Star Death ( ) High Mass Star. Red Supergiant. Supernova + Remnant. Neutron Star

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1 Star Death High Mass Star Red Supergiant A star with mass between 8 M and 20 M will become a red supergiant and will subsequently experience a supernova explosion. The core of this star will have a mass between 1.4 M and 3 M. It will continue to collapse until its density is high enough for protons and electrons to interact to form neutrons. The result is a 10 km radius remnant composed mostly of neutrons. Neutron degeneracy pressure prevents gravity from compressing it further. Estimate the average density of this neutron star assuming a radius of 10 km and a mass of 2 M. Supernova + Remnant M D = V 4 3 V = πr 3 4 r = 10 km = 10 m Neutron Star M = ( 2 10 kg) = 4 10 kg kg 18 3 D = = 10 kg / m m 4 3 ( ) m = 4 10 m 4 V = π 3 One cubic centimeter would weigh about 10 9 tons on Earth! 1

2 Pre-Supernova Layering in a High Mass Star As a high mass star uses up one nuclear fuel after another, it develops a layered structure with the heavier elements concentrated closest to the center. The following diagram shows the approximate structure of the star just before it explodes. Hydrogen Fusing Shell Iron Core Silicon Fusing Shell Helium Fusing Shell Carbon Fusing Shell Magnesium Fusing Shell Neon Fusing Shell Oxygen Fusing Shell Hydrogen and helium originated in the Big Bang. All heavier elements were created by fusion in the cores of stars or in supernova explosions. Iron and elements heavier than iron are the result of nuclear reactions during the collapse of the iron core and the ensuing supernova explosion. Burning Stages in High Mass Stars Core Burning 9M Star 24 M Star Typical Core Stage Temperatures H burning 20 million years 7 million years (3-10) 10 7 K He burning 2 million years 700,000 years (1-7.5) 10 8 K C burning 380 years 160 years ( ) 10 9 K Ne burning 1.1 years 1 year ( ) 10 9 K O burning 8 months 6 months ( ) 10 9 K Si burning 4 days 1 day (2.8-4) 10 9 K 2

3 Events Preceding a Type II Supernova Silicon fuses to form an iron core; this takes about 1 day. Iron cannot release energy by fusion, so gravity causes the iron core to collapse. Atomic nuclei break up into helium nuclei as they absorb high energy gamma rays. The removal of the gamma rays cools the iron core and thereby further reduces the core pressure and accelerates collapse. The density is so high that electrons combine with protons, converting them into neutrons and producing neutrinos. As a result of this, the core collapses in less than 0.1 second. This rapid collapse results in the rapid conversion of gravitational potential energy into heat and light; the collapsing core heats up. The collapse of the core suddenly leaves the envelope of the star without support, and it falls toward the core. This rapid convergence of the envelope towards the core produces a shock wave that moves outward through the envelope. The power of the shock wave is enhanced dby the flood of neutrinos coming from the core and turbulent convection in the material that hits the core and rebounds from it. The shock wave bursts through the surface and an amount of energy comparable to a megaton nuclear bomb is released. The remains of the star consist of a supernova remnant (a nebula made of the material ejected in the explosion) and a neutron star (if the mass of the core is less than about 3 M ) or black hole (if the mass of the core is greater than about 3 M ). Very High Mass Star A star with mass between 8 M and 20 M will become a red supergiant and will subsequently experience a supernova explosion. The core of this star will have a mass greater than about 3 M. Red Supergiant Supernova + Remnant Black Hole There is no force that can prevent gravity from squeezing this mass into zero volume. The compact object that results is called a black hole. 3

4 Supernovae and Novae Supernova Classification Based on Spectra at Maximum Light No H Lines H Lines Type I Type II No SiII Lines SiII Lines Type Ia Type Ib 4

5 Type Ia Supernova Material from the normal star passes through the inner Lagrangian point and forms a disk around the white dwarf. The matter from the disk spirals inward and falls onto the surface of the white dwarf. Eventually, the mass of the hydrogen on the white dwarf exceeds the Chandrasekhar limit and the white dwarf collapses. Thermonuclear reactions ignite explosively and destroy the star. Visual Light Curve for SN1995AL A Type Ia Supernova Rapid rise to maximum brightness ( L sun). Gradual decline. Spectrum does not contain hydrogen emission lines. Maximum brightness greater than that of a type II supernova. 5

6 Type Ia Supernova in Centaurus A Supernovae are rare, so few have been observed in our galaxy. The video below shows a supernova in the galaxy Centaurus A. HST Image of Eta Carinae The mass of the star that produced this nebula is between 100 and 150 M. Its luminosity is about 4 million times that of the Sun. Sometime in the next few million years, it will undergo a type II supernova explosion. 6

7 Visual Light Curve of SN1987A A Type II-P Supernova Unlike most type II supernovae, SN1987A resulted from the collapse of a blue supergiant instead of a red supergiant. Rapid rise to maximum brightness ( L Sun ) Initial rapid decline slows down to form a shoulder in the light curve. Shoulder followed by gradual decline. Spectrum contains strong hydrogen emission lines. Tarantula Nebula in the Large Magellanic Cloud 7

8 Supernova 1987A Before and After Collapse When SN1987A occurred, there had not been another naked eye supernova in 383 years. Supernovae are rare, so most of those observed are in distant galaxies After Iron Core Collapse Before Iron Core Collapse Anglo-Australian Observatory Globular Cluster M13 More than 100,000 stars About 20,000 ly away About 12,000,000,000 years old About 150 ly across Credit & Copyright: Yuugi Kitahara 8

9 Estimating the Age of a Globular Cluster HR Diagram for 47 Tucanae B-V m Series B V 0.60 at the "turn off point". spectral class is G0 M= 1.1M years t = = ( 1.1) years M45, The Pleiades More than 3000 stars About 420 ly away About 100,000,000 years old 13 ly across 9

10 Finding the Age of M45 Hertzsprung -Russel Diagram for M B - V B V 0 at the"turnturn off point". spectral class A0 M = 3.6 M years t = = years m 8 10 Series Cepheid Variable Stars Supergiant or bright giant stars. Spectral type F or G. Periods between 2 and 60 days. Magnitude changes between 0.5 and 1. Type II are about 1.5 magnitudes fainter than type I. Period Luminosity relation (Type I Cepheids) Mv = 2.81logP

11 Cepheid Instability Strip Cepheids in M

12 Calculation of the Distance to M100 M V = log 10 P d 1.43 P d = 46 days m = = M V = log 10 (46) 1.43 = (m M + 5) / 5 ( ) / 5 d = 10 = 10 = pc = 23.5Mpc d = 74M ly The Pulsation Valve Mechanism Random radial inward fluctuation of a layer. Gravitational PE converted into heat and photons. Most of the converted energy causes ionization of HeII instead of increasing pressure to restore equilibrium.. HeII ionization complete, pressure increases and stops collapse, but now the layer is below the equilibrium point and rebounds toward it. Due to the cooling that results from expansion, the HeII ions recombine with electrons, releasing photons and increasing the pressure. When recombination is complete, the layer falls back through the equilibrium position. The layer s inertia carries it past the equilibrium point. As it rises, it cools, and pressure is reduced. 12