# Astronomy 192-401 Stars, Galaxies, and the Universe We’ll be starting with a quiz today. There will be five questions. You can get one wrong without being.

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Astronomy 192-401 Stars, Galaxies, and the Universe We’ll be starting with a quiz today. There will be five questions. You can get one wrong without being penalized. In other words, the quiz will scored as follows: Number correctGrade 4,5100% 375% 250% 125% 00% Please take out a sheet of paper and put your name and today’s date on it. Remove all other papers and books from the top of your desk.

1) A white dwarf in a close binary system can acquire hydrogen from its companion through an ______________ disk that swirls toward the white dwarf’s surface. 2) True or False: If the Sun suddenly became a black hole then the Earth would be sucked in. 3) True or False: The Schwarzschild radius is the distance from the center of a black hole to its surface. 4) True or False: A neutron star resembles a giant atomic nucleus. 5) True or False: The event horizon is the at the surface of a black hole. Quiz 8

Test 2 Test 2 will be on Wednesday, March 19. The test will follow the same format as Test 1. All material from chapter 15 through section 18.2, and all material covered in class since Feb. 11 is fair game for the exam.

Homework 2 Homework 2 is now available on the class website. It is due on March 17.

Nomenclature Massive does not mean Big. Massive means having a lot of mass. Big means large in size. For a white dwarf being more massive actually means being smaller.

White Dwarfs Remnant core of a dead star. Composition: Carbon, Oxygen(low-mass-star remnant) Oxygen, Neon, Magnesium(intermediate-mass-star) Mass – up to 1.4 M Sun (Chandrasekhar limit) Size – roughly Earth sized. Smaller white dwarfs have higher mass.

White Dwarfs White dwarfs in a binary system can collect matter from a companion star. The material leaving the star has angular momentum. It will form an accretion disk around the white dwarf. This disk is similar to a protostellar disk. What are the differences? A white dwarf is smaller and denser than a protostar. Therefore, the accretion disk has a smaller radius, is moving faster, and is hotter.

Accretion Disk Simulation Credit: Michael Owen, John Blondin (North Carolina State Univ.)Michael OwenNorth Carolina State Univ.

Mira: The Wonderful Star Credit: X-ray Image: M. Karovska (Harvard-Smithsonian CfA) et al., CXC / NASA Illustration: M.Weiss(CXC)Harvard-Smithsonian CfAet al.CXCNASA

White Dwarfs What happens as material is added to the white dwarf? Fusion can start to occur in the accreted layer of hydrogen. The white dwarf will then become a nova while this lasts (typically for a couple of weeks). Most of the accreted material will be expelled from the white dwarf. A nova is typically only as bright as 100,000 Suns. A supernova is typically only as bright as 10 billion Suns.

White Dwarfs What happens as material is added to the white dwarf? After the nova subsides the white dwarf can start accreting material again. A white dwarf can become a nova multiple times. Whether this process results in an increase of the mass of the white dwarf is unknown.

White Dwarfs What happens as material is added to the white dwarf? A white dwarf cannot exist with a mass greater than the Chandrasekhar limit (approx. 1.4 M Sun ). Some white dwarfs accrete enough material from a companion star that they approach this limit. When this happens, the temperature in the core increases enough that carbon fusion occurs catastrophically. The white dwarf becomes a type Ia supernova, and is destroyed.

Four Supernova Remnants Credit: NASA, CXC, SAONASACXCSAO

V838 Mon: Mystery Star Credit: Lisa Crause (Univ. Cape Town),Lisa Crause Warrick Lawson (Australian Defence Force Academy)Warrick Lawson Originally appeared to be a nova. Now appears to be a supergiant.

White Dwarfs What is the density of a white dwarf? Assume: mass= M Sun = 2 x 10 30 kg radius= R Earth = 6 x 10 6 m density= mass / volume

Supernova What are the differences between type Ia and type II supernova? Type II supernova leave a neutron star or black hole. Type Ia supernova don’t. The luminosities of the two types of supernova vary differently as a function of time. Fig 18.5

Neutron Stars A neutron star may be created when a high-mass star becomes a supernova. Where do all the neutrons come from? Electrons and protons can combine to form a neutron and a neutrino. This happens as the core of the high-mass star collapses. Neutrinos travel at close to the speed of light. We can detect a burst of neutrinos before we see the supernova.

Neutron Stars Composition: Almost entirely neutrons(interior) Electrons and atomic nuclei(crust) Mass – probably up to 2.5 M Sun Size – about 10 km Smaller neutron stars have higher mass.

Neutron Stars How come a neutron star is denser than a white dwarf? The size of a white dwarf is determined by electron-degeneracy pressure. The size of a neutron star is determined by neutron-degeneracy pressure. Neutrons can be packed more closely together than electrons. Neutrons are more massive than electrons.

Pulsars Neutron stars rotate rapidly. Why? Neutron stars have a strong magnetic field. The intense magnetic field directs radiation out along the magnetic poles. If the magnetic poles are not aligned with the axis of rotation then the beams of radiation sweep around. Such neutron stars are called pulsars. The rate of rotation decreases gradually as does the strength of the magnetic field. All pulsars are neutron stars, but not all neutron stars are pulsars.

Pulsars How do we know that pulsars are neutron stars? No other type of highly-massive object (other than a black hole) can rotate as rapidly.

X-Ray Binaries What can happen when a neutron star is in a close binary system? Matter can be transferred from the companion star to the neutron star. When this happens the material being accreted by the neutron star gets so hot that it emits x-rays. In such a situation the neutron star is called an X-Ray Binary. The rate of rotation of X-Ray Binaries tends to increase with time.

X-Ray Binaries X-Ray Binaries have a thin, hydrogen-burning shell. Below the hydrogen-burning shell is a layer of helium. The layer of helium can eventually get hot enough to fuse. This process repeats itself (typically every few hours to few days). All of the helium and surrounding hydrogen fuse in the course of a few seconds. This results in a burst of x-rays. Neutron stars undergoing this process are known as X-Ray Bursters.

Mathematical Interlude Suppose Betelgeuse (approx. 400 ly away) became a supernova. How would its brightness (don’t worry about the distribution of wavelengths) compare to that of the Sun? B = brightness L = luminosity D = distance

Mathematical Interlude

What is L Supernova ? L Supernova = 10 10 L Sun

Mathematical Interlude How many light years from the Earth is the Sun?

Mathematical Interlude

Suppose one star is orbiting another star. If I know the velocity of the star and how long it takes to make one orbit, can I determine the separation between the two stars? s = d / t s = speed d = distance traveled t = time d = 2  r r = the separation between the two stars Mathematical Interlude

s = speed d = distance traveled t = time r = separation between stars Mathematical Interlude

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