Introduction ? Recall that White Dwarfs are the second most common type of star. ? They are the remains of medium-sized stars - hydrogen fused to helium Neutron Stars - failed to ignite carbon - drove away their envelopes to from planetary nebulae & - collapsed and cooled to form White Dwarfs ? The more massive a White Dwarf, the smaller its radius Black Holes ? Stars more massive than the Chandrasekhar limit of 1.4 solar masses cannot be White Dwarfs Formation of Neutron Stars As the core of a massive star (residual mass greater than 1.4 ?Supernova 1987A solar masses) begins to collapse: (arrow) in the Large - density quickly reaches that of a white dwarf Magellanic Cloud was - but weight is too great to be supported by degenerate the first supernova electrons visible to the naked eye - collapse of core continues; atomic nuclei are broken apart since 1604. by gamma rays - Almost instantaneously, the increasing density forces freed electrons to absorb electrons to form neutrons - the star blasts away in a supernova explosion leaving behind a neutron star. Properties of Neutron Stars Crab Nebula ? Neutrons stars predicted to have a radius of about 10 km ? In CE 1054, Chinese and a density of 1014 g/cm3 . astronomers saw a ? This density is about the same as the nucleus supernova ? A sugar-cube-sized lump of this material would weigh 100 ? Pulsar is at center (arrow) million tons ? It is very energetic; pulses ? The mass of a neutron star cannot be more than 2-3 solar are detectable at visual masses wavelengths ? Neutron stars are predicted to rotate very fast, to be very ? Inset image taken by hot, and have a strong magnetic field. Hubble Space Telescope ? Such objects have been identified as pulsars, sources of pulsed radio energy. 1 Pulsars ? Pulsars are spinning ? A few pulsars have been neutron stars found in binary systems ? They emit beams of ? Astronomers can estimate radiation from their the masses of the pulsars magnetic poles ? In some binary systems, ? As they spin, the beams such as Hercules X-1,mass sweep through the sky like flows into a hot, accretion a lighthouse diskaround the neutron star ? X rays are emitted Black Holes ?If the core of a star collapses and contains more ? Escape Velocity is the than 3 solar masses, no force can stop it. initial velocity an object needs to escape from a ?As object shrinks, its density and gravitational celestial body. strength increase. ? For Earth, escape velocity ?If it shrinks to zero radius, its density and gravity is 11 km/s (25 000 mph) become infinite. ?Such a point is called a Singularity ?A black hole is formed Entering a Black Hole ? A black hole forms when ? If we were to leap, feet an object collapses to a first, into a black hole, small size ( perhaps to a outside observers would singularity) notice two relativistic ? The escape velocity in its effects. neighborhood is so great ? Time dilation- they would light cannot escape see our clock slow down ? This boundary is the ? Our light would seem to be Event Horizon red-shifted to longer ? Radius R S is wavelengths Schwarzschildradius ? We would not notice these ? Events inside event effects horizon cannot be seen 2 Entering a Black Hole Entering a Black Hole ?Entering feet first ? As we fell closer, tidal toward event horizon, forces would crush our we would feel our feet bodies laterally, and being pulled more stretch it longitudinally, strongly than our long before reaching the heads. event horizon ?This is a tidal force ? Friction from distortion would heat up our ?It will at first be minor bodies to millions of degrees ? We would emit X rays & Gamma rays Search for Black Holes Search for black Holes ?Do black holes exist? ? Cygnus X-1 was first candidate ? Consists of supergiant O star ?A black hole alone is totally invisible. with strong stellar wind, ?If a gas is flowing rapidly into it, the gas will heat carrying gas into a hot accretion up and emit X-rays before it enters event disk around a compact object. horizon. ? Doppler shift suggests that mass of compact object is about 10-15 ?Consider a black hole in a close binary system. solar masses ?Black hole could drain mass from companion ? Good candidate, but not 100% star, and form a hot accretion disk that would conclusive ? A few dozen more candidates, emit X-rays such as LMC X-3, V616 Mon, QZ Vul, … Misconceptions of Black Holes ? Black holes exist only in theory. ? Black holes are giant cosmic vacuum cleaners that swallow Recent observations obtained by the Hubble Space up everything around them. Telescope have given considerable support to the presence In reality, at a given distance from its center a black hole of supermassive black holes at the centers of some galaxies, creates the same gravity as would a normal object with the but there is not yet absolute proof. There has been very same mass. Therefore, black holes at a distance do not attract good evidence for the existence of stellar black holes for at matter more strongly than ordinary stars do. However, another least 20 years, based on the effects of an unseen companion important factor that determines the magnitude of the force of in the double-star system, Cygnus X-1. The existence of gravity is the radius of the object. The mass of a black hole is black holes seems likely, given our understanding of so compressed that it is possible to get very close to its center physical processes, but it is not absolutely confirmed. where gravity will be enormously strong. 3 ?Black holes can be detected visually. ?Our Sun will become a black hole. We observe black holes indirectly by the effect Only stars that are more massive than our Sun they have on material around them, but by might become black holes when they run out of definition a black hole cannot be "seen." fuel at the end of their lives. The Sun is not massive enough to become a black hole. Model of a Black Hole 4.