{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

CH13 - Astronomy 1F03 2010/11 Fall Term 2010/11...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 13 Neutron Stars and Black Holes Neutron Stars Neutron Neutron stars are very small, perhaps 20 km in diameter 20 They can be found at or near the centre of Supernova remnants of They are so small – how do you see They how them? them? Radio Signals from Space from Jocelyn Bell (Cambridge, 1967) detected regular radio signals from space space Initially the sources were labelled LGM labelled LGM 1, LGM 2, etc. 1, LGM was short for Little Green Men – no-one knew what they were Radio Pulses explained Radio Anthony Hewitt (Nobel prize 1974) realized that a small, rapidly spinning magnetized object could create such radio waves radio The only thing small, spinning fast enough and sufficiently magnetized is a Neutron Star Neutron Pulsars The Neutron Star radio sources are called pulsars pulsars Pulsars can send radio pulses as fast as every millisecond every Pulsar Model: Pulsar Interstellar Lighthouse The enormous magnetic field of the Neutron star accelerates particles close to the speed of light from the magnetic poles of These particles emit radio waves ahead of them like a lighthouse beam them The spin of the Neutron Star sweeps the beam across the sky to strike earth regularly several times per second several Pulsar Model Pulsar Where did the spin and strong Magnetic Field come from? Magnetic The spin and magnetic field are the result of the extreme collapse that formed the Neutron Star formed Much of the angular momentum of the initial star is compressed into a region only 20 km in size – it must spin fast The star magnetic field is also compressed and grows much stronger compressed Are all Neutron Stars seen as Pulsars? Pulsars? No Over time Pulsar spins slow down and the radio pulses get weaker the Some Pulsars probably never point their radio beams at Earth radio Neutron Stars and Pulsars Neutron The number of Pulsars detected can be corrected for those not seen corrected The number is consistent with the idea that nearly every high mass star left behind a rapidly spinning Neutron star – a Pulsar Pulsar Crab Nebula Pulsar Pulsar A supernova supernova exploded in 1054 AD 1054 Leaving the Leaving Crab Nebula with a spinning neutron star at the middle the Visible Pulse Pulsar winds Pulsar glows in X-rays glows X-ray Pulse Supernova Kicks Supernova The supernova The explosion is often lopsided lopsided This can kick the This remnant (Neutron star or Black Hole) through space at very high speeds very 100 km/s or more Neutron star at 110 km/s Neutron Star-Ordinary Neutron Ordinary Star Binaries Star An ordinary star in a close binary orbit with a Neutron star can transfer material to it material Material builds up on the Neutron star until it rapidly undergoes nuclear fusion until X-rays are emitted in a massive burst Similar to nova outbursts for white dwarfs dwarfs Millisecond Pulsars Millisecond Neutron stars can be spun up to rotate 1000 times per second! 1000 This requires a companion to add mass and angular momentum – a binary Millisecond pulsars produce X-rays from Millisecond rays the infalling material as well infalling Neutron Star-Neutron Star Neutron Neutron Binaries Binaries Two very large stars can both evolve to become Neutron Stars become The orbital motion can leak out via gravity waves gravity Eventually the Neutron Stars may get close enough to merge close The high energy explosion could release Gamma Rays and explain Gamma Ray Bursts Bursts LIGO Gravity Waves Gravity Nothing moves faster than light – this this includes gravity includes When gravitational forces change, waves travel out to signal the change waves Gravitational waves are very weak and hard to detect hard The LIGO experiment has yet to detect anything anything Gamma Ray Gamma Bursts Discovered by Nuclear Test monitoring satellites satellites Uniform on the sky – must be at great must distances distances Energies of possibly 100 supernova in milliseconds milliseconds Must comes from a small region Must What are What Gamma Ray Bursts… Very rare events in distant galaxies: Magnetars (Soft Gamma Ray repeaters) Neutron star binary merger (once) Neutron Hypernova: A highly energetic beamed Hypernova highly supernova (once) supernova Gamma Ray Bursts Gamma Black Holes… Black Even a Neutron Star can’t resist gravity Even resist if it weighs more than 3 Solar Masses if What happens next? What Escape Speeds Escape Escape speed is the minimum speed required to leave from an object against the pull of its gravity the As you compact an object, the escape speed gets higher speed If you compressed Earth to 1 cm then the escape speed is the speed of light… the The Speed Limit The The speed of light is the fastest speed possible possible If the escape speed equals or exceeds the speed of light then nothing leaves the object, ever the Light can’t lleave – the object is eave the Light invisible: invisible: A black hole The Event Horizon The The radius where light can’t escape is called The escape the Event Horizon the This name refers to the fact that no events occuring beyond this point will ever be seen occuring beyond or detected or The Event Horizon exists when all the mass of the object is put within its Schwarzschild radius. The Event Horizon then occurs at the Schwarzschild radius. the The Event Horizon The The Schwarzschild radius is about 3 km times the mass of the object in Solar Masses Masses A 1.4 Solar Mass Neutron Star has: 1.4 Radius 10 km Radius Schwarzschild Radius 4.2 km Neutron Stars are very close to the limit Neutron Aside: Aside: Micro Black Holes In very energetic events small masses could be pushed into their event horizon – micro black holes The Large Hadron Collider Hadron Collider could make them could However, they evaporate very quickly very Cosmic ray events are more common and more energetic and we are still here here Aside: Aside: Evaporating Black Holes: Hawking Radiation Quantum field theory indicates particles Quantum are created continuously in a vacuum using borrowed energy (uncertainty principle) principle) - Proven via Casimir Effect Proven Casimir The are created in particle – anti-particle The particle pairs. Hawking radiation is when one of them falls into black hole and the other escapes. The energy to make the particle is lost by the Black Hole (decreased mass) (decreased Eventually the black hole evaporates Black Holes and Curved Space Curved Einstein’s General Theory of Einstein General Relativity supposes that matter causes space to curve causes There are no Newton style gravitational forces needed – everything travel straight ahead everything through a curved spacetime spacetime Thus even light can be trapped by the curved spacetime near a black spacetime near hole hole Testing General Relativity (GR) Testing Gravitational Lensing: Light is bent twice Gravitational Lensing Light as much was expected by Newton as Testing General Relativity (GR) Testing Precession of the Precession perihelion of Mercury: Mercury: Newton’s Gravity Gravity gives repeating Keplerian ellipses. Keplerian ellipses. The ellipses naturally precess iin precess n GR GR Non-Newtonian Gravity Non Newtonian Gravity gives perfect repeating Newtonian ellipses – nothing is lost In Einstein’s General Relativity, orbits are General not perfect, energy can be lost not These imperfect orbits have energy These carried away by gravitational waves – ripples in spacetime ripples spacetime Aside: Relativity Aside: 2 kinds: both due to Einstein Answers to two questions: Special Relativity (1905): What happens Special at high speeds near the speed of light? at General Relativity (1915): What is the General fundamental nature of Gravity? fundamental Special Relativity Special Michelson-Morley experiment 1887 – If Morley If light moves in a medium, how fast is Earth moving relative to that medium? Earth Answer: No medium – no absolute Answer: no reference frame exists reference The speed of light in vacuum is fixed (c = The 300000 km/s) in all reference frames 300000 What are the consequences? Observers Observers For all observers to see c the only way to do it is if a moving observer experiences time more slowly (time dilation) and sees lengths shorter (length contraction) contraction) 2 v Aside: The factor is Aside: 1− c Events that are simultaneous to one observer are not to others – depending on relative speed Twin paradox, GPS Twin Aside: Testing Special Relativity: Muons Muons Muon’s decay very quickly. decay Even at 0.98 c they’d mostly Even mostly disappear before hitting Earth. We only see them because their time moves slowly 0.2 seconds per 1 second for us In their reference frame, the length is short (2 km instead of 10 km) of Both observers agree the speed is 294,000 km/s speed Space Travel near Space Black Holes The tides near black holes would be enormous unless the black holes is very large (super massive) large No normal matter could resist being torn apart and superheated torn Matter falling into black holes releases copious X-rays which can be detected copious Gravitational Redshifts Redshifts It takes energy to leave regions near a black hole black For light, this means it converts to longer wavelength light – a redshift redshift Time Dilation Time Time appears to slow for something moving very fast (close to the speed of light) or falling into a black hole light) You never see it ever cross the Event Horizon Horizon It just fades from view ever more slowly as light from it gets redshifted redshifted What’s in a Black Hole What Nothing can get out, not even light and certainly no signal light You can go in to find out but never tell anyone never The most simple and direct application of GR says it’s a application singularity of infinite density (all the mass at a point of zero size). “Seeing” a black hole 1. The effects of its gravity 1. Something enormously heavy, small and invisible and 2. Seeing X-rays from material falling into rays the black hole the Cygnus X-1 Cygnus X-ray source Artists conception 25 Solar Mass B Star 25 10 Solar Mass very compact binary companion Best direct proof you can get – something something dark, incredibly massive in a small space space Black Hole in the Milky Way Using Kepler’s Laws on the Kepler Laws orbits of stars – determine determine mass of black hole mass ~ 2 x 106 Solar masses In a region < 30 AU Intermediate Mass Black Holes Black Good evidence for stellar Good mass (10’s Msun) and mass Msun and supermassive (millions of supermassive (millions Msun) black holes – what Msun black what about 1000 Msun? Msun Perhaps wandering X-ray ray sources are black holes accreting from the interstellar medium? interstellar M82 ...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online