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© Sierra College Astronomy Department 1 Stellar Remnants
© Sierra College Astronomy Department2 Stellar Remnants White Dwarfs Discovery for White Dwarfs The first white dwarf was discovered when the star Sirius (the Dog Star) was suspected to have an unseen binary companion in 1850. It was discovered in 1862 and measured to be 10,000 times fainter than Sirius and became known as Sirius B (or the Pup). However, its mass was 0.98 M ⊙ and appeared highly under- luminous according to mass-luminosity relationships for main sequences stars. In the early 20 th century, the temperature of Sirius B was measured to be that of an A star (10,000 K) and therefore was warmer per square meter than the Sun. The only way it could be this faint and still be so hot was that the star was very small and hence was named a white dwarf.
© Sierra College Astronomy Department3 Stellar Remnants White Dwarfs White Dwarfs – Basic Properties White dwarfs have observed surface temperatures between 4,000 K and 85,000 K. Their masses range from perhaps 0.5 solar masses up to 1.4 M ⊙. When the nature of white dwarfs were realized, astronomers realized that the gas making up white dwarfs was extremely dense and was likely degenerate. In 1930, S. Chandrasekar calculated that a star made of pure degenerate material could support the gravity of the entire star if it were about the size of the Earth.
© Sierra College Astronomy Department4 Stellar Remnants White Dwarfs White Dwarfs - Strange Properties Chandrasekar found other surprising properties of white dwarfs If you add heat to this degenerate gas it does not expand (unlike gas in this room) As you add more mass to the white dwarf, it gets smaller! Theory predicts that white dwarfs radius will go to zero when the mass of the white dwarf becomes 1.4 M ⊙. This is the Chandrasekhar limit Most isolated stars lose enough mass to avoid this
© Sierra College Astronomy Department5 Stellar Remnants White Dwarfs End State of White Dwarfs A typical white dwarf has 1.0 M ⊙, a 12,000 km diameter (90% of Earth’s), and a teaspoon of white dwarf material would weigh 2 tons. Larger stars form more massive white dwarfs After it forms, no nuclear reactions are possible and the white dwarf simply cools off. A black dwarf is the theorized “final” state of a star with a main sequence mass less than about 8 solar masses, in which all of its energy sources have been depleted so that it emits no radiation. However, this could take billions of years: a 0.6 M ⊙ white dwarf would become 1/10,000 as luminous as the Sun after 6 billion years. White dwarfs less then 0.6 M ⊙ are rare since single stars would take over 10 billion years to these type of white dwarfs.
© Sierra College Astronomy Department6 Stellar Remnants White Dwarfs in Binary Systems An Alternate End-Game for White Dwarfs A close binary system of a white dwarf and a newly formed red giant will result in the formation of an accretion disk around the white dwarf. Hydrogen build-up on the white dwarf can ignite an explosive fusion reaction blowing off a gas shell that causes the white dwarf to brighten by 10 mags in a few days - the brightening is called a nova. The explosion does not disrupt the binary system. Infalling H ignition can recur with periods ranging from months to thousands of years.
© Sierra College Astronomy Department7 Stellar Remnants Supernova – Type I A White Dwarf Supernova Event If accretion brings the white dwarf mass above the Chandrasekhar limit, electron degeneracy can no longer support the star, and it collapses. The collapse raises the core temperature and runaway carbon-fusion begins, which ultimately leads to the star’s explosion. Such an exploding white dwarf is called a Type I supernova. (or white dwarf supernova) While a nova may reach an absolute magnitude of -8 (about 150,000 Suns), a Type I supernova attains a magnitude of -19 (5 billion Suns).
© Sierra College Astronomy Department8 Stellar Remnants Neutron Stars Neutron Stars – Just Theory A neutron star is the end result of a 6-12 M ⊙ star where its core has collapsed during a massive star supernova. Most of the star is composed of neutrons in a superfluid state and supported by neutron degeneracy. The outer crust of a neutron star is largely electrons and positively charged atomic nuclei. The diameter of a typical neutron star is only 0.2% of the diameter of a white dwarf (about 20 km diameter) and the neutron star is a billion times more dense. Neutron stars have masses between 1.4 and 3 solar masses.
© Sierra College Astronomy Department9 Stellar Remnants Neutron Stars Observation - The Discovery of Pulsars In 1967, Jocelyn Bell discovered an unknown source of rapidly pulsating radio waves. Subsequent discoveries of similar sources gave rise to the name pulsar. A pulsar is a celestial object of small angular size that emits pulses of radio waves with a regular period between about 0.0015 and 8.5 seconds, though nearly all fall between 0.1 and 2.5 seconds.
© Sierra College Astronomy Department10 Stellar Remnants Neutron Stars Back to Theory Objects that emit pulsing signals with a duration of 0.001 sec cannot have a diameter any greater than 0.001 light- secs, which is a few hundred kms. Such a small size ruled out white dwarfs (Earth-sized objects), leaving the hypothesized neutron star as the explanation for pulsars. The lighthouse model is a theory that explains pulsar behavior as being due to a spinning neutron star whose (synchrotron) radiation beam we see as it sweeps by. The high spin rate of a neutron star is obtained from the original star’s spin as a result of angular momentum conservation.
© Sierra College Astronomy Department11 Stellar Remnants Neutron Stars Data and Expectations More than 1000 pulsars have been discovered. The Crab pulsar spins so rapidly because it formed so recently. Over time it will lose rotational energy, slow down, and emit less energy. The Crab pulsar is slowing down because of the “drag” of the electrons propelled out into the nebula surrounding the pulsar.
© Sierra College Astronomy Department12 Stellar Remnants Neutron Stars Neutron Stars in Close Binaries Accretion disks may form as with white dwarfs in close binaries. X-ray Binaries Huge amount of energy release of in-falling matter heats inner regions of accretion disk to point of x-ray emission – 100,000 times more the Sun’s combined wavelength energy output. Pulse rate similar to radio pulsation, but accelerates with time (to millisecond pulses) as neutron star gains angular momentum. X-ray Bursters Accreted hydrogen fuses to helium and builds up. Helium fuses rapidly, an x-ray burst occurs, and may repeat every few hours to every few days with durations of seconds.
© Sierra College Astronomy Department13 Stellar Remnants Black Holes Creation of Black Holes A black hole is the end result of a supernova explosion of a star with initial mass greater than about 12 M ⊙. Neutron degeneracy cannot support a neutron star whose mass is greater than about 3 solar masses.
© Sierra College Astronomy Department14 Stellar Remnants Black Holes Black Hole Structure The Schwarzschild radius is the radius of a spherical region in space within which no light can escape: R S = 3M (R S in km; M in solar masses) The size of the Schwarzschild radius depends on the mass within the sphere. A black hole is a spherical volume of space with a radius given by the Schwarzschild formula above and with an escape velocity that exceeds the speed of light.
© Sierra College Astronomy Department15 Stellar Remnants Black Holes More Structure The event horizon is the spherical surface of radius R S around a black hole from which nothing can escape. Inside a black hole, an object will eventually be subjected to extreme tidal forces pulled apart. The final destination of an object (or so it is thought) in a black hole is to be crushed out of existence at a central singularity. Spinning and charged black holes are more complex.
© Sierra College Astronomy Department16 Stellar Remnants Black Holes Detecting Black Holes If a black hole has a close binary companion, material may be pulled from the companion to form an accretion disk around the black hole. The accretion disk radiates x-rays and gamma rays as the gas is heated to very high temperatures as it approaches the event horizon. From their x-ray emissions, Cygnus X-1 and AO620-00 are two very good black hole candidates. V404 Cygni - Observations of X-rays and and in the visible have shown that this is a binary system in which a late G or early K star revolves every 6.47 days around a compact companion with a mass between 8 to 15 solar masses.
© Sierra College Astronomy Department17 Stellar Remnants Black Holes Black Holes Forever? Jacob Bekenstein discovered that the black holes can be assigned a temperature. Soon thereafter in 1974, Stephen Hawking showed that this temperature meant that a black hole emits thermal radiation through a quantum/gravity energy exchange. Hawking radiation means black holes radiate away, although a one-solar-mass black hole will take 10 67 years to do so.
© Sierra College Astronomy Department18 Stellar Remnants Black Holes Visiting a Black Hole The physics of black holes, both inside and outside is best understood through Einstein’s General Theory of Relativity The theory employs spacetime: a 4-dimensional construct that blends the 3 dimensions of space with the dimension of time Objects with mass bend spacetime The curvature of spacetime then dictates how objects move Consequences as you approach the event horizon of a black hole feet first: You age at a slower rate relative to someone far from the black hole (although you do not sense such a slowing yourself) To an outside observer, you never appear to cross the event horizon as your emissions shift from the visible to the infrared to the radio due to gravitational redshift You are stretched in length and squeezed thinner, and eventually the tidal forces overwhelm the molecular bonds in your body (i.e., you die!)
© Sierra College Astronomy Department19 Stellar Remnants Gamma-Ray Bursts Observations of Gamma-Ray Burst First detected in 1960s by U.S. military using satellites to detect gamma-ray signatures of nuclear bomb tests. Gamma-ray bursts (GRBs) with dramatic fluctuations over just a few seconds were found to be non-terrestrial. By 1991, GRBs were found to be randomly distributed and not correlated to the Milky Way’s disc as X-ray bursts were. And by 1997, correlations between numerous GRBs and galaxies very far away was established – making GRB sources the most powerful bursts of energy we observe in the universe.
© Sierra College Astronomy Department20 Stellar Remnants Gamma-Ray Bursts Two Possible Causes of Gamma-Ray Bursts The required energy to create the burst suggests that they are created when certain kinds of black holes are formed. They may be created during unusually powerful supernova explosions that create black holes Or perhaps they are caused when two neutron stars collide
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