Neutron Capture

Snapshot of Final Evolution Sequence
(on a time scale of 100 milliseconds)

The ejecta is enriched with neutrons that form
heavier and heavier elements due to neutron capture

Two processes:

• S-process = slow (neutrons allowed to decay into protons before another neutron is captured

• R-process = rapid (neutrons keep being captured until extremely unstable nuclei form)

Note: S-process material predicts proton rich heavy nuclei
R-process material predicts neutron rich heavy material.

On earth, ²³⁵U (1/2 life of 750 million years) comes from S-process and is reactor core fuel.

but ²³⁸U (1/2 life of 4.5 billion years) comes from R-process and this drives plate tectonics.

The Solar system has both R and S process material in it.

Sometimes (rarely) direct proton capture can occur:

As a result or repeated neutron capture the remnants of supernova contain the entire periodic table of elements.

Supernova in external galaxies are relatively easy to detect with modern imaging technology.

A supernova will radiate luminosity at about a level of 10% that of its host galaxy for a period of 2-3 weeks.

The following is an image galaxy of some recent SN occurences in nearby galaxies:

PMO Image

Implementing a supernova search program is relatively straightforward to do and can be done with automated robotic telescopes.

As a result, SN detection efficiency over time is strongly increasing.

Historical Supernova In Our Galaxy

Year	Peak Mag.	Constellation	Distance Light Years
AD 185	-6	Centaurus	4500
AD 386	-3	Scorpius	16000
AD 1006	-6	Lupus	4500
AD 1054	-10	Taurus	6500
AD 1181	-1	Cassiopeia	8500
AD 1572	-4	Caseopeia	10000
AD 1604	-3	Ophiuchus	14000
AD 1671	+6	Cassiopeia	9000

Candidate stars for next supernova:

• Betelgeuse (in Orion) at 430 light years,
• Antares (in Scorpius) at 600 light years
• Rasalgethi (in Hercules) at 380 light years

These are all a lot closer than past Galactic SN and therefore will be quite bright and clearly visible in the daytime sky. Radiation damage is a small possibility as well.

There is some evidence (but weak) that 5 million years ago, there was a small extinction event induced by a supernova explosion nearby.

The first extragalactic supernova ever discovered was SN 1885A near the nucleus of M31 (the famous "Andromeda Galaxy") on 20 August 1885. SN 1885A had an apparent visual magnitude of 5.85 - it would have been just barely visible to the naked eye had not the glow from M31 overwhelmed it

Supernova Rates:

• Very difficult to estimate accurately.
• 1970's estimate was 1 every 26 +/10 years for a galaxy like ours clearly wrong
• More recent estimates are 1 every 50 years per 10¹¹ stars.

• Note in the entire Universe the SN rate is about 1 per second.

To first order, one would think that the occurence of SN is correlated to where most of the stars are. But anecdotal evidence is strange.

For instance, the Coma Cluster

which has at least 10¹⁴ stars in it, hasn't had a supernova occur since 1935. This is a factor of 1000 below the canonical rate.

The galaxy NGC 5253, which has 10⁹ stars in it, has had 3 SN occur in the last 20 years.

Supernova Light Curves:

These light curves are powered by radioactive decay in two stages.

Note also that current astrophysical lore argues that the Peak luminosity of type Ia is a "constant". If this is true, then such objects are useful cosmological probes (see below).

There is, however, a strong minority viewpoint that the assumption of constant peak luminosity is bullshit there is no strong theoretical expectation for this.

Stage 1 is the decay of ⁵⁶Ni to ⁵⁶Co This has a 1/2 life of 6.1 days and predicts that the SN luminosity decays at the rate of 11% per day.

Stage 2 is the decay of ⁵⁶Co to ⁵⁶Fe This has a 1/2 life of 77 days and predicts that the SN luminosity decays at the rate of 1% per day.

These two stages are reflected in this light curve:

Distant Supernova as Cosmological Probes

We will more critically examine this data in the second half of the couse as we move away from stellar evolution and onto more cosmological issues.