23 The Death of Stars 1

Transcription

1 23 The Death of Stars 1

2 23.1 Death of Low-Mass Stars W hen last we left off, the star of our show was in dire straits. Life was getting rough, as the star had evolved off the main sequence into a red giant -- not just once but twice! What will happen next to our star? Is it doomed, or will something happen at the last minute to save it from its ultimate fate? We now rejoin our story

3 23.1 Death of Low-Mass Stars A.Star evolves from the main sequence into a red giant. Surface temperature decreases, luminosity increases. C. Brief period of stability during which helium is fused to carbon and oxygen in the core (in the process the star becomes hotter and less luminous). B. Helium flash occurs at this point, leading to a readjustment of the star s internal structure D. After the helium in core is exhausted, star becomes giant again and moves to higher luminosity and lower temperature. By this time, however, star has exhausted inner resources and will soon begin to die.

4 23.1 Death of Low-Mass Stars At the end, helium fusion exhausted Star loses a great deal of its mass during formation of planetary nebula Equilibrium is lost - gravity takes over Core collapses due to lack of outward pressure from nuclear fusion Finally reaches equilibrium again, but only after core has attained incredible density Nearly 1 million times the density of water! Such hot, dense gas is said to be degenerate

5 23.1 Death of Low-Mass Stars A: Red giant. Loses mass as core begins to collapse. Planetary nebula formed. B: Star becomes hotter during collapse. Luminosity constant as temp rises. C: Star begins to cool very slowly. D: In billions of years, all heat will radiate away

6 23.1 Death of Low-Mass Stars Degenerate stars Material packed together so solidly that nothing can move Electrons cannot be squeezed together any more tightly Pressure exerted by electrons in their orbits stop collapse End result: a white dwarf A white dwarf with the mass of the Sun would equal the Earth in diameter On Earth, a teaspoon of the material would weigh several tons

7 23.1 Death of Low-Mass Stars White dwarf The larger the mass of the WD, the smaller its radius Figure 23.2

8 23.1 Death of Low-Mass Stars White dwarf Maximum mass= 1.4M Sun Chandrasekhar Limit In other words, the maximum mass that a star can still have after red giant phase and still evolve into a white dwarf is 1.4M Sun Anything greater would have a radius of zero Stars that are more massive do not end as white dwarfs Subrahmanyan Chandrasekhar ( )

9 23.1 Death of Low-Mass Stars The Ultimate Fate of White Dwarfs Eventually, all energy will be radiated away -- takes billions of years: Fig 23.4 Leftover corpse: black dwarf Not to be confused with a black hole Made of cooled, compressed oxygen and carbon Ultimate fate: a huge diamond "Twinkle, twinkle little star, How I wonder what you are. Up above the world so high, Like a diamond in the sky..."

10 23.1 Death of Low-Mass Stars The largest diamond ever found is not on Earth, but February 2004

11 23.1 Death of Low-Mass Stars Evidence That Stars Can Shed a Lot of Mass as They Evolve Chandrasekhar Limit: 1.4M Sun Doesn't say anything about a star's original mass How massive can a star be and still evolve into a white dwarf? Studies of star clusters provide answer

12 23.1 Death of Low-Mass Stars Mass Loss Leading up to White Dwarf NGC 1818 HR diagram reveals stars 6M Sun are just evolving off main sequence Spectra also reveals at least one white dwarf in cluster Chandrasekhar Limit says that star must have lost more than 4.6M Sun during giant phases. Why? That s because it s current mass must be <1.4M Sun

13 23.2 Evolution of Massive Stars Studies show that stars up to 8 M Sun will evolve into white dwarfs But, we know that stars can have masses up to 150 M Sun How will they ultimately die?

14 23.2 Evolution of Massive Stars Nuclear Fusion of Heavier Elements After helium in core is exhausted, massive stars contract until core becomes hot enough to fuse oxygen and carbon into silicon, sulfur, calcium, and argon. Silicon, when heated to a still-higher temperature, can combine to produce iron. Iron is the last step in the sequence of nonexplosive element production Star continues 3-stage evolution RAPIDLY! Nuclear fuel used up Star contracts Fusion of heavier nuclei begins

15 23.2 Evolution of Massive Stars Nuclear Fusion of Heavier Elements Eventually, an iron core develops Only in stars >8 M Sun 8-10 solar mass not hot enough - core = oxygen, neon, magnesium Surrounded by shells of silicon, sulfur, oxygen, neon, carbon, helium, and hydrogen Fusion continues in shells, but not core Iron absorbs energy (not emits), so stops producing

16 23.2 Evolution of Massive Stars Nuclear Fusion of Heavier Elements Figure 23.6 Structure of an Old Massive Star

17 23.2 Evolution of Massive Stars Collapse into a Ball of Neutrons As each finishes fusion process, leftover material falls onto the core, increasing its mass After density hits a certain point (400 billion times the density of water), atoms crushed so tightly that electrons combine with protons to form neutrons Result: Neutron star Collapse of electrons absorbed into nucleus to create neutrons is very rapid In less than a second, the core (size of Earth) is crushed to < 20 km (12 miles) in diameter Equivalent density, squeeze all the people in the world into a single sugar cube!

18 23.2 Evolution of Massive Stars Collapse and Explosion Collapse halts suddenly when density of core exceeds density of an atomic nucleus (densest form of matter known) Sudden halt sets up shock waves that reverberate throughout outer layers Causes outer layers to be blown off violently Supernova (Type II) During the collapse, each neutron generates a neutrino (energetic subatomic particles) Neutrinos carry away about 90% of supernova energy Review chapter 16 if you don t recall what a neutrino is

19 23.2 Evolution of Massive Stars Crab Nebula movie X-ray light by Chandra (left, blue) and optical light by Hubble (right, red)

20 23.2 Evolution of Massive Stars

21 23.2 Evolution of Massive Stars What happens next depends on the mass of the core Table 23.1

22 23.3 Supernova Observations Max brightness = 10 million times luminosity of Sun Material ejected at 44 million miles per hour Material is recycled into space to eventually form new stars and planets What about being too close to a supernova? Within 50 LY of Earth -- all life destroyed Within 100 LY, radiation would also have drastic consequences Closest massive star to Earth is Spica LY

23 23.3 Supernova Observations Supernova in Large Magellanic Cloud, 1987 (shorthand: SN 1987A)

24 23.3 Supernova Observations SN 1987A in Large Magellanic Cloud Original star Spectral type O Mass = 20 M Sun Age = 10M years old L = 60,000 x Sun

25 23.3 Supernova Observations The Change in the Brightness of SN 1987A over Time Rate of decline of the supernova s light slowed between days 40 and 500. During this time, light generated from energy emitted by newly formed (and fast-decaying) radioactive elements. Example: Ni-56, which decays to Co-56, which decays to Fe-56 (stable)

26 23.4 Pulsars and the Discovery of Neutron Stars After a type II supernova fades, all that is left is a neutron star Densest objects in universe First discovered in 1967 Source of rapid, regular bursts of radio noise (static) First thought to be intelligent radio beacons: LGM But, too many were found, so concluded they were too common to be civilizations Now we know: remnants of some supernovae Jocelyn Bell Burnell

27 23.4 Pulsars and Neutrinos Rapidly rotating neutron star emit beams of energy Rapidly spinning magnetic field Typical speeds between 1,000/sec to 1 rotation in 10 seconds We can detect the beam of energy if we are facing its sweep Eventually, pulsars will slow down as energy is depleted Lifespan: 10 million yrs

28 23.5 Evolution of Binary Star Systems Companions will evolve at different rates, greatly influencing each other Material can flow between stars The result? Mass of donor star decreases Mass of recipient star increases Artist s rendition of close binary star system

29 23.5 Evolution of Binary Star Systems White-Dwarf Explosion: The Mild Kind System: White dwarf and companion Companion slowly transfers material to WD Eventually, WD surface temp rises to H-He fusion Outer layer blown away, causing star to flare in brightness: Nova New star appears where none was seen before

30 23.5 Evolution of Binary Star Systems White-Dwarf Explosions: The Violent Kind If white dwarf accumulates mass rapidly, its mass can exceed 1.4M Sun Begins to collapse New nuclear reactions in degenerate core Sudden, complete destruction of WD Supernova Type Ia 30

31 23.6 The Mystery of Gamma-ray Bursts Discovered in the 1960s by military satellites looking for nuclear explosions 1991: Compton Gamma-Ray Observatory Detected flashes of gamma rays about once a day somewhere in the sky Each lasted from a fraction of a second to several hundred seconds 2008: extremely bright gamma-ray burst was detected Lasted about 30 seconds So bright, it could have been seen by the unaided eye More amazing, the burst originated 8 billion light-years from Earth! 31

32 Two Distinct Types of Gamma-ray Bursts Long-Duration Gamma-Ray Bursts: Exploding Stars Last more than 2 seconds, typically about a minute Originate in distant galaxies that are still actively making stars Thought to be caused when a massive star supernovas and is simultaneously stripped of its outer hydrogen layer: Type 1c supernova Sudden collapse produces swirling jets of particles and powerful beams of radiation 32

33 Two Distinct Types of Gamma-ray Bursts Short-Duration Gamma-Ray Bursts: Colliding Stellar Corpses Last less than 2 seconds Leading theory: merger of two compact stellar corpses: two neutron stars, or perhaps a neutron star and a black hole. Evidence: In 2017, gravitational waves were observed for the first time. Source aligned with a gamma-ray burst. This observation not only confirms the theory of the origin of short gamma-ray bursts, but also is a spectacular demonstration of the validity of Einstein s theory of general relativity. To be continued in the next chapter 33

34 24 Black Holes and Curved Spacetime 34

35 24.5 Black Holes What is a black hole? A black hole is an object whose gravity is so powerful that not even light can escape it.

36 24.5 Black Holes Origin of theory: Pierre-Simon Laplace: 18 th century Karl Schwarzschild: 20 th century He did the math!

37 24.5 Black Holes

38 24.5 Black Holes Events leading up to a black hole If the mass of a stellar remnant (neutron star) exceeds about 3M Sun, no known pressure (even degenerate neutron pressure) can halt the collapse of the star to a geometrical point. Object literally devours itself

39 24.5 Black Holes Geometry of a Black Hole Two parts Singularity (center) Event horizon Schwarzschild radius r = 2mG 2 c

40 24.5 Black Holes Schwarzschild radius 2mG r = 2 c Value depends on mass Earth = 1 centimeter Jupiter = 3 meters Sun = 3 kilometers

41 Spacetime Special relativity showed that space and time are not absolute. Instead, they are inextricably linked in a four-dimensional combination called spacetime.

42 Rubber Sheet Analogy Matter distorts spacetime in a manner analogous to how heavy weights distort a rubber sheet.

43 Curvature Near Sun Sun's mass curves spacetime near its surface.

44 Curvature Near Sun If we could shrink the Sun without changing its mass, curvature of spacetime would become greater near its surface, as would strength of gravity.

45 Curvature Near Black Hole Continued shrinkage of Sun would eventually make curvature so great that it would be like a bottomless pit in spacetime: a black hole.

46 Curvature Near Black Hole Spacetime is so curved near a black hole that nothing can escape. The "point of no return" is called the event horizon. Event horizon is a threedimensional surface.

47 Black holes don't suck! Well outside the Schwarzschild radius, the influence of a black hole is no different than for any other source of gravity. If the Sun were replaced by a black hole of equal mass, any planets that lie well outside the Schwarzschild radius would continue in their orbits as if nothing had happened.

48 What would it be like to visit a black hole? Death by Black Hole

49 24.5 Black Holes Evidence for Black Holes Although black holes emit no light, matter falling toward the black hole will be heated by friction before it passes the Schwarzschild radius. If one star in a binary system becomes a black hole, the other can expand in size later in life and dump material onto the black hole. When this happens, frictional heating is so intense that the infalling gas emits X-rays and gamma rays.

50 24.6 Evidence for Black Holes Studies show that a number of strong X-ray sources are in binary systems (which also conveniently allows us to measure their mass) Several systems are known where the mass of the unseen companion exceeds 3 M Sun

51 24.6 Evidence for Black Holes One example: Cygnus X kpc from Earth System: Type O9 blue supergiant and an unseen companion orbiting in 5.6 days. Mass of unseen companion = 15 M Sun One of the brightest x-ray sources in the sky

52 24.7 Gravitational Wave Astronomy Einstein's Theories of Relativity Special Theory of Relativity (1905) Usual notions of space and time must be revised for speeds approaching light speed (c). General Theory of Relativity (1915) Expands the ideas of special theory to include gravity

53 24.7 Gravitational Wave Astronomy Black hole mergers should produce strong gravitational waves.

54 Gravitational Waves explained on the Late Show