The life cycle of a star u All stars go through four main stages u Nebulae u Protostar u Main sequence u Red giant

Nebulae (nebula, singular) u Clouds of gas (mostly Hydrogen and Helium) and dust where stars form u Gas and dust collect together as gravity pulls inward u It heats up in the center from the pressure u Emits bright infrared radiation

Protostar u When the center of the material heats up to 10,000 degrees K, a protostar is formed and the new star glows. (Celcius = Kelvin) u The gas and dust continue to collect and pressure and temperature build up within the protostar as it becomes denser

Main sequence star u When the core reaches a critical temperature, hydrogen fusion begins u 4 hydrogen atoms are fused into one helium atom u A small amount of matter turns into a large amount of energy ( E = mc 2 ) u E = energy in joules u M = the mass converted to energy u C = the speed of light in m/s u The energy from the fusion pours out from the core and balances the force of gravity that would make the star collapse

Main sequence con’t u How long the star stays on the main sequence fusing Hydrogen depends on its mass u Larger stars (up to 20x the size of our sun) are hotter and burn up their fuel faster, have shorter lives (millions of years). u Our sun, a medium yellow star, is expected to spend 10 billion years here u An x-ray image of the sun is shown righthttp://chandra.harvard.edu/xray_so urces/solar_system.html

Red Giant (from main sequence stars of.8 to 8 solar masses u When the hydrogen in the core is used up, fusion stops and the star collapses inward due to gravity u This heats up the core and the star fuses Helium to Carbon (100 million degrees K) u The outer layers of the star expand outward from the energy becoming cooler and redder(surface temp drops to K) u Brigtness increases 1000 to 10,000 X u The star is now up to 100x bigger and is called a red giant

Planetary nebula and White Dwarfs (the fate for stars.8 to 8 times the mass of our sun ) u The red giant can only fuse Carbon for a short time (100 million years) u Some stars go through a pulsating stage called a Mira Variable Star where outer layers expand and contract u Eventually the outer layers are shed as a cloud called a planetary nebula u These are only visible for 50,000 years because the gases become so thin

White Dwarfs u About 20% the stars mass will be left behind cooling and shrinking to a few thousand miles in diameter. This is a white dwarf u Extremely hot (temp about 20,000 K) but low magnitude due to small size u Made primarily of Carbon u White dwarfs cannot exceed 1.4 solar masses u The electron cloud repulsive forces in the atom are strong enough to stop any further collapse due to gravity when the size is 1.4 solar masses or less

A Planetary Nebula “The Cat’s Eye”

Black dwarf u When all the heat is radiated out into space, after about 12 billion years, the star is a dark cold mass called a black dwarf. u The universe is not old enough for any black dwarfs to exist yet.

Brown dwarfs u If a protostar forms with a mass less than.08 solar masses, its internal temperature never gets hot enough for fusion to occur. u This is called a brown dwarf and is a failed star u Stars must have a mass about 75x the mass of Jupiter or 8% the mass of the sun to get mot enough (3 million K) in its core to start the fusion reaction. u Brown dwarfs emit infrared energy due to the potential energy of collapse converted into kinetic energy. It can shine for 15 million years

Red dwarfs u Proxima Centauri, the nearest star to our sun is a red dwarf u Stars less than.8 solar masses do not evolve u They use up their H fuel very, very slowly, beyond the current age of the universe or 14 billion years.

Chandra image -Milky Way u The images of the constellation Sagittarius show a change in light generated by a black hole at the center of our galaxy u Astronomers think a mass the sizes of Mercury was devoured by the black hole about 50 years ago, causing the x-ray outburst which reflected off gas clouds near Sagittarius A.