Introduction to Stellar Evolution

1. Introduction to Stellar Evolution 13.7 billion years ago, the Universe as we know it came to being as a result of what we call the “Big Bang”. At that time the Universe contained only protons and electrons. Eventually those protons and electrons combined to form Hydrogen and Helium. Where did everything else come from?

2. How Do Stars Shine? Stars shine by nuclear fusion: the process of extracting energy from the fusion of lighter elements into heavier elements. Helium is made from 4 Hydrogen atoms, but one Helium atom is slightly less massive than the 4 Hydrogen atoms combined.

3. Nuclear Fusion/Hydrostatic Equilibrium The “extra” mass is converted into energy according to Einstein’s famous formula: E = mc2 Fusion generates heat, which keeps stars from collapsing under their own weight.

4. Next we are going to talk about the changes that stars undergo with time. But before that let’s look at the intrinsic differences between stars.

5. The 30 closest stars to the Sun: Based on their brightness, color, etc., we can group stars into various classes.

6. Stellar Brightness/Distance • Ancient astronomers assumed that all stars were the same distance away. • We now know that the stars are not all the same distance away. • A star’s distance from the Earth affects how bright it appears to be. • Farther stars appear fainter; closer stars appear brighter

7. The Inverse Square Law Move 2x as far from a light and it gets 4x dimmer. The light is simply spread out over a larger area.

8. Luminosity vs. Brightness Therefore, two stars that appear equally bright might be a closer, dimmer star and a farther, brighter one:

9. Group Question Which star is brightest? A B A and B are equal Can’t tell A B Which star is the most luminous? A B C Can’t tell C

10. Stellar Temperature/Color The color of a star is indicative of its temperature. Red stars are relatively cool, while blue ones are hotter.

11. Stellar Colors/Temperature The color of an object is inversely proportional to temperature. If we know the color of a star, we know the temperature.

12. Stars: Color vs. Temperature Red star Yellow star Blue star

13. The Hertzsprung-Russell (H-R) Diagram Any plot of (intrinsic) brightness versus color or temperature is called an HR diagram.

14. Classes of Stars Stars aren’t scattered randomly on the HR diagram, rather they fall in certain clumps. In our neighborhood, about 90% of stars lie on the “main sequence”; 9% are “red giants” and 1% are “white dwarfs”.

15. Main Sequence Stars • Also called dwarfs • Normal, run-of-the-mill stars • About 90% of nearby stars are MS • In H-R Diagram: band from upper left (bright and hot) to lower right (faint and cool) • Sun is a MS star (type G2) • Cool MS stars are more common than hot MS stars.

16. Giants • MUCH bigger than the Sun (10 to 100x) • Red Giants – Coolest (~4000 K) giant stars; appear very red • Supergiants – Both bigger and brighter than the average giant

18. White Dwarfs • MUCH smaller than the Sun • Very hot, but not very bright • Actually remnants of dead or dying normal stars

20. OK, now that we know that there are hot, blue massive stars and cooler, red less massive stars, let’s talk about what happens to them over time.

21. Star Formation and Lifetimes Tutorial

22. Hydrostatic Equilibrium Fusion keeps stars from collapsing under their own weight. Pressure from the outflowing hot gas balances the pressure of gravity. This process is called hydrostatic equilibrium

23. Stellar Evolution At first, all stars process hydrogen into helium. In low-mass stars this can take billions of years.

24. The Death of Stars Eventually, as hydrogen in the core is consumed, the star begins to die. But stars don’t go down easy! Its evolution from then on depends very much on the mass of the star: Low-mass stars go quietly High-mass stars go out with a bang!

25. Evolution of a Low-Mass Star As the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse. That collapse releases gravitational energy and the layer just outside the core heats up. Hydrogen begins to fuse outside the core:

26. Evolution of a Low-Mass Star Initial giant stage: the red giant branch -- shell hydrogen fusion As the core continues to shrink, the outer layers of the star expand and cool. The star, now a red giant, is as big as the orbit of Mercury. Despite its cooler temperature, its luminosity increases enormously due to its large size.

27. Evolution of a Low-Mass Star Next giant stage: the horizontal branch -- core helium burning Once the core temp has risen to 100,000,000 K, the helium in the core starts to fuse into carbon Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium

28. Evolution of a Sun-like Star, Cont As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster. Soon all the helium in the core has been converted to carbon. Again the core contracts and collapses, releasing gravitational energy.

29. Evolution of a Low-mass Star Final giant stage: asymptotic giant branch (supergiant stage) -- shell helium fusion Helium is fused in the first shell, hydrogen in the next.

30. Interior of an Old Low Mass Star

31. Death of a Low-Mass Star • Low mass stars are not massive enough to turn carbon and oxygen cores into heavier elements (not hot enough) • These stars eject most of their outer layers • Only the core is left, which “lights-up” the gas that the star has been ejecting – causing a planetary nebula

32. Death of a Low-Mass Star The core continues to contract, but never gets hot enough to burn Carbon. Meanwhile, the outer layers of the star expand to form a planetary nebula.

33. The Helix Nebula

34. NGC 6826 and 7027

35. Death of a Low-Mass Star Even after the nebula has dispersed into space, the core remains. It is extremely dense and extremely hot, but quite small. This stage is called a white dwarf. White dwarfs are very hot, but not very luminous.

37. Group Question The force of gravity acts to make a star Larger Smaller Cooler None of these

38. Group Question • 1) hotter • 2) smaller • 3) larger • 4) cooler • 5) identical in size More massive white dwarfs are ______ compared with less massive white dwarfs.

39. Group Question • 1) hotter • 2) smaller • 3) larger • 4) cooler • 5) identical in size More massive white dwarfs are ______ compared with less massive white dwarfs. Chandrasekhar showed that more mass will squeeze a white dwarf into a smaller volume, due to electron degeneracy pressure.

40. White Dwarfs White Dwarf – the burned out carbon-oxygen core of a “dead” star • Supported by electron degeneracy pressure • Less than 1.4x MSun (Chandrasekhar Limit) • Form from MS stars between 0.8x and 8x MSun

41. A White Dwarf The small star Sirius B is a white-dwarf companion of the much larger and brighter Sirius A: As a white dwarf cools, its size does not change much; it simply gets dimmer and dimmer

42. The Hubble Space Telescope has detected white dwarf stars (circled) in globular clusters:

43. Low-Mass Star Summary • Giants – after core hydrogen burning • First: shell hydrogen burning (normal giants) • Second: core helium burning (horizontal branch giants) • Third: shell helium burning (supergiants) • Most go through an unstable variable stage • White Dwarfs – collapsed remnants of low-mass stars.

44. Stellar EvolutionSequence(Low-Mass Stars)

45. Universe Video Ch 2. 3:50 – 7:30. 11:44- eoc

46. Evolution of High-Mass Stars High-mass stars, like all stars, leave the Main Sequence when there is no more hydrogen fuel in their cores. The first few events are similar to those in lower-mass stars – first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells. But instead of stopping at Carbon, a star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate.

47. Evolution of High-Mass Stars Stars more massive than the Sun follow very different paths when leaving the Main Sequence:

48. High Mass Star Cores Star has layers of lighter and lighter elements, like an onion.

49. High Mass Stars Continued • Eventually the core is composed entirely of Iron (Fe) • Iron is heaviest element that can be created by fusion • Core now supported only by electron degeneracy pressure • Once core becomes more than 1.4x MSun, the core collapses

50. Supernovae Core collapses until the density reaches the neutron degeneracy limit Core collapse comes to a sudden halt and bounces back (core bounce) Outward moving core slams into infalling outer layers of star Star is blown to smithereens (supernova) Elements heavier than iron are formed during the explosion.