Two types of supernovae

• Type I Supernova - hydrogen poor, light curve similar to a nova.

• Type II Supernova - lots of hydrogen, shows a characteristic “bump” in the light curve a few months after the maximum.

Why are there two types?

A recurrent nova follows an accretion-explosion cycle.

All the matter collected on

a white dwarf in a binary system due to accretion may not be expelled by the nova explosion. The dwarf will increase in mass with each nova cycle.

A white dwarf is held up by the pressure of the electrons compressed until they are effectively in contact. However, there is a limit to the pressure the electrons can withstand.

The maximum mass for a white dwarf is 1.4 solar masses. This is called the Chandrasekhar Mass. The electrons cannot withstand the pressure when this mass is exceeded.

If an accreting white dwarf exceeds the Chandrasekhar mass, it collapses so fast that carbon fusion begins everywhere in the star at once. This is a carbon detonation supernova.

Also, it is possible that two white dwarfs in a binary system can collide and thus exceed the Chandrasekhar mass.

This carbon detonation of a white dwarf is a Type I supernova. The implosion-explosion of a massive star that we discussed earlier is a Type II supernova.

The remains of a supernova is called a supernova remnant. An example of this is the Crab Nebula.

No supernova has been seen in our area of the galaxy since 1604. We should see one, visible with the naked eye, every 100 years or so. We are way overdue, so there could be one any day.

Supernovae are examples of standard candles, objects of known absolute brightness.

Supernovae only occur at well a defined critical mass (the C. mass) and composition, so all those of the same type (I or II) have the same absolute brightness.

Therefore, supernovae in distant galaxies can be used to find the distance to the galaxies where they occur.

Evolution of the Elements Hydrogen and helium are “primordial” elements. This means they are the original elements of the universe. All other elements are formed by stellar nucleosynthesis.

All elements except hydrogen and helium are formed by nuclear fusion in stars.

Remember that the proton-proton chain produces helium:4 H ---> He (+ 2 positrons + 2 neutrinos + energy)

The triple-alpha reaction produces carbon:3 4He ---> 12C + energy

Two carbons can then combine to form magnesium:2 12C ---> 24Mg + energy

But carbon is more likely to form oxygen by helium capture:12C + 4He ---> 16O + energy

(Less energy is required fro helium capture.)

Then:2 16O ---> 32S + energyBut: 16O + 4He ---> 20Ne + energy

(Helium capture) is more likely (Again, less energy required).

Heavier elements tend to form by helium capture, not by combining like molecules.

Also some elements combine with protons and neutrons to form intermediate elements by proton- or neutron-capture.

As these heavier elements form, heat increases and this causes some new elements to break apart. Some break apart into 4He, which can combine with other nuclei forming 32S, 36Ar, 40Ca, 44Ti, 48Cr, 52Fe, and 56Ni.

This photodisintegration and combining with 4He is called the alpha process. (This is not the same as alpha capture.)

At this point, 56Ni decays to 56Co, then to 56Fe. Remember, fusion involving iron consumes energy (because 56Fe is so stable), so no elements beyond nickel can be formed by the alpha process.

Producing elements beyond iron begins with iron gaining three neutrons as follows:

56Fe + n ---> 57Fe 57Fe + n ---> 58Fe 58Fe + n ---> 59Fe

59Fe is unstable, and decays to 59Co. 59Co can gain a neutron and then decay to heavier nuclei.

This neutron gain and decay continues up to 209Bi and is called the s-process. “S” stands for “slow.”

209Bi decays too rapidly to gain a neutron, so the s-process stops at bismuth.

In a supernova explosion, the rapid production of neutrons during the neutronization of the core lets larger nuclei gain neutrons faster than they can decay.

In this way, elements heavier than 209Bi can be produced. This rapid formation of all elements heavier than bismuth, the r-process, occurs only in an exploding star, a supernova.