The Star Cycle

Birth Stars begin in a DARK NEBULA (cloud of gas and dust)… aka the STELLAR NURSERY The nebula begins to contract due to gravity in response to some interstellar disturbance One or more PROTOSTARS are formed

Birth The protostar continues to collapse until its centre reaches a few million degrees K Nuclear fusion begins. Hydrogen is converted to Helium.

Brown Dwarf Brown Dwarf - a protostar that does not have enough mass to reach the required temperature and pressure for nuclear fusion

Main Sequence Pressure, from energy released during fusion, balances gravitational contraction Low mass stars take about 2 million years to reach main sequence High mass stars only take 10, 000 years

Main Sequence Lowest mass main sequence stars are Red Dwarfs. Surface temp: 3000K High mass main sequence stars are Blue Giants. Surface temp: 30,000K +

Aging: Low Mass When hydrogen in the core is all used up: –The core of helium collapses –The temp in the core increases –The area around the core expands Becomes a Red Giant

Aging: Low Mass Eventually, the helium core ignites Carbon and Oxygen are produced from “Helium burning”

Aging: Low Mass Late in the life of a Red Giant, oxygen and carbon can be “dredged up” by convection currents. A star with enough carbon on its surface is called a Carbon Star

Aging: Low Mass If the mass is less than 8 solar masses, nuclear fusion stops with the production of Carbon and Oxygen This is the fate of our Sun

Death: Low Mass Low mass stars eject their outer layers in the form of Planetary Nebula The carbon and oxygen core is left behind as a White Dwarf

Death: Low Mass The White Dwarf is very hot and very small Small white dwarfs are more massive than large white dwarfs

Death: Low Mass The White Dwarf eventually consumes the last of its fuel Becomes a cold, dense, non-luminous Black Dwarf

Aging: High Mass If a star is greater than 8 solar masses, fusion does not stop with Helium burning The carbon and oxygen core contracts and temp increases

Aging: High Mass Surface expands further. Becomes a Red Supergiant. Surface as large as the orbit of Jupiter (1.5 billion km in diameter). Core the size of Earth

Aging: High Mass Carbon and oxygen burning produce Neon, Magnesium, Silicon, and Sulphur If it is massive enough, these will later form nickel, iron and other elements of similar atomic weight

Aging: High Mass When the core is full of iron, no more fusion is possible.

Death: High Mass Supernova –Core reaches a density of 4x10 17 kg/m 3. –Regions surrounding the core rush inward at unbelievable speeds –Increased pressure and temperature force the material back out –When shock waves reach the surface of the star, the outer layers explode

Death: High Mass Depending on the Mass, the supernova explosion will leave behind one of three things: 1. White Dwarf –Glowing core similar to the remains of a low mass star. Its high mass will mean it is very small.

Death: High Mass 2. Neutron Star –Incredibly compact stellar corpse –Escape velocity is about half the speed of light –All protons and electrons are converted to neutrons Fast spinning neutron stars send out radio waves and are called PULSARS

Death: High Mass 3. Black Hole –The burned out core presses inward to create a density even greater than the neutron star –Escape velocity is greater than the speed of light –The gravity of the mass alters the fabric of spacetime

Death: High Mass 3. Black Hole –It forms a rapidly rotating disk around itself, called the ACCRETION DISK –X-rays are given off and matter is ejected in jets –Video: shows/other-shows/videos/how-the- universe-works-birth-of-a-black- hole.htm