Stellar Remnants

Stellar Remnants © Sierra College Astronomy Department

Stellar RemnantsWhite Dwarfs Discovery for White Dwarfs • The firstwhite 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 20th 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 Department

Stellar RemnantsWhite 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 Department

Stellar RemnantsWhite 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 zerowhen 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 Department

Stellar RemnantsWhite 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 Department

Stellar RemnantsWhite 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 Department

Stellar RemnantsSupernova – 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 Department

Stellar RemnantsNeutron 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 Department

Stellar RemnantsNeutron 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 Department

Stellar RemnantsNeutron 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 Department

Stellar RemnantsNeutron 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 Department

Stellar RemnantsNeutron 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 Department

Stellar RemnantsBlack 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 Department

Stellar RemnantsBlack Holes Black Hole Structure • The Schwarzschild radius is the radius of a spherical region in space within which no light can escape: RS = 3M (RS 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 Department

Stellar RemnantsBlack Holes More Structure • The event horizon is the spherical surface of radius RS 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 Department

Stellar RemnantsBlack 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 Department

Stellar RemnantsBlack 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 1067 years to do so. © Sierra College Astronomy Department

Stellar RemnantsBlack 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 Department

Stellar RemnantsGamma-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 Department

Stellar RemnantsGamma-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 © Sierra College Astronomy Department

Stellar Remnants