Stars and Stellar EvolutionStars and Stellar Evolution

Unit 6: AstronomyUnit 6: AstronomyUnit 6: AstronomyUnit 6: Astronomy

What are stars?What are stars?

Stars = spheres of very hot gasStars = spheres of very hot gasNearest star to Earth is the sunNearest star to Earth is the sunConstellations = group of stars named Constellations = group of stars named

for a mythological charactersfor a mythological characters 8888

Stars = spheres of very hot gasStars = spheres of very hot gasNearest star to Earth is the sunNearest star to Earth is the sunConstellations = group of stars named Constellations = group of stars named

for a mythological charactersfor a mythological characters 8888

Characteristics of StarsCharacteristics of Stars

Star color and Star color and temperaturetemperature Color can give us a Color can give us a

clue to star’s clue to star’s temperaturetemperature

Very hot (above Very hot (above 30,000 K) = blue30,000 K) = blue

Cooler = redCooler = red In-between (5000-In-between (5000-

6000 K) = yellow6000 K) = yellow

Star color and Star color and temperaturetemperature Color can give us a Color can give us a

clue to star’s clue to star’s temperaturetemperature

Very hot (above Very hot (above 30,000 K) = blue30,000 K) = blue

Cooler = redCooler = red In-between (5000-In-between (5000-

6000 K) = yellow6000 K) = yellow

Characteristics of StarsCharacteristics of Stars

Binary stars and stellar Binary stars and stellar massmass Binary star = stars that Binary star = stars that

orbit each other (Pair)orbit each other (Pair) Because of gravityBecause of gravity 50% of all stars50% of all stars

Can calculate mass of Can calculate mass of starstar

Equal mass --> center Equal mass --> center of mass halfway of mass halfway between starsbetween stars

Size of orbits known --> Size of orbits known --> masses can be masses can be calculatedcalculated

Binary stars and stellar Binary stars and stellar massmass Binary star = stars that Binary star = stars that

orbit each other (Pair)orbit each other (Pair) Because of gravityBecause of gravity 50% of all stars50% of all stars

Can calculate mass of Can calculate mass of starstar

Equal mass --> center Equal mass --> center of mass halfway of mass halfway between starsbetween stars

Size of orbits known --> Size of orbits known --> masses can be masses can be calculatedcalculated

Distances to StarsDistances to Stars

Light-year = distance Light-year = distance light travels in a year light travels in a year (9.5 trillion kilometers)(9.5 trillion kilometers)

Parallax = slight shifting Parallax = slight shifting in the apparent position in the apparent position of a nearby star due to of a nearby star due to the orbital motion of the the orbital motion of the EarthEarth Photographs Photographs

(comparisons) --> angle(comparisons) --> angle Nearest = largest angles; Nearest = largest angles;

distant = too small to distant = too small to measuremeasure

Only a few thousand stars Only a few thousand stars are knownare known

Light-year = distance Light-year = distance light travels in a year light travels in a year (9.5 trillion kilometers)(9.5 trillion kilometers)

Parallax = slight shifting Parallax = slight shifting in the apparent position in the apparent position of a nearby star due to of a nearby star due to the orbital motion of the the orbital motion of the EarthEarth Photographs Photographs

(comparisons) --> angle(comparisons) --> angle Nearest = largest angles; Nearest = largest angles;

distant = too small to distant = too small to measuremeasure

Only a few thousand stars Only a few thousand stars are knownare known

How bright is that star?How bright is that star?

Brightness = Brightness = magnitudemagnitude

Apparent magnitude Apparent magnitude = a star’s brightness = a star’s brightness as it appears from as it appears from EarthEarth How big it isHow big it is How hot it isHow hot it is How far away it isHow far away it is Larger number = Larger number =

dimmerdimmer

Brightness = Brightness = magnitudemagnitude

Apparent magnitude Apparent magnitude = a star’s brightness = a star’s brightness as it appears from as it appears from EarthEarth How big it isHow big it is How hot it isHow hot it is How far away it isHow far away it is Larger number = Larger number =

dimmerdimmer

-1.4

How bright is that star?How bright is that star?

Absolute magnitude = how bright a star Absolute magnitude = how bright a star actually isactually is Magnitude of star if I was a distance of Magnitude of star if I was a distance of

32.6 light-years32.6 light-years Ex: Sun = apparent magnitude: -26.7, Ex: Sun = apparent magnitude: -26.7,

absolute magnitude: 5absolute magnitude: 5 More negative = brighter, more positive = More negative = brighter, more positive =

dimmerdimmer

Absolute magnitude = how bright a star Absolute magnitude = how bright a star actually isactually is Magnitude of star if I was a distance of Magnitude of star if I was a distance of

32.6 light-years32.6 light-years Ex: Sun = apparent magnitude: -26.7, Ex: Sun = apparent magnitude: -26.7,

absolute magnitude: 5absolute magnitude: 5 More negative = brighter, more positive = More negative = brighter, more positive =

dimmerdimmer

Hertzsprung-Russell DiagramHertzsprung-Russell Diagram

H-R diagram shows the H-R diagram shows the relationship between the relationship between the absolute magnitude and absolute magnitude and temperature of starstemperature of stars Can also help us infer Can also help us infer

distance, life span massdistance, life span mass Stars are plotted Stars are plotted

according to their according to their temperature and temperature and absolute magnitudeabsolute magnitude

Interpret stellar evolutionInterpret stellar evolution Birth, age, deathBirth, age, death

H-R diagram shows the H-R diagram shows the relationship between the relationship between the absolute magnitude and absolute magnitude and temperature of starstemperature of stars Can also help us infer Can also help us infer

distance, life span massdistance, life span mass Stars are plotted Stars are plotted

according to their according to their temperature and temperature and absolute magnitudeabsolute magnitude

Interpret stellar evolutionInterpret stellar evolution Birth, age, deathBirth, age, death

H-R diagramH-R diagram

Bright stars are near Bright stars are near the top and dimmer the top and dimmer stars are near the stars are near the bottombottom

About 90% are About 90% are main-sequence main-sequence starsstars Hottest = brightestHottest = brightest Coolest = dimmestCoolest = dimmest

Bright stars are near Bright stars are near the top and dimmer the top and dimmer stars are near the stars are near the bottombottom

About 90% are About 90% are main-sequence main-sequence starsstars Hottest = brightestHottest = brightest Coolest = dimmestCoolest = dimmest

H-R diagramH-R diagram

Brightness of main-Brightness of main-sequence stars are sequence stars are related to massrelated to mass Hottest blue stars are 50 Hottest blue stars are 50

times more massive than times more massive than the sunthe sun

Coolest red stars and Coolest red stars and only 1/10 as massiveonly 1/10 as massive

Main-sequence stars Main-sequence stars appear in decreasing appear in decreasing orderorder

Hotter, more massive Hotter, more massive blue stars --> cooler, blue stars --> cooler, less massive red starsless massive red stars

Brightness of main-Brightness of main-sequence stars are sequence stars are related to massrelated to mass Hottest blue stars are 50 Hottest blue stars are 50

times more massive than times more massive than the sunthe sun

Coolest red stars and Coolest red stars and only 1/10 as massiveonly 1/10 as massive

Main-sequence stars Main-sequence stars appear in decreasing appear in decreasing orderorder

Hotter, more massive Hotter, more massive blue stars --> cooler, blue stars --> cooler, less massive red starsless massive red stars

H-R diagramH-R diagram

Red giantsRed giants Above and to right of Above and to right of

main-sequence starsmain-sequence stars Size --> compare Size --> compare

them with stars of them with stars of known size that have known size that have same temperaturesame temperature

Supergiants = Supergiants = biggerbigger Ex: BetelgeuseEx: Betelgeuse

Red giantsRed giants Above and to right of Above and to right of

main-sequence starsmain-sequence stars Size --> compare Size --> compare

them with stars of them with stars of known size that have known size that have same temperaturesame temperature

Supergiants = Supergiants = biggerbigger Ex: BetelgeuseEx: Betelgeuse

Betelgeuse

H-R diagramH-R diagram

White dwarfsWhite dwarfs Lower-central partLower-central part Fainter than main-Fainter than main-

sequence stars of sequence stars of same temperaturesame temperature

White dwarfsWhite dwarfs Lower-central partLower-central part Fainter than main-Fainter than main-

sequence stars of sequence stars of same temperaturesame temperature

Variable StarsVariable Stars

Stars might fluctuate Stars might fluctuate in brightnessin brightness Cepheid variables = Cepheid variables =

brighter and fainter in brighter and fainter in regular patternregular pattern

Nova = sudden Nova = sudden brightening of a starbrightening of a star

Outer layer ejected at Outer layer ejected at high speedhigh speed

Returns to original Returns to original brightnessbrightness

Binary systemsBinary systems

Stars might fluctuate Stars might fluctuate in brightnessin brightness Cepheid variables = Cepheid variables =

brighter and fainter in brighter and fainter in regular patternregular pattern

Nova = sudden Nova = sudden brightening of a starbrightening of a star

Outer layer ejected at Outer layer ejected at high speedhigh speed

Returns to original Returns to original brightnessbrightness

Binary systemsBinary systems

How does the H-R diagram predict stellar evolution?How does the H-R diagram predict stellar evolution?

Illustrate changes Illustrate changes that take place in a that take place in a star in its lifetimestar in its lifetime

Position on H-R Position on H-R diagramdiagram Represents color Represents color

and absolute and absolute magnitude at various magnitude at various stages of evolutionstages of evolution

Illustrate changes Illustrate changes that take place in a that take place in a star in its lifetimestar in its lifetime

Position on H-R Position on H-R diagramdiagram Represents color Represents color

and absolute and absolute magnitude at various magnitude at various stages of evolutionstages of evolution

Stellar EvolutionStellar Evolution

How stars are born, age and dieHow stars are born, age and dieStudy stars of different agesStudy stars of different ages

How stars are born, age and dieHow stars are born, age and dieStudy stars of different agesStudy stars of different ages

Life of a StarLife of a Star

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Star BirthStar Birth

Born in nebula = Born in nebula = dark, cool, dark, cool, interstellar clouds of interstellar clouds of gas and dustgas and dust Milky Way = 92% Milky Way = 92%

hydrogen, 7% heliumhydrogen, 7% helium Dense --> contracts Dense --> contracts

--> gravity squeezes --> gravity squeezes particles toward particles toward center --> energy center --> energy converted into heat converted into heat energyenergy

Born in nebula = Born in nebula = dark, cool, dark, cool, interstellar clouds of interstellar clouds of gas and dustgas and dust Milky Way = 92% Milky Way = 92%

hydrogen, 7% heliumhydrogen, 7% helium Dense --> contracts Dense --> contracts

--> gravity squeezes --> gravity squeezes particles toward particles toward center --> energy center --> energy converted into heat converted into heat energyenergy

Protostar StageProtostar Stage

Protostar = a developing star not yet hot Protostar = a developing star not yet hot enough to engage in nuclear fusionenough to engage in nuclear fusion Contraction continues --> collapse causes Contraction continues --> collapse causes

the core to heat much more intensely than the core to heat much more intensely than the outer layerthe outer layer

When is a star born?When is a star born? Core of protostar reaches about 10 million Core of protostar reaches about 10 million

K --> nuclear fusion of hydrogen startsK --> nuclear fusion of hydrogen starts

Protostar = a developing star not yet hot Protostar = a developing star not yet hot enough to engage in nuclear fusionenough to engage in nuclear fusion Contraction continues --> collapse causes Contraction continues --> collapse causes

the core to heat much more intensely than the core to heat much more intensely than the outer layerthe outer layer

When is a star born?When is a star born? Core of protostar reaches about 10 million Core of protostar reaches about 10 million

K --> nuclear fusion of hydrogen startsK --> nuclear fusion of hydrogen starts

Balanced ForcesBalanced Forces

Hydrogen fusionHydrogen fusion Gases increase motion --Gases increase motion --

> increase in outward > increase in outward gas pressuregas pressure

Outward pressure from Outward pressure from fusion balances inward fusion balances inward force of gravityforce of gravity

Becomes main-sequence Becomes main-sequence star (stable)star (stable)

Hydrogen fusionHydrogen fusion Gases increase motion --Gases increase motion --

> increase in outward > increase in outward gas pressuregas pressure

Outward pressure from Outward pressure from fusion balances inward fusion balances inward force of gravityforce of gravity

Becomes main-sequence Becomes main-sequence star (stable)star (stable)

Main-sequence stageMain-sequence stage

Balanced between forces of gravity (trying to Balanced between forces of gravity (trying to squeeze into smaller space) and gas squeeze into smaller space) and gas pressure (trying to expand it)pressure (trying to expand it)

Hydrogen fusion for few billion yearsHydrogen fusion for few billion years Hot, massive blue stars deplete fuel in only few Hot, massive blue stars deplete fuel in only few

million yearsmillion years Least massive main sequence remain stable for Least massive main sequence remain stable for

hundreds of billions of yearshundreds of billions of years Yellow star (sun) = 10 billion yearsYellow star (sun) = 10 billion years

90% of life as main-sequence star90% of life as main-sequence star Runs out of hydrogen fuel in core --> diesRuns out of hydrogen fuel in core --> dies

Balanced between forces of gravity (trying to Balanced between forces of gravity (trying to squeeze into smaller space) and gas squeeze into smaller space) and gas pressure (trying to expand it)pressure (trying to expand it)

Hydrogen fusion for few billion yearsHydrogen fusion for few billion years Hot, massive blue stars deplete fuel in only few Hot, massive blue stars deplete fuel in only few

million yearsmillion years Least massive main sequence remain stable for Least massive main sequence remain stable for

hundreds of billions of yearshundreds of billions of years Yellow star (sun) = 10 billion yearsYellow star (sun) = 10 billion years

90% of life as main-sequence star90% of life as main-sequence star Runs out of hydrogen fuel in core --> diesRuns out of hydrogen fuel in core --> dies

Red Giant StageRed Giant Stage

Zone of hydrogen fusion moves outward --> Zone of hydrogen fusion moves outward --> helium corehelium core All hydrogen in core is used up (no fusion in core) All hydrogen in core is used up (no fusion in core)

--> still taking place in outer shell--> still taking place in outer shell Not enough pressure to support itself against force of Not enough pressure to support itself against force of

gravity --> core contractsgravity --> core contracts Core gets hotter --> hydrogen fusion in outer shell Core gets hotter --> hydrogen fusion in outer shell

increases --> expands outer layer --> giant bodyincreases --> expands outer layer --> giant body Surface cools --> redSurface cools --> red Core keeps heating up and converts helium to carbon to Core keeps heating up and converts helium to carbon to

produce energyproduce energy

Zone of hydrogen fusion moves outward --> Zone of hydrogen fusion moves outward --> helium corehelium core All hydrogen in core is used up (no fusion in core) All hydrogen in core is used up (no fusion in core)

--> still taking place in outer shell--> still taking place in outer shell Not enough pressure to support itself against force of Not enough pressure to support itself against force of

gravity --> core contractsgravity --> core contracts Core gets hotter --> hydrogen fusion in outer shell Core gets hotter --> hydrogen fusion in outer shell

increases --> expands outer layer --> giant bodyincreases --> expands outer layer --> giant body Surface cools --> redSurface cools --> red Core keeps heating up and converts helium to carbon to Core keeps heating up and converts helium to carbon to

produce energyproduce energy

Burnout and Death of StarsBurnout and Death of Stars

Low-mass starsLow-mass stars 1/2 mass of sun1/2 mass of sun Consume fuel slowly --> main sequence for Consume fuel slowly --> main sequence for

up to 100 billion yearsup to 100 billion years Consume all their hydrogen --> collapse Consume all their hydrogen --> collapse

into white dwarfsinto white dwarfs Eventually ends up as a black dwarfEventually ends up as a black dwarf

Low-mass starsLow-mass stars 1/2 mass of sun1/2 mass of sun Consume fuel slowly --> main sequence for Consume fuel slowly --> main sequence for

up to 100 billion yearsup to 100 billion years Consume all their hydrogen --> collapse Consume all their hydrogen --> collapse

into white dwarfsinto white dwarfs Eventually ends up as a black dwarfEventually ends up as a black dwarf

Burnout and Death of StarsBurnout and Death of Stars

Medium-mass starsMedium-mass stars Similar to sunSimilar to sun Turn into red giants --> once fuel is gone, Turn into red giants --> once fuel is gone,

collapse as white dwarfs --> eventually collapse as white dwarfs --> eventually black dwarfblack dwarf

Medium-mass starsMedium-mass stars Similar to sunSimilar to sun Turn into red giants --> once fuel is gone, Turn into red giants --> once fuel is gone,

collapse as white dwarfs --> eventually collapse as white dwarfs --> eventually black dwarfblack dwarf

Burnout and Death of StarsBurnout and Death of Stars

Burnout and Death of StarsBurnout and Death of Stars

Massive starsMassive stars Shorter life spansShorter life spans End lives in supernovas End lives in supernovas

= becomes 1 million = becomes 1 million times brighter (rare)times brighter (rare)

Consumes most of its Consumes most of its fuel -> gas pressure does fuel -> gas pressure does not balance gravitational not balance gravitational pull --> collapses --> pull --> collapses --> huge implosion --> shock huge implosion --> shock wave moves out and wave moves out and destroys the star (outer destroys the star (outer shell blasted into space)shell blasted into space)

Massive starsMassive stars Shorter life spansShorter life spans End lives in supernovas End lives in supernovas

= becomes 1 million = becomes 1 million times brighter (rare)times brighter (rare)

Consumes most of its Consumes most of its fuel -> gas pressure does fuel -> gas pressure does not balance gravitational not balance gravitational pull --> collapses --> pull --> collapses --> huge implosion --> shock huge implosion --> shock wave moves out and wave moves out and destroys the star (outer destroys the star (outer shell blasted into space)shell blasted into space)

Formation and Destruction of StarsFormation and Destruction of Stars

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Stellar RemnantsStellar Remnants

All stars collapse into one of the three: white All stars collapse into one of the three: white dwarf, neutron star, or black holedwarf, neutron star, or black hole

White dwarf = remains of low-mass and medium-White dwarf = remains of low-mass and medium-mass starsmass stars Extremely small with high densities Extremely small with high densities Surface becomes very hotSurface becomes very hot

No energy --> becomes cooler and dimmerNo energy --> becomes cooler and dimmer Last stage of white dwarf = black dwarf (small, cold Last stage of white dwarf = black dwarf (small, cold

body)body) Smallest = most massiveSmallest = most massive

Collapse of larger starsCollapse of larger stars Largest = least massiveLargest = least massive

Collapse of less massive starsCollapse of less massive stars

All stars collapse into one of the three: white All stars collapse into one of the three: white dwarf, neutron star, or black holedwarf, neutron star, or black hole

White dwarf = remains of low-mass and medium-White dwarf = remains of low-mass and medium-mass starsmass stars Extremely small with high densities Extremely small with high densities Surface becomes very hotSurface becomes very hot

No energy --> becomes cooler and dimmerNo energy --> becomes cooler and dimmer Last stage of white dwarf = black dwarf (small, cold Last stage of white dwarf = black dwarf (small, cold

body)body) Smallest = most massiveSmallest = most massive

Collapse of larger starsCollapse of larger stars Largest = least massiveLargest = least massive

Collapse of less massive starsCollapse of less massive stars

Stellar RemnantsStellar Remnants

Neutron star = Neutron star = smaller and more smaller and more massive than white massive than white dwarfsdwarfs Remnants of Remnants of

supernovassupernovas Composed entirely of Composed entirely of

neutronsneutrons

Neutron star = Neutron star = smaller and more smaller and more massive than white massive than white dwarfsdwarfs Remnants of Remnants of

supernovassupernovas Composed entirely of Composed entirely of

neutronsneutrons

Stellar RemnantsStellar Remnants

Supernovae = outer Supernovae = outer layer of star is layer of star is ejected --> collapse ejected --> collapse into hot neutron starinto hot neutron star

Pulsar = emits short Pulsar = emits short bursts of radio bursts of radio energyenergy Remains of Remains of

supernovasupernova

Supernovae = outer Supernovae = outer layer of star is layer of star is ejected --> collapse ejected --> collapse into hot neutron starinto hot neutron star

Pulsar = emits short Pulsar = emits short bursts of radio bursts of radio energyenergy Remains of Remains of

supernovasupernova

Stellar RemnantsStellar Remnants

Black hole = a massive Black hole = a massive star that has collapsed star that has collapsed to such a small volume to such a small volume that its gravity prevents that its gravity prevents the escape of the escape of everythingeverything Cannot be seenCannot be seen Evidence of matter being Evidence of matter being

rapidly swept into an rapidly swept into an areaarea

AnimationAnimation

Black hole = a massive Black hole = a massive star that has collapsed star that has collapsed to such a small volume to such a small volume that its gravity prevents that its gravity prevents the escape of the escape of everythingeverything Cannot be seenCannot be seen Evidence of matter being Evidence of matter being

rapidly swept into an rapidly swept into an areaarea

AnimationAnimation

Where did the elements of the universe come from?Where did the elements of the universe come from? After the universe became cool enough for After the universe became cool enough for

atoms to form, they began to clump together atoms to form, they began to clump together into clouds of gasinto clouds of gas First stars made up of mostly hydrogen with a First stars made up of mostly hydrogen with a

small amount of heliumsmall amount of helium Heavier elements like iron and silicon not yet made Heavier elements like iron and silicon not yet made

inside starsinside stars More and more stars formed, became main-More and more stars formed, became main-

sequence, grew old, and diedsequence, grew old, and died More and more matter was fused into heavier elements More and more matter was fused into heavier elements

and expelled back into interstellar space by supernovas and expelled back into interstellar space by supernovas and dying red giant starsand dying red giant stars

Eventually our sun and its planets formed from this Eventually our sun and its planets formed from this interstellar gas and dustinterstellar gas and dust

After the universe became cool enough for After the universe became cool enough for atoms to form, they began to clump together atoms to form, they began to clump together into clouds of gasinto clouds of gas First stars made up of mostly hydrogen with a First stars made up of mostly hydrogen with a

small amount of heliumsmall amount of helium Heavier elements like iron and silicon not yet made Heavier elements like iron and silicon not yet made

inside starsinside stars More and more stars formed, became main-More and more stars formed, became main-

sequence, grew old, and diedsequence, grew old, and died More and more matter was fused into heavier elements More and more matter was fused into heavier elements

and expelled back into interstellar space by supernovas and expelled back into interstellar space by supernovas and dying red giant starsand dying red giant stars

Eventually our sun and its planets formed from this Eventually our sun and its planets formed from this interstellar gas and dustinterstellar gas and dust

CitationsCitations

TLC Elementary School: The Moon and TLC Elementary School: The Moon and Beyond. Discovery Channel School. Beyond. Discovery Channel School. 2004.unitedstreaming.2004.unitedstreaming.

Science Investigations: Earth Science: Science Investigations: Earth Science: Investigating Astronomy. Discovery Investigating Astronomy. Discovery Channel School. 2004. unitedstreaming. Channel School. 2004. unitedstreaming.

TLC Elementary School: The Moon and TLC Elementary School: The Moon and Beyond. Discovery Channel School. Beyond. Discovery Channel School. 2004.unitedstreaming.2004.unitedstreaming.

Science Investigations: Earth Science: Science Investigations: Earth Science: Investigating Astronomy. Discovery Investigating Astronomy. Discovery Channel School. 2004. unitedstreaming. Channel School. 2004. unitedstreaming.