Understanding the Crescent Nebula

[symesie04]

Inspired by another post on star ingredients and then fuelled by APODs pic of the day I realised im a little confused on a matter. In the stunning image below we see the red hydrogen rich shell encased in a coating of oxygen rich gas both of which are being discarded by the dying Wolf Ryat at the centre.

Now my understanding of star life and death is this. The massive star burns hydrogen at its core fusing it into Helium. However at some stage so much Helium is present it almost chokes itself and fusion begins to to slow down. This prompts the star to start to collapse, the result massive pressure at the core causes the core to heat up. The extra heat in the form of energy allows the fusion process to start fusing Helium into heavier elements such as Oxygen and this process is repeated for each of the heavier elements. Its a little basic i know but is in the right ball park i think. So what puzzles me then is how is there Oxygen discarded in the outer layer in one of the early hiccups before the big boom. Surely the heavier Oxygen was created in the core and therefore should be trapped deep within the star. Would the huge mass and gravity of a massive star simply not allow the oxygen to escape to the outer regions over any period of time, the only release coming at supernova? 

[Tiki]

2 hours ago, symesie04 said:

 Surely the heavier Oxygen was created in the core and therefore should be trapped deep within the star. Would the huge mass and gravity of a massive star simply not allow the oxygen to escape to the outer regions over any period of time, the only release coming at supernova? 

 

Huge stars typically have huge convectional regions resulting in a rich stellar atmosphere of heavier elements.

[andrew s]

Another point to consider is that the emission from various neutral elements and there ions depends very much on the details of the physical conditions (temperature, pressure etc.)  and not just on concentration. If you look at the strength of the lines in the solar spectrum you would be hard pushed to casually convince yourself that hydrogen was the most common element in the Sun.

Regards Andrew