So, that is the general pattern of 
evolution for stars with mass over eight 
solar masses. 
We've had that down. 
before we finish the dramatic end we need 
to make some comments on stars that are 
actually really big. 
masses over, say twenty solar masses, 
these are the O-type stars. 
They are rare, but their evolution is 
interesting. 
first distinction we notice is that when 
these guys depart the main sequence, they 
do not brighten, 
there's no red giant phase, 
they're already giants. 
They do redden, but they don't increase 
their luminosity so much. 
What's going on is there's no core 
collapse going on. 
The helium core does not collapse. 
temperatures and pressures are high 
enough in the core of these huge stars 
for helium fusion to begin without core 
collapse. 
So there's no red giant giantism phase 
such as it is. 
They're red super giants, but that's 
because they were already large. 
The other thing that happens in these 
large stars is that the growth of the 
envelope as the star cools, even 
maintaining its luminosity they become 
huge, the envelope is poorly bound and 
rates of mass loss become humongous. 
they lose as much as a 10,000th of a 
solar mass per year. 
This is an amazing amount of stellar wind 
coming off at high velocities. 
what the spectral signature of this is, 
is that their spectrum is dominated by 
not absorption lines in the atmosphere 
but emission lines in the atmosphere at 
this point in their evolution. 
We get a spectrum that's dominated by, by 
the admission lines with a characteristic 
so called P-Cygni signature of mass loss 
you won't play with the mass with, with 
the line shape for lack of time. 
And, in addition, in general, O-type 
stars show evidence that they rotate more 
rapidly than other stars. 
And the very recent result which may be 
pertinent to understanding how all this 
comes about is that over three quarters 
of O-type stars are in binary systems. 
Remember, this to be compared with 
somewhere between about a third or a 
fifth of the general star population is 
in a binary system. 
And about half of them have partners 
close enough to have undergone mass 
transfer as we discussed in the past. 
What this tells us is perhaps that O 
stars are rare, it's difficult to make an 
o-star perhaps the only reason we find so 
many of them is that many of them were 
not initially o-stars. 
And suddenly, this will effect the way 
they evolve the research on that is 
definitely still ongoing but these high 
mass loss stars 
class the, the, the, the technical term 
is Wolf-Rayet stars for the people who 
classified them. 
And these are these stars with these 
large emission lines in their spectrum 
and the nature of the emission lines is 
as well interesting. 
We find typically very little hydrogen 
emission. 
We find classification by the spectrum, 
WN stars are dominated by nitrogen 
emission lines, WC stars by carbon 
emission lines, and the rare WO stars by 
oxygen emission lines. 
And the understanding of the mechanism is 
that essentially these Wolf-Rayet stars 
have already lost their hydrogen and 
helium envelope. 
And what we're seeing is a stellar wind 
essentially composed of the dredged up 
interiors of the stars and these stars 
losing huge quantities of 
heavier fusion products into the 
interstellar medium are very important 
for producing the seeding of the 
interstellar medium with these heavy 
elements. 
And this phase of evolution, the it is 
typically thought, is followed by a core 
collapse supernova. 
We'll talk about what happens when the 
core collapses in a bit 
but many of these stars do not survive to 
an actual horizontal branch. 
They go off into Wolf-Rayet and emit, 
lose their envelope and at this point the 
core collapses. 
though as we will see where this is not 
completely true, we have evidence that 
some of them do manage a return to blue 
supergiant status. 
in stars that are even more massive with 
masses above, say 50 solar masses, 
these don't even make it to the red side 
of the spectrum. 
They never even red significantly, 
these stars are very poorly understood. 
the most famous and in some sense, 
enigmatic of the classes of it are 
Carinae. 
this is a star that famously in 1837 for 
long, extended period of time brightened 
to immense luminosity. 
We can see 20,000,000 solar luminosities 
compared to its current quiescent mere 
5,000,000 solar luminosities. 
So this is a monster of a star, this is a 
recent Hubble Space Telescope image 
showing dramatic evidence of the mass 
loss. 
You see the polar flow of the ejector, as 
well as some kind of strange 
fragmentary disk structure that's not 
well understood. 
these, as they're called luminous blue 
variables, are very poorly understood 
stars. but, again, the critic clearly 
play a critical role because of the large 
amounts of heavy elements that they emit 
in the driving the structure of 
interstellar medium. 
And as we shall see when we talk about 
galaxy structure in driving the dynamics 
of star formation in the galaxy.