Lecture 7

1. Lecture 7 SaravanaKumar.S Department of Physics

2. Hertzsprung–Russell diagram The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosity versus their spectral types or classifications and effective temperatures.

4. Stellar evolution • Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years (for the most massive) to trillions of years (for the least massive, which is considerably more than the age of the universe). • Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at the various points in their life, and by simulating stellar structure with computer models.

5. Star Birth –Condensation of Inter stellar medium Stars are created from condensations in the interstellar medium (ISM). For a star to be created gravity must overcome the internal pressure of the medium, and this is most likely to happen in regions where the ISM is relatively dense and temperatures are relatively low. The colder temperatures mean less internal pressure and hence a greater chance of condensation occurring

6. Dark Nebulae

8. Star birth - Protostar • Stellar evolution begins with the gravitational collapse of a giant molecular cloud (GMC). • Typical GMCs are roughly 100 light-years (9.5×1014 km) across and contain up to 6,000,000 solar masses (1.2×1037 kg). • As it collapses, a GMC breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar.

9. Star birth • Protostars with masses less than roughly 0.08 M⊙ (1.6×1029 kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs and they shine dimly and die away slowly, cooling gradually over hundreds of millions of years. • For a more massive protostar, the core temperature will eventually reach 10 million kelvins, initiating the proton-proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium. • The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core exerts a "radiation pressure" balancing the weight of the star's matter, preventing further gravitational collapse. The star thus evolves rapidly to a stable state, beginning the main sequence phase of its evolution. • A new star will fall at a specific point on the main sequence of the Hertzsprung-Russell diagram, with the main sequence spectral type depending upon the mass of the star.

10. This shows the progression from a cold nebula (maybe T = 30 K) with a total mass of about 1 solar mass into a star like our Sun.

11. This shows that the collapse of larger clouds takes less time than that for smaller clouds. Okay, bigger clouds, more gravity. Where a star is on the main sequence is determined by its mass.

12. Life of a star • Main sequence stars all fuse hydrogen into helium in their cores. • This generates heat and gas pressure sufficient to balance the contracting force of gravity, what we call “hydrostatic equilibrium”. • Stars are huge balls of gas, they have lots of hydrogen and can do this fusion for very long periods of time. • But not forever, slowly the core will fill with helium produced by the hydrogen fusion.

13. Life of a star • The main-sequence stars convert hydrogen into helium in their cores, what happens when the stars run low or run out of hydrogen?

14. Life of star Core Temperature Increases • The core of the star gradually contracts as the hydrogen is used up, then contracts much faster when the hydrogen shortage becomes severe. • The core actually becomes hotter as what hydrogen there is left is fused faster and faster. • Much of the hydrogen fusion occurs in a shell surrounding the mostly-helium core. • It is odd, the fusion rate actually increases despite the fact that the hydrogen fuel is in shorter supply.

15. Red Giants and Supergiants • Because the core has become hotter, the core pressures increase and the outer layers of the star expand. • Those outer layers cool as they lose energy (opposite of the gain of energy when things fall inward). • Stars with initial masses <8 times mass of sun will be converted into red giants and those with initial masses >8 times mass of sun will be converted into supergiants.

16. Same old Sun with a new look • The transition into a red giant is underway even today; our Sun is gradually (very gradually) getting hotter and larger. • But it will still be classified as a main sequence star for at least another 5 billion years (astronomers estimate, though, that the warming Sun will cause all Earth’s oceans to boil away about one billion years from now). • The Sun will grow physically larger in size but will still contain the same amount of material.

17. Red Giants are Huge • The core will become denser than it is currently, the outer layers will expand tremendously and become much less dense. • The Sun, in 5 or 6 billion years, will expand and swallow up Mercury and Venus! • They will be engulfed by the growing Sun and vaporized by its heat. • The Earth will be the closest planet to the red giant Sun, orbiting just above its bloated surface.

18. The Sun’s changing properties as it becomes a red giant shown here on an H-R Diagram.

19. Changes after red giant stage for stars with mass < 8 solar mass • Eventually, the core heats up to the point where a new nuclear fusion reaction can take place: 3 He => C + 2 g • Three helium nuclei fusing together to form a carbon nucleus, with two photons (gamma rays) emitted as by-products. • This is called the triple-alpha process, because helium nuclei are also known as “alpha particles”. • Like hydrogen fusion, this helium fusion converts some mass into energy.

20. Helium Flash and yellow giants • Unlike the gradual transition from main sequence to red giant, the onset of helium fusion starts in a sudden burst called helium flash. • Once helium fusion begins in the core, it causes changes in the rest of the star. • The star becomes more yellow in color (which is a hotter surface than the previous red) and usually starts to pulsate (it puffs in and out). • These are yellow giant stars, they are also pulsating stars.

21. Death of Stars like the Sun • When the core is completely filled with carbon, there will be no more fusion. • The final frantic fusion that occurs along with a gravitational collapse of the core produces a final burst of energy. • This last pulse causes the outer layers of the star to be blown out into space, above the escape velocity creating what is called a planetary nebula.

23. Planetary Nebula Aftermath • After the planetary nebula phase, a star like our Sun will lose almost half of its mass (larger stars may lose 80-90% of their material), blown off into space never to return. • The remaining core settles into a small, dense object called a white dwarf.

24. Changes in massive stars (with mass > 8 solar mass) after supergiant stage • Helium fusion starts in these stars but not with the sudden helium flash as in low-mass stars. • Helium fusion produces carbon and carbon quickly begins to accumulate in the core. • A star like our Sun ends its life made mostly of carbon, but these stars have such extreme conditions in their cores that new fusion reaction occur, carbon into neon, oxygen, and nitrogen. • These new fusions release energy, keep up core pressures, and prevent gravitational collapse.

25. Onion Skin • And when the core is filled with Ne, O, and N? • Core contraction turns up the temperature even higher and new fusion reactions of these elements produce still heavier elements, like Silicon (Si) and Iron (Fe). • The star does not fuse one element at a time, it develops shells of materials with different fusions occurring predominantly in different shells. • Astronomers often refer to this as the “onion skin” or “onion core” due to the similarity of shells.

26. After Iron core? • And after iron fills the core of the supergiant star? • Iron nuclei turn out to be the most stable nuclei, nuclear reactions favor converting other elements into iron. • The core is full of iron nuclei, flying around, maybe undergoing some fusion and fission, but not generating the new energy needed to maintain the pressures in the core. • These stars suffer a sudden decrease in core pressure which had been balancing gravity. • Both the core and the outer layers of the star will have catastrophic collapses.

27. Core Collapse • Gravity causes the core to shrink. • But mixed in with all those iron nuclei are electrons which had been largely unimportant previously. • The core collapse brings the electrons to their degenerate limit (electron degeneracy), the electrons cannot be squeezed together any more than that.

28. Inverse Neutron Decay • Astronomers believe that the extreme conditions within the core will result in this reaction occurring: p + e- => n + n proton + electron becomes neutron + neutrino • [This reaction is the opposite of the “neutron decay” process that happens to neutrons in certain situations so it is called “inverse neutron decay”.] • This reaction requires energy to make it go but astronomers still believe it will happen.

29. Neutron Star • The core will end up a tightly packed ball of neutrons. • In less than a second of time, maybe two solar masses worth of iron is mashed into a ball of neutrons about 10 kilometers across. • This surviving object has been named a neutron star.

30. Supernova Explosion • The surface materials basically go into free fall, accelerating over an AU or more of distance, pulled towards the core by gravity. • That materials crashes onto the core with stunning violence and mostly rebounds/blasts back into space. • This is a (Type II) Supernova explosion • After supernova explosions, black holes are formed

31. Other important points Strömgren sphere • A Strömgren sphere is the sphere of ionized hydrogen (H II) around a young star of the spectral classes O or B. • Very hot stars, those of the spectral classes O or B, emit very energetic radiation, especially ultraviolet radiation, which is able to ionize the neutral hydrogen (H I) of the surrounding interstellar medium (ISM), i.e. the hydrogen atom loses its single electron. This state of hydrogen is called H II.

32. Size of the Strömgren sphere can be calculated from the following equation where r = radius of stromgren sphere, N* = total ionization rate, α = interaction probability and ne =density of free electrons

33. Mass-luminosity relation • A relationship between luminosity (intrinsic brightness) and mass for stars that are on the main sequence. Averaged over the whole main sequence (i.e. for stars of all masses), it has been found that L = M3.5, where both L and M are measure in solar units. • A more detailed examination shows that the relationship is different in different mass regimes. • For stars of less than 0.43 solar mass (in which convection is the sole energy transport process), L = 0.23 M2.3. • For a mass greater than this and up to several solar masses, L varies as the fifth power of M • For the very massive stars, L varies as M cubed. These relationships are empirical ones based largely on observations of binary stars.