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So, that is the general pattern of 
evolution for stars with mass over eight 

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solar masses. 
We've had that down. 

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before we finish the dramatic end we need 
to make some comments on stars that are 

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actually really big. 
masses over, say twenty solar masses, 

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these are the O-type stars. 
They are rare, but their evolution is 

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interesting. 
first distinction we notice is that when 

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these guys depart the main sequence, they 
do not brighten, 

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there's no red giant phase, 
they're already giants. 

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They do redden, but they don't increase 
their luminosity so much. 

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What's going on is there's no core 
collapse going on. 

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The helium core does not collapse. 
temperatures and pressures are high 

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enough in the core of these huge stars 
for helium fusion to begin without core 

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collapse. 
So there's no red giant giantism phase 

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such as it is. 
They're red super giants, but that's 

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because they were already large. 
The other thing that happens in these 

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large stars is that the growth of the 
envelope as the star cools, even 

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maintaining its luminosity they become 
huge, the envelope is poorly bound and 

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rates of mass loss become humongous. 
they lose as much as a 10,000th of a 

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solar mass per year. 
This is an amazing amount of stellar wind 

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coming off at high velocities. 
what the spectral signature of this is, 

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is that their spectrum is dominated by 
not absorption lines in the atmosphere 

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but emission lines in the atmosphere at 
this point in their evolution. 

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We get a spectrum that's dominated by, by 
the admission lines with a characteristic 

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so called P-Cygni signature of mass loss 
you won't play with the mass with, with 

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the line shape for lack of time. 
And, in addition, in general, O-type 

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stars show evidence that they rotate more 
rapidly than other stars. 

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And the very recent result which may be 
pertinent to understanding how all this 

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comes about is that over three quarters 
of O-type stars are in binary systems. 

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Remember, this to be compared with 
somewhere between about a third or a 

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fifth of the general star population is 
in a binary system. 

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And about half of them have partners 
close enough to have undergone mass 

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transfer as we discussed in the past. 
What this tells us is perhaps that O 

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stars are rare, it's difficult to make an 
o-star perhaps the only reason we find so 

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many of them is that many of them were 
not initially o-stars. 

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And suddenly, this will effect the way 
they evolve the research on that is 

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definitely still ongoing but these high 
mass loss stars 

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class the, the, the, the technical term 
is Wolf-Rayet stars for the people who 

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classified them. 
And these are these stars with these 

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large emission lines in their spectrum 
and the nature of the emission lines is 

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as well interesting. 
We find typically very little hydrogen 

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00:02:55,484 --> 00:02:59,012
emission. 
We find classification by the spectrum, 

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00:02:59,012 --> 00:03:03,781
WN stars are dominated by nitrogen 
emission lines, WC stars by carbon 

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00:03:03,781 --> 00:03:08,066
emission lines, and the rare WO stars by 
oxygen emission lines. 

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00:03:08,066 --> 00:03:13,733
And the understanding of the mechanism is 
that essentially these Wolf-Rayet stars 

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have already lost their hydrogen and 
helium envelope. 

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And what we're seeing is a stellar wind 
essentially composed of the dredged up 

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interiors of the stars and these stars 
losing huge quantities of 

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heavier fusion products into the 
interstellar medium are very important 

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00:03:33,542 --> 00:03:39,509
for producing the seeding of the 
interstellar medium with these heavy 

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00:03:39,509 --> 00:03:43,298
elements. 
And this phase of evolution, the it is 

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00:03:43,298 --> 00:03:47,432
typically thought, is followed by a core 
collapse supernova. 

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00:03:47,432 --> 00:03:51,980
We'll talk about what happens when the 
core collapses in a bit 

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but many of these stars do not survive to 
an actual horizontal branch. 

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They go off into Wolf-Rayet and emit, 
lose their envelope and at this point the 

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00:04:02,537 --> 00:04:06,462
core collapses. 
though as we will see where this is not 

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00:04:06,462 --> 00:04:12,147
completely true, we have evidence that 
some of them do manage a return to blue 

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00:04:12,147 --> 00:04:16,451
supergiant status. 
in stars that are even more massive with 

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00:04:16,451 --> 00:04:22,264
masses above, say 50 solar masses, 
these don't even make it to the red side 

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00:04:22,264 --> 00:04:26,193
of the spectrum. 
They never even red significantly, 

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00:04:26,193 --> 00:04:31,928
these stars are very poorly understood. 
the most famous and in some sense, 

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00:04:31,928 --> 00:04:35,385
enigmatic of the classes of it are 
Carinae. 

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00:04:35,385 --> 00:04:42,141
this is a star that famously in 1837 for 
long, extended period of time brightened 

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00:04:42,141 --> 00:04:46,753
to immense luminosity. 
We can see 20,000,000 solar luminosities 

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00:04:46,753 --> 00:04:51,766
compared to its current quiescent mere 
5,000,000 solar luminosities. 

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00:04:51,766 --> 00:04:57,053
So this is a monster of a star, this is a 
recent Hubble Space Telescope image 

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00:04:57,053 --> 00:05:00,211
showing dramatic evidence of the mass 
loss. 

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00:05:00,211 --> 00:05:05,430
You see the polar flow of the ejector, as 
well as some kind of strange 

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00:05:05,430 --> 00:05:09,607
fragmentary disk structure that's not 
well understood. 

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00:05:09,607 --> 00:05:15,591
these, as they're called luminous blue 
variables, are very poorly understood 

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00:05:15,591 --> 00:05:21,266
stars. but, again, the critic clearly 
play a critical role because of the large 

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00:05:21,266 --> 00:05:27,247
amounts of heavy elements that they emit 
in the driving the structure of 

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00:05:27,247 --> 00:05:32,231
interstellar medium. 
And as we shall see when we talk about 

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00:05:32,231 --> 00:05:38,060
galaxy structure in driving the dynamics 
of star formation in the galaxy.