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Cosmology and astronomy
Course: Cosmology and astronomy > Unit 2
Lesson 1: Life and death of stars- Birth of stars
- Accreting mass due to gravity simulation
- Challenge: Modeling Accretion Disks
- Becoming a red giant
- White and black dwarfs
- Star field and nebula images
- Lifecycle of massive stars
- Supernova (supernovae)
- Supernova clarification
- Black holes
- Supermassive black holes
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Birth of stars
Birth of Stars. Created by Sal Khan.
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- So lets say a hydrogen atom fuses with another hydrogen atom and creates deuterium. Then that deuterium fuses with maybe another deuterium atom and creates helium. Wouldn't it be possible for that helium atom to fuse with another hydrogen atom to create lithium? Or is it just hydrogen and helium here?(741 votes)
- You're right, fusion can actually continue to occur up until a star begins to create Iron! This is because fusing Iron (or heavier elements) costs more energy than it releases. Only the largest stars get this far, and it doesn't tend to happen until the lighter elements are used up because heavier elements require more and more energy to fuse. When/if this happens, btw, the star is seconds away from going super nova.(554 votes)
- Where did matter originate?(123 votes)
- Matter originated roughly 10^-35 seconds after the big bang, the reason the universe created matter is because of a symmetry violation. For every billion antimatter particles created, there were a billion and one particles of matter created. At first the energy from the big bang formed quarks and electrons then heavier particles after. So we do have an explanation as to how matter originated (for the most part, physicists are still experimenting to explain the gaps) and energy in the universe. We just can't see before the big bang and thus do not know at this time where that came from, if it came from anything at all.(146 votes)
- I am confused, how do the hydrogen atoms 'fuse' to form helium?(77 votes)
- The protons need to get close enough together for the strong nuclear force to effectively power out the repulsion of the electro-magnetic force. Once this happens, the H nuclei fuse to form a new He nucleus. Gravitational pressure, once it grows strong enough, speed up the movement of the nuclei so that they have a chance to collide and get close enough for this to occur.(55 votes)
- Is it possible that one day a star the size as the sun will form in our solar system one day(12 votes)
- The majority of mass in the solar system is already in the Sun. There isn't enough mass left over to form another star.(21 votes)
- But what did all the elements that were used to do the star come from.....(12 votes)
- Most of the hydrogen and helium in our universe formed in the initial stages of the big bang, after the forces separated, matter won out over antimatter, and a brief period of nucleosynthesis allowed for some fusion to occur.(15 votes)
- Will it not be possible for the hydrogen atoms to bond with each other and form hydrogen molecules? Sal never mentions anything but atoms, so I wanted to ask this question.(11 votes)
- No, the conditions inside stars are too extreme for molecular bonds to form.(13 votes)
- Is it possible to 'build' a star here on earth? And if it was, what would happen?(6 votes)
- It is possible to create nuclear fusion here on Earth. There are even websites that can show how to make a homemade nuclear fusion machine that teenagers have successfully built. https://www.theguardian.com/science/2015/jan/09/nuclear-fusion-young-scientist-jamie-edwards-star-in-jar(5 votes)
- Why does one of the hydrogen nuclei degrade into a neutron and lose its positive charge?(5 votes)
- when the 2 H atoms are forced together, it creates a more stable situation if one of them becomes a neutron.(4 votes)
- I just watched the red giant video, and it left me a little curious. The creation and the evolution of stars, have scientists been able to observe these events? Or did we determine this through theory and computer simulation?(4 votes)
- The short answer is both. We have a good theory of star formation, supported by observations and computer simulations.
The longer answer is that star formation takes tens of thousands of years to tens of millions of years, depending on the mass of the star. Massive stars take less time to form than main sequence stars, but they still take tens of thousands of years to form. With that length of time, there is no way for us to observe the entire formation of a single star.
What we can observe is stellar formation in various stages in different stars. We can observe the incipient or beginning phase in infrared wavelengths, which can see through the dust cloud which hides the newborn star. Other observations in optical wavelengths show the progress of star formation.
https://en.wikipedia.org/wiki/Star_formation(3 votes)
- So is a nebula not a star, just where many stars are born? Or is that what it is called when they are first born?(2 votes)
- Nebulae are simply clouds of dust and gas. As stars are formed from gas compressing, they are formed in nebulae, but not all nebulae form stars.
Kind of like hailstones form in clouds, but not all clouds form hail.(4 votes)
Video transcript
Let's imagine we
have a huge cloud of hydrogen atoms
floating in space. Huge, and when I say huge
cloud, huge both in distance and in mass. If you were to combine
all of the hydrogen atoms, it would just be this
really, really massive thing. So you have this huge cloud. Well, we know that gravity
would make the atoms actually attracted to each other. It's-- we normally don't think
about the gravity of atoms. But it would slowly
affect these atoms. And they would slowly
draw close to each other. It would slowly condense. They'd slowly move
towards the center of mass of all of the atoms. They'd slowly move in. So if we fast
forward, this cloud's going to get denser and denser. And the hydrogen atoms
are going to start bumping into each other and
rubbing up against each other and interacting with each other. And so it's going to get
denser and denser and denser. Now remember, it's a huge
mass of hydrogen atoms. So the temperature is going up. And it'll-- they'll
keep condensing. They'll just keep
condensing and condensing until something really
interesting happens. So let's imagine that they've
gotten really dense here in the center. And there's a bunch of
hydrogen atoms all over. It's really dense. I could never draw the
actual number of atoms here. This is really to
give you an idea. There's a huge amount of
inward pressure from gravity, from everything
that wants to get to that center of mass
of our entire cloud. The temperature here is
approaching 10 million Kelvin. And at that point,
something neat happens. And to kind of realize the
neat thing that's happening, let's remember what a
hydrogen atom looks like. A hydrogen-- and
even more, I'm just going to focus on
the hydrogen nucleus. So the hydrogen
nucleus is a proton. If you want to think
about a hydrogen atom, it also has an electron orbiting
around or floating around. And let's draw another
hydrogen atom over here. And obviously this
distance isn't to scale. This distance is
also not to scale. Atoms are actually--
the nucleus of atoms are actually much,
much, much, much smaller than the actual
radius of an atom. And so is the electron. But anyway, this just
gives you an idea. So we know from
the Coulomb forces, from electromagnetic forces,
that these two positively charged nucleuses will
not want to get anywhere near each other. But we do know from our-- from
what we learned about the four forces-- that if they did get
close enough to each other, that if they did get-- if
somehow under huge temperatures and huge pressures you were able
to get these two protons close enough to each other,
then all of a sudden, the strong force will overtake. It's much stronger
than the Coulomb force. And then these two
hydrogens will actually-- these nucleuses would actually
fuse-- or is it nuclei? Well, anyway, they would
actually fuse together. And so that is what
actually happens once this gets hot
and dense enough. You now have enough pressure
and enough temperature to overcome the
Coulomb force and bring these protons close enough
to each other for fusion to occur, for fusion ignition. And the reason why-- and
I want to be very careful. It's not ignition. It's not combustion in
the traditional sense. It's not like you're
burning a carbon molecule in the presence of oxygen. It's not combustion. It's ignition. And the reason why
it's called ignition is because when two
of these protons, or two of the nucleuses
fuse, the resulting nucleus has a slightly smaller mass. And so in the first
stage of this, you actually have two protons
under enough pressure-- obviously, this would not happen
with just the Coulomb forces-- with enough pressure
they get close enough. And then the strong interaction
actually keeps them together. One of these guys
degrades into a neutron. And the resulting mass
of the combined protons is lower than the mass
of each of the original. By a little bit, but
that little bit of mass results in a lot of
energy-- plus energy. And this energy is why
we call it ignition. And so what this energy
does is it provides a little bit of
outward pressure, so that this thing
doesn't keep collapsing. So once you get pressure
enough, the fusion occurs. And then that energy
provides outward pressure to balance what is now a star. So now we are at
where we actually have the ignition at the center. We have-- and we still have all
of the other molecules trying to get in providing the pressure
for this fusion ignition. Now, what is the hydrogen
being fused into? Well, in the first step of
the reaction-- and I'm just kind of doing the most basic
type of fusion that happens in stars-- the hydrogen
gets fused into deuterium. I have trouble spelling. Which is another way of
calling heavy hydrogen. This is still
hydrogen because it has one proton and
one neutron now. It is not helium yet. This does not have two-- it
does not have two protons. But then the deuterium
keeps fusing. And then we eventually
end up with helium. And we can even see that
on the periodic table. Oh, I lost my periodic table. Well, I'll show
you the next video. But we know hydrogen
in its atomic state has an atomic number of 1. And it also has a mass of 1. It only has one
nucleon in its nucleus. But it's being fused. It goes to hydrogen-2,
which is deuterium, which is one neutron, one proton
in its nucleus, two nucleons. And then that
eventually gets fused-- and I'm not going into the
detail of the reaction-- into helium. And by definition, helium has
two protons and two neutrons. So it has-- or we're
talking about helium-4, in particular, that
isotope of helium-- it has an atomic mass of 4. And this process
releases a ton of energy. Because the atomic mass of
the helium that gets produced is slightly lower than
four times the atomic mass of each of the
constituent hydrogens. So all of this energy, all this
energy from the fusion-- but it needs super high pressure, super
high temperatures to happen-- keeps the star from collapsing. And once a star
is in this stage, once it is using hydrogen--
it is fusing hydrogen in its core, where the
pressure and the temperature is the most, to form helium--
it is now in its main sequence. This is now a main
sequence star. And that's actually where
the sun is right now. Now there's questions of,
well, what if there just wasn't enough mass to get
to this level over here? And there actually
are things that never get to quite
that threshold to fuse all the way into helium. There are a few things
that don't quite make the threshold of stars
that only fuse to this level. So they are generating
some of their heat. Or there are even
smaller objects that just get to
the point there's a huge temperature and pressure,
but fusion is not actually occurring inside of the core. And something like Jupiter
would be an example. And you could go several
masses above Jupiter where you get
something like that. So you have to reach
a certain threshold where the mass, where the
pressure and the temperature due to the heavy mass,
get so large that you start this fusion. And-- but the smaller you
are above that threshold, the slower the
fusion will occur. But if you're super
massive, the fusion will occur really, really fast. So that's the general idea
of just how stars get formed and why they don't
collapse on themselves and why they are
these kind of balls of fusion reactions
existing in the universe. In the next few
videos, we'll talk about what happens once that
hydrogen fuel in the core starts to run out.