13 Stellar Explosions

1. Life After Death for White Dwarfs

Most stars shine steadily (year after year)
Some stars show dramatic changes in their brightness:
- variation time scale: very short period wrt life-span
- Nova (meaning "new" star)
- Star's brightness increases more than 10 000 times
- Event happens in a matter of days
- It is not a new star but a faint White Dwarf

Nova

Explosion on WD's surface:
- Causes a rapid temporary increase in L
- WD fades back to normal in few days/months
On average: 2-3 nova per year
Recurrent Nova:
- Repeats nova outburst over a few decades

White Dwarf

WD phase ➤ star cools ➤ becames Black Dwarf

White Dwarf in a Binary

To convert a WD into a Nova, WD has to be in a binary and binary separation has to be small. Then ...

WD's gravity pulls matter (H & He) from the giant.
System becomes a mass-transferring binary
Matter builds up on WD's surface
WD becomes hotter & denser
- T exceeds 107 K
- H ignited ➤ He
Brief & violent outburst of energy (luminosity)
Material keeps being transferred to the WD and the process repeats

Accretion Disk

Because stars rotate both around themselves (axial) and around each other (orbital):
- matter doesn't fall directly onto WD
  - instead it misses the companion and
  - loops around behind it
- Therefore, an Accretion Disk is created
Due to the friction within the gas, the matter in the disk drifts gradually inward.
- Its temperature increase
- Inner part of the becomes hot: in Visible, UV, X-ray

Nova Matter Ejection

(a) The ejection of material from a star’s surface can clearly be seen in this image of Nova Persei, taken some 50 years after it suddenly brightened by a factor of 40,000 in 1901.

(b) Nova Cygni erupted in 1992 (10 000 ly away).

At left, more than a year after the blast, a rapidly billowing bubble is seen.
At right, 7 months after that, the shell continued to expand and distort.

2. End of a High-Mass Star

A low-mass star (< 8 M⊙ ) fuses only H and He and ends up as C-O White Dwarf.
A high-mass star can continue to fuse elements in its core right up to iron
After iron the fusion reaction is energetically unfavored.
As heavier elements are fused, the reactions go faster and the stage is over more quickly.
A 20 M⊙ star will burn carbon for about 10,000 years.
But its iron core lasts less than a day

Iron and Nuclear Masses

The figure shows the relative stability of nuclei.

On the left, nuclei gain energy through fusion.
- the mass per particle decreases and energy is released.
On the right they gain it through fission.
- the total mass again decreases and energy is again released

Iron is the crossing point:

when the core has fused to iron, no more fusion can take place.
because it can be neither fused nor split to release energy.

Gravity vs Pressure

The inward pressure is enormous, due to the high mass of the star.
There is nothing stopping the star from collapsing further
- GRAVITY > PRESSURE
It does so very rapidly, in a giant implosion.
As it continues to become more and more dense, the protons and electrons react with one another to become neutrons:

p + e → n + neutrino

The neutrinos escape;
The neutrons are compressed together until the whole star has the density of an atomic nucleus, about 1015 kg/m3.
The collapse is still going on
- it compresses the neutrons further until they recoil in an enormous explosion as a supernova.

Supernova 1987A

(Left) Before the event. (Right) After supernova explosion.

3. Supernovae

A supernova is a one-time event.
- Once it happens, there is little or nothing left of the progenitor star.
There are two different types of supernovae, both equally common:
- Type I, which is a carbon-detonation supernova.
- Type II, which is the death of a high-mass star

Type I

WD that has accumulated too much mass from binary companion.
If the white dwarf’s mass exceeds 1.4 M⊙, electron degeneracy can no longer keep the core from collapsing.
Carbon fusion begins throughout the star almost simultaneously, resulting in a carbon explosion.

Type II

It occurs when the core of a high-mass star collapses and then rebounds in a catastrophic explosion.

Supernova Light Curves

In both cases, the maximum luminosity can sometimes reach that of a billion suns.
But there are characteristic differences in the falloff of the luminosity after the initial peak.
- Type I light curves somewhat resemble those of novae but the total release of energy is much larger.
- Type II curves have a characteristic plateau during the declining phase.
All High-Mass stars ➤ ➤ Type II SN
Only tiny fraction of Low-Mass stars ➤ ➤ WD ➤ ➤ Type I SN
But since Nlow-mass ≫ Nhigh-mass
- NSN-type-I ~ NSN-type-II

4. Supernova Remnants

Supernovae leave remnants

- the expanding clouds of material from the explosion.

Crab Supernova Remnant

This remnant of an ancient Type II supernova.
It has an angular diameter about 1/5 of the full Moon.
Because its debris is scattered over a region of “only” 2 pc, the Crab is considered to be a young supernova remnant.
In A.D. 1054, Chinese astronomers observed this supernova explosion.

Vela Supernova Remnant

It spreads across 6° of the sky.
The inset shows more clearly some of the details of the nebula’s extensive filament structure.

SN1987A Supernova Remnant

Through the years the remnant revealed itself. Now an expanding ring illuminated by the internal matter can easily be seen in both visible and X-ray wavelength bands.

5. Formation of the elements

115 elements in total
There are 81 stable
10 radioactive (natural) elements - having long decays
19 radioactive (artificial) - having quick decays

Where do they come from?

Stellar Nucleosynthesis

Formed during normal stellar fusion: three helium nuclei fuse to form carbon.

This process requires 108 K and it is names as triple-alpha process.

Carbon can then fuse, either with itself or with alpha particles, to form more nuclei:

(a) This process requires 6 x 108 K and it is uncommon.

(b) This process requires 2 x 108 K and it is favorable.

Alpha Process

(a) At high temperatures, heavy nuclei (such as silicon, shown here) can be broken apart into helium nuclei by high-energy photons.

(b) Other nuclei can capture the helium nuclei (or alpha particles) thus produced, forming heavier elements by the so-called alpha process.

This process continues all the way to the formation of nickel-56 (in the iron group). Nickel-56 is unstable and therefore decays into cobalt-56 which decays into iron-56 which is the final element in this process

However, within the cores of the most massive stars, neutron capture can create heavier elements, all the way up to bismuth-209.

The heaviest elements are made during the first few seconds of a supernova explosion.

Beyond Iron

Alpha process stops at Iron (Fe)
So, what about other heavier elements?
- Neutron Capture:
  - Neutrons are produced as by-products of nuclear reactions
  - There are many neutrons to interact with Fe
  - Neutrons have no charge

➤ No repulsion barrier to overcome in combining with positively charged nuclei

➤ Therefore, mass of Iron nucleus continuously grows

So, neutron capture will be in action after Iron:

56Fe + n ⇒ 57Fe (relatively stable)

57Fe + n ⇒ 58Fe (relatively stable)

58Fe + n ⇒ 59Fe (radioactively unstable)

... ⇒ decays into ⇒ 59Co (stable)

59Co + n ⇒ 60Co

... ⇒ decays into ⇒ 60Ni

╚══════════════════════╝ ➤ s-process

s-process: each neutron capture takes about a year.

6. Cycle of Stellar Evolution

Stellar Recycling

The cycle of star formation and evolution continuously replenishes the Galaxy with new heavy elements and provides the driving force for the creation of new generations of stars.

Clockwise from the top are

an interstellar cloud (Barnard 68),
a star-forming region in our Galaxy (RCW 38),
a massive star ejecting a “bubble” and about to explode (NGC 7635),
a supernova remnant and its heavy-element debris (N49).

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