Stellar Evolution Stellar Evolution Or How and Where do stars come from?

Stellar Stellar EvolutionEvolution

Or How and Where do Or How and Where do stars come from?stars come from?

Let’s follow the evolution Let’s follow the evolution of a star the same size as of a star the same size as the sunthe sun

End product G class starEnd product G class star

1 solar mass – 2 x 101 solar mass – 2 x 103030 kg kg

Surface temp. ~ 6000 KSurface temp. ~ 6000 K

Where did the gas clouds Where did the gas clouds come from?come from?

The big bangThe big bang

Supernova explosionsSupernova explosions

Where did the sun come Where did the sun come from?from?

The sun is a third generation starThe sun is a third generation star It formed from a supernova It formed from a supernova

explosion that happened about 5 explosion that happened about 5 billion years agobillion years ago

How does a nebula form a How does a nebula form a star?star?

A nebula, or interstellar cloud, is A nebula, or interstellar cloud, is the beginning stage of a star the beginning stage of a star (Stage 1)(Stage 1)

Witch’s Head NebulaWitch’s Head Nebula

These are stars in the middle of the Rosetta Nebula

Stage 2Stage 2

Gas cloud starts to contract due to Gas cloud starts to contract due to gravitygravity

Stage 3Stage 3•Gas starts to heat upGas starts to heat up•Near the end of stage 3, the star is Near the end of stage 3, the star is called a “Protostar”called a “Protostar”

Stage 4Stage 4

Begins around 100,000 years after Begins around 100,000 years after the cloud started contractingthe cloud started contracting

The protostar’s density and The protostar’s density and temperature increase temperature increase

Stage 5Stage 5•The “star” is heading toward the The “star” is heading toward the main sequencemain sequence•These are often called T Tauri starsThese are often called T Tauri stars

This is a protostar This is a protostar blowing away blowing away excess gas and excess gas and dustdust

This is a T Tauri This is a T Tauri star formingstar forming

Stage 6Stage 6

At 10 million years the protostar has At 10 million years the protostar has shrunk shrunk

-the core density and pressure are high -the core density and pressure are high enough for nuclear enough for nuclear fusionfusion to start to start

It is now called a “Star”It is now called a “Star” For 30 million more years the star For 30 million more years the star

contracts a little more and settles down contracts a little more and settles down to the main sequenceto the main sequence

And then finally, after a total of 40-50 And then finally, after a total of 40-50 million years…million years…

Stage 7, main sequence. The star Stage 7, main sequence. The star has “arrivedhas “arrived”

What about other size What about other size stars?stars?

Larger massLarger mass (>1 solar (>1 solar mass)mass)

Get to main sequence Get to main sequence quickerquicker

Burn much brighter Burn much brighter and fasterand faster

Largest can become a Largest can become a star in <1 million star in <1 million yearsyears

Smaller massSmaller mass (<1 solar (<1 solar mass)mass)

Take longer to get to Take longer to get to main sequencemain sequence

Burn dimmer and Burn dimmer and longerlonger

<.08 solar masses <.08 solar masses never become stars, never become stars, these are called these are called “Brown Dwarfs”“Brown Dwarfs”

Once again let’s follow the Once again let’s follow the Sun’s evolutionSun’s evolution

Right now about halfway through Right now about halfway through its 10 billion year lifespanits 10 billion year lifespan

Burning Hydrogen in its coreBurning Hydrogen in its core

Middle aged looking forward Middle aged looking forward toward retirementtoward retirement

From Hydrogen to HeliumFrom Hydrogen to Helium

Inside core Hydrogen keeps burningInside core Hydrogen keeps burning A Helium “ash” forms in the very centerA Helium “ash” forms in the very center As the helium core grows a hydrogen As the helium core grows a hydrogen

burning shell surrounds itburning shell surrounds it The core density increases as it shrinks The core density increases as it shrinks

under gravity’s pressureunder gravity’s pressure Core temp. increases causing H burning Core temp. increases causing H burning

to increaseto increase This causes the star to expand against This causes the star to expand against

gravity trying to pull it ingravity trying to pull it in

Stage 8Stage 8

As the star burns more Hydrogen, As the star burns more Hydrogen, it starts to swell. This is Stage 8-it starts to swell. This is Stage 8-the Subgiant phasethe Subgiant phase

The star continues to become a The star continues to become a Red GiantRed Giant

Stage 9Stage 9

If it were simple, the star would If it were simple, the star would continue to expand and cool offcontinue to expand and cool off

It’s not that simple, the core is in a It’s not that simple, the core is in a “degenerate” state“degenerate” state

When helium burning starts, the core When helium burning starts, the core heats with no change in pressure or size heats with no change in pressure or size in the corein the core

This increase in temp causes a “Helium This increase in temp causes a “Helium Flash,” the star burns uncontrollably for Flash,” the star burns uncontrollably for a few hoursa few hours

This is a red giant star This is a red giant star with a dust ring around itwith a dust ring around it

Stage 10Stage 10

The core now is steadily burning The core now is steadily burning heliumhelium

The star resides in a stable area off The star resides in a stable area off the main sequence called the the main sequence called the “Horizontal branch”“Horizontal branch”

It remains here for 50-100 million It remains here for 50-100 million yearsyears

Stage 11Stage 11

As the temperature in the core As the temperature in the core increases, helium starts to burn and increases, helium starts to burn and form carbonform carbon

The core now becomes carbon ashThe core now becomes carbon ash This core is surrounded by a helium This core is surrounded by a helium

burning shell and around that a burning shell and around that a hydrogen burning shellhydrogen burning shell

Like before, the star swells, this time Like before, the star swells, this time becoming a Red Supergiantbecoming a Red Supergiant

Which Red Supergiant is Which Red Supergiant is this?this?

Betelgeuse Betelgeuse Betelgeuse

This is what will happen to This is what will happen to the sunthe sun

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Where do we go from Where do we go from here?here?

The core is now building up carbon The core is now building up carbon ashash

Temp. is approaching 300 million KTemp. is approaching 300 million K This isn’t hot enough for the next This isn’t hot enough for the next

step in fusionstep in fusion The core becomes more dense and The core becomes more dense and

the outer shell expands. This is the outer shell expands. This is stage 12. It continues until the final stage 12. It continues until the final part of this stage.…part of this stage.…

The outer shell is thrown The outer shell is thrown off as a planetary nebulaoff as a planetary nebula

The star that is left is a white dwarf, this is The star that is left is a white dwarf, this is Stage 13Stage 13

This will eventually cool to become a black This will eventually cool to become a black dwarf, this is the final stage, Stage 14dwarf, this is the final stage, Stage 14

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Here is an example from Here is an example from start to finishstart to finish

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That’s all folksThat’s all folks

You might have a questionYou might have a question

What is a white dwarf?What is a white dwarf? What happens to smaller mass What happens to smaller mass

stars?stars? What happens to larger mass What happens to larger mass

stars?stars?

White DwarfsWhite Dwarfs

The carbon core made during The carbon core made during helium burning is all that is lefthelium burning is all that is left

It is extremely dense and hotIt is extremely dense and hot The only thing stopping it from The only thing stopping it from

collapsing is electron pressurecollapsing is electron pressure It is only the size of the earthIt is only the size of the earth One teaspoon would weigh about One teaspoon would weigh about

20 million tons20 million tons

These are cooling White These are cooling White DwarfsDwarfs

Black DwarfBlack Dwarf

The white dwarf is extremely The white dwarf is extremely brightbright

Since there is no longer fusion, it Since there is no longer fusion, it will eventually coolwill eventually cool

It will become a black dwarfIt will become a black dwarf

Smaller mass StarsSmaller mass Stars

Smaller mass stars follow almost Smaller mass stars follow almost exactly the same sequence as the exactly the same sequence as the sunsun

It takes them longer to burn all It takes them longer to burn all their fuel, so they exist in the their fuel, so they exist in the different stages for longer timesdifferent stages for longer times

Sometimes something Sometimes something cool happenscool happens

A white dwarf can output a huge A white dwarf can output a huge amount of light amount of light

This is called a NOVAThis is called a NOVA

How does this happen if a white How does this happen if a white dwarf is no longer generating dwarf is no longer generating energy?energy?

It happens when the white It happens when the white dwarf is not alonedwarf is not alone

It has a companion starIt has a companion star Matter is taken from the companion and Matter is taken from the companion and

builds up on the white dwarf, this is mostly builds up on the white dwarf, this is mostly Hydrogen and HeliumHydrogen and Helium

When enough builds up, fusion starts in this When enough builds up, fusion starts in this shell and it violently burns and explodes shell and it violently burns and explodes

This causes the white dwarf to flare into a This causes the white dwarf to flare into a Nova Nova

Nova means “new” and to early observers Nova means “new” and to early observers these stars appeared newthese stars appeared new

This is what is happening This is what is happening in a Novain a Nova

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A Nova in AquillaA Nova in Aquilla

This is the shell blown off This is the shell blown off from a Nova in Cygnus from a Nova in Cygnus that happened in 1992that happened in 1992

This nebula only became This nebula only became visible after the Nova in visible after the Nova in CygnusCygnus

This tiny Ball is theGas fromThe Nova

What about those larger What about those larger mass Stars?mass Stars?

They can become neutron starsThey can become neutron stars

Or if even more massive, “Black Or if even more massive, “Black Holes”Holes”

But, that’s a story for another timeBut, that’s a story for another time

SourcesSources www.cdnow.com www.wavcentral.com www.tidalwaves.com www.nonstick.com SmashmouthSmashmouth JourneyJourney Hubble Space Telescope CenterHubble Space Telescope Center Astronomy A Beginners guide to theAstronomy A Beginners guide to the UniverseUniverse, ,

Chaisson and McMillanChaisson and McMillan Paul for his helpPaul for his help Julie for her Sting musicJulie for her Sting music