1. Lives of Stars
Stellar Evolution

2. Learning Goals Students will:
Understand stellar evolution (how stars are formed, live and die).
Understand how stars produce energy.

3. Success Criteria
Students will show their understanding of learning goals by:
explaining how nuclear fusion works to produce a stars energy.
Predicting the stellar evolution and death of a star based on its mass.

4. Star Formation

5. Stellar Evolution
The formation and lifetimes of stars is dependant on their size.
I will focus on stars approximately the same size as our sun.
At one time the sun was considered to be a very ordinary average sized star, however work done in the past decade has revealed an enormous number of stars called brown dwarves which generally have solar masses of 0.2 (The sun = 1)
Now astronomers feel that our sun is larger than 80-85% of the galaxies’ stars.

6. Stage 1: Protoplanetary Nebula
All stars form from diffuse clouds of gas and dust found in the interstellar medium (the space between stars)
These clouds which are mostly composed of hydrogen (often called a molecular cloud) are found within the arms of the Milky Way Galaxy (and likely within all galaxies).
GMC’s or Giant Molecular Clouds have been located by astronomers that are opaque to telescopes and have masses exceeding 1000 solar masses.

7. Stage 1: Protoplanetary Nebula
As any gas cloud begins to collapse, the angular momentum causes it rotate.
As a result a flat disk is formed from the original diffuse cloud.

8. YouTube Video
(The Birth of Stars - NASA)
(Life Cycle of Stars ) - 8:54
(The Life Cycle of Stars - 4:58)
(Mr. Robinson - 17:57 - lecture on Star Formation)
(Beta Pictoris - an actual protoplanetary disk)

9. Stage 1: Protoplanetary Nebula
Since any object with mass has gravity, the particles of gas and dust begin to attract each other.
Despite the very weak gravitational pull, given millions or even hundreds of millions of years, the dust clouds will coalesce (many astronomers use the word “collapse”) into a protostar
The gravitation collapse accelerates as the particles draw ever closer.

10. 2) The Protoplanetary Disk
As gases collapse into a star, the remainder of the dust forms a “disk” that rotates around the newly forming star.
This process may take as long as 100,000 years and the accretion of gas into the star may take another 10 million years.
The disks are typically a few hundred AU’s but may be as great as 1000 AU in width.
This disk often contains the debris that accretes into planets.
Radiation from the newly formed star often blows away much of the gas in the protoplanetary disk - this radiation is called the stellar wind.
An actual protoplanetary disk 450 ly from Earth. The red stellar jet suggests that the star is still forming.

11. 2) The Protoplanetary Disk
At right: An actual spectra of a proto-planetary disk from the Spitzer Space telescope which collects data in the Infrared range.

12. 2) The Protoplanetary Disk
The protoplanetary disk is filled with gas, rocky and icy debris, and newly forming planets.
This is an important scenario to remember when thinking of the formation of the Earth in the early Solar system.

13. Stage 3: The Protostar
With enough mass, the gravitational collapse reaches a point where the extreme velocity and energy of the gas and the gravitational pressure cause the cloud to begin to glow with heat.
As the cloud collapses further, there comes a point at which temperatures (and pressures) become great enough for nuclear fusion.
Once this point has been achieved the protostar has reached star status.
The new protostar is often accompanied by a jet of radiation and particles
The newly formed star also blows away the envelope of gas surrounding it.

14. Stage 3: The Protostar
Protostars/Young stars are typically more luminous than more mature stars that have reached the Main Sequence (remember the Harzsprung-Russell diagram).
The stars are often found in the middle of a swirling disk of gas.
Planets, commas and asteroids often condense from this swirling disk of gas.

15. Nuclear Fusion
(Stars, Nuclear Fusion, & The Elements - Cody Lovern - 20:02) This is a detailed university level discussion, but it is well done.
(Solar Energy)
(Nuclear Fusion)

16. Nuclear Fusion
Nuclear fusion is the energy that fuels the sun – hence all life on planet Earth ultimately derives its energy from a fusion reaction in the core of the sun.
It is the opposite of fission – in fusion, two light atoms are fused into larger atoms – typically two isotopes of hydrogen, deuterium (H-2) and tritium (H-3) are fused into a helium.
Since the mass of the products is slightly less than the mass of the reactants, some mass has been converted to energy (remember the famous E=mc2 equation)

17. Nuclear Fusion in Stars
Nuclear fusion in stars is slightly different - two protons (hydrogen atoms) fuse, with one proton converting to a neutron to form deuterium (an isotope of hydrogen with 1 neutron).
The deuterium fuses with another proton to form Helium-3. Two He-3 nuclei fuse to form the stable He-4 and 2 protons which start the whole process again.
Since the mass of the products is slightly less than the mass of the reactants, some mass has been converted to energy (remember the famous E=mc2 equation)
In heavier mass stars the He nuclei can fuse to form carbon and these elements can even fuse heavier elements right up to iron.
Our sun will produce very few elements heavier than helium.

18. Nuclear Fusion It takes temperatures exceeding 15 million degrees K.
In the Universe this only occurs in the cores of stars.
All elements heavier than hydrogen were created by stars and spread through the universe by novas (stellar collapses and explosions).
All elements heavier than iron were created in supernovas (explosions of massive stars). Hence, much of the Earth and the solar system were created from the remnants of a supernova.

19. Nuclear Fusion
Since you are mostly made of C, H, N and O; you are “star dust”! Listen to the words of the famous song “Woodstock” written by Canadian Joni Mitchell and sung by Crosby, Stills, Nash and Young.
This classic “hippie” song is mostly written about the famous Woodstock concert of 1967, but it references the fact that all humans are made from carbon produced billions of years ago.

20. Size matters
Stars that have solar masses less than about 0.08 solar masses cannot ignite a fusion reaction and form brown dwarves.
Brown dwarves are basically hot Jupiters which remain glowing for extremely long lifetimes and slowly fade out after as many as 100 billion years.
Stars that have solar masses of about 0.20 solar masses ignite a fusion reaction and form a red dwarf, the most common type of star.

21. Size matters
Protostars with solar masses close to the size of our sun form stars which last billion years, spending most of that time on the main sequence.
If the planetary nebula contains a large amount of mass, a protostar will form more rapidly (More Mass results in a faster Gravitational collapse)
Protostars with masses more than 5 times the size of our sun form Giant and Supergiant stars which burn out very quickly and meet a more violent end.
Red Giants tend to be stars at the final phase of their lives (and have lower solar masses) while Blue Giants are hot, supermassive stars that are younger, but destined for short lives.

22. Review H-R Diagram (Herzsprung-Russell Diagram)
Remember that the vast majority of stars are located on the Main Sequence of the HR Diagram.
Generally the stars outside of the Main Sequence are dying (Red Giants and Supergiants) or are remnants of a nova (White Dwarves)

23. Stars: A balance between 2 forces
At all times, the immense gravity of a star is trying to collapse all of the matter inward into the core of the star.
Counteracting this force, is the fusion reaction created at the core. A tremendous amount of energy in the form of electromagnetic (EM) radiation and particles push outwards.
In a stable star like our own sun, these two forces are in balance and the star remains the same size.
However as fusion energy increases, the star will expand or if fusion energy decreases the star will collapse.

24. Stars: A balance between 2 forces
The nuclear fusion reaction produces an outward force (pressure) as energy and particles are released.
Gravity draws the mass of the star inwards.
A balance between the pressure and gravity allows a stable main sequence star to maintain its size.
This balance is lost when nuclear “fuel” is used up.

25. The Layers of the Sun

26. Stage 3: Star Lives and Stage 4: Deaths

27. Star Lives and Deaths
Once again, these stages are dependant on a star’s mass
We will group stars into 5 different sizes
Sun-like stars (about 1 solar mass) - these stars will spend most of their lives as Main Sequence Stars
Red Dwarves
Brown Dwarves
Giants
Supergiants

28. 1) Sun-like
Stars close to the same mass as the sun will “burn” their hydrogen fuel into helium over a period of about 10 to 12 billion years.
The stars luminosity changes from hotter blue/white to cooler yellow/red in the very early part of its life.
When the hydrogen in the core is used up, a new type of fusion, triple fusion, fuses 3 helium atoms into carbon atoms. (hydrogen still remains in outer layers of the star).
At this point, the star leaves the main sequence and moves up and to the right.

29. 1) Leaving the Main Sequence
At this stage, more energy is created in the core of the star and it expands. The surface temperature is actually lower (3000K vs 10000K) and the star appears red.
Thus, a red giant star is formed. This is the predicted future of our sun. At this point the sun will expand to engulf all of the inner planets.
The He to C fusion occurs rapidly and the He is used up quickly. When this type of fusion ceases, the star collapses, shedding its outer layers of gas. (A Nova). I have simplified this step!
The remnant of the star’s core becomes a white dwarf - a tiny, hot star composed in large part of carbon that is extremely dense.
The white dwarf eventually cools completely into a black dwarf. Since compressed carbon is diamond - theoretically a black dwarf is a big glowing diamond.

30. 1) Main Sequence Stars
This diagram clearly shows the life cycle of the Sun – a classic main sequence star.
Notice that after 10 billion years (about 5 ½ billion years from now) the sun will expand into a red giant.
WHY IS THIS?
The sun will then produce a Nova and collapse into a white dwarf surrounded at first by a planetary nebula.

31. The Life of a Star plotted on a H-R diagram
A star will spend the vast majority of its life on the “main sequence”
As it begins to run out of fuel (Hydrogen), the star begins to expand into a red giant and moves off the main sequence
It briefly moves back to a yellow giant as it (furiously burns helium) and then moves back into a larger red giant. (Hyashi Contraction)
The star goes nova, ejecting its outer layer of gases and becomes a white dwarf.
The star slowly dims
Note the path on the H-R diagram and the amount of time spent at each phase.

32. 1) Sun-Like Stars Final Stage: Nova & Planetary Nebula
F and G stars with similar masses to our own sun exhaust their fuel in approximately billion years.
A this point all of the hydrogen in the core has been fused into helium.
Helium in the core starts to contract and the star begins to burn hydrogen in the outer shell.
The burning of hydrogen in the outer shell causes the star to expand, but the surface cools resulting in the formation of a Red Giant.
The helium in the core will then fuse into carbon (He + He + He = C) in a process called the Helium flash. The helium fusion causes the star to burn hotter (and therefore more brightly) for a short period of time until the helium is used up and the colour dims again.

33. 1) Sun-like Stars Final Stage: Nova & Planetary Nebula
The helium core burns out relatively quickly and the star goes Nova. The star is not large enough to ignite carbon fusion
There is no explosion, the star sheds its outer H and He layers and core of mostly carbon is left behind. The layers of gas and dust form a ring nebula around the star that expands and become less dense over time.
The Carbon (and some Oxygen) core become a white dwarf star. Initially this star burns very hot and is thus very luminous (white or blue)
The white dwarf slowly cools into a red dwarf over a period that can last 5 billion years. The red dwarf will eventually cool further until it is a non-luminous body of solid carbon called a black dwarf.

34. 1) Main Sequence Stars The Type Ia Supernova
This type of supernova involves binary stars in which one member of the binary is a white dwarf.
It is believed that 55% of star systems are binary systems. Of stars larger than one solar mass, 80% of stars belong to binary systems.
White dwarves can only remain stable if they have a mass of less than 1.4 solar masses (The Chandrasehkar Limit).
In a Type IA supernova, a smaller but more massive white dwarf begins to accrete material from its partner. (I have simplified this process – see the 9-step diagram at left for a more detailed explanation)
When its so much mass that it reaches the Chandrasehkar Limit, the star explodes into a thermonuclear explosion (supernova) that leaves no remnant star.

35. Brown Dwarfs on the HR Diagram
Brown dwarves are barely visible due to their cool surfaces.
They live long lives billion years + due to the slow “burning” of nuclear fuel.
They are now thought to be the most abundant stars in the galaxy
Only with recent technology have we been able to determine their abundance.

36. 2) Red Dwarf Stars
Red dwarves are K or M spectral classes. 20 out of the 30 closest stars to the sun are red dwarves. Proxima Centauri (our closest stellar neighbour) is a red dwarf.
Stars of 0.2 to 0.8 solar masses burn much more slowly than Sun-sized stars and thus have much longer lives – up to 100 billion years.
Larger red dwarves will go through a red giant stage (a smaller red giant to be sure), but they are not large enough to initiate He fusion into C.
They do not experience the helium flash but simply shed their outer layers and leave a white dwarf.

37. 3) Brown Dwarf Stars
These are basically failed stars which do not have enough mass to sustain a nuclear fusion reaction.
Brown dwarves are difficult to detect. Only close brown dwarves can be detected with IR or longer wavelengths.
However these stars are somewhat warmer than large gas giant planets like Jupiter – begging the question, what is the source of the heat?
Is the heat a result of the contraction of gases or was there briefly a fusion reaction at the core?

38. 4) Giant Stars
Red Giants, typically start out as sun-like stars and are in the last phase of their lives before collapsing into a nova
Red giants have cooler surface temperatures and appear red.
Blue Giants are very massive, hot stars. It is these stars which are of interest in this section. They use up their fuel quickly and meet a more violent end as a supernova.
Giant stars produce a supernova which is a true explosion – a nova is really just a collapse and a shedding of outer layers of gases.
Supernova 1987A and its expanding wave of gases.

39. 4) Giant Stars
A supernova releases more energy than the sun produces in its lifetime.
Intense gamma radiation can be collected from these events.
The event is so luminous that it will outshine all other objects in a galaxy.
The blast produces a shock wave that can destroy all objects within 10 – 100 ly of the explosion.
The remnant of a Giant star is a neutron star - very small (the size of a small moon or less but with incredible mass (and density)).

40. 4) Giant Stars

41. 5) Supergiant Stars
Stars above 8 solar masses can continue to fuse heavier and heavier elements as seen in the chart to the right.
The supermassive star has several layers or shells in cross-section – each corresponding to a different type of fusion.
However, once iron (Fe) is reached, fusion no longer releases energy, but requires energy!
The iron catastrophe is reached and the star explodes in a massive supernova.
Unlike Giant stars the collapse is so intense that a black hole is produced – a region of space where gravity is so intense that nothing, not even light can escape.
(Supernovas)

42. 5) Supergiant Stars
This diagram shows the inner structure of a supergiant star (generally with at least 8 solar masses.
Due to the incredible gravity created by the extreme mass of these stars, core pressures and temperatures are much higher than the sun.
This allows for fusion of heavier elements as iron

43. 5) Supergiant Stars
Elements larger than iron can ONLY be created by the extreme energies produced in supernovas
Hence all of these elements found on Earth must have been created in a supernova and this material must have been found in our sun’s protoplanetary disk.

44. 5) Supergiant Stars
If the sun were one foot (30 cm) in diameter, then Betelguese would be 22 miles in diameter (35 km)

45. Supernova 1987A
Core Collapse Supernovae probably occur about once every 50 years in our galaxy, but most of them are hidden by the dust of the galaxy.
In 1987 a supernova occurred in the closest galaxy to the Milky Way called the Large Magellenic Cloud. This was an explosion of a blue supergiant star.
Canadian Ian Sheldon and partner Oscar Duhalde were the first to witness this event while working at the Los Campanas Observatory in Chile (see the image below right)
(Supernova 1987A and supernovas)
The class B3 star before the supernova
The class B3 star after the supernova - notice the luminosity

46. Supernova as seen by Hubble
Supernova seen by the Hubble Space Telescope over a two year period.
Note the expanding cloud of gas ejected from the star and the remnant star in the middle.
The explosion is incredibly bright.

47. The Life of a Star plotted on a H-R diagram
Stars that start at the upper left of the H-R diagram tend to be larger and found near the core of a galaxy and have much shorter lives (as little as million years - a galactic blink of an eye)
Stars at the bottom right tend to be smaller, have simpler paths and much longer and less violent lives.

48. Mass vs. Evolution

49. Comparison of the Life Cycle of Large Stars

50. Evolution of Larger Stars
Sun-like Star ( solar masses): star → red giant → nova → white dwarf
Large Mass Star ( to 8.0 solar masses): star → red giant → supernova → neutron star
Supermassive Star (4.0 to ? solar masses): star → red supergiant → supernova → black hole

51. Our Solar System Where did these heavy elements come from?
There is a problem that has challenged astronomers for decades:
Most protoplanetary nebulae contain only light elements - H and He
A planetary nebulae that is produced from the nova of a sun-like star wil also only produce light elements - H, He, Li, C, some O and a few other elements.
If so, the planets produced from the Protostars circumstellar disk would only contain light elements and only gas giant planets could condense.
Our solar system contains 4 rocky planets composed mostly of heavy elements like Fe, Ni, Si, O, Mg, etc. and even contains very heavy elements like U, Pb, Au etc.
Where did these heavy elements come from?
(think about what you have just learned)

52. The Solar System’s Future

53. The Prevailing Theory
The answer to the problem is that only a supergiant star can fuse elements as large as iron and only a supernova can produce elements that are even larger.
Thus a supernova must have seeded the Solar System’s protoplanetary nebula and protoplanetary disk with heavy elements.
Perhaps a supergiant star was created in the same star cluster as the Sun OR a nearby supernova expelled its remnants into the sun’s protoplanetary nebula.
Since most living things contain a fair share of heavy elements - this was a critical event for us!
Go to YouTube or download the famous song “Woodstock” sung by Crosby, Stills and Nash and written by famous Canadian folk singer Joni Mitchell. It contains the line “We are stardust, we are golden, we are billion year old carbon and we’ve got to get ourselves back to the garden” - Joni is right - we are star dust!

54. Homework
Describe two types of stars that are not on the main sequence.
Explain the difference between apparent magnitude and absolute magnitude.
What are the main properties of stars?
What are the two fundamental Properties that are being plotted on the H-R diagram?
What is the significance of the main sequence?

55. Extra Images: Stellar Evolution

56. Extra Images: Stellar Evolution

57. Extra Images: Stellar Evolution