Lecture 24: Life as a High-Mass Star

Review from Last Time: life for low-mass stars molecular cloud to proto-star main sequence star (core Hydrogen burning) core Hydrogen exhausted (sub-giant) shell Hydrogen burning (red giant) core Helium burning (Helium Flash) shell Helium burning (double-shell burning red giant) planetary nebula white dwarf

Life as a High Mass Star basic story from proto-star to main sequence is the same as for low-mass stars – everything just happens faster. the high temperature inside a high mass star make additional kinds of fusion reactions possible for example, the CNO cycle is a different way that Hydrogen can be fused into Helium – it is much faster than the proton- proton chain.

Quick review of the p-p chain…

The CNO cycle

remember low mass stars can never get hot enough to fuse Carbon… High mass stars can continue the fusion process: Carbon  Neon, Neon  Oxygen, Oxygen  Silicon, Silicon  Iron

Helium flash

High Mass stars: No Helium Flash

Mass-loss from a supergiant star

Mass per nucleon fusion of elements lighter than Iron releases energy because the mass of the product is less than the mass of the components another way of saying this is that the mass per nucleon is smaller for Helium than for Hydrogen, smaller for Carbon than for Helium, etc…

Iron: the end of the road

The death of a high mass star for a while, the star is supported by degeneracy pressure in the Iron core when the temperature gets high enough, the electrons combine with the protons to form neutrons and neutrinos…

Supernova! the collapse releases a tremendous amount of energy, about J! Most of this energy is carried out of the star by neutrinos. the neutrinos produce a shock wave that blows the outer envelope of the star away at huge speeds. the star becomes as bright as 10 billion Suns, as bright as a whole galaxy!

The Crab Nebula

Supernova 1987A

Supernova Remnants if the degeneracy pressure of the neutrons is enough to balance the force of gravity, the core of the star is left behind as a giant ball of neutrons called a neutron star otherwise, the core of the star collapses into a black hole…

Star Clusters Many stars are born in clusters Open clusters are loose, irregular groups of young stars, found mainly in the disk of the Galaxy. Globular clusters are round, regular balls of old stars, found mainly in the halo of our Galaxy.

Star Clusters Open Cluster Globular Cluster

Star Clusters are useful laboratories for studying stars: All the stars in the cluster are at about the same distance from us All the stars in the cluster formed at about the same time (so they are about the same age) the H-R diagram of a cluster represents stars at all stages of their evolution

Temperature Luminosity main sequence turnoff Pleiades

Palomar 3 What is the age of this cluster?

The Age of the Universe Stars in the oldest clusters have ages of billion years From the expansion rate of the universe, we can estimate the time since the Big Bang. Current values are around 13 billion years. Are there stars older than the Universe???