Re: Can you tell me about the life stages of the star Alnitak (an O star)

Bob

First allow me to apologise for the late reply to this question. 
As you asked about a specific star so I have been trying to find 
details on that star. Unfortunately no reference work I have has 
the specifics I wanted and my maths is too rusty to attempt the 
solution of the stellar models. So I am afraid I can only give 
you some general details about the evolution of high mass stars. 

Alnitak is part of the Orion Association: a large gas/dust cloud 
from which all stars in Orions Belt formed. Alnitak's vital 
statistics are:

RA                    05 40 45.5 
DEC                  -01 56 34 
V                     2.05 
B-V                   -0.21 
Spectral Type         O9.7Ib 
Distance              1400 Lyr
Mass:                 approx 30 solar mass
Radius:               approx 5 Solar Radii (unsure of this value)
Effective Temperature 33,600K


Armed with this info we can understand something about Alnitak. 
It is a hot blue giant predominantly made of hydrogen and helium 
with a fraction of heavier elements present, like all stars.

Some few hundred million years ago the gas cloud Alnitak lives in
started to collapse. This may have been formed by a nearby supernovae 
or possibly a spiral density wave (from the Lin-Shu Density wave 
model 1963). As the cloud collapses it would start spinning to 
conserve angular momentum. Also, the mass in the middle of the 
cloud would get more and more dense as more material falls inward. 
Eventually the density in the middle would be great enough to start 
nuclear fusion. This collapse would have occured over some  tens 
of millions of years.  When fusion  stars, Alnitak would still be 
surrounded in a dense cloud of gas. So the light given off would 
be highly scattered into the infra-red spectrum by this cloud. 
This is what is called a T-Tauri star.  When nuclear fusion starts 
we also say the star has entered the Zero Age Main Sequence (ZAMS).


As the star collapses further and the rate of nuclear fusion increases 
more and more particles (its stellar wind) and light are given off. 
This effectively blows away the remaining gas and dust to reveal 
a true star and it is said to evolved onto the main sequence. This 
process is what was seen last year by the Hubble Space Telescope
when it viewed the Eagle Nebulae. For a star like Alnitak it oly takes
about 5 million years to evolve from its ZAMS stage to the main sequence. 
I have no exact figures to rely on but I think Alnitak has only been 
on the main sequence for a few tens of millions of years. It should 
last for another hundred million years or so yet. 


The major factor that decides how long a star lives is its mass. Larger 
mass stars have higher core densities that increase the rate of nuclear 
reactions. It also allows heavier elements to be fused. So, larger mass 
stars last for only a few hundred million years as opposed to our sun, 
which will last for thousands of millions of years or M class stars that 
last for hundreds of thousands of millions of years. 

Once on the main sequence Alnitak will rapidly start fusing hydrogen to 
helium and then helium to beryllium and boron and even heavier elements. 
Eventually iron is formed in the core. It takes more energy to fuse 
iron than you get out of the fusion. What happens next is that as  
more and more iron is created, the core cools down. This means that the
star's stability will rapidly deterioate. 

When on the main sequence the gravitational collapse of the star mass 
is exactly counteracted by the pressure of particles emitted from its 
core due to nuclear fusion. If the core cools the star starts collapsing 
under its mass and the core density increases which will increase the 
rate of iron production. In a relatively short space of time, tens of 
thousands of years,  the core will have no material left to stop a 
catastrophic collpase.

Eventually the gravitational energy of the star exceeds the pressure 
from the core. The star starts collapsing rapidly. As the core density 
increases the atoms are packed close enough for their electron shells 
to overlap. The pressure increases and the electrons are stripped from 
their orbits to form a plasma of atomic nuclei and elecrtons. The
pressure still increases and electrons and protons in the nuclei start 
interacting to from neutrons.  The pressure still increases and the nuclei 
are forced to touch each other and split apart. At this stage the core is 
made of only neutrons stripped from their atoms and packed in next to each 
other. The core desnity is now millions of millions of kilograms per 
cubic meter. As the core pressure still increases, it exceeds strong 
nuclear force holding the neutrons together. Even they are split apart 
into their constituent quarks. At this point the core collapses totally 
and completely to form a black hole. Current physics can not model the 
conditions inside a black hole or what they are made of. We do know 
they probably exist as the HST has found several candidates for them. 

To put life into context. The process described in the last paragraph 
occurs in under a second or so. The star goes from being 10 times larger 
than our sun to an object only a kilometer in diameter in a second. 
This is a supernovae. The rate of fusion reactions and atomic reactions 
in the core is great that the whole star heats up rapidly and all 
material starts fusing to create heat and light. The light of the 
supernovae is so great that it can outshine a Galaxy of over 100 million 
solar masses.

I hope this gives you roughly what you asked for. Stellar evolution is 
reasonably well understood but as I say I do not have specifics about 
the numbers to hand. If you have any further questions please E-Mail 
me and I will do the best I can.

Ref: 

Astrophysics I, Bowers and Deeming
Structure and evolution of the Stars, Schwarchsild

Dave Barlow