Life Cycle of a Star Star Life Cycle: Stars are like humans. They are born, live and then die.

Interstellar Medium – the total mass of the gas and dust between the stars. What exists BEFORE stars form?

Nebula – a dense cloud of gas and dust What is a Nebula?

The nebula begins to contract due to gravity and the pressure and temperature increase and becomes a protostar What is a Protostar?

When the temperature gets hot enough, nuclear fusion begins and a main sequence star is born. Inward force of gravity = Outward force of nuclear fusion 90% of all stars What is a Main Sequence Star?

Quick Check The fusion reaction that fuels a main- sequence star converts hydrogen into: A. Carbon B. Iron C. Neon D. Helium

A star loses mass during fusion as energy is released. This decreases the star’s gravity. A star will expand, becoming a Red Giant when the outward force of fusion is greater than the inward force of gravity What is a Red Giant?

As the fuel runs out in a star, fusion slows down. When the outward force of fusion is less than the inward force of gravity, the star will shrink in size, becoming a white dwarf. Pulsar – a rotating white dwarf emitting radio waves. White Dwarf What is a White Dwarf?

Quick Check There are many sizes and colors of stars. Which of the following best describes the category in which the Sun would be placed? A. blue supergiant stars B. red giant stars C. yellow main sequence stars D. white dwarf stars

Supernova – an explosion that marks the end of a very massive star’s life. When it occurs, the exploding star can outshine all of the other stars in the galaxy in total for several days and may leave behind only a crushed core. What is a Supernova?

The life cycle of stars depends on their mass. Small and medium stars become black dwarves once they die. Larger stars become novae and die as: neutron stars A neutron star is an imploded core of an exploded star made up almost entirely of neutrons. A teaspoon of their material would weigh more than all of automobiles in the U.S. together How does a star’s life end?

The life cycle of stars depends on their mass. A black hole is an extremely massive remnant from which light can not escape The most massive stars become supernovae and die as a black hole How does a star’s life end?

Quick Check All stars end up as a supernova and black hole A. True B. False

Large, more massive stars have much more gravity than the sun. – This greater internal pressure causes fusion reactions to occur quickly. – This causes the largest stars to burn their fuel, and eventually run out, much more quickly Larger stars live shorter lives. Bigger stars are brighter and hotter due to the rapid rate of fusion. Why do larger/hotter stars burn their fuel faster and live shorter lives than the Sun?

Where do we get the elements?? All stars spend the majority of their lives fusing hydrogen into helium: the main sequence. When all of the hydrogen in the central regions is gone and converted to helium, the star will begin to “burn” helium into carbon. Massive stars: Stars heavier than about 5 times the mass of the sun can do this with no problem: they burn hydrogen and then helium and then carbon, oxygen, silicon, and so on…until Iron.

Iron is the lightest element that doesn’t release energy when you attempt to fuse it together. You actually end up with less energy than you started with! So instead of generating pressure to hold up the outer layers, the iron fusion actually takes pressure out of the core. Thus, there is nothing left to combat gravity from the outer layers. The result: collapse! ALWAYS REMEMBER THE BALANCE BETWEEN FUSION AND GRAVITY!! This collapse (supernova) happens very, very quickly: about 15 seconds. During the collapse (supernova), the nuclei in the outer parts of the star are pushed together, so close that elements heavier than Iron are formed. Where do we get the elements??

What is the Hertzsprung-Russell Diagram? The stars in the universe are at different stages in their life cycles. Some stars are young and hot; others are older and colder. The Hertzsprung- Russell Diagram (or H-R Diagram) gives us a picture of a star’s life.

What is the Hertzsprung-Russell Diagram? The H-R diagram plots the luminosity (brightness) or absolute magnitude of stars against their surface temperatures. Most stars fall into 4 distinct groups in the H-R diagram, because the groups represent stages in the life cycles of the stars.

What is the Hertzsprung-Russell Diagram? X axis: Plots surface temperature Decreases as you move from left to right

What is the Hertzsprung-Russell Diagram? Y axis: Plots luminosity (brightness) or absolute magnitude Increases as you move from bottom to top

APPARENT MAGNITUDE VS. ABSOLUTE MAGNITUDE Apparent magnitude Absolute magnitude Measure of the amount of a star’s light received on Earth Measure of the amount of light a star actually gives off

What does COLOR indicate about a star? Hot stars are…….. Cool stars are…….. Medium temperature stars are…….. Blue Red Yellow

White dwarfs small, hot stars leftover centers of old stars

Red-Giants When star runs out of energy, the center shrinks and the outer part expands outward. It will grow very large and cool.

Supergiants Very massive cool giant star

Main Sequence Stretches diagonal from the top left to the right bottom Our sun is an average main sequence star

Blue Supergiants Very massive blue stars Quickly use up energy and turn into Giants or Supergiants

Red Dwarfs Low-mass stars Oldest stars in galaxy