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Space Phenomenon
A main sequence star like our sun keeps burning for many thousands of millions of years. The star synthesises helium molecules via a nuclear reaction, but this uses up the hydrogen.
p+p® d+e++v
This process creates deuterium. This first step is very difficult but when it is completed the reactions required to create helium proceed much more rapidly.
P+d® 3He+g
Which is followed by:
3He+3He ® 4He+p+p
This sequence is called the proton proton cycle. In younger and more massive stars this processes may be replaced by the Bethe Carbon Cycle where carbon atoms are used as a catalyst to create helium.
Hey and Walters 1987
These reactions are the beginning of a cycle that will eventually exhaust all the stars fuel and lead to its death. If we examine Sanduleak –69° 202 as a rule of thumb then we can see how these processes led to the explosion observed over the night of 23-24 February 1987. The star was born 11 million years prior to its death. Our sun will last around 1000 times longer than this so it is clear Sanduleak –69° 202 must have burned its fuel very vigorously. The star shone 40,000 times brighter than our sun.
Main Sequence Life Times.
| Mass in multiples of solar masses | Surface Temperature in Kelvin | Luminosity in multiples of solar luminosity | Time on main sequence 106 years |
| 25 | 35,000 | 80,000 | 3 |
| 15 | 30,000 | 10,000 | 15 |
| 3 | 11,000 | 60 | 500 |
| 1.5 | 7,000 | 5 | 3,000 |
| 1 | 6,000 | 1 | 10,000 |
| 0.75 | 5,000 | 0.5 | 15,000 |
| 0.5 | 4,000 | 0.03 | 200,000 |
Kaufmann 1991
After ten million years the hydrogen burning was exhausted and the star could no longer support the gravitational attraction attempting to pull the star inwards:
Sanduleak –69° 202 began collapsing when its fuel was exhausted. The core fell in upon its self and the outer shell expanded becoming very large but tenuous. This is known as a supergiant. The collapsing core raised temperatures and pressures high enough that helium burning could begin and this lasted for another 1 million years and created carbon in the core. After this, a similar process to before occurred whereby the core shrank and the carbon began to synthesise neon, magnesium and oxygen. This process lasted for 12,000 years before it too could no longer support the star. Neon burning happened immediately afterwards but for only 12 years before giving way to oxygen burning for about 4 years. The final week of the life of the star involved fusion reaction converting the other elements into silicon. Unfortunately, silicon gave way to iron, which is very stable and would not synthesise other materials.
The Valley of Stability.
At this time elements heavier than iron can be created from the s process (slow process). The s process involves capturing neutrons into a nucleus and allowing beta decay, see the Cerenkov radiation site and the neutrino discussion for more information on this.
As the silicon processes finished and at the moment the core became unstable for the last time the star collapsed. For Sanduleak –69° 202 this meant that in a few tenths of a second the core went from the size of our sun to an object only a few kilometres across. The outer cloud now began to collapse at a speed of almost ¼ c. However, core gave off intense gravitational energy as it collapsed in the form of heat causing an outward moving shockwave. The shockwave and the infalling matter collide and the r process is initiated whereby elements are synthesised very rapidly. Neutrinos travelling at the speed of light overtake the slowing shockwave and the remaining star collapses to form a neutron star.
Gribbin 1996
After the core helium burning begins these mature stars move across the main sequence and during this transition the star may become unstable and pulsate. This is known as an instability strip.
Kaufmann 1991