0:00:00.000,0:00:00.650


0:00:00.650,0:00:03.140
We've already talked about[br]the life cycle of stars

0:00:03.140,0:00:06.070
roughly the same mass as our[br]sun, give or take a little bit.

0:00:06.070,0:00:07.710
What I want to do[br]in this video is

0:00:07.710,0:00:09.260
talk about more massive stars.

0:00:09.260,0:00:12.372


0:00:12.372,0:00:14.080
And when I'm talking[br]about massive stars,

0:00:14.080,0:00:17.180
I'm talking about stars that[br]have masses greater than 9

0:00:17.180,0:00:20.960
times the sun.

0:00:20.960,0:00:23.330
So the general idea[br]is exactly the same.

0:00:23.330,0:00:27.170
You're going to start off[br]with this huge cloud of mainly

0:00:27.170,0:00:27.910
hydrogen.

0:00:27.910,0:00:29.410
And now, this cloud[br]is going to have

0:00:29.410,0:00:34.070
to be bigger than the clouds[br]that condensed to form stars

0:00:34.070,0:00:34.741
like our sun.

0:00:34.741,0:00:36.240
But you're going[br]to start with that,

0:00:36.240,0:00:40.440
and eventually gravity's[br]going to pull it together.

0:00:40.440,0:00:43.330
And the core of it is going[br]to get hot and dense enough

0:00:43.330,0:00:47.090
for hydrogen to ignite, for[br]hydrogen to start fusing.

0:00:47.090,0:00:49.534
So this is hydrogen,[br]and it is now fusing.

0:00:49.534,0:00:50.200
Let me write it.

0:00:50.200,0:00:51.630
It is now fusing.

0:00:51.630,0:00:53.550
Hydrogen fusion.

0:00:53.550,0:00:55.360
Let me write it like this.

0:00:55.360,0:01:01.000
You now have hydrogen[br]fusion in the middle.

0:01:01.000,0:01:03.280
So it's ignited,[br]and around it, you

0:01:03.280,0:01:05.820
have just the other[br]material of the cloud.

0:01:05.820,0:01:06.990
So the rest of the hydrogen.

0:01:06.990,0:01:09.640
And now, since it's so[br]heated, it's really a plasma.

0:01:09.640,0:01:12.280
It's really kind of a soup[br]of electrons and nucleuses

0:01:12.280,0:01:15.957
as opposed to well-formed atoms,[br]especially close to the core.

0:01:15.957,0:01:17.290
So now you have hydrogen fusion.

0:01:17.290,0:01:19.820
We saw this happens at[br]around 10 million Kelvin.

0:01:19.820,0:01:21.276
And I want to make[br]it very clear.

0:01:21.276,0:01:23.150
Since we're talking[br]about more massive stars,

0:01:23.150,0:01:25.650
even at this stage,[br]there's going

0:01:25.650,0:01:31.710
to be more gravitational[br]pressure, even at this stage,

0:01:31.710,0:01:34.020
during the main[br]sequence of the star,

0:01:34.020,0:01:35.560
because it is more massive.

0:01:35.560,0:01:38.460
And so this is going to[br]burn faster and hotter.

0:01:38.460,0:01:42.650
So this is going to be faster[br]and hotter than something

0:01:42.650,0:01:43.670
the mass of our sun.

0:01:43.670,0:01:46.350


0:01:46.350,0:01:50.850
And so even this stage[br]is going to happen over

0:01:50.850,0:01:54.470
a much shorter period of[br]time than for a star the mass

0:01:54.470,0:01:55.100
of our sun.

0:01:55.100,0:01:58.747
Our sun's life is going to be[br]10 or 11 billion total years.

0:01:58.747,0:02:00.580
Here, we're going to[br]be talking about things

0:02:00.580,0:02:02.840
in maybe the tens of[br]millions of years.

0:02:02.840,0:02:05.579
So a factor of 1,000[br]shorter life span.

0:02:05.579,0:02:07.370
But anyway, let's think[br]about what happens.

0:02:07.370,0:02:09.500
And so far, just the[br]pattern of what happens,

0:02:09.500,0:02:11.083
it's going to happen[br]faster because we

0:02:11.083,0:02:13.850
have more pressure, more[br]gravity, more temperature.

0:02:13.850,0:02:16.460
But it's going to happen[br]in pretty much the same way

0:02:16.460,0:02:20.000
as what we saw with a[br]star the mass of the sun.

0:02:20.000,0:02:23.060
Eventually that helium--[br]sorry, that hydrogen

0:02:23.060,0:02:25.930
is going to fuse into[br]a helium core that's

0:02:25.930,0:02:28.164
going to have a hydrogen[br]shell around it.

0:02:28.164,0:02:30.080
It's going to have a[br]hydrogen shell around it,

0:02:30.080,0:02:31.520
hydrogen fusion shell around it.

0:02:31.520,0:02:34.170
And then you have the rest[br]of the star around that.

0:02:34.170,0:02:35.870
So let me label it.

0:02:35.870,0:02:41.740
This right here is[br]our helium core.

0:02:41.740,0:02:43.420
And more and more[br]helium is going

0:02:43.420,0:02:45.970
to be built up as this[br]hydrogen in this shell fuses.

0:02:45.970,0:02:49.930
And in a star the size of our[br]sun or the mass of our sun,

0:02:49.930,0:02:52.020
this is when it starts[br]to become a red giant.

0:02:52.020,0:02:56.020
Because this core is getting[br]denser and denser and denser

0:02:56.020,0:02:58.670
as more and more[br]helium is produced.

0:02:58.670,0:03:01.110
And as it gets denser[br]and denser and denser,

0:03:01.110,0:03:03.370
there's more and more[br]gravitational pressure

0:03:03.370,0:03:06.490
being put on the hydrogen,[br]on this hydrogen shell

0:03:06.490,0:03:09.600
out here, where we have[br]fusion still happening.

0:03:09.600,0:03:14.440
And so that's going to release[br]more outward energy to push out

0:03:14.440,0:03:17.836
the radius of the actual star.

0:03:17.836,0:03:19.210
So the general[br]process, and we're

0:03:19.210,0:03:21.430
going to see this as the star[br]gets more and more massive,

0:03:21.430,0:03:23.888
is we're going to have heavier[br]and heavier elements forming

0:03:23.888,0:03:25.142
in the core.

0:03:25.142,0:03:26.600
Those heavier and[br]heavier elements,

0:03:26.600,0:03:28.370
as the star gets[br]denser and denser,

0:03:28.370,0:03:31.170
will eventually ignite,[br]kind of supporting the core.

0:03:31.170,0:03:33.900
But because the core itself[br]is getting denser and denser

0:03:33.900,0:03:36.920
and denser, material is getting[br]pushed further and further out

0:03:36.920,0:03:38.010
with more and more energy.

0:03:38.010,0:03:39.710
Although if the star[br]is massive enough,

0:03:39.710,0:03:41.460
it's not going to be[br]able to be pushed out

0:03:41.460,0:03:44.330
as far as you will have[br]in kind of a red giant,

0:03:44.330,0:03:45.824
with kind of a sun-like star.

0:03:45.824,0:03:47.740
But let's just think[br]about how this pattern is

0:03:47.740,0:03:48.490
going to continue.

0:03:48.490,0:03:52.280
So eventually, that helium,[br]once it gets dense enough,

0:03:52.280,0:03:55.340
it's going to ignite and it's[br]going to fuse into carbon.

0:03:55.340,0:03:57.760
And you're going to have[br]a carbon core forming.

0:03:57.760,0:03:59.350
So that is carbon core.

0:03:59.350,0:04:00.840
That's a carbon core.

0:04:00.840,0:04:02.810
Around that, you[br]have a helium core.

0:04:02.810,0:04:05.650


0:04:05.650,0:04:07.980
And near the center[br]of the helium core,

0:04:07.980,0:04:10.180
you have a shell[br]of helium fusion--

0:04:10.180,0:04:13.150
that's helium, not hydrogen--[br]turning into carbon, making

0:04:13.150,0:04:15.310
that carbon core[br]denser and hotter.

0:04:15.310,0:04:18.680
And then around that,[br]you have hydrogen fusion.

0:04:18.680,0:04:20.139
Have to be very careful.

0:04:20.139,0:04:21.180
You have hydrogen fusion.

0:04:21.180,0:04:23.925
And then around that, you[br]have the rest of the star.

0:04:23.925,0:04:27.172


0:04:27.172,0:04:29.380
And so this process is just[br]going to keep continuing.

0:04:29.380,0:04:31.700
Eventually that carbon[br]is going to start fusing.

0:04:31.700,0:04:33.090
And you're going to[br]have heavier and heavier

0:04:33.090,0:04:34.048
elements form the core.

0:04:34.048,0:04:36.130
And so this is a[br]depiction off of Wikipedia

0:04:36.130,0:04:39.440
of a fairly mature massive star.

0:04:39.440,0:04:41.760
And you keep[br]forming these shells

0:04:41.760,0:04:43.760
of heavier and heavier[br]elements, and cores

0:04:43.760,0:04:46.380
of heavier and heavier[br]elements until eventually, you

0:04:46.380,0:04:47.310
get to iron.

0:04:47.310,0:04:52.650
And in particular, we're[br]talking about iron 56.

0:04:52.650,0:04:54.920
Iron with an atomic mass of 56.

0:04:54.920,0:04:57.980
Here on this periodic table[br]that 26 is its atomic number.

0:04:57.980,0:04:59.680
It's how many protons it has.

0:04:59.680,0:05:03.010
56, you kind of view it[br]as a count of the protons

0:05:03.010,0:05:05.940
and neutrons, although[br]it's not exact.

0:05:05.940,0:05:08.720
But at this point, the reason[br]why you stop here is that you

0:05:08.720,0:05:12.290
cannot get energy[br]by fusing iron.

0:05:12.290,0:05:15.210
Fusing iron into heavier[br]elements beyond iron

0:05:15.210,0:05:16.720
actually requires energy.

0:05:16.720,0:05:19.070
So it would actually be[br]an endothermic process.

0:05:19.070,0:05:23.200
So to fuse iron actually[br]won't help support the core.

0:05:23.200,0:05:25.980
So what I want to do in this--[br]So just to be very clear,

0:05:25.980,0:05:28.630
this is how the heavy[br]elements actually formed.

0:05:28.630,0:05:31.500
We started with[br]hydrogen, hydrogen fusing

0:05:31.500,0:05:34.090
into helium, helium[br]fusing into carbon,

0:05:34.090,0:05:36.979
and then all of these things[br]in various combinations--

0:05:36.979,0:05:38.520
and I won't go into[br]all the details--

0:05:38.520,0:05:40.380
are fusing heavier[br]and heavier elements.

0:05:40.380,0:05:42.650
Neon, oxygen, and you[br]see it right over here.

0:05:42.650,0:05:43.700
Silicon.

0:05:43.700,0:05:45.300
And these aren't the only[br]elements that are forming,

0:05:45.300,0:05:47.820
but these are kind of the main[br]core elements that are forming.

0:05:47.820,0:05:50.236
But along the way, you have[br]all this other stuff, lithium,

0:05:50.236,0:05:51.070
beryllium, boron.

0:05:51.070,0:05:53.520
All of this other[br]stuff is also forming.

0:05:53.520,0:05:57.330
So this is how you form[br]elements up to iron 56.

0:05:57.330,0:06:00.980
And also, this is actually how[br]you can form up to nickel 56,

0:06:00.980,0:06:03.600
just to be exact.

0:06:03.600,0:06:05.470
There will also[br]be some nickel 56,

0:06:05.470,0:06:07.770
which has the same[br]mass as iron 56,

0:06:07.770,0:06:11.400
just has two fewer neutrons[br]and two more protons.

0:06:11.400,0:06:15.590
So nickel 56 will[br]also form, can also

0:06:15.590,0:06:17.420
be, it'll be like[br]a nickel-iron core.

0:06:17.420,0:06:19.250
But that's about[br]how far a star can

0:06:19.250,0:06:23.940
get, regardless of how massive[br]it is, at least by going

0:06:23.940,0:06:26.150
through traditional[br]fusion, through

0:06:26.150,0:06:28.280
the traditional[br]ignition mechanism.

0:06:28.280,0:06:30.310
What I want to do[br]is leave you there

0:06:30.310,0:06:33.180
just so you can think about[br]what might happen next,

0:06:33.180,0:06:36.400
now that we can't fuse[br]this star anymore.

0:06:36.400,0:06:39.890
And what we're actually going to[br]see is that it will supernova.

0:06:39.890,0:06:40.427