Stellar Remnants

Stellar Remnants

1. Under-mass Nebula

   • less than 0.08 solar mass (80 Jupiter's masses)
     • the nebula will NOT become hot enough to initiate hydrogen fusion
       • the nebula will FAIL to form a star
         • => Brown Dwarf

2. Contracting Nebula

   • a star begins to form when a nebula starts to collapse under the influence of its own gravity
     • as the nebula contracts, it becomes hotter
   • as the carob core contracts, the electrons in it are squeezed close tgt, raising the electron degeneracy pressure
     • eventually, the electron degeneracy pressure balances the gravity, & the contraction of the nebula ceases

3. Mass of nebula > 0.08 solar mass

   • the nebula will become hot enough to initiate H fusion
     • the nebula will form a main-sequence star
       • H fusion occurs in the core of a main-sequence star
         • as H fusion proceeds, eventually, a helium core starts to grow
           • this non-burning He core will contract under the inward pull of gravity & heat up
             • if the star's mass is lower than approx 0.5 solar mass, the He core will NOT become hot enough to initiate He fusion
             • as the He core contracts, eventually, the electron degeneracy pressure balances the gravity, & the contraction ceases
               • the He core will evolve to become a He white dwarf
                 • Eg) at the centre of Helix nebula
             • if the star's mass is between approx 0.5-8 solar masses, the He core will become hot enough to fuse He into C & O
               • eventually, the core evolves to become a carbon-oxygen white dwarf

4. Chandrasekhar Limit

   • the physics of degeneracy yields max mass for a non-rotating white dwarf, the Chandrasekhar limit
     • approx 1.4 solar masses - beyond which it canNOT be supported by electron degen pressure
   • the modern value of the limit was first published in 1931 by Subrahmanyan Chnadrasekhar
     • tgt with William Alfred Fowler, Chandrasekhar received the Nobel prize for this & other work in 1983

5. High-mass Star

   • if the star's mass is HIGHER than approx 8 solar mass
     • the star's core will exceed the Chandrasekhar limit
       • electron degeneracy pressure is overcome & the star's core collapses further
         • eventually, a core of non-burning pure NEUTRON starts to grow
           • Neutron Degeneracy Pressure
             • as the neutron core contracts under its own gravity
               • the neutron degeneracy pressure eventually halts the neutron core's gravitational contraction
                 • neutron core will evolve to become a neutron star
                   • Neutron star
                     • =dense & tiny
                       • mass comparable to that of the Sun, but it is only about 10km in radius
                         • neutron stars are virtually undetectable
                           • HST observed 1 thermally radiating NS
                     • some neutron stars rotate very rapidly & emit beams of radiation as pulsars
                       • Formation of Pulsar
                         • pulsar = NS that is formed with very high rotation speed
                           • beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the NS
                             • the magnetic axis is NOT the same as its rotational axis
                               • this misalignment causes the beam to be seen once for every rotation of the NS, which leads to the "pulsed" nature of its appearance
                                 • pulsar is the short for puslating radio star
                                   • this radiation can only be observed when the beam of emission is pointing toward the Earth
                         • the 1st pulsar was observed on November 28, 1967 by Jocelyn Bell Burnell & Antony Hewish
                           • they observed pulses separated by 1.33 seconds
                           • in 1974, Antony Hewish & Martin Ryle became the first astronomers to be awarded the Nobel Prize in physics, with the Royal Swedish Academy of Sciences noting that Hewish played a "decisive role in the discovery of pulsars"
                           • Jocelyn did not share the 1974 Nobel Prize for Physics with Hewish, she has been honoured by many other organisations

6. TOV Limit

   • (Tolman-Oppenheimer-Volkoff Limit)
   • =upper bound to the mass of NS
     • approx 3.0 solar masses, corresponding to an original stellar mass of 20 solar masses

7. Black Hole

   • if the mass of the stellar remnant is high enough, the neutron degen pressure will be INsufficient to prevent collapse
     • the stellar remnants -> black hole
       • BH = region of spacetime exhibiting such a strong grav pull that NO particles/radiation can escape from it
         • the Theory of GR predicts that a sufficiently compact mas can deform spacetime to form a BH
           • by their nature, BH do NOT directly emit any signals (the hypothetical Hawking radiation is predicted to be very weak)
             • astrophysicists searching for BH thus have to rely on indirect observations
               • BH's existence can be inferred by observing its grav interactions w its surroundings
                 • Eg) X-ray Binaries
                   • =binary star systems that are luminous in the X-ray part of the specturm
                     • Eg) Cygnus X-1 (one of the strongest X-ray sources seen from Earth)
                   • X-ray emissions are generally thought to be caused by one of the component stars being a BH accreting matter from the other (regular) star
                     • Accretion Disc
                       • due to conservation of angular momentum, gas falling into the grav well created by a BH will typically form a disc-like structure around the object
                         • friction w/i the disc increases the temp of the gas
                           • the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes
   • predicted by Theory of General Relativity
     • according to classical general relativity, NO matter/light can flow from the interior of a black hole to an outside observer
       • (although quantum effects may allow deviations from this strict rule)
         • General Relativity & current description of gravitation in modern physics - Albert Einstein @ 1915
           • Gen Rel provides a unified description of gravity as a geometric property of space & time, or spacetime
             • in particular, the curvature of spacetime is directly related to the E of whatever matter is present

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