Star Death

Question 1

Post Main Sequence evolution of a relatively low mass star (<4M_O) like our Sun.

Answer 1

Asymptotic Giant Branch (AGB) star – the star is “asymptotically” approaching the red giant branch of the HR diagram for the second time.

Question 2

Low mass AGB Star structure and Future of our Solar System

Answer 2

The structure of an old, low-mass AGB star. Thermonuclear reactions in the helium-fusing shell are so rapid that the star’s luminosity is at least 1000 times our present-day Sun.

When the Sun becomes an AGB star in 12 billion years the increase in luminosity of factors of 1000 – 10,000 will evaporate most of the planets out to, and including, Jupiter.

The outer layers of the star will reach the Earth’s orbit, swallowing up Mercury & Venus on the way….

Question 3

AGB Shell Ejection

Answer 3

As our star evolves through the AGB phase it ejects shells of material off into space, until all that is left is the hot core.

Up to 40% of the stars mass may be lost by this process

The result of all this mass ejection is to form a Planetary Nebula (PN)

Question 4

White Dwarf Stars

Answer 4

• The residual star of a Planetary Nebula is a white dwarf.
• This is a hot carbon-oxygen core, but not hot enough to ignite either element and to undergo further fusion.
• The WD will thus gradually cool down leaving a burnt out relic.
• Because the core is so dense, the degenerate electron pressure will prevent further collapse.

Question 5

White Dwarf - Mass-Radius relationship

Answer 5

The surprising mass to radius relationship for white dwarf stars:

Smaller is more massive!

This result is because as you increase the mass, the more degenerate the matter becomes, and hence more compact.

The maximum theoretical mass for a WD is 1.4M₀ (known as the Chandrasekhar limit).

Question 6

White Dwarf - cooling curves

Answer 6

Question 7

Higher Mass stars (>4M₀)

Answer 7

• The evolutionary path for massive star (M > 4M₀) is somewhat different from low mass stars.
• The outcomes from this branch of evolution results in neutron stars and black holes.
• Because we now have much more mass available, these stars can continue burning elements beyond the carbon-oxygen limit. As a result we can now have carbon → silicon fusion occurring. With each new stage of core collapse, the temperature rises and the next stage goes even faster.

Question 8

Higher Mass stars - final stage of fusion

Answer 8

• Ultimately, the final stage of fusion occurs, that of making iron. Beyond iron in the periodic table, the thermonuclear fusion process requires energy rather than liberating energy.
• The final extremely rapid stage of collapse–the Super Nova explosion - creates pressures so great that electrons are forced to merge with protons producing a neutron core.

e- + p → n + ν (neutrino

)

Question 9

Neutrinos (ν)

Answer 9

• The production of neutrinos is critical in this stage.
• They react very weakly with matter and hence are able to carry away most of the heat energy in the final 1⁄4 second.
• The loss of heat leads to the final collapse, in which the structure of the core changes to become very rigid at these phenomenally high densities.
• This causes the inner core to reflect the collapsing pressure wave and the outer shells of star are propelled into space with the resulting shock wave.
• It is the neutrino blast that people search for as one of the signatures of a SN explosion.

Question 10

Two types of Supernova

Answer 10

Type 1 occurs in binary systems

A carbon-rich white dwarf pulls matter onto itself from nearby red-giant or Main Sequence star.

Type 2

The second type occurs due to the collapse of a single massive star

The core of a high mass star collapses then re- bounds in a catastrophic explosion

Question 11

Cosmic Abundance of Elements

Answer 11

Question 12

Summary of Stellar Evolution Pathways

Answer 12