Star Ch. 21: Novae, Supernovae, and the Formation of the Elements Stellar Explosions Star Ch. 21: Novae, Supernovae, and the Formation of the Elements

Standards Understand the scale and contents of the universe, including stars. Describe how stars are powered by nuclear fusion, how temperature and luminosity indicate their age, and how stellar processes create heavier and stable elements that are found throughout the universe.

Life After Death for White Dwarfs Becomes a nova – a type of star that may increase enormously in brightness, by a factor of 10,000 or more, in a matter of days. Means “new” in Latin. So named because they seemed to appear suddenly in the sky.

Life After Death for White Dwarfs The white dwarf undergoes an explosion on its surface that results in a rapid, temporary increase in luminosity. It fades back to normal in a few weeks or months.

a) Nova Persei b) Nova Cygni

Life After Death for White Dwarfs Recurring novae occur in binary systems. The dwarf’s gravitational field pulls matter (hydrogen & helium) from the surface of a main sequence or giant companion star. The stolen gas builds up on the surface of the white dwarf and becomes hotter & denser, eventually reaching a high enough temperature to ignite.

White dwarf pulls matter from a red giant

End of a High-Mass Star As high mass stars evolve, they fuse heavier & heavier elements. As temperature in the star increases with depth, the ash of each burning stage becomes fuel for the next stage. The inner core of the star is made of iron, which is surrounded by shells of silicon, magnesium, neon, oxygen, carbon, helium & hydrogen

Layers of star as it fuses heavy elements

Collapse of the Iron Core Once the inner core begins to change into iron, the star is in trouble. Nuclear fusion involving iron does not produce energy because the nuclei are so compact. So, iron plays the role of fire extinguisher, damping the inferno in the stellar core.

Collapse of the Iron Core The central fires cease for the last time, internal supports dwindle & equilibrium is gone forever. Gravity overwhelms the outward pressure from fusion and the star implodes, falling in on itself.

Collapse of the Iron Core Core temperature rises to 10 billion Kelvin, which gives photons enough energy to split iron into lighter nuclei – called photodisintegration In less than 1 second, the collapsing core undoes all the effects of nuclear fusion that occurred during the previous 10 million years.

Collapse of the Iron Core As core density rises, protons & electrons are crushed together to form neutrons & neutrinos: p+ + e-  no + neutrino: called neutronization of the core. Neutrinos hardly react with matter. They escape into space carrying away energy.

Collapse of the Iron Core Neutrino escape and electron disappearance make matters worse for core stability. There is now nothing to prevent collapse to the point at which neutrons come in contact with one another. Contact halts the collapse, but not until core has overshot point of equilibrium.

Collapse of the Iron Core Core becomes compressed, stops, and then rebounds with a vengeance. It takes only 1 second from the start of the collapse to the “bounce” at neutron contact. An energetic shock wave sweeps through the star at high speed, blasting all overlying layers, including heavy elements outside iron inner core, into space.

Collapse of the Iron Core The star explodes in one of the most energetic events known, and will shine as brightly as the entire galaxy in which it resides for a period of a few days. This death of a high mass star is a core-collapse supernova.

Supernova 1987A near the nebula 30 Doradus

Supernova Explosions Novae and supernovae are driven by very different underlying physical processes. A supernova is more than 1 million times brighter than a nova. The total amount of energy radiated by a supernova is equal to the amount of energy our sun will radiate during its entire 10 billion year lifetime.

Supernova Explosions A star can nova many times, but a supernova only happens once to a star. Two types of supernovae: Type I: hydrogen poor, formed from the detonation of a carbon white dwarf Type II: hydrogen rich, formed by the implosion-explosion of the core of a massive star (core-collapse supernova)

Supernova Remnants Crab nebula: original explosion in 1054 A.D. (observed by Chinese astronomers) Could be seen in broad daylight for a month. Is a type II supernova that is expanding into space at several thousand km/s.

Crab Supernova Remnant

Supernova Remnants Vela supernova remnant: expansion velocities imply it exploded around 9000 B.C. It is only 500 parsecs from Earth, and may have been as bright as the moon for several months.

Vela Supernova Remnant

Supernova Remnants Last observed supernova in our galaxy was Tycho’s supernova in 1572. Helped shatter Aristotelian idea of an unchanging universe.

Tycho’s Supernova in x-rays

Formation of the Elements Stars’ processes are responsible for creating much of the world in which we live. We currently know of 118 different elements. The 81 stable elements found on Earth make up the bulk of matter in the universe. 10 radioactive elements also occur naturally on our planet. 19 radioactive elements have been artificially produced in nuclear laboratories.

Formation of the Elements Hydrogen and most of the helium in the universe are primordial, that is they date from the earliest times. All other elements in the universe are a result of stellar nucleosynthesis: they were formed by nuclear fusion in the heart of stars. (also by processes occurring in supernovae)

Formation of the Elements Galaxies continuously recycle their matter. Each new round of formation creates stars with more heavy elements than proceeding generations had. The sun is a product of many such cycles. We ourselves are another. We are, literally, made of stars. Without the heavy elements synthesized in the hearts of stars, life on Earth would not exist.

Stellar recycling