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

Lets follow the evolution of a star the same size as the sun End product G class star End product G class star 1 solar mass – 2 x kg 1 solar mass – 2 x kg Surface temp. ~ 6000 K Surface temp. ~ 6000 K

Where did the gas clouds come from? The big bang The big bang Supernova explosions Supernova explosions

Where did the sun come from? The sun is a third generation star The sun is a third generation star It formed from a supernova explosion that happened about 5 billion years ago It formed from a supernova explosion that happened about 5 billion years ago

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

Witchs Head Nebula

These are stars in the middle of the Rosetta Nebula

Stage 2 Gas cloud starts to contract due to gravity Gas cloud starts to contract due to gravity Stage 3 Gas starts to heat upGas starts to heat up Near the end of stage 3, the star is called a ProtostarNear the end of stage 3, the star is called a Protostar

Stage 4 Begins around 100,000 years after the cloud started contracting Begins around 100,000 years after the cloud started contracting The protostars density and temperature increase The protostars density and temperature increase Stage 5 The star is heading toward the main sequenceThe star is heading toward the main sequence These are often called T Tauri starsThese are often called T Tauri stars

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

This is a T Tauri star forming

Stage 6 At 10 million years the protostar has shrunk At 10 million years the protostar has shrunk -the core density and pressure are high enough for nuclear fusion to start -the core density and pressure are high enough for nuclear fusion to start It is now called a Star It is now called a Star For 30 million more years the star contracts a little more and settles down to the main sequence For 30 million more years the star contracts a little more and settles down to the main sequence And then finally, after a total of million years… And then finally, after a total of million years…

Stage 7, main sequence. The star has arrived Stage 7, main sequence. The star has arrived

What about other size stars? Larger mass (>1 solar mass) Get to main sequence quicker Get to main sequence quicker Burn much brighter and faster Burn much brighter and faster Largest can become a star in <1 million years Largest can become a star in <1 million years Smaller mass (<1 solar mass) Take longer to get to main sequence Burn dimmer and longer <.08 solar masses never become stars, these are called Brown Dwarfs

Once again lets follow the Suns evolution Right now about halfway through its 10 billion year lifespan Right now about halfway through its 10 billion year lifespan Burning Hydrogen in its core Burning Hydrogen in its core Middle aged looking forward toward retirement Middle aged looking forward toward retirement

From Hydrogen to Helium Inside core Hydrogen keeps burning Inside core Hydrogen keeps burning A Helium ash forms in the very center A Helium ash forms in the very center As the helium core grows a hydrogen burning shell surrounds it As the helium core grows a hydrogen burning shell surrounds it The core density increases as it shrinks under gravitys pressure The core density increases as it shrinks under gravitys pressure Core temp. increases causing H burning to increase Core temp. increases causing H burning to increase This causes the star to expand against gravity trying to pull it in This causes the star to expand against gravity trying to pull it in

Stage 8 As the star burns more Hydrogen, it starts to swell. This is Stage 8-the Subgiant phase As the star burns more Hydrogen, it starts to swell. This is Stage 8-the Subgiant phase The star continues to become a Red Giant The star continues to become a Red Giant

Stage 9 If it were simple, the star would continue to expand and cool off If it were simple, the star would continue to expand and cool off Its not that simple, the core is in a degenerate state Its not that simple, the core is in a degenerate state When helium burning starts, the core heats with no change in pressure or size in the core When helium burning starts, the core heats with no change in pressure or size in the core This increase in temp causes a Helium Flash, the star burns uncontrollably for a few hours This increase in temp causes a Helium Flash, the star burns uncontrollably for a few hours

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

Stage 10 The core now is steadily burning helium The core now is steadily burning helium The star resides in a stable area off the main sequence called the Horizontal branch The star resides in a stable area off the main sequence called the Horizontal branch It remains here for million years It remains here for million years

Stage 11 As the temperature in the core increases, helium starts to burn and form carbon As the temperature in the core increases, helium starts to burn and form carbon The core now becomes carbon ash The core now becomes carbon ash This core is surrounded by a helium burning shell and around that a hydrogen burning shell This core is surrounded by a helium burning shell and around that a hydrogen burning shell Like before, the star swells, this time becoming a Red Supergiant Like before, the star swells, this time becoming a Red Supergiant

Which Red Supergiant is this? Betelgeuse

This is what will happen to the sun __

Where do we go from here? The core is now building up carbon ash The core is now building up carbon ash Temp. is approaching 300 million K Temp. is approaching 300 million K This isnt hot enough for the next step in fusion This isnt hot enough for the next step in fusion The core becomes more dense and the outer shell expands. This is stage 12. It continues until the final part of this stage.… The core becomes more dense and the outer shell expands. This is stage 12. It continues until the final part of this stage.…

The outer shell is thrown off as a planetary nebula The star that is left is a white dwarf, this is Stage 13 The star that is left is a white dwarf, this is Stage 13 This will eventually cool to become a black dwarf, this is the final stage, Stage 14 This will eventually cool to become a black dwarf, this is the final stage, Stage 14 __

Here is an example from start to finish __

Thats all folks

You might have a question What is a white dwarf? What is a white dwarf? What happens to smaller mass stars? What happens to smaller mass stars? What happens to larger mass stars? What happens to larger mass stars?

White Dwarfs The carbon core made during helium burning is all that is left The carbon core made during helium burning is all that is left It is extremely dense and hot It is extremely dense and hot The only thing stopping it from collapsing is electron pressure The only thing stopping it from collapsing is electron pressure It is only the size of the earth It is only the size of the earth One teaspoon would weigh about 20 million tons One teaspoon would weigh about 20 million tons

These are cooling White Dwarfs

Black Dwarf The white dwarf is extremely bright The white dwarf is extremely bright Since there is no longer fusion, it will eventually cool Since there is no longer fusion, it will eventually cool It will become a black dwarf It will become a black dwarf

Smaller mass Stars Smaller mass stars follow almost exactly the same sequence as the sun Smaller mass stars follow almost exactly the same sequence as the sun It takes them longer to burn all their fuel, so they exist in the different stages for longer times It takes them longer to burn all their fuel, so they exist in the different stages for longer times

Sometimes something cool happens A white dwarf can output a huge amount of light A white dwarf can output a huge amount of light This is called a NOVA This is called a NOVA How does this happen if a white dwarf is no longer generating energy? How does this happen if a white dwarf is no longer generating energy?

It happens when the white dwarf is not alone It has a companion star It has a companion star Matter is taken from the companion and builds up on the white dwarf, this is mostly Hydrogen and Helium Matter is taken from the companion and builds up on the white dwarf, this is mostly Hydrogen and Helium When enough builds up, fusion starts in this shell and it violently burns and explodes When enough builds up, fusion starts in this shell and it violently burns and explodes This causes the white dwarf to flare into a Nova This causes the white dwarf to flare into a Nova Nova means new and to early observers these stars appeared new Nova means new and to early observers these stars appeared new

This is what is happening in a Nova -

A Nova in Aquilla

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

This nebula only became visible after the Nova in Cygnus This tiny Ball is the Gas from The Nova

What about those larger mass Stars? They can become neutron stars They can become neutron stars Or if even more massive, Black Holes Or if even more massive, Black Holes But, thats a story for another time But, thats a story for another time

Sources Smashmouth Smashmouth Journey Journey Hubble Space Telescope Center Hubble Space Telescope Center Astronomy A Beginners guide to the Universe, Chaisson and McMillan Astronomy A Beginners guide to the Universe, Chaisson and McMillan Paul for his help Paul for his help Julie for her Sting music Julie for her Sting music