Stellar Evolution - Chapter 12 and 13. The Lives and Deaths of Stars White dwarfs, neutron stars and black holes

Transcription

1 Stellar Evolution - Chapter 12 and 13 The Lives and Deaths of Stars White dwarfs, neutron stars and black holes

2 During the early stages of a star formation the objects are called a protostars. The internal temperature is not high enough to produce fusion. These objects radiate energy away in the form of light. That energy comes from gravitational energy converted to heat. Once they reach the Main Sequence, The temperature is high (10 million K) and fusion starts. The contraction stops because gravity and internal pressure caused by the energy released from fusion exactly balance each other (hydrostatic equilibrium) Nuclear reactions occur at exactly the right rate to balance gravity. Remember that a stars mass determines its luminosity

3 Evolution of a Sun-like star (A star with the mass similar to the Sun) Nuclear reactions slowly convert H to He in the core. That is called core hydrogen burning. Burning here means fusion Newly formed stars are typically: ~91% Hydrogen (H) ~9% Helium (He) In the Sun s core, the conversion of H to He will take ~10 billion years (its Main Sequence lifetime). Composition of a Sun-like star.

4 What happens when the core Hydrogen is used up? Nuclear reactions stop. Core pressure decreases. Core contracts and gets hotter - heating overlaying layers. 4H 1He burning moves from the core to a hot shell surrounding the core.

5 Post-Main Sequence evolution 4H 1He reactions occur faster than before; the shell is at a higher temperature. The hot shell causes the outer layers to expand and cool! The star gets brighter (more luminous)! The star moves off the Main Sequence, up the Red Giant branch. Ascension up the red giant branch takes ~100 million years.

6 What happens in the core as it continues to contract and get hotter? Remember why Hydrogen burning requires 10 7 K (10 million K)? (Protons repel each other.) Helium nuclei (2 protons) repel each other even more In order to collide and fuse it is necessary higher temperatures (The nuclei need higher velocities!) Helium begins to fuse into Carbon at >10 8 K (100 million K). (Helium nuclei have 2 protons and two neutrons. Carbon nuclei have 6 protons + 6 neutrons.) This reaction is called triple alpha = 3He C (Alpha particle: Nucleus of He)

7 The Helium Flash: Post-Main Sequence evolution After the core reaches 10 8 K (100 million K) (stage 9), Helium ignites to make Carbon. The onset of this burning causes the temperature to rise sharply in a runaway explosion it is called Helium Flash Eventually the core expands, density drops and equilibrium is re-established Core structure is now readjusted during Helium core burning and total luminosity is actually decreased (Radius decrease,d temperature increases During core Helium burning, the star is on the Horizontal Branch (Stage 10).

8 Relative sizes of main-sequence, red giant, and horizontal branch stars. Stage 7 Stage 9 Stage 10

9 Structure of Stars in Different Evolutionary Stages

10 Stage 10 - Helium-to-Carbon burning occurs stably in core, with Hydrogen-to- Helium burning in shell Carbon Helium until the core Helium runs out. in just million years Carbon Helium Carbon Helium

11 The increased shell burning causes the outer layers to expand and cool (again). The star moves up the asymptotic giant branch (in only ~10,000 years!) to stage 11. becoming a red supergiant

12 During this phase, Heliumto-Carbon burning creates a Carbon core, which starts to contract and heat up. Then He burning moves to a shell With H burning in an outer shell. What do you suppose happens next in the core? For a Sun-like star: nothing. Why? Solar mass stars cannot squeeze and heat the core enough to ignite Carbon. It will need to reach a temperature of 600 million K to ignite carbon

13 So what does happen? With no more production of energy in the core, the carbon core continues to contract and heat. Shell He burning grows more intense. He flashes occur in the shell. Surface layers pulsate and are finally ejected (slowly, at ~10s of km/s). The hot core collapse into a tiny object called White Dwarf. White Dwarf And a Planetary Nebula appears! (The expanding emission line nebula heated by intense radiation from the hot white dwarf)

14 Planetary nebula White Dwarf Planetary Nebulae have nothing to do with planets. It is the last stage of the evolution of a star with a mass close to the Sun. The white dwarf has a temperature of about 120,000 K. It radiate UV emission. The emission excite the gas and the gas glows The nebula emit line emission (hot, low pressure gas). In size they are much smaller than the emission nebulae (HII regions). Example:Orion nebula They are important sources of heavy elements (C, N and O), which contaminate interstellar clouds and will go into the next generation of stars.

15 White dwarfs have about the Sun s mass (the rest of the mass was expelled). The white dwarf is an object about the size of the Earth! (~0.01 solar radii, 12,000 km) The collapse is stopped due to electron degeneracy pressure Density: ~1 million g/cm 3!! Very low luminosities (small radius) (L = 4 R 2 T 4 ) What eventually happens to a white dwarf? It gets cooler and fainter (at the same radius).

16 This is the End White Dwarf fades away. It doesn t produce more energy, it dissipate the energy stored in its mass. Because of the small radius (small surface), it takes a long time to cool off. Planetary Nebula dissipates into interstellar space. End of story for stars like the Sun.

17 Summary:

18 What happens to higher mass stars? Gravity squeezes and heats the core. The temperature needs to increase enough to be able to ignite Carbon, around 600 million K Then it will continue fusing Oxygen, then Neon as each fuel gets exhausted in the core, its burning moves to a shell. Concentric fusion shells form an onion skin structure. Important! The formation of an Iron core is the last stage

19 Reaching the conversion to Iron marks the end of the life of a massive star Creating elements heavier than Iron requires energy! Why is Iron formation the end of the line? Hydrogen With no more sources of energy, and the Iron fusion taking energy from the core The pressure that support the star s core is lost The core quickly collapses under its own weight.

20 The mass is too large and the electron degeneracy pressure cannot stop the collapse Protons and electrons are crushed together in the collapsing core, making neutrons. Eventually, the neutrons are so close together they touch. They generate the neutron degeneracy pressure The body that results from the collapse is a Neutron star The densities reach 100 trillion g/cm 3 (at those densities the whole Earth would fit in a football stadium!!) The collapsing neutron core then bounces! Supernova An explosive shock wave propagates outward, expelling all outer layers.

21 The Crab Nebula This supernova was recorded by Chinese astronomers in 1054 AD. It was BRIGHT! It was seen in daylight! Crab Nebula (Supernova remnant) The Crab Nebula (M1) is located in the Taurus constellation. The distance is about 6,500 ly The age is about 1,000 years The relative magnitude is +8.4 Supernova ejecta like this spread heavy elements throughout the Galaxy. Heavier elements, heavier than iron are created during the supernova explosion

22 A composite image of the Crab nebula from images of Chandra (X-rays), Hubble (Visual) and Spitzer (IR) The central object (core of star) is a rotating neutron star with a strong magnetic field. The object is called a pulsar The Crab pulsar (first detected at radio wavelength) has a rotation period of 33 milliseconds!

23 Neutron Star size compared to New York City What happen to the core? Composition of the core: Neutrons Mass: 1 3 M Sun Radius: ~10 km (6 miles) Density: kg/m 3 The core rotates fast 1 cm 3 weighs as much as Mt. Everest!

24 Neutron stars also spin very rapidly Why? Why the core (neutron star) rotates fast? The angular momentum L is conserved: L before = L after mvr = mvr Mass stays the same Most neutron stars rotate in less than a second! The range of rotational periods of pulsars is from a few milliseconds to about 4 seconds Slow rotation v Large radius R Fast rotation V Small radius r

25 Neutron stars have very strong magnetic fields Why? Magnetic field of neutron stars Why neutron star have a very intense magnetic field? When the star collapses into a neutron star, the plasma (electrically charged ) that make up the core of the star carries and intensify the magnetic field The result is that a neutron star rotates fast and have a very intense magnetic field

26 The combination of these two characteristics leads to an interesting phenomenon: PULSARS RA Dec The first pulsar, PSR was discovered in 1967 by then a graduate student, Jocelyn Bell in England who measured this periodic radio signal from an unresolved source (They were studying scintillation of radio sources): The observations were made with a radio telescope at 82 MHz (3.7 m wavelength). The period of the pulsar is seconds In 1974, her advisor, Anthony Hewish, won the Nobel Prize in physics for explaining it.. She was not a co-recipient of the award

27 A pulsar model The Lighthouse model of a pulsar 1) Strong magnetic fields lead to hot spots at the magnetic poles. 2) Accelerated particles at hot spots emit beamed radiation. For most pulsars the emission is in the radio wavelengths 3) If the rotation axis is not aligned with the magnetic field axis, then this searchlight rotates. 4) If Earth is in the path of the rotating beam, for every rotation, a pulse is detected

28 An animation of the lighthouse model of a pulsar

29 Are neutron stars the densest, most compact object in the universe? Can a neutron star be crushed further? YES!

30 Neutron stars are also stable structures, supported by neutron degeneracy pressure. What do you suppose happens if the mass exceeds 3 solar masses? The structure is crushed, and now there is nothing to halt the complete gravitational collapse!! The structure becomes a Black Hole

31 The end of the life of stars of different masses Diameter of a white dwarf ~ 12,000 km diameter (~Earth diameter) Diameter of a neutron star ~20 km across???

32 Black holes are thought to be the endpoints of stars that exceed solar masses on the main sequence They are concentrations of mass where gravity is so strong that nothing (including light) can escape! In terms of theory of relativity, the mass of the black hole distort and curve the space-time and creates an extremely deep gravitational well The mass will collapse into what is called in relativity as a SINGULARITY. The radius and density of the resulting object cannot be determined. The state of the matter inside cannot be described.

33 Due to the presence of the mass, the trajectory of the marble is a curve more gravity. A small mass creates a small curvature Einstein s General Theory of Relativity Masses curve the space around them. More mass more curvature. Roll a marble A way to visualize the curvature of space

34 Eventually (squeezing the mass even more) the escape velocity would be equal to the speed of light (300,000 km/sec). The trajectory of a light beam will be so distorted. The beam of light will remain inside the gravity well Nothing could get out, including light! That s a Black Hole. The critical radius at which the escape velocity equals the speed of light is called the Schwarzschild Radius. This is considered the radius of a black hole. There is no solid surface at that radius. The sphere around the black hole at the Schwarzschild Radius is called the event horizon, because no event inside that sphere can ever be seen, heard or known by anyone outside.

35 A simulation of the distortion of the images of stars in the background by a supermassive black hole (Gravitational lensing) (Supermassive Blackhole: Masses in the range of million to billion solar masses) Event horizon

36 Schwarzschild radii for different masses of black hole: 1 earth mass: 1 cm 1 solar mass: 3 km 10 6 solar masses: 3 x 10 6 km (0.02 AU) 10 9 solar masses: 3 x 10 9 km e:what would happen to the orbit of the Earth if the Sun suddenly became a Black Hole? Would the Earth get sucked in? No! The Earth will be at the same distance, at one AU from a solar mass black hole, it will feels the same gravitational attraction from the Sun at 1 AU. The gravitational force depends on the product of the masses and is inversely proportional to the distance squared. The masses of the Sun (or black hole) and the Earth are the same and the distance remain the same!!!

37 An example: The presence of a supermassive black hole in the center of the Milky Way An animation of the observation (in IR) of the center of the Milky Way showing the movement of stars around the central supermassive black hole. The black hole (Located by the star symbol in the animation) is located in the center of the Milky Way, in the constellation Sagittarius, at a distance of about 26,000 ly. How can we determine the mass of the black hole? Newton version of Kepler 3 rd Law! M BH = a 3 / P 2 The mass is about 4.3 million (4.3 x 10 6 ) solar masses P = Orbital period of a star orbiting the black hole a = Radius of the orbit of the star Mass of Black Hole = M BH