Chapter 25Chapter 25Stars and GalaxiesStars and Galaxies

Section 2Section 2

Stellar EvolutionStellar Evolution

Notes 25-2Notes 25-2

NebulaNebula

First stage of a starFirst stage of a star’’s lifes life Cloud of dust and gasCloud of dust and gas

70% H, 28% He, 2% heavy elements70% H, 28% He, 2% heavy elements Begins to contract when a star near by Begins to contract when a star near by

explodes or another nebula collides with itexplodes or another nebula collides with it Gravity increases as the matter gets closerGravity increases as the matter gets closer

Region of dense matter shrinks and begins to Region of dense matter shrinks and begins to spinspin

Rate of spin increases as shrinking continuesRate of spin increases as shrinking continues

NebulasNebulas

ProtostarProtostar

Shrinking, spinning gases flatten into a Shrinking, spinning gases flatten into a disk with a central portion called a disk with a central portion called a protostarprotostar

Pressure builds and temperature buildsPressure builds and temperature buildsEventually the gas heats to the point it Eventually the gas heats to the point it

becomes plasma (millions of years)becomes plasma (millions of years)Temp. increases to 10,000,000Temp. increases to 10,000,000°C°CNuclear fusion beginsNuclear fusion beginsA star is bornA star is born

ProtostarProtostar

Main Sequence StarsMain Sequence Stars

Second stage of life for a starSecond stage of life for a starLongest stage of lifeLongest stage of life

Fusion releases energy as hydrogen Fusion releases energy as hydrogen atoms fuse to become helium atomsatoms fuse to become helium atoms

Energy moves outward but is kept in Energy moves outward but is kept in check by gravitycheck by gravity

Star maintains a stable size while there is Star maintains a stable size while there is enough hydrogen to keep fusion goingenough hydrogen to keep fusion going

Sun (Main Sequence Star)Sun (Main Sequence Star)

Giants and SupergiantsGiants and Supergiants

Third stage in the life of a starThird stage in the life of a starWhen almost all of the hydrogen has fused When almost all of the hydrogen has fused

into heliuminto heliumCore of star contracts and causes increased Core of star contracts and causes increased

temperaturestemperaturesThis allows for the fusion of helium atoms into This allows for the fusion of helium atoms into

carbon atomscarbon atoms

He and H fusion cause lots of energy that He and H fusion cause lots of energy that causes the outer shell to expandcauses the outer shell to expand

Giants and SupergiantsGiants and Supergiants

As it expands it becomes coolerAs it expands it becomes cooler It is no longer a main sequence starIt is no longer a main sequence star

It is now a red giant or red supergiantIt is now a red giant or red supergiantGiants 10 times bigger than the sunGiants 10 times bigger than the sunSupergiants 100 times bigger than the sunSupergiants 100 times bigger than the sun

Scientist estimate that the sun has only Scientist estimate that the sun has only converted 5% of its original hydrogenconverted 5% of its original hydrogenStill lots of time until the giant stageStill lots of time until the giant stage

SupergiantSupergiant

White DwarfWhite Dwarf

Last stages of a giant starLast stages of a giant starStar loses its outer gasesStar loses its outer gases

Core showsCore showsA planetary nebula occurs when the outer A planetary nebula occurs when the outer

gases are heated by the core and escapegases are heated by the core and escapeThe core collapses and leaves a hot, The core collapses and leaves a hot,

dense mass – white dwarfdense mass – white dwarf

White Dwarf in Planetary NebulaWhite Dwarf in Planetary Nebula

White Dwarf

Black DwarfBlack Dwarf

A white dwarf can cool and become faintA white dwarf can cool and become faintOnce it no long gives off energy it is deadOnce it no long gives off energy it is dead

Called a black dwarfCalled a black dwarfDonDon’’t exist yett exist yetThe universe isnThe universe isn’’t old enough to have t old enough to have

produced any yetproduced any yet

NovaNovaA white dwarf can have a different endingA white dwarf can have a different ending It can explodeIt can explode

Called a Nova Called a Nova May become a nova more than onceMay become a nova more than once

Becomes 1 million times brighter than the Becomes 1 million times brighter than the sunsunBrightness lasts only a few days then it Brightness lasts only a few days then it

returns to normalreturns to normalBelieved only to happen when a white Believed only to happen when a white

dwarf revolves around a main sequence or dwarf revolves around a main sequence or giant stargiant star

SupernovaSupernova

Last stage of a supergiantLast stage of a supergiantSupergiant explodes up to 100 times Supergiant explodes up to 100 times

brighter than a novabrighter than a novaBlows itself apartBlows itself apart

Fusion begins againFusion begins againCarbon fuses into heavier elements like Carbon fuses into heavier elements like

magnesiummagnesiumThese fuse into ironThese fuse into iron

SupernovaSupernova

Fusion continues until the core is ironFusion continues until the core is iron Iron absorbs energy from the gravity Iron absorbs energy from the gravity

contracting the mattercontracting the matterCore collapses and causes the outer layers to Core collapses and causes the outer layers to

explodeexplode

Neutron StarNeutron Star

The core of a supernova contracts to form The core of a supernova contracts to form a neutron stara neutron starMost dense star in the universeMost dense star in the universe

One spoonful of the star would weigh 100 million One spoonful of the star would weigh 100 million tons on the earthtons on the earth

Neutron star rotates quicklyNeutron star rotates quicklyCan give off two beams of radiation Can give off two beams of radiation

Called pulsarsCalled pulsars

Black HoleBlack Hole If the core of the supernova is too If the core of the supernova is too

massive, it wonmassive, it won’’t make a neutron star but t make a neutron star but instead a black holeinstead a black hole

It is so massive and gravity so strong that It is so massive and gravity so strong that not even light can escape itnot even light can escape it

Can only detect black holes by observing Can only detect black holes by observing the matter and stars around itthe matter and stars around itXrays are given off just before matter is pulled Xrays are given off just before matter is pulled

into the black holeinto the black holeCan see the XraysCan see the Xrays

More SitesMore Sites

http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html

http://amazing-space.stsci.edu/resources/explorations/blackholes/

Very cool site!Very cool site!

http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html