Chapter 12 Star Stuff The birth and dead of stars. Remember that without this life cycle we wouldn’t exist, either.

12.1 Star Birth Our Goals for Learning • How do stars form? • How massive are newborn stars?

We are “star stuff” because the elements necessary for life were made in stars.

How do stars form?

Stars are born in molecular clouds consisting mostly of hydrogen molecules. Clouds are pretty cold, 10-30K (it is 300K on Earth), and not very dense. In the picture: star-forming cloud of molecular hydrogen gas (the dark region; gas obscures our view of distant stars). Blue-white blotches contain newly formed stars.

Each shrinking fragment of molecular gas heats up as it contacts Each shrinking fragment of molecular gas heats up as it contacts. Question: what is the source of this heat?

…Conservation of energy Contraction can continue if thermal energy is radiated away, preventing temperature and pressure from building enough to resist gravity. The temperature of the cloud remains below 100K. Question: What kind of light do this clouds emit? Collape_of_solar_nebula.swf

Star-forming clouds emit infrared light! In infrared light Download a movie of optical-to-IR transformation from the Spitzer Space Teelscope site The same picture in visible light Star-forming clouds emit infrared light!

Orion Nebula is one of the closest star-forming clouds Infrared light from Orion; yellow and white show hotter regions.

Solar-system formation is a good example of star birth

As gravity forces a cloud to become smaller, it begins to spin faster and faster

…Conservation of angular momentum Gas settles into a spinning disk. Why? The central region, which heats up more and more, grows dense enough to trap infrared radiation (which used to be radiated away). The central temperatures rise dramatically and the rising pressure pushes against gravity, slowing down the contraction – protostar is born!

Angular momentum leads to: Rotation of protostar Disk formation … and sometimes … Fragmentation of a protostar into binary Sometimes, protostar emits jets – probably due to strong magnetic fields.

Protostar to Main Sequence Protostar contracts and heats until core temperature is sufficient for hydrogen fusion. Contraction ends when energy released by hydrogen fusion balances energy radiated from surface. Takes 50 million years for star like Sun (less time for more massive stars)

Summary of Star Birth Gravity causes gas cloud to shrink and fragment Core of shrinking cloud heats up When core gets hot enough, fusion begins and stops the shrinking New star achieves long-lasting state of balance

How massive are newborn stars?

A cluster of many stars can form out of a single cloud.

Very massive stars are rare Luminosity Low-mass stars are common Temperature

Stars more massive than 100 MSun would blow apart Luminosity Stars less massive than 0.08 MSun can’t sustain fusion Temperature

Pressure Gravity If M > 0.08 MSun, then gravitational contraction heats core until fusion begins If M < 0.08 MSun, degeneracy pressure stops gravitational contraction before fusion can begin

Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying same state in same place

Thermal Pressure: Depends on heat content The main form of pressure in most stars Degeneracy Pressure: Particles can’t be in same state in same place Doesn’t depend on heat content

Brown Dwarf An object less massive than 0.08 MSun Radiates infrared light Has thermal energy from gravitational contraction Cools off after degeneracy pressure stops contraction

What have we learned? • How do stars form? Stars are born in cold, relatively dense molecular clouds. As a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas. The protostar may also fire jets of matter outward along its poles. Protostars rotate rapidly, and some may spin so fast that they split to form close binary star systems.

What have we learned? • How massive are newborn stars? Newborn stars come in a range of masses, but cannot be less massive than 0.08MSun. Below this mass, degeneracy pressure prevents gravity from making the core hot enough for efficient hydrogen fusion, and the object becomes a “failed star” known as a brown dwarf.

12.2 Life as a Low-Mass Star Our Goals for Learning • What are the life stages of a low-mass star? • How does a low-mass star die? In short: low mass stars (like our Sun) shine steadily for billions of years burning hydrogen. At the ends of their lives, they swell up to become red giants and then die by expelling their outer layers into space. All that remains of a low mass star is a dead core that we know as a white dwarf.

What are the life stages of a low mass star?

High-Mass Stars > 8 MSun Intermediate-Mass Stars Low-Mass Stars < 2 MSun Brown Dwarfs

A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. Main-seq_lifetime_and_mass.swf

Red giant stage Question: how does the equilibrium between gravity and core pressure in the Sun work? What happens when nuclear fusion ceases?

Red giant stage Question: how does the equilibrium between gravity and core pressure in the Sun work? What happens when nuclear fusion ceases? The core is no longer able to stand the crush of gravity and it will begin to shrink. As it shrinks, what happens to its temperature?

Red giant stage Question: what is gravitational equilibrium in the Sun based on? What happens when nuclear fusion ceases? The core is no longer able to stand the crush of gravity and it will begin to shrink. As it shrinks, what happens to its temperature? So the star shrinks and heats up.

What is the core now made of? Fusion of two helium nuclei doesn’t work, helium fusion must combine three He nuclei to make carbon. Helium fusion requires much higher temperatures than hydrogen fusion because larger charge leads to greater repulsion. That is why it the core has to shrink substantially before the He could start to fuse.

In the mean time, while core shrinks… Thought Question What happens as a star’s inert helium core starts to shrink? A. Hydrogen fuses in shell around core B. Helium fusion slowly begins C. Helium fusion rate rapidly rises D. Core pressure sharply drops

In the mean time, while core shrinks… Thought Question What happens as a star’s inert helium core starts to shrink? A. Hydrogen fuses in shell around core B. Helium fusion slowly begins C. Helium fusion rate rapidly rises D. Core pressure sharply drops

In this phase, the Sun works as a broken thermostat: Core continues to shrink; But the high fusion rate in the shell keeps on expanding outer layers of the Sun! – the Sun grows in size and luminosity becoming red giant (subgiant) star!

Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A. Helium fusion slowly starts up B. Hydrogen fusion stops C. Helium fusion rises very sharply Hint: Degeneracy pressure is the main form of pressure in the inert helium core

Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A. Helium fusion slowly starts up B. Hydrogen fusion stops C. Helium fusion rises very sharply Hint: Degeneracy pressure is the main form of pressure in the inert helium core

Helium Flash Thermostat is broken in low-mass red giant because degeneracy pressure supports core Core temperature rises rapidly when helium fusion begins Helium fusion rate skyrockets until thermal pressure takes over and expands core again

Helium burning stars neither shrink nor grow because thermostat is temporarily fixed. Helium will continue burning for about 100 million years, before it gets all used up.

Thought Question What happens when the star’s core runs out of helium? A. The star explodes B. Carbon fusion begins C. The core cools off D. Helium fuses in a shell around the core

Thought Question What happens when the star’s core runs out of helium? A. The star explodes B. Carbon fusion begins C. The core cools off D. Helium fuses in a shell around the core

In the last stage we have a star which has: Carbon core Shell burning He Shell burning H The star gets very big, and it starts loosing is outer layers making a planetary nebula! Only a white dwarf (exposed core of a star) is left behind.

Life stages of a low-mass star like the Sun Death_sequence_of_sun.swf

How does a low mass star die?

Only a white dwarf is left behind A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind Great planetary-nebula movies can be downloaded from Press Release 2003-11 (Helix Nebula) of the Space Telescope Science Institute (see hubble.stsci.edu)

Thought Question What happens to Earth’s orbit as Sun loses mass late in its life? A. Earth’s orbit gets bigger B. Earth’s orbit gets smaller C. Earth’s orbit stays the same

Thought Question What happens to Earth’s orbit as Sun loses mass late in its life? A. Earth’s orbit gets bigger B. Earth’s orbit gets smaller C. Earth’s orbit stays the same

Stages_low-mass_death_sequence.swf

What have we learned? • What are the life stages of a low-mass star? A low-mass star spends most of its life generating energy by fusing hydrogen in its core. Then it becomes a red giant, with a hydrogen shell burning around an inert helium core. Next comes helium core burning, followed by doubleshell burning of hydrogen and helium shells around an inert carbon core. c In the late

What have we learned? • How does a low-mass star die? A low-mass star like the Sun never gets hot enough to fuse carbon in its core. It expels its outer layers into space as a planetary nebula, leaving behind a white dwarf.

12.3 Life as a High-Mass Star Our Goals for Learning • What are the life stages of a high mass star? • How do high-mass stars make the elements necessary for life? • How does a high-mass star die? Both, low-mass and high-mass stars are crucial for our existence. Why we need low mass stars for life to thrive? Why are the high mass stars important?

What are the life stages of a high mass star?

High-Mass Stars > 8 MSun Intermediate-Mass Stars Low-Mass Stars < 2 MSun Brown Dwarfs

High-Mass Star’s Life Early stages are similar to those of low-mass star: Main Sequence: H fuses to He in core Red Supergiant: H fuses to He in shell around inert He core Helium Core Burning: He fuses to C in core (no flash) What is the main difference in the way these stages proceed, when compared to a low mass star?

Lets look at a life of a star 25 times heavier than the Sun. It is also born from a molecular cloud. At what moment we say that the star is born from a protostar in a center of shrinking molecular cloud? Nuclear fusion however proceeds differently in 25 Msun star. How does the fusion of H to He proceeds in the Sun?

Lets look at a life of a star 25 times heavier than the Sun. It is also born from a molecular cloud. At what moment we say that the star is born from a protostar in a center of shrinking molecular cloud? Nuclear fusion however proceeds differently in 25 Msun star. How does the fusion of H to He proceeds in the Sun? Through proton-proton chain…

In 25 Msun star fusion of H into He goes through CNO cycle! CNO cycle is just another way to fuse H into He, using carbon, nitrogen, and oxygen as catalysts CNO cycle is main mechanism for H fusion in high mass stars because core temperature is higher. How come there are elements heavier than N and He present in a star? Overall interaction is however, the same.

High-mass stars become supergiants after core H runs out (in few million years). He will burn for few hundred thousand years, then burning of C will start (and last for few hundred years)… In the mean time lighter elements are fusing in shells – star is getting larger and larger. It zig-zags on HR diagram!

How do high mass stars make the elements necessary for life?

Big Bang made 75% H, 25% He – stars make everything else

Helium fusion can make carbon in low-mass stars

Helium-capture reactions add two protons at a time

Helium capture builds C into O, Ne, Mg, …

Advanced nuclear fusion reactions require extremely high temperatures Only high-mass stars can attain high enough core temperatures before degeneracy pressure stops contraction

Advanced reactions make heavier elements

Advanced nuclear burning occurs in multiple shells - star resembles an onion…

Iron is dead end for fusion because nuclear reactions involving iron do not release energy (Fe has lowest mass per nuclear particle)

Evidence for our understanding of heavier element production: Helium capture: higher abundances of elements with even numbers of protons There is few elements heavier than Fe, produced in rare fusion processes at the end of star’s life Younger stars have more heavier elements (2,3%) than early stars (0.1%). Why?

How does a high mass star die?

Core then suddenly collapses, creating supernova explosion. Iron builds up in core until degeneracy pressure can no longer resist gravity Core then suddenly collapses, creating supernova explosion. Death_seq_of_high-mass_star.swf Download a good supernova explosion movie from the Chandra Science Center (chandra.harvard.edu)

Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star. Neutrons have degeneracy pressure of their own so the collapse halts here.

In less than a second, an iron core with a mass of our sun and radius a little bigger than the Earth, shrinks into a ball of neutrons, so densely packed that they are just few kilometers across! Question: how come that so much mass can be packed in a ball few kilometers across. This gravitational collapse of a core will release an enormous amount of energy (more than 100 times the energy Sun will radiate over its entire life). This energy drives the outer layers off into space in one of the greatest explosions – supernova. What is left is a ball of neutrons – neutron star. The ejected gases (ejected initially with 10,000 km/s speeds) slowly cool, but continue to expand outward until they eventually mix with other gases in interstellar space. Waiting to become parts of new stars/planets.

Energy and neutrons released in supernova explosion enables elements heavier than iron to form

Elements made during supernova explosion when some of neutrons produced in a core slam into shell nuclei making heavier elements out of them.

Crab Nebula: Remnant of supernova observed in 1054 A.D.

before after Supernova 1987A is the nearest supernova observed in the last 400 years. For about a week, a supernova blazes as powerfully as 10 billion Suns, with the luminosity of a moderate size galaxy. We have seen this supernova in 1987. But when did it really happen? (hint: Large Magellanic cloud is 150,000 light years away)

The next nearby supernova?

What have we learned? • What are the life stages of a high-mass star? A high-mass star lives a much shorter life than a low-mass star, fusing hydrogen into helium via the CNO cycle. After exhausting its core hydrogen, a high-mass star begins hydrogen shell burning and then goes through a series of stages burning successively heavier elements. The furious rate of this fusion makes the star swell in size to become a supergiant.

What have we learned? • How do high-mass stars make the elements necessary for life? In its final stages of life, a high-mass star’s core becomes hot enough to fuse carbon and other heavy elements. The variety of different fusion reactions produces a wide range of elements— including all the elements necessary for life—that are then released into space when the star dies.

What have we learned? • How does a high-mass star die? A high-mass star dies in the explosion of a supernova, scattering newly produced elements into space and leaving a neutron star or black hole behind. The supernova occurs after fusion begins to pile up iron in the high-mass star’s core. Because iron fusion cannot release energy, the core cannot hold off the crush of gravity for long. In the instant that gravity overcomes degeneracy pressure, the core collapses and the star explodes.

12.4 Summary of Stellar Lives Our Goals for Learning • How does a star’s mass determine its life story? • How are the lives of stars with close companions different?

How does a star’s mass determine its life story?

Low-Mass Star Summary Main Sequence: H fuses to He in core Red Giant: H fuses to He in shell around He core Helium Core Burning: He fuses to C in core while H fuses to He in shell Double Shell Burning: H and He both fuse in shells 5. Planetary Nebula leaves white dwarf behind Not to scale!

Reasons for Life Stages Core shrinks and heats until it’s hot enough for fusion Nuclei with larger charge require higher temperature for fusion Core thermostat is broken while core is not hot enough for fusion (shell burning) Core fusion can’t happen if degeneracy pressure keeps core from shrinking Not to scale!

Life Stages of High-Mass Star Main Sequence: H fuses to He in core Red Supergiant: H fuses to He in shell around He core Helium Core Burning: He fuses to C in core while H fuses to He in shell Multiple Shell Burning: Many elements fuse in shells 5. Supernova leaves neutron star (or black hole!) behind Not to scale!

Life of a 20 MSun star Life of a 1 MSun star

How are the lives of stars with close companions different?

Thought Question The binary star Algol consists of a 3.7 MSun main sequence star and a 0.8 MSun subgiant star. What’s strange about this pairing? How did it come about?

Stars in Algol are close enough that matter can flow from subgiant onto main-sequence star

Star that is now a subgiant was originally more massive As it reached the end of its life and started to grow, it began to transfer mass to its companion Now the companion star is more massive

What have we learned? • How does a star’s mass determine its life story? A star’s mass determines how it lives its life. Low-mass stars never get hot enough to fuse carbon or heavier elements in their cores, and end their lives by expelling their outer layers and leaving a white dwarf behind. High-mass stars live short but brilliant lives, ultimately dying in supernova explosions. Sun

What have we learned? • How are the lives of stars with close companions different? When one star in a close binary system begins to swell in size at the end of its hydrogen-burning life, it can begin to transfer mass to its companion. This mass exchange can then change the remaining life histories of both stars. Sun