Ep. 783: Cataclysmic Variable Stars

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Astronomy cast episode 783 cataclysmic variable stars welcome to astronomy cast are weekly facts based journey through the cosmos where we help you understand not only what we know but

how we know what we know i'm for zircane i'm the publisher of the university with me as always is dr. Pamela gay a senior scientist for the planetary science institute and the director of cause request hey Pamela you done I am much less sniffly than I was last time we recorded yeah better living through chemistry yes yeah so so rich you're my hero there was so much sniffling that I had to move from our last episode I broke down I got the actual suit of fed and thank you everyone who sent me well

wishes over on patreon that really meant the world to me my phone just kept going alert alert and it made me happy while feeling miserable so thank you everyone for your kindness

there are many types of variable stars today we're going to talk about cataclysmic variable stars which are the result of a white dwarf stealing material from a companion star

and this whole process makes super villain Pamela happy it's true we're talking about a second but it's time for break

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and we're back yeah so so I have anticipated here that we're going to talk about something which

that if you were experiencing it would be a nightmare that just seemed to never end but from an outside observer

ooh that star is getting brighter and dimmer and oh why it's happening is very interesting it's so cool.

uh-huh and it involves relativity and descriptors like vampire or black widow yeah there was actually a point in graduate school

where McDonald Observatory I don't know if it still does but it used to have this really amazing library

in the dome for the 82 inch telescope up there and I it was cloudy it's the story of my dissertation

yes of all astronomers everywhere yeah yeah so I went pillaging for something to read and there

was a conference proceeding on cataclysmic variables and I didn't really know that much about them at that

point in my career and I ended up just like casually reading an entire conference proceedings

that night because I could and it turns out this stuff is just cool yeah yeah it's like it's

come on it's right there in the name cataclysmic yes like that's not good that's not like a nice day

head by all this is a cataclysm I know and you like it like stare to this like it was a reference

manual all night it was glorious imagining the dreadful horrors that are that are befalling some

poor civilization so I just again putting it on the record you're a monster so here here's my

favorite random fact that the stars in Tantoine system and Star Wars yeah are two bright yellowy stars

which means they're massive they are a close-in binary one of them will finish evolving before

the other they're not going to be the exact same mass oh future it's gonna it's gonna be future

cataclysmic variables that's so cool and you like is this been discussed or you just like

watching the movie like wait a minute that one that one that's amazing to write this up as an

totally should that's so cool okay well so let's explain like we've gotten we got away from

the actual explanation so so what is a cataclysmic variable star all right so backing all the way up

you you find when you look out across the the galaxy that a lot not the majority we thought

that for a while but we were wrong it turns out a lot of the stars out there are formed in

binary and multi-star system the big ones the big ones are yeah yeah um and when you have two stars

that are close enough together and one of them finishes evolving first that's always gonna be the

case the one that is more massive blows through its fuel first um when it's done it's gonna form

some sort of a compact object the gentlest form of death is two form of planetary nebula uh

exhaling the outer layers of the star and leaving behind the core as either a carbon or a hydrogen

helium white dwarf star right and that core is so dense that that the the atoms are essentially

forming a crystal latiss a a crystal lattice okay that's the word I meant yeah um and so you have

electron degeneracy pressure with all the electrons going poly exclusion principle we're done

in the same level pushing out against each other and and physics is going this is as small an

object right as we can make with normal matter now when you make normal matter absolutely as dense

as possible it means you can have massive surface gravity now white dwarfs are roughly the

size of between the moon and the earth and and when you have something that is like solar

mast and that tiny yeah it can snuggle up to a friend with all of its gravity and go friend I

am taking your atmosphere from you right right so like just to put some numbers on this that the

surface gravity of a white dwarf is in the thousands of kilometers per second when you compare to

earth is like eleven and the sun I forget what it is it's like hundreds for the sun you're looking

you're in the you're in the thousands um and and that a star like our sun will sort of at the end

of its life it's gonna puff off half of its mass and so you're left with the really the core the

burned out core of the where the fusion once happened that is now uh the white dwarf and it is

it is a fraction of the size of the interval volume of the of the star so it's much much smaller

and and yet as you say has just this enormous gravity and so if it does have a friend nearby victim

nearby uh then interesting things it start to happen especially when that other friend reaches

the end of its life and pops out it's outer layer so what kind of distance are we talking about

here when when do you just get a white dwarf orbiting another star and when will you get a white

dwarf feeding feasting on another star so when the separation between the surface of the white

dwarf and the surface of the frenemy is that the correct turn victim victim was the one that I used

yeah okay the and the surface of the victim are roughly a stellar diameter apart

is when you start seeing this happening and that's close you can get there through several different

scenarios so there is the star one shut its outer layers there's now all of this material hanging

out and this can cause them to dynamically move closer and closer over time and there are instances

of main sequence stars white dwarfs and that white dwarf feeds um uh the other scenario you can end

up with is star b and star a start out sufficiently far away but then a star like our sun is fully

capable of loading up to have an outer radius that is about the earth's orbit radius or larger if

it's a larger star and so you can start out with with the two of them separated just fine no dynamical

motion required no shrinking of orbits required it's just evolution causes the one star to make the

literally fatal mistake of loading up in size and um yeah it then gets victimized so I think about

like a classic example say is the is alpha centauri so you've got two stars that are sort of

sunlight that are in orbit around one another but they're 11 AU apart right is that too far that's

really far like that is that is the sun and Jupiter ish yeah it's it's more than Jupiter yeah so

that's pretty far that's too far and at that distance it would require migration and I'm not ever

going to say that migration isn't possible yeah um but that would require significant unfortunate

coupling of factors to migrate them together right though those two should be safe right but once

you get a little bit closer than that then when the one star puffs up and gives you that red

giant then the other star is plowing through the atmosphere of this other star which is a

source of friction as you said the mass distribution changes that causes migration yeah things get

planets get eaten things get weird um okay well we're going to talk about what this looks like

visually to astronomers but it's time for another break help melwood expand opportunities for

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and we're back so so we've described sort of the in the situation that happens but but tell me

that can be the sort of the history lesson for how we first identified these and and realize what

it was that we were looking at and and why do we see them as as variable so the the first observations

of them were the origin of the word Nova in parts so the word Nova it came about because of supernovae

but then it got reused anytime astronomers noticed something new lighting up in the sky

and there are a number of stars out there that are some form of not supernova but Nova

either because they are going off at recurring rates they go off once and then maybe never again

but whatever their regularity or lack thereof these objects increase anywhere from a few magnitudes

to 20 magnitudes yeah and that that's a lot we notice that when it happens yeah yeah yeah I mean

I forget what what how does it go like four magnitudes are 10 times like it's logarithmic right

yeah it's it's logarithmic but it it's not a perfect log there's a factor of 2.5 in there

okay okay but the point being like it's not like 20 is not 20 orders of magnitude but it's still

many orders of magnitude and something is doing a lot brighter but you know now I'm like we've

done a whole show on on Novae classical Novae the regular old planal vanilla Novae which is a

white dwarf feeding on a companion star so what is the the sort of distinction between a

cataclysm variable and an actual Nova or they just versions of the same thing the the way to think

of it is we used to have c for 1 c for 2 quasars all these different names for what we now

understand are active galactic nuclei that are being seen either at different powers different

viewing angles or both usually both with cataclysm variables what we're looking at is systems

undergoing mass transfer and then having a variety of different outcomes to that mass transfer

in some cases you have white dwarf grads material off of companion star gravitationally

and we've talked about this before we'll talk about it again angular momentum is the enemy

of quick actions and and this material can't flow directly except in special cases with magnetic

fields involved it can't flow directly from the the non evolved star onto the compact object so

it forms a disk now the disk allows angular momentum to get shed through other factors frictional

heating things like that and it will fall onto the surface of the star along the way you can end

up with brightening mean caused by two different kinds of things one is material builds up on the

surface that white dwarf builds up on the surface that white dwarf and then it eventually ignites

this is unprocessed material and you pile enough of it up with enough gravity and that gravity is

going to say hi would you like to react and the material is going to go thermonuclear as it does

right but that's a rare event like that is a once every you know many year sometimes many

century event depending on how actively the this white dwarf is feeding off of its partner it

depends on transfer rates it can be faster than that it all comes down to transfer rates yes

yeah but i'm like there's just one the we're all waiting for this nova to appear uh in the sky

and it's disappointing it has disappointed yeah but it's been calculated that happens every

whatever it is 10 to 80 years yeah exactly many decades apart between those and that is the

nova and that is i guess a type of variable star but that's different from the cataclysmic variable

behavior which is a separate thing so cataclysmic variables is the term that encompasses all of these

things so so supernovae are not cataclysmic variables just to be clear um but classical novae

recurrent novae dwarf novae all of those are forms of cataclysmic variables so yes there there are

all these kinds of of specialty names like sw sextantis is a dwarf novae with accretion

disk and study states so they don't outburst but we see the disk so they appeared very in brightness

because of that glowing disk um there's all sorts of weird side ways that we look at these things

we classify these things but when you have the compact object non-volved star and mass transfer

leading to brightening that is a cataclysmic variable okay and and it's not just material in the

surface the star igniting you can also have the accretion disk itself build up enough material

that the accretion disk can ignite so when we look at quasi-stolar objects when we look at active

galactic nuclei QSO as agn's um those are situations where you have a disk material that is

completely lit up undergoing thermonuclear reactions glowing brighter than the rest of the galaxy

it's located in that's the special case of supernovae in the center when you have the the less

exciting case of compact stellar objects so white dwarf these can happen with neutron stars

or a whole lot more rare and can happen with black holes as well yeah yeah even more rare um

but in these scenarios that accretion disk can build up sufficient material that it will ignite

an undergo thermonuclear reactions right yeah temporarily it blows itself apart

well so it's a micro quasar which is which is the term for these things so I love this idea that

that you can have a black hole or a neutron star or a white dwarf that is that is accreting

off material off of a partner that gets us accretion disk around it and then in that accretion

disk you get the same kind of spin magnetic field polar jets and even nucleosynthesis that's

happening in the disk in a way that mirrors or mimics what happens at the the giants supermassive

black holes at a reduced scale in a way that scales up beautifully from the tiny to the gigantic

and so astronomers were able to study these micro quasars in the galaxy make predictions about their

behavior and then scale that up to what you would expect to see on the larger scales and this

this sort of this scaling law works um so I want to talk about the future of these objects the part

that makes super villain panel happy but first it's time for another break and we're back so

this you've got a white dwarf that is feeding on material from a partner star this can't last

no no see so you have a variety of different situations the the most common is a white dwarf with

companion star that has become a red giant and as that red giant expands it fills the technical

term is rochlobe that gravitational equal potential surface and when it overflows material flows

from the the companion star onto the white dwarf and at a certain point the star no longer

has enough stuff to fill its rochlobe and it sort of settles down it's a smaller star

oh weird like it yeah it almost prevents it from dying well it's still going to die it's just

going to die as a lower mass object and take maybe a little longer and there's weird cases that

we learned about when I was in graduate school where there's ideas and we see this with blue

stragglers of a star is able to recover from collapsing into a white dwarf by gnawing enough material

off of its companion that it just becomes a star again so that's crazy now his companion is

quite dead at this point yeah that's that's a problem exactly exactly so so there there is

vampirism yes and and I have to admit one of my favorite things is this is one of the rare cases

where astronomers can almost name things well so for instance when when you have pulsars neutron

stars that are eating material off of tiny companions they're called black wings yes I love it

like portal pulsars yeah yeah and and so we have all these different cases of depending on the

difference in mass between the two objects the difference in evolutionary stage between the two

objects we call them different things because matter is able to flow at different rates if you have

a main sequence star which has a much denser atmosphere that wanders close to its companion star

it's going to have a whole lot more stuff that can be grabbed off of that higher density atmosphere

than that extremely diffuse extremely evolved red giant and so we see different flow rates we see

different potentials depending on just how young your victim might be right right um and so

like you think about this the the compact object that is drawing material and as you said you know

maybe it stops drawing material and the the companion object just stops like it's no longer

on the Rochelle oven so yeah it's feeding time is over yeah um but what if they're so close that

feeding time can never be over so in general like where where the star is orbiting even potentially

within the other star well blow it up as a red giant so so we we've seen a couple of different

results my my favorite result is the white dwarf that literally bit off more than it can chew

it orbited into the atmosphere of its companion that was bloating up fed fed fed went supernova

type 1a inside of the other star yeah that that was a wild one uh in general what's more likely is

that white dwarf is going to be outside of its companion when it overeats it exceeds uh there's

this barrier between something being supported by electron degeneracy pressure and the electrons

going nope can't do it anymore and the electrons and protons combine under the force of gravity

to be neutrons which take up less space and and that barrier going from electron degeneracy to

neutron degeneracy pressure uh there's a lot of energy given off when something collapses and

it's just gonna blow itself apart type 1a supernova events occur and it's interesting because the

the sort of series events that lead up to the creation of a type 1a supernova which we you know

astronomers use as this cosmic yardstick sounds very similar to the series of events that happen

from a white dwarf feeding from a partner star you've got a white dwarf in both cases it has a

companion object in both cases it is feeding from this companion object and it reaches a certain

point in the case of a nova it flashes off cleans off this surface and it starts from square 1 again

supernova builds up the material builds up the material and it just detonates as a supernova

and it's interesting you know we've been talking about this for since the beginning of astronomy

and 19 years and i've gotten kind of obsessed about this question about like what is the

delineation way to get one and the other and it appears that it's sort of like this perfect flow

rate that that if you don't have enough flow then the you get a nova but if you have too much flow

then um then you get another kind of supernova but if you get just the right rate that the super

that the white dwarf can pull in the material and accumulate it then you can get this this type

one is supernova and this sort of helps explain why they're so rare is that you actually need

very specific set of conditions for you to get the the one that blows the star up entirely as opposed

to the other possible outcomes that you can get but it's still a bit of an ongoing mystery

yeah right because the the the the series sequence of events sound identical right and it feels

like there should be situations where something goes from cataclysm variable to supernova as the

end point and and we just haven't seen that yet but we've only been looking for so long uh this

is the kind of science where having a ribbon online we're just going to keep promoting ribbon it's

what we do yeah um having the ribbon telescope online may it launch quickly um is kind to if we don't

see anything like this with it over the length of its survey that's going to put limits on whether

everything's like this can happen yeah and so you can never prove that something is impossible

but you can start to put limits on frequency and just how common or uncommon it something is yeah um

I'm about to just enthusiastically talk about Rubin here and I think I'm not going to suck myself so

um astronomers know of about twenty five hundred type one a supernova yeah throughout the history

of all of all the time the astronomers have been doing this astronomers know of about like maybe

fifteen hundred sixteen hundred cataclysm variable stars throughout identify throughout the entire

history of astronomy they're not as bright so we can only see them more locally just to be clear

this is a distance issue yeah yeah yeah your Rubin is going to find millions of supernova

it's going to find so much more of these claduclysm variables because it's going to it's going to

notice the ones that are just dimming a little bit that are yeah you're that you're catching these

these these subtle brightening that other telescopes would have missed because it's just

watching the same spots over and over and over again and that all of these things and I think

you nailed it perfectly which is it is it's at the very edges is the places where the rare things

happen that give you the insights into the underlying structure of what these things are and and boy

Rubin can't arrive soon enough now these are the things that I am deaf spread for and

the fact that that Cisco is naming some of its new systems after the Rubin Observatory

says just like how much computing emphasis had to go into building stuff for this telescope yeah

this is a Rubin class router that's crazy so there you go cataclysm variable stars another

you know another tool in the toolkit of the supervillains out there thanks bella

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