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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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Lifecycle of massive stars
Lifecycle of Massive Stars. Created by Sal Khan.
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- How then are even heavier elements formed? Earth contains a lot of uranium for example. By what process was it formed if not from a solar reaction?(28 votes)
- When a massive star dies, it doesn't flicker out like the Sun, it goes out with a bang. Watch the next video to learn more about their fiery death but that process generates the heat needed to fuse even heavier elements. Every ounce of iron, gold, copper, lead, and uranium was produced to the scorching heat of a dieing star.(32 votes)
- How do scientists know what a star is made up of? i.e.. iron core, silicon etc.(16 votes)
- Scientists use a spectroscope to determine the absorption/emission lines(the frequencies the stars/celestial objects emit at) which tells us exactly which element or molecule is present in the body. Since each element and molecule is unique, their emission and absorption lines are also unique. Spectroscopic lines can also tell us about other properties of the star. For more about this, you should study the wave-particle duality of light.(4 votes)
- does the star have a life span?(6 votes)
- Yes, all stars will die eventually. Small stars like red dwarfs have life spans that range trillions of years. Our Sun will survive live around 10 or 11 billions years, it is currently 4.5 billion years old. A bright blue giant will live only a few million years.(9 votes)
- In the picture of a super giant, why does neon fuse before oxygen?(4 votes)
- The picture isn't showing the order they are formed. It is displaying the order in which they are burned. Neon burns in a reaction that allows it to be used up at pressures/temperatures lower than the reaction that burns oxygen. Thus the order in which they are displayed.(6 votes)
- You told that Fe can't be fused in a massive star. Then how elements larger than Fe is produced? Is there any other process to produce bigger elements?(5 votes)
- Yes, the process is called neutron capture, where neutrons can build up on nuclei creating unstable isotopes which decay into heavier elements.(2 votes)
- What is the average known lifespan of a massive star?(4 votes)
- Not a few million, but a few billion, actually. The sun, an average sized star, will "live" to be 9 to 10 billion years, while tiny dwarf stars will last even longer. In fact, some of the first stars that formed are still here! Only the largest stars will be in the millions. For example, Betelgeuse, a supermassive star, will only last about 11 million years.(4 votes)
- At, how does the other elements, such as lithium, are formed? in the exact same process as carbon, for example? is there a reason why these elements are not noted in the diagram? 5:45(3 votes)
- Lithium, Beryllium and Boron can be produced in stars. But they are easily burned at temperatures less than stars burn at (brown dwarfs are even capable of lithium burning), so they are quickly burned up after being generated. Most of the elemental occurrences of these elements aren't generated in stars but via other nucleosynthesis process, primarily cosmic ray spallation.(6 votes)
- I have noticed that in all the videos which I have watched, the birth of stars always starts with massive masses of hydrogen atoms. Does this mean that hydrogen is the only gas which is present in the universe? Is there not helium or atoms of other elements? If there were, why can't stars start forming from atoms of helium directly without passing the "hydrogen fusing into helium" stage?(4 votes)
- There is helium available. However, as the gasses compress, hydrogen fuses at a lower temperature than helium, so that point is reached first. Once the hydrogen starts fusing, it temporarily halts the increase in pressure/temperature while the hydrogen is fused in the core. Once the hydrogen in the core is exhausted and the fusion there stops, the matter can further collapse to build up the pressure/temp until the helium can finally start fusing.(1 vote)
- Why does most of the helium fuse into carbon rather than lithium? Do 3 heliums fuse together to form carbon?(1 vote)
- It will form lithium. However, lithium burns up easily at the conditions even hydrogen will fuse. So no significant amount will build up in the star.(7 votes)
- How hot is a massive star?(2 votes)
- It varies. The red hypergiants can have a surface as low as 3000 K while the blue hypergiants can be up to 54,000 K on the surface. A hypergiant will have a core temperature of 2.7 to 3.5 billion K.(3 votes)
Video transcript
We've already talked about
the life cycle of stars roughly the same mass as our
sun, give or take a little bit. What I want to do
in this video is talk about more massive stars. And when I'm talking
about massive stars, I'm talking about stars that
have masses greater than 9 times the sun. So the general idea
is exactly the same. You're going to start off
with this huge cloud of mainly hydrogen. And now, this cloud
is going to have to be bigger than the clouds
that condensed to form stars like our sun. But you're going
to start with that, and eventually gravity's
going to pull it together. And the core of it is going
to get hot and dense enough for hydrogen to ignite, for
hydrogen to start fusing. So this is hydrogen,
and it is now fusing. Let me write it. It is now fusing. Hydrogen fusion. Let me write it like this. You now have hydrogen
fusion in the middle. So it's ignited,
and around it, you have just the other
material of the cloud. So the rest of the hydrogen. And now, since it's so
heated, it's really a plasma. It's really kind of a soup
of electrons and nucleuses as opposed to well-formed atoms,
especially close to the core. So now you have hydrogen fusion. We saw this happens at
around 10 million Kelvin. And I want to make
it very clear. Since we're talking
about more massive stars, even at this stage,
there's going to be more gravitational
pressure, even at this stage, during the main
sequence of the star, because it is more massive. And so this is going to
burn faster and hotter. So this is going to be faster
and hotter than something the mass of our sun. And so even this stage
is going to happen over a much shorter period of
time than for a star the mass of our sun. Our sun's life is going to be
10 or 11 billion total years. Here, we're going to
be talking about things in maybe the tens of
millions of years. So a factor of 1,000
shorter life span. But anyway, let's think
about what happens. And so far, just the
pattern of what happens, it's going to happen
faster because we have more pressure, more
gravity, more temperature. But it's going to happen
in pretty much the same way as what we saw with a
star the mass of the sun. Eventually that helium--
sorry, that hydrogen is going to fuse into
a helium core that's going to have a hydrogen
shell around it. It's going to have a
hydrogen shell around it, hydrogen fusion shell around it. And then you have the rest
of the star around that. So let me label it. This right here is
our helium core. And more and more
helium is going to be built up as this
hydrogen in this shell fuses. And in a star the size of our
sun or the mass of our sun, this is when it starts
to become a red giant. Because this core is getting
denser and denser and denser as more and more
helium is produced. And as it gets denser
and denser and denser, there's more and more
gravitational pressure being put on the hydrogen,
on this hydrogen shell out here, where we have
fusion still happening. And so that's going to release
more outward energy to push out the radius of the actual star. So the general
process, and we're going to see this as the star
gets more and more massive, is we're going to have heavier
and heavier elements forming in the core. Those heavier and
heavier elements, as the star gets
denser and denser, will eventually ignite,
kind of supporting the core. But because the core itself
is getting denser and denser and denser, material is getting
pushed further and further out with more and more energy. Although if the star
is massive enough, it's not going to be
able to be pushed out as far as you will have
in kind of a red giant, with kind of a sun-like star. But let's just think
about how this pattern is going to continue. So eventually, that helium,
once it gets dense enough, it's going to ignite and it's
going to fuse into carbon. And you're going to have
a carbon core forming. So that is carbon core. That's a carbon core. Around that, you
have a helium core. And near the center
of the helium core, you have a shell
of helium fusion-- that's helium, not hydrogen--
turning into carbon, making that carbon core
denser and hotter. And then around that,
you have hydrogen fusion. Have to be very careful. You have hydrogen fusion. And then around that, you
have the rest of the star. And so this process is just
going to keep continuing. Eventually that carbon
is going to start fusing. And you're going to
have heavier and heavier elements form the core. And so this is a
depiction off of Wikipedia of a fairly mature massive star. And you keep
forming these shells of heavier and heavier
elements, and cores of heavier and heavier
elements until eventually, you get to iron. And in particular, we're
talking about iron 56. Iron with an atomic mass of 56. Here on this periodic table
that 26 is its atomic number. It's how many protons it has. 56, you kind of view it
as a count of the protons and neutrons, although
it's not exact. But at this point, the reason
why you stop here is that you cannot get energy
by fusing iron. Fusing iron into heavier
elements beyond iron actually requires energy. So it would actually be
an endothermic process. So to fuse iron actually
won't help support the core. So what I want to do in this--
So just to be very clear, this is how the heavy
elements actually formed. We started with
hydrogen, hydrogen fusing into helium, helium
fusing into carbon, and then all of these things
in various combinations-- and I won't go into
all the details-- are fusing heavier
and heavier elements. Neon, oxygen, and you
see it right over here. Silicon. And these aren't the only
elements that are forming, but these are kind of the main
core elements that are forming. But along the way, you have
all this other stuff, lithium, beryllium, boron. All of this other
stuff is also forming. So this is how you form
elements up to iron 56. And also, this is actually how
you can form up to nickel 56, just to be exact. There will also
be some nickel 56, which has the same
mass as iron 56, just has two fewer neutrons
and two more protons. So nickel 56 will
also form, can also be, it'll be like
a nickel-iron core. But that's about
how far a star can get, regardless of how massive
it is, at least by going through traditional
fusion, through the traditional
ignition mechanism. What I want to do
is leave you there just so you can think about
what might happen next, now that we can't fuse
this star anymore. And what we're actually going to
see is that it will supernova.