Stellar EvolutionStellar Evolution

Basic Structure of StarsBasic Structure of Stars

Mass and composition of stars determine Mass and composition of stars determine nearly all of the other properties of starsnearly all of the other properties of stars

More massive a star is, the greater the More massive a star is, the greater the gravity is, and the hotter and denser the gravity is, and the hotter and denser the star is insidestar is inside

Temperature inside stars determines rate Temperature inside stars determines rate of nuclear reactions, which in turn affects of nuclear reactions, which in turn affects the energy output, or luminosity of the starthe energy output, or luminosity of the star

FusionFusion

Density and temperature increase toward Density and temperature increase toward the center, where energy is generated by the center, where energy is generated by nuclear fusion (hydrogen into helium)nuclear fusion (hydrogen into helium)

Stars not on main sequence either fuse Stars not on main sequence either fuse different elements in their core, or do not different elements in their core, or do not undergo fusion at allundergo fusion at all

Stellar Evolution and Life CycleStellar Evolution and Life Cycle A star changes as it ages A star changes as it ages

because its internal composition because its internal composition changes as nuclear fusion changes as nuclear fusion reactions convert one element reactions convert one element into anotherinto another

As the star’s core composition As the star’s core composition changes, its density increases, its changes, its density increases, its temperature rises, and its temperature rises, and its luminosity increasesluminosity increases

When nuclear fuel runs out, star’s When nuclear fuel runs out, star’s internal structure and mechanism internal structure and mechanism for producing pressure must for producing pressure must change to counteract the force of change to counteract the force of gravitygravity

Star FormationStar Formation

Begins with Begins with cloud of cloud of interstellar interstellar gas and dust, gas and dust, called a called a nebulanebula that that collapses in collapses in on itself as a on itself as a result of its result of its own gravityown gravity

Star FormationStar Formation

As the nebula cloud As the nebula cloud contracts, its contracts, its rotation forces it into rotation forces it into a disk shape with a a disk shape with a hot condensed hot condensed object at the center, object at the center, called a called a protostarprotostar

The condensed The condensed object will become a object will become a new starnew star

Star FormationStar Formation

Eventually, temperature inside protostar is Eventually, temperature inside protostar is hot enough for nuclear fusion reactions to hot enough for nuclear fusion reactions to occuroccur

Once fusion of hydrogen to helium occurs, Once fusion of hydrogen to helium occurs, the star becomes stable because it has the star becomes stable because it has sufficient internal heat to produce the sufficient internal heat to produce the pressure needed to balance the pressure pressure needed to balance the pressure of gravityof gravity

Life Cycle of Average Stars (Our Sun)Life Cycle of Average Stars (Our Sun)

What happens during a star’s life cycle depends What happens during a star’s life cycle depends on its masson its mass

As a star like the Sun converts hydrogen to As a star like the Sun converts hydrogen to helium in its core, it gradually becomes more helium in its core, it gradually becomes more luminous because the core temperature and luminous because the core temperature and density rise and increase the fusion reaction ratedensity rise and increase the fusion reaction rate

Takes 10 billion years for average-sized star to Takes 10 billion years for average-sized star to convert all the hydrogen in its core into heliumconvert all the hydrogen in its core into helium

Life Cycle of Average Stars (Our Sun)Life Cycle of Average Stars (Our Sun)

Only innermost 10% of a star’s Only innermost 10% of a star’s mass can undergo nuclear mass can undergo nuclear reactions because temperatures reactions because temperatures outside the core never get hot outside the core never get hot enough for nuclear reactions to enough for nuclear reactions to occuroccur

When all the hydrogen in a star’s When all the hydrogen in a star’s core is gone, star has a helium core is gone, star has a helium center and the outer layers have center and the outer layers have mostly hydrogen gasmostly hydrogen gas

Some hydrogen will continue to Some hydrogen will continue to undergo reactions in the undergo reactions in the outermost layer of the helium outermost layer of the helium core; energy produced in this core; energy produced in this layer forces out layers of the star layer forces out layers of the star to expand and coolto expand and cool

The star has now become a The star has now become a red red giantgiant

Life Cycle of Average Stars (Our Sun)Life Cycle of Average Stars (Our Sun) While the star is a red giant, While the star is a red giant,

it loses gas from its outer it loses gas from its outer layerslayers

Core becomes hot enough to Core becomes hot enough to for helium to react and form for helium to react and form carboncarbon

Star contracts to a more Star contracts to a more smaller size where it is more smaller size where it is more stablestable

Star never becomes hot Star never becomes hot enough for carbon to react, enough for carbon to react, so star’s energy production so star’s energy production ceases at this pointceases at this point

Outer layers of gas expand Outer layers of gas expand and are driven offand are driven off

This outer layer of gas is This outer layer of gas is called a called a planetary nebulaplanetary nebula

Core of star becomes Core of star becomes exposed as a small, hot exposed as a small, hot object the size of Earth object the size of Earth called acalled a white dwarf white dwarf

White dwarf remains

Life Cycle of Massive StarsLife Cycle of Massive Stars More massive star More massive star

begins its life begins its life same way as same way as average sized average sized stars, but the stars, but the star’s lifetime is star’s lifetime is much shorter much shorter because the star because the star is so luminous and is so luminous and uses its fuel up uses its fuel up quicklyquickly

Undergoes more Undergoes more reaction phases reaction phases and produces and produces more elements in more elements in its interiorits interior

Life Cycle of Massive StarsLife Cycle of Massive Stars Star becomes a red Star becomes a red

giant several times as it giant several times as it expands following the expands following the end of each fusion end of each fusion reactionreaction

As more shells are As more shells are formed, the star expands formed, the star expands to larger size and to larger size and becomes a supergiant becomes a supergiant (EX: Betelgeuse)(EX: Betelgeuse)

Light Echo  Illuminates Dust Around

Supergiant Star V838 Monocerotis (V838 Mon)

Size Comparison of StarsSize Comparison of Stars

Life Cycle of Massive StarsLife Cycle of Massive Stars

Some massive stars lose enough mass to Some massive stars lose enough mass to become white dwarfsbecome white dwarfs

Stars that don’t lose that much mass come to a Stars that don’t lose that much mass come to a more violent endmore violent end

Once reactions in the core have produced iron, Once reactions in the core have produced iron, no future reactions can occur and the core the no future reactions can occur and the core the star quickly collapses on itselfstar quickly collapses on itself

Neutron starNeutron star is formed while outer gas layers is formed while outer gas layers are blown off in a massive explosion called a are blown off in a massive explosion called a supernovasupernova

Life Cycle of Massive StarsLife Cycle of Massive Stars

Neutron Star

Supernova explosion

Life Cycle of Massive StarsLife Cycle of Massive Stars

When they are formed When they are formed neutron stars rotate in neutron stars rotate in space. As they compress space. As they compress and shrink, the rotation and shrink, the rotation occurs faster. Those occurs faster. Those bodies that are still bodies that are still spinning rapidly may emit spinning rapidly may emit radiation that from Earth radiation that from Earth appears to blink on and appears to blink on and off as the star spins, like off as the star spins, like the beam of light from a the beam of light from a turning lighthouse. This turning lighthouse. This "pulsing" appearance "pulsing" appearance gives some neutron stars gives some neutron stars the name the name pulsarspulsars. .

The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration. Credit: Casey Reed

Life Cycle of Massive StarsLife Cycle of Massive Stars

Some stars too massive to form neutron Some stars too massive to form neutron stars. The core of these stars collapses stars. The core of these stars collapses forever, compacting matter into smaller forever, compacting matter into smaller and smaller volumesand smaller volumes

The small, extremely dense object that The small, extremely dense object that remains is called a remains is called a black holeblack hole

Called a black hole because its gravity is Called a black hole because its gravity is so great that nothing, not even light, can so great that nothing, not even light, can escape escape

Black HolesBlack Holes Estimated that black holes will Estimated that black holes will

consume other stars at a rate of consume other stars at a rate of about once every ten thousand about once every ten thousand years in a typical galaxyyears in a typical galaxy

Journey to a Black Hole!Journey to a Black Hole! What happens if you fall into a What happens if you fall into a

black hole?black hole?

Life Cycle of Stars – Summary Life Cycle of Stars – Summary