Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•Why do massive stars become supernovae?•What kind of star is left behind after a massive star supernova, and why?•Why do neutron stars rotate rapidly?•What are pulsars and how are they related to neutron stars?•What are black holes?•How does the escape velocity depend on mass and radius?•What is the curvature of space?1Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWAdministrative Stuff• Midterm this Wednesday!• 50 multiple choice questions in 50 minutes.• Bring a scantron form!• Office hours tomorrow 9:30am-1:20pm• CLUE review session TOMORROW NIGHT.• Multiple choice strategies:– All questions worth the same # of points, so if you get stuck, move on!– Cross out answers that you know can’t be right.– Read all the answers before deciding.22Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Let’s review:•Massive Star collapses•But, M > 1.4M!!!!! Can’t be supported by electron degeneracy pressure•Electrons combine with protons.•Star becomes solid neutrons!Neutron Degeneracy Pressure Supports the “Neutron Star”Can neutron degeneracy pressure fail too?G: Yes B: No3Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4Massive star SN should leave behind a SN remnant and a neutron star.Outer LayersCore4Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5Let’s review:•Massive Star collapses•But, M > 1.4M!!!!! Can’t be supported by electron degeneracy pressure•Electrons combine with protons.•Star becomes solid neutrons!Neutron Degeneracy Pressure Supports the “Neutron Star”Can neutron degeneracy pressure fail too?G: Yes B: No5Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6What happens if neutron degeneracy pressure can’t support the core either?•Collapses some more! (Mneutron>3M! or so?)•Nothing (we know of) can stop it!A black hole is “black” because light cannot escape from itBLACK HOLE!Any mass smaller than a particular size becomes a black hole! 6Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7What is a Black Hole???•Not quite…7Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8All objects have an “Escape Velocity”•“Escape Velocity” = how fast something on the surface of an object has to go to escape the object’s gravitational grip•Depends on mass & size ! Vescape2=2GMRs8Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9Escape velocity depends on mass and size!•Moon: 2 km/s•Earth: 11 km/s•Sun: 620 km/s•White Dwarf: 7,600 km/s•Neutron Star: 160,000 km/s When the escape velocity is larger than the speed of light, NOTHING CAN ESCAPE!c!300,000 km/s ! Vescape2=2GMRs9Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10So how smooshed do you have to get to be a black hole?•“Rs” = “Schwarzschild Radius”•If an object’s mass is compressed within Rs, the escape velocity is greater than the speed of light, nothing can get out, and it’s a BLACK HOLE! ! Vescape2=2GMRs= c210Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11What’s Your Schwarzschild Radius?•M ~ 70 kg•Rs ~ 10-25 m!Smaller than a nucleus!11Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12What about the Sun? •M ~ 2x1030 kg•Rs ~ 3 km!Still tiny! Smaller than a neutron star!12Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13What does the Schwarzschild radius mean?•If you’re inside Rs, you can never leave!“EVENT HORIZON”Since not even light can get past the event horizon, there is no way of knowing what goes on inside.13Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14•It doesn’t matter what’s inside the event horizon.Only things that matter are the mass, electrical charge, and rotation speed of the black hole.14Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15•If you’re well outside Rs, the black hole acts like any other mass.Q: If the Sun turns into black hole tomorrow, what will happen to the Earth?Yellow: The Earth will be sucked in.Pink: Nothing.15Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16If you see a black hole, DON’T JUMP!!!•Gravity increases with decreasing distance!•The force on your feet is much, much, much stronger than the force on your head! •“Tidal Forces” – pull you apart!16Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17Both sides of your body fall straight towards the black hole, stretching and smooshing you.•The friction heats you to millions of degrees.•The peak wavelength of a thermal spectrum with T=106K is in the X-ray!17Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18Black holes are rarely black!•When anything falls into a black hole, it will probably emit X-raysBlack holes can be some of the brightest objects in the universe!1.X-ray binaries2.Quasars & Active Galactic Nuclei18Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19How to light up a black hole:1.In a binary star:- Gas can jump from companion star “X-ray binary”2.In the center of a galaxy:- Gas can spiral into the center.- “Quasar” or “Active Galactic Nucleus” (“QSO” or “AGN”)19Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20Cygnus X1: A bright X-ray source with a companion O starThe O star is probably:G: More massiveB: Less massivethan 10 M!.This is a binary star -- 2 stars that were born at the same time!20Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21Cygnus X1: A bright X-ray source with a companion O starThe O star must be:G: More massiveB: Less massivethan the other star that evolved off the main sequenceThe progenitor of the O star’s companion must have been over 10 M!21Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXCygnus X1: A bright X-ray source with a companion O star• Progenitor of the x-ray source was too massive to be supported by neutron degeneracy pressure• Probably formed a black hole!The X-rays probably come from a whole disk of material pulled off the O star (a disk because of ang. mom.)22Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22Likely Black Hole CandidatesToo massive to be a neutron starFast orbit! Must orbit something small!All are main
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