A1199 Are We Alone? " The Search for Life in the Universe

Transcription

1 ! A1199 Are We Alone? " The Search for Life in the Universe Instructor: Shami Chatterjee! Summer 2018 Web Page: HW2 now posted...! So far: Cosmology, galaxies, star formation! Now: The lives and deaths of stars!

2 Stars: Birth, Life, and Death BIRTH: Gravitational Collapse of interstellar clouds. Hayashi Contraction. LIFE: Stability on Main Sequence. Energy from nuclear reactions in stellar cores (E = mc 2 ). DEATH: Lack of nuclear fuel. Instability, variability, expansion (giants, supergiants). Spectacular explosions!

3 H-R diagram

4 Stellar Evolution Interstellar Cloud! Proto-Star! Hayashi Contraction! Main Sequence! Red Giant! Variable Star!! Explosion! White Dwarf

5 Shell burning in massive stars: a layer cake of elements

6 Nuclear fusion: The Iron Limit Peak in binding energy per nucleon around 56 Fe.! Lighter nuclei release energy by fusion.! Heavier nuclei release energy by fission, not fusion.

7 Deaths of Stars When nuclear reactions cease, hydrostatic equilibrium cannot be sustained, so the core of the star collapses. What must accompany collapse of the core? Release of energy into radiation, in some cases explosively. What can stop the collapse of the core? Other sources of pressure Degenerate electron pressure. Degenerate neutron pressure. Are there stars for which no known source of pressure can push back against gravity? Yes: black holes.

8 Stars: from cradle to grave

9 Planetary nebulae

10 Supernovae (other galaxies)

11 Supernova explosion: SN 1998dh 07/29 08/04 08/30 11/14

12 Supernova explosion: SN 1998dh 07/29 08/04 08/30 11/14

13 Supernova Types

14 Supernova Types" Collapse of accreting white dwarf or merger of two white dwarfs Collapse of core of star to produce either a neutron star or black hole

15 This is one possible scenario. The other is where two white dwarfs in a binary spiral in and merge

16 Supernova Energetics The energy released in a supernova is the binding energy of the core of a star 0.1M core c 2 ~ 0.1M " c 2 = 0.1 x 2x10 33 g (3 x cm/s) 2 ~ 2x10 53 erg. Compare to total energy output of the Sun during its lifetime: L " x 10 Gyr = 4 x erg/s x 3 x s ~ erg. Most of the energy released is in the form of neutrinos. About 1% = erg is in kinetic energy imparted to the outer envelope of the star to form a blast wave + EM radiation. EM radiation stays bright for months so the luminosity is comparable to that of an entire galaxy. Hans Bethe, who pioneered our understanding of SN, invented an energy unit called the foe, for 10 to the fifty-one (1 followed by 51 zeros) ergs.

17 Supernova 1987A in the Large Magellanic Cloud SN1987A BEFORE Wide angle view of the Large Magellanic Cloud galaxy. Close-up view of small region. Observed in January Star suddenly appeared, visible to naked eye. Event actually occurred 160,000 years ago!

18 SN 1987A

19 Supernova Sequence When the nuclear fusion in the core stops, the hydrostatic equilibrium balance ceases; gravity wins. Within about 0.1 sec, the core collapses. After about 0.5 sec, a burst of neutrinos is emitted when p + (proton) + e - (electron) n (neutron) + ν (neutrino) A burst of neutrinos was actually observed in SN1987A.

20 Super Kamiokande

21 Super Kamiokande 50,000 tons of pure water in a cylindrical stainless steel tank (41.4 m tall, 39.3 m diameter), surrounded by 11,146 photomultiplier tubes. Detect Cerenkov radiation from (very rare) neutrino interactions with water molecules.

22 Supernova Sequence When the nuclear fusion in the core stops, the hydrostatic equilibrium balance ceases; gravity wins. Within about 0.1 sec, the core collapses. After about 0.5 sec, a burst of neutrinos is emitted. Within 2 hours, the envelope of the star is explosively ejected. When the photons reach the surface of the star, it brightens by 100 million times! In SN1987A, the burst of neutrinos occurred ~2 hr before the star brightened.

23 Supernova Sequence When the nuclear fusion in the core stops, the hydrostatic equilibrium balance ceases; gravity wins. Within about 0.1 sec, the core collapses. After about 0.5 sec, a burst of neutrinos is emitted. Within 2 hours, the envelope of the star is explosively ejected. When the photons reach the surface of the star, it brightens by 100 million times. Over a period of months, the expanding remnant emits X-rays, radio, visible light, but starts to fade. The outer enriched layers carry off the heavy elements into the interstellar medium.

24 The evolution of SN 1987A has been observed at every available band X-ray, optical, radio

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26 The evolution of SN 1987A has been observed at every available band X-ray (Chandra), optical (HST), radio (ATCA)

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28 Crab Nebula (M1) Optical image VLT Explosion seen in 1054 by Chinese astronomers and Native Americans (petroglyph in Chaco Canyon, NM)

29 Kepler s Supernova Chandra X-ray Observatory Blue: 4-6 kev Green: kev Hubble Space Telescope Yellow: Optical Spitzer Space Telescope Red: IR Discovered Oct 9, 1604 as a naked-eye object (before the telescope). This is the most recent Galactic SN.

30 Cas A Supernova Remnant APOD 2011 Mar 5 Distance = 11,000 light years Composite X-ray (Chandra) + optical (HST)

31 Supernova Remnants The star can blow off most of its original mass. The expanding remnant contains products of fusion (heavy elements) # Pollutes (or fertilizes) surroundings. The expanding remnant can collide with other clouds nearby, triggering star formation: Death # Birth. Crab Supernova Remnant hot rarefied Cas A Ambient interstellar gas

32 Stellar remnants and stellar masses In the post Main Sequence phases (including planetary nebula or supernova), a star can lose a significant fraction of its original mass (i.e. the mass it had when it was a Main Sequence star). The final remnant depends on the mass of the star s core after the planetary nebula or supernova event. White dwarf: if the remnant mass is less than 1.4 solar masses (the Chandrasekhar limit). Neutron star: if the remnant mass is more massive than 1.4 solar masses and less than (about*) 3 solar masses. Stellar black hole: if the remnant mass is more massive than (about*) 5 solar masses. * We aren t sure what happens to a remnant of between 3 and 5 solar masses

33 Endpoints of Stellar Evolution Diameter ~ 10 4 km Density ~ 10 6 g cm -3 Diameter ~ 10 km Density ~ g cm -3 Degeneracy pressure: Pauli exclusion principle: no two particles (Fermions) can be in same quantum state Diameter ~ 2 x GM/c 2 or 0 Density #

34 What is left of the star s core? 1. A white dwarf: Collapse is halted by electron degeneracy. Works if the core remnant is less than 1.4 solar masses (Chandrasekhar limit). 2. A neutron star: Collapses is halted by neutron degeneracy. Works if the core remnant is less than (about) 3 solar masses (Volkoff-Oppenheimer limit). 3. A (stellar) black hole: Nothing stops the collapse! Happens if the core remnant is more than (about) 5 solar masses. During the last phases of a star s life, it can blow off a significant fraction of its mass; the remnant core is much less massive than the total mass of the star when it was on the Main Sequence.

35 The final outcome of a star s evolution Depends critically on 1. Initial mass (when on Main Sequence). 2. How much mass it loses in its post-main Sequence phases. Mass Radius Density Support Ordinary star White dwarf Neutron star Stellar black hole M! 1.4 M! < 3 M! >5 M!? km (Sun/star) ~10 4 km (Earth) ~10 km (Ithaca) gm cm -3 Gas pressure gm cm -3 Electron degeneracy pressure gm cm -3 Neutron degeneracy pressure ~15 km > gm cm -3 No equilibrium

36 Neutron Stars, Radio Pulsars Surface quantities:! B Gauss g NS g Φ volts

37 Neutron Stars, Radio Pulsars B B

38 Nucleosynthesis: where did it all come from? Big Bang: Hydrogen, Helium, Lithium, Stellar fusion: Hydrogen fusion (p-p, CNO); then successively heavier elements (till Fe). Seeded by planetary nebulae, supernovae. Elements beyond Iron typically created in SN explosions. All these elements are available for later generations of stars and planetary systems! The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff. (Cosmos, Carl Sagan)

39 This is one possible scenario. The other is where two white dwarfs in a binary spiral in and merge

40 Origin of gold is likely in rare neutron-star collisions" (Washington Post, 17 July 2013)