PCS 581 Lecture Notes - Lecture 4: Retina, Brown Dwarf, Cosmic Distance Ladder
Lecture 4:
Last Class: White Dwarfs
Death of a sun
At the end, h and he in core, and eventually burns carbon – carbon is dead and tries its
best to hold itself together using inflation and gravity
And when becomes white dwarf - the electrons start running around, and their motion
causes pressure
Density
WD are extremely dense
The bigger they are, the smaller they will actually be
The more material, the more condensed it will be, and denser it will be, and the motion
will be slowed down, and it will crush down and become smaller
Gravity to Speed
If its’s larger, it will have more gravity, compressing it more
This makes the electrons to move faster
Is there a limit to how fast they move? Yes
WD shrink if more stuff is added
Limited space and they move faster and faster until no space
find more resources at oneclass.com
find more resources at oneclass.com
The Chandrasekhar Limit
Mass limit for WD: 1.4 MSun
Depends slightly on composition
We’ve never found a WD with 1.5MSun
oThis limits easily “falsifiable” we can prove something wrong easily – if we found
WD that is 1.5 MSun than we wouldn’t have that
Add stuff that reaches a point when it can’t hold itself together – collapses on itself
Stellar Mass –> Core Mass
The Sun will leave 0.6MSun white dwarf
Stars up to 8Msun could leave a WD, depending on how much mass they lose to winds
If the star’s remains are > 1.4Msun, it won’t leave a WD
60% of suns mass left on white dwarf
Anything 2 to 8MSun can leave a white dwarf
Breaking the Limit
All White dwarf star out with < 1.4MSun. Is there any way that limit can be crossed? Yes,
if it finds a source of mass
What will happen if it is exceeded? It collapses
Binaries
IF sun leaves WD that is the end
About half of all stars in the sky are binaries/ triples/ etc.?
Shouldn’t they influence their neighbour’s life?
oOnly if they’re close…. But some of them are very close
oClose binaries can influence their neighbour’s life
The Generous Neighbour
Biggest possible WD is 1.4MSun
But what if a star leaves a corpse close to that size, and then mass transfer occurs?
find more resources at oneclass.com
find more resources at oneclass.com
Overflowing the Roche Lobe
Mass transfer happens when one star when one-star overflowing the roche lobe
Roche lobe - Where gravity dominated by a single object
When starts start to die and expand and stars material crosses the dotted line and
crosses its own roche lobe and enter the white dwarfs roche lobes and becomes more
attracted to the white dwarf
The whole thing rotating – why not whole thing just go into the other all at once– both
stars are moving at different speed and
Everything is spinning and as materials and materials falls off and the materials are
going everywhere
Everything is always in motion and nothing is ever going to be in straight line
They are both orbiting and entering the accretion
Lageranian point is where gravity equals but at the point when mass transfer the white
dwarf will be more heavy
Disks are required by law… what law? Conservation of angular momentum
Hot spots stand out in spectra
Never seen it but the spectra tell the lot of the story (tale)
Everything is moving and it won’t go straight to the white dwarf – its due to
conservation of angular motion – where the material swirls in
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
At the end, h and he in core, and eventually burns carbon carbon is dead and tries its best to hold itself together using inflation and gravity. And when becomes white dwarf - the electrons start running around, and their motion causes pressure. The bigger they are, the smaller they will actually be. The more material, the more condensed it will be, and denser it will be, and the motion will be slowed down, and it will crush down and become smaller. If its"s larger, it will have more gravity, compressing it more. Wd shrink if more stuff is added. Limited space and they move faster and faster until no space. We"ve never found a wd with 1. 5msun: this limits easily falsifiable we can prove something wrong easily if we found. Wd that is 1. 5 msun than we wouldn"t have that. Add stuff that reaches a point when it can"t hold itself together collapses on itself.
