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.