How are the lives of stars with close companions different? How could this strange pairing have come about?
Stars in Algol are close enough that matter can flow from the subgiant (which just left the main sequence) onto the mainsequence star Lefthand star is now a subgiant (just leaving the MS), but was originally more massive, say 4.5 solar masses, than its companion (which started with, say, 0.5 solar masses). These originalmass stars are shown at top, on MS. Lefthand star is now a subgiant (just leaving the MS), but was originally more massive, say 4.5 solar masses, than its companion (which started with, say, 0.5 solar masses). As the lefthand star reached the end of its MS life and expanded, it began to transfer mass to its companion. Lefthand star is now a subgiant (just leaving the MS), but was originally more massive, say 4.5 solar masses, than its companion (which started with, say, 0.5 solar masses). As it reached the end of its MS life and expanded, it began to lose mass to its companion. Now the companion star is more massive (it went from 0.5 to 4.3 solar masses), while the masslosing star (now a subgiant) went from 4.5 to 0.7 solar masses. Eventually the mass losing subgiant star (the star on the left) will become a white dwarf. What happens after that? Role reversal! When the star on the right becomes a giant, the white dwarf gains matter from it. Chapter 13 The Bizarre Stellar Graveyard What is a white dwarf?
White dwarfs are the leftover cores of dead stars, usually made mostly of carbon (some are made mostly of helium; others of oxygen or other elements heavier than carbon, up to and including iron).
Their name comes from the fact they are 'born' glowing whitehot with high temperatures (remember that the core of a normal star has a higher temperature than the surface of the star). In this Hubble space telescope photo we see Sirius A, the visually brightest star in the sky, and the white dwarf Sirius B as a tiny dot at the lower left. (The spikes of light are artifacts of the camera.) In Xrays (photo at left), Sirius B, the white dwarf, is brighter than its binary companion Sirius A, the visually brightest star in the sky. Electron degeneracy pressure supports white dwarfs against gravity, and doesn't depend on temperature. So a white dwarf has the same temperature inside as on its surface (unlike normal stars or planets). White dwarfs cool off and grow dimmer with time Hubble space telescope photo of white dwarfs in a globular cluster… they’re very dim! White dwarfs cool off and grow dimmer with time. So not all white dwarfs are white: they have colours from bluewhite (young) to orange red (old). A solarmass white dwarf is about the same size as Earth White dwarfs shrink when you add mass to them because their gravity gets stronger. Temperature also increases. Shrinkage of White Dwarfs • White dwarfs shrink when they get ‘heavier’!
• Quantum mechanics says that electrons in the same place cannot be in the same state
• Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space Shrinkage of White Dwarfs • Quantum mechanics says that electrons in the same place cannot be in the same state
• Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space
• In order to avoid being in the same state in the same place some of the electrons need to move faster. That increases the temperature, but not the pressure degeneracy pressure doesn't depend on temperature
• Is there a limit to how much you can shrink a white dwarf? (That is, how much mass a WD can have?) The White Dwarf Mass Limit
Einstein’s theory of relativity says that nothing can move faster than light. [The speed of limit is the same relative to all observers.]
When electron speeds in a white dwarf approach the speed of light, electron degeneracy pressure can no longer support the white dwarf.
Chandrasekhar found (at age 20!) that this happens when a white S. Chandrasekhar dwarf’s mass reaches 1.4 MSun What can happen to a white dwarf in a close binary system? White dwarf’s gravity pulls matter off of giant companion, but angular momentum prevents the matter from falling straight in; instead, it forms an accretion disk around the white dwarf. Friction in disk makes it hot, causing it to glow
Friction also removes angular momentum from inner regions of disk, allowing them to sink onto white dwarf Hydrogen that accretes onto a white dwarf builds up in a shell on the surface
When base of shell gets hot enough, hydrogen fusion suddenly begins and causes a nova Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space Two Types of Supernova
Massive star supernova: (Type II)
Iron core of massive star reaches white dwarf limit and collapses into a neutron star; rest of star 'bounces' off neutron star and explodes
White dwarf supernova: (Type Ia)
As white dwarf in close binary system reaches white dwarf limit, carbon fusion begins suddenly, throughout the white dwarf (uniform temperature) … complete explosion of white dwarf into space One way to tell supernova types apart is through their light curves (showing how luminosity changes with time) Nova or White Dwarf Supernova?
• Supernovae are MUCH, MUCH more luminous (about 10 million times)
• Nova: H to He fusion in a surface layer, white dwarf left intact
• White dwarf Supernova: complete explosion of white dwarf, nothing left behind Supernova Type: Massive Star or White Dwarf?
• Light curves differ (brightness changes over time are different)
• Spectra differ (exploding white dwarfs don’t have hydrogen absorption lines they're made of carbon and some oxygen, but essentially no hydrogen) What have we learned? • How are the lives of stars with close companions different? • When one star in a close binary system begins to swell in size at the end of its hydrogenburning life, it can begin to transfer mass to its companion. This mass exchange can then change the remaining life histories of both stars. • Sun What have we learned? What is a white dwarf? • A white dwarf is the core left over from a lowmass star, supported against the crush of gravity by electron degeneracy pressure. • What can happen to a white dwarf in a close binary system? • It can acquire hydrogen from its companion through an accretion disk. As hydrogen builds up on the white dwarf’s surface, it may ignite with nuclear fusion to make a nova, or compress the white dwarf until carbon fusion creates a supernova.