February 29, 2020 0

Astronomy: Stellar Evolution


hi and welcome to the day’s webcast I’m
professor Kenny Tapp and today we’re talking about stellar
evolution which is the evolution the life and death of a star a star much like our own Sun indeed about four-and-a-half to five-and-a-half
billion years ago our own Sun was created and through that
process it evolves it goes through its own life span
and in about another 4.5-5.5 billion years from now it will go through its death
phase where it will end its life into something
dramatic what that is is what we’re going to be
learning about in this module in fact when we look at different stars across our own galaxy and beyond we find
that it really comes down to three different types of stars there are low mass stars medium mass stars and high-mass stars our
own sun is an example of a medium mass star so the mass plays a
major role in the life cycle the evolution of a
star the way I look at it is like an analogy of
driving a big SUV remember those hummers verses like a
Toyota Prius right if I were to give each of those
vehicles 10 gallons of gasoline which vehicle would be able to travel the farthest on the 10 gallons of gasoline I think we all vote
the Toyota Prius is probably going to go pretty far compared to the large hummer
right and what we get from that example we get
that the larger the vehicle the more mass there is the faster it burns
through the fuel and that’s exactly how we look at stars
stars are nothing more than a ball of gas that is burning and depending on
the initial mass of that star will determine its lifetime its longevity of life our
own Sun is a medium mass star it has the projected
lifetime of about 10 billion years so if you were to dramatically reduce
the mass that particular star a low-mass star could last for tens of billions of years if not a
hundred billion years or more but if you were to even just double the
mass of our own sun you cut the lifetime down to tens of millions of years so the lifetime
of the star itself is purely dependent on its mass
now in all of the stages of evolution there are
a lot of similarities there are only some minor differences when we get to the end phase the end stage the death stage
of a star based on its mass alright so let’s walk through this when we
look at the initial stages of a star being born we need to recognize
that there are two dominate gases that exist out in space hydrogen and helium now hydrogen is
the number one gas that exists out there so here we are we are looking at what we
call a nebula a nebula is a cloud of gas and dust and
when I say cloud i’m talking about. a huge cloud something larger than our
own solar system I mean we humans are conditioned when we
think of clouds you’re thinking of the big white puffy things up in the sky that you probably have found yourself
looking up and looking at imaginary animals right we all do that from time to time those
clouds are small and so when I say the word cloud you
might have subconsciously already limited your thinking to that small thing but I’m
talking about something huge i’m talking about a cloud bigger than our solar system and inside
that cloud of gas and dust we find hydrogen gas now I mentioned dust now the processes vary and the theories
vary but ultimately what happens inside that
cloud of gas and dust we get dust particles that collide and stick together and the more particles that stick together
of dust you start building up some mass you start building up gravity gravity is
a force what we call an attractive force gravity is
a force that exist between any two pairs of objects and under the universal law of gravitation
it says basically that any pair of objects that are around in
space will attract each other and be attracted by
each other so that means that if I were floating in
space and let’s say there is a wrench floating out just beyond arm’s
reach we are a pair of objects that means that I
would attract the wrench and the wrench would attract me now however the force of gravity of an object is tied
exclusively to its mass and we kind of got a feel for that when we
talked about the jovian versus terrestrial planets right jovian planets have tremendously more
gravitational forces because they are much more massive
than our smaller terrestrial planets so we already have a picture of that so as
these dust particles start to clump together and gather together they start building up gravity gravity
starts to build up and build up and build up and it starts to compress and squeeze the
cloud of gas and dust and it will start to compress it
and compress it and compress it down until the point where you have actually
compressed hydrogen gas together to ignite it in a process called fusion
its like lighting the fuse we ignite the clout thats are very official first stage of
stellar evolution called the Protostar stage and the Protostar
stage is basically when we’ve lit that fuse and you’ve got gravity that has
compressed the cloud fusing the hydrogen atoms causing it to
burn now some real cool stuff is going on now
because now we’ve got hydrogen burning if you’ve ever watched an action film right you’ve seen maybe in an action
film you’ve got the good guy walking casually and in the background a massive
explosion right and typically what happens in those action films the good guy flings flies away right
what do we notice we notice that every time there’s an explosion there is an
outward force that ball a burning gas in a protostar thats hydrogen burning thats a
thermonuclear reaction that’s a thermonuclear explosion so
here’s what’s going on you’ve got an outward force that is being
projected outward because the hydrogen gas is burning but remember gravity was there to begin
with gravity is there at the core of that newborn star gravity is an attractive
force always pulling inward so now we have this game of tug-o-war we’ve got gravity pulling inward
everything is pulling inwards towards the center of the star but the stars now burning and it’s
causing an outward force of energy at some point at some point
there is enough energy released outward that will
balance the gravitational attraction inward and that gets us to our second stage a stage of equilibrium which is what
we call a main-sequence star I can look at it like this if we take
the football team at the university and put them on one side to the game a
tug-o-war and let’s say we take the chess team now I’m not playing stereotypes I guess I am
but I’m going to guess the chess team is not pumping weights everyday so
there’s going to be an automatic disadvantage and we get that but let’s say you’ve got
twelve guys from the football team but you’ve got thirty guys from the chess team and you keep adding more and more and more to the chess team at some point the game of tug-o-war won’t be so disadvantaged it will even out thats what we’re
looking at here the hydrogen burning is causing forces
to go outward while gravity’s pulling forces in
at some point it equals out and that’s the main sequence star ninety
percent of a stars life time is spent in the main sequence stage our own sun
is a main sequence star which is great because we want to wake
up every day and our star is doing exactly what it did the day
before you do not want to live and be on the planet next to a star that is
either being born to the Protostar or going through its death stages alright, so
ninety-percent stars lifetime spent as a main sequence star now this particular star is burning
burning burning one of the consequences of burning hydrogen
is that you produce helium so the core the star is starting to fill up
with helium helium is not the name of the game for a star it’s burning its burning
hydrogen at some point the fuel is starting to run low so the star in order to keep burning has
to go towards the availability of fuel
remember this ball of burning gas is inside a larger
cloud so what happens then is that the ball
simply expands the layers of burning migrate
outward toward the availability of fuel if I
were to leave a trail of lighter fluid and light a match on one side of it is the
match going to stay here and just suck in all the fuel or is the match going to burn the fuel leave a
residue and travel toward the availability of fuel you’re right it’s going to travel down
the availability of fuel right we’ve seen that in action films that’s what’s happening in a
main-sequence star at the end of the main sequence stage
equilibrium is still there however you’re starting
to fill the core with helium gas and helium gas is not the name of the game here burning hydrogen is so you’ve got this three-dimensional sphere this ball that
starts to expand and get bigger because the layers of burning are migrating outward to get to more fuel this clicks
over to our third stage called a red giant stage two things are going on here the stars
getting bigger so it turns into a giant and check this out when you have burning
you you are exerting a certain amount of
heat right when the star gets bigger you’re taking
that same amount to heat the same amount of burning and you’re distributing it over a larger
surface area if you can imagine going from a golf
ball to a soccer ball right there are differences
they’re bigger right the the soccer ball is much
bigger it has a larger surface area which translates to mean that the
surface is going to cool down its sort of like if I were to take a small piece
a metal about the size of an index card and hold a blowtorch next to it for 5 minutes and I said here take this piece a metal you would probably say no thank you
that’s gonna burn my hands if I were to take a large three foot by three
foot piece a metal and hold the same blowtorch for the same
5 minutes to one corner then say hey can you move this piece of metal you would probably say yeah I actually probably could pick up the other end because you know that the heat would be distributed out
across the larger surface area and thus be cooler than it would be if
distributed over a smaller area the same thing happens with the star we take
the surface area of a small star and make it bigger we take the heat and we distribute it over
larger surface area and it distributes out less so the
actual outer surface of the star cools down slightly now when I say cool
down slightly it’s still really hot compared us 98 degrees here it’s still really hot but it will
change color it will turn a red color in fact if
you’ve ever cooked on a natural gas stove you’ve noticed
likely that the flame is not red orange flames that we see from
candles and campfires it’s actually a blue color right and
what do we know about colors on burning temperatures blue signifies a
higher degree of burning a hotter temperature red would
be technically cooler that’s why you can
always boil water faster over a natural gas or propane powered stove versus over your campfire because its just
hotter so when we see something turn red we know its a cooler temperature than
if something were burning simply blue so in the third stage of stellar
evolution the red giant stage its red because the outer surface has
expanded distributing out the same amount of heat over a larger surface area causing the outer layer to drop in temperature ever so slightly and it’s a giant
because the outer layers of burning had a migrate outward toward the
availability of fuel well not everything in space has an
infinite amount of quantity there is a limit a finite limit to the amount of
hydrogen gas available for that star to burn alright so we’ve got this red giant star and it has expanded at some point it
hits the limit of fuel it reaches what’s left of the fuel there’s nothing more to burn and in a
very dramatic fashion some cool stuff happens that represents
the death of that star now all of the stages for
every star goes through the same we go through the nebula the Protostar the
main sequence into the red giant here at the end game is where it differs
based on mass here are the three possible scenarios based
on the mass of a star for a low-mass star that red giant will simply flick lights out and
collapse now why would it collapse because
remember when hydrogen is burning it is exerting a force outward but gravity is always there at
the core of the star pulling inward this is a game of tug-o-war if the chess
team simply walks away from the other side of the game a tug-o-war the red flag in the middle is
going to move to one side because gravity is still there
Gravity is still there and left unchecked when the hydrogen burning
turns off because it has exhausted the fuel supply so it simply takes this huge star and it
collapses it into what we call a white dwarf in fact you can take a
star much larger than our own Sun and cram it
into something smaller than our own moon and thats about the size of a white
dwarf it looks white because of the incredible amount of heat
that’s there now there’s no heat source it’s just left over heat and over time it
will cool down it will lose its heat and eventually
reach the temperature of absolute zero which then we will call it a black dwarf you can’t see it anymore
you can only see things that emit a temperature in space now thats the first scenario, 2nd scenario is for a medium mass star it will collapse and become a white
dwarf but something cool happens when it
collapses because it’s just got enough mass when
the three-dimensional sphere starts to collapse on itself there is a conservation of momentum that
occurs when the two sides of the sphere meet each other at the center and it
translates into causing the thinner lesser mass outer
layers of that star to eject outward into what we call a
planetary nebula for quick demo here’s what I did in
class to demonstrate that with a basketball and tennis ball>>alright so what do we see we saw
that a tennis ball was ejected out when we dropped the two objects that is
exactly what’s going on for medium mass stars, it creates this beautiful ring of cloud-like
gases that we call a planetary nebula now the
naming convention is unfortunate because the word nebula would imply that it gives birth to something and planetary nebula could give birth to other planets, no it does not it does not absolutely give birth to other
planets it’s just an unfortunate naming
convention here are some great cool pictures that the Hubble Space
Telescope has captured of such planetary nebula and check out
the center of those you’ll see a white dot that’s
the white dwarf… a planetary nebula is a phase of stellar evolution that the
Sun should experience several billion years from now when it
expands to become a red giant it will then shed most of its outer
layers leaving behind a hot core that contracts to
form a dense white dwarf star a wind from the hot
core will ram into the ejected atmosphere
creating beautiful shell like structures seen with optical telescopes this
gallery shows four planetary nebulas from the first
systematic survey of such objects in the solar
neighborhood made with Nasa’s Chandra x-ray Observatory x-ray emission from Chandra is colored purple and optical emission
from the Hubble Space Telescope is colored red green and Blue the diffuse x-ray emission is caused by
shock waves as the wind collides with the ejected
atmosphere alright the third scenario in the death phase of stellar evolution
is for a high-mass star a really highly massive star doesn’t
simply collapse instead it terminates its life in a
brilliant explosion called a supernova in a single
supernova explosion will release more energy than all of the
energy combined when that thing was burning hydrogen that’s pretty insane a massive explosion a massive explosion call a supernova there are two outcomes when a supernova happens either you have caused an explosion so severe that has combined electrons and protons into neutrons and
you get what’s called a neutron star or the second scenario is that gravity is
left just completely insanely out of check and you have out of that supernova
explosion created a black hole so the stellar remnants of a high mass star is either a neutron star or a black hole
and a stellar remnant for a medium mass star and a low-mass star are just simply a white dwarf
that in a nutshell is stellar evolution so enjoy exploring the stages of stellar
evolution learning a bit more about it as you go through this module
I’m Professor Kenny Tapp we’ll see you next time

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