Introduction to Astrophysics Lecture 11: The life and death of stars Eta Carinae

Stellar physics and evolution Stars generate energy by nuclear fusion. In most stars, the main reaction is the conversion of hydrogen to helium. It takes four hydrogen nuclei to fuse together to form a helium nucleus. The nuclear reactions take place deep in the star, where the temperatures are extremely high. The energy slowly leaks out, because the environment is so dense. It is estimated that a photon of light experiences so many collisions that it take 10 million years to escape the Sun.

Stellar structure So far we have only considered the surface properties of the star, such as its temperature. A complete model will tell us how the properties vary within the star. It must include computation of where and how much energy is generated by nuclear reactions, and study how that energy is transported out of the star. Needless to say, it is the Sun which has been most studied.

We can divide the Sun up into zones: The core: in the inner one third of its radius, nuclear fusion is taking place, generating energy which heats the core to between five and fifteen million degrees. The radiation zone: for the next one third energy transport is mostly by radiation, bringing the temperature down to around one million degrees. The convection zone: energy transport is primarily by convection, with the temperature falling to just 5800K at the Sun’s surface. The photosphere: this is the surface where light escapes from. The chromosphere: this is the region above the visible surface of the Sun, visible mainly during eclipses.

The Sun in X-rays PhotospherePhotosphere Sunspot close-up

Stellar evolution Evolution of a star’s properties can be represented as a track in the Hertzsprung-Russell diagram. The main feature in the evolution is when the star exhausts its hydrogen fuel and starts to burn helium. At this stage it becomes a Red Giant.

The lifetime of stars The main sequence has a relation between mass and luminosity of approximately L ∝ M 4 and the rate at which fuel is used up is proportional to the luminosity, with the amount of fuel proportional to the mass. This gives the crucial relation Main sequence lifetime ∝ 1/M 3 The more massive stars are more short lived!

Some sample main sequence lifetimes

Evolutionary stages When a star’s hydrogen runs out it becomes a red giant, burning helium in the core. Later on it goes through cycles as it is forced to burn heavier and heavier elements. Eventually, it exhausts its supply of fuel completely. What happens next depends on the mass of the star.

Low mass stars For stars with mass up to about twice the solar mass, when the fuel is exhausted the outer envelope of the star is ejected leaving behind a dense core. Computer simulation of a red giant star This core is known as a white dwarf. It is heated by energy released by the gravitational contraction and may initially be very hot. After that it cools and fades.

White dwarf properties  They are extremely dense, perhaps up to a million times the density of water. Despite having a mass comparable to the Sun, their size can be comparable to the Earth!  They are prevented from total collapse because of the electrons. Quantum mechanics does not allow electrons to be compressed into a smaller volume.  The more massive they are, the smaller their radius.  The highest mass they can have is 1.4 solar masses, known as the Chandrasekhar limit.

High-mass stars A high mass star can burn heavier and heavier elements, until it creates Iron at its core. Iron is the most stable element there is; it cannot be burned to create anything else. Deprived of energy to support it, the core collapses and the star explodes!

Supernova!! Close up of supernova 1987a A supernova explosion is one of the Universe’s most spectacular events. Briefly, the explosion of a single star can be as bright as all the stars in a galaxy put together. In a typical galaxy there are a few supernovae every century.

We are all made from supernovae remnants! A supernova is the main way in which the heavy elements, such as oxygen, carbon and iron, escape the stars in which they are created and are returned to the interstellar dust. Without supernovae, the elements from which we are made would not exist outside the cores of stars.

What’s left behind? The supernova explosion throws off the outer shell of the star. What’s left behind depends on the initial mass. Either  A neutron star, or  A black hole Chandra satellite X-ray image of Cassiopeia A

Neutron stars Towards the lower end of the mass range, what’s left is a neutron star. Neutron stars are...  Composed of neutrons. The intense force of gravity is so strong that it forces electrons and protons together to form neutrons. Being much more massive that electrons, these allow the star to become even more dense.  A neutron star is in effect a giant atomic nucleus!  They spin quickly. Some emit radio waves and are known as pulsars.  They have masses up to about three times the Sun’s mass.

Computer animation of a pulsar in action

Black holes If the mass of the core that remains is more than about three solar masses, even neutrons are not able to survive the gravitational attraction. Gravitational collapse is so powerful that nothing, not even light, can escape. Black holes can therefore only be identified by their gravitational effects on nearby objects. We’ll explore the astrophysics of black holes in a later lecture.

Monday, usual time, usual place Guest lecture by science writer John Gribbin on Extraterrestrial Life