of angular momentum creates an accretion disk.
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
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.
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
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?
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)
Veil Nebula NGC 6960
Type Ib = Type II in which the massive star lost its atmosphere…
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
0The 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.
“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.
In 1054 AD, Chinese astronomers recorded a “guest star” in the constellation Taurus.
The “new star” was bright enough to see during daytime!
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.