Both of the processes involve producing final products that are more stable than the initial ingredients. Fission happens when a heavy nucleus breaks up into two or more lighter nuclei, releasing lots of energy, because the lighter nuclei are more stable. Fusion goes the other way. Two very light nuclei are combined to give a heavier, more stable nucleus, releasing even more energy per gram, because the difference in stability between the “parents” and the “child” is even greater.

The most stable nuclei have 50-60 neutrons plus protons in them (iron and nickle, are typical in this range). So the final product at the end of the life of a star, for example, is a large chunk of this material. The lighter elements (nuclei) have fused together into these medium-heavy nuclei.

For comparison, nuclear fission starts with very heavy nuclei, Uranium, for example, with 238 neutrons and protons, and splits up into products frequently in this same range of medium-heavy nuclei.

So, to review, a star spends something like 99% of its life converting Hydrogen into Helium in the fusion process, then it changes character, spending the rest of its life (much shorter time scale) combining Helium into heavier nuclei, and those products into heavier ones still, until the stability maximum is reached, then it “dies”.

Since fusion is more efficient than fission, then why haven’t fusion reactors been made successfully for man’s use? The answer is, because it is very difficult to get the Hydrogen nuclei close enough to combine. They naturally repel each other because the protons are positively charged (remember, like charges repel).

Because a star is so massive, the strong gravitational force overcomes the repulsion between like charges, forcing them close enough to fuse. This kind of force has been very difficult to produce in the laboratories on earth. Only in the Hydrogen bomb has fusion been consistently possible. The atomic bomb is a fission process.

So that is a brief account of how a star “burns”. Another article will talk about the sequence of the life of a star including the time scale involved.

What are these stellar objects?  Well, astronomers think they all started out as regular stars and these are the results of their”deaths”.  But these are theories that may or may not be true.  (I think they are true, but I can’t prove them).

As in most all things created, we cannot do experiments in the present to see what really happened in the past. The best we can do is make up theories that seem to explain the evidence, and then compare other evidence as it is found against the theories.

So here is a brief summary of the theories as I understand them.

Red Giants = stars that are nearing death.  They have used up all the Hydrogen in the core, in the fusion process from Hydrogen to heavier elements, and there is a outer shell of hydrogen which continues the fusion process, but expands the diameter of the star greatly – typically 200 times its original size.  (Our sun would grow large enough to engulf the earth, it is predicted).

White Dwarfs = The white hot core of the star after it has “died” and blown off the outer shells.  It takes millions of years to finally cool off and become a cold, black dwarf, and no longer visible.

Black Holes = Many types, but one type is a star that had so much mass at the end of its life, that it collapsed under its own gravity to such a size where the density was so great that not even light could escape.  And this is the one I want to talk about more because a lot of the physics needs explaining.

The history line says “The original Galaxy Zoo was launched in July 2007, with a data set made up of a million galaxies imaged with the robotic telescope of the Sloan Digital Sky Survey. With so many galaxies, the team thought that it might take at least two years for visitors to the site to work through them all. Within 24 hours of launch, the site was receiving 70,000 classifications an hour, and more than 50 million classifications were received by the project during its first year, from almost 150,000 people.”

I just joined and hope to spend some time contributing. Anyone can do it. A tutorial is included. And your classification is checked against other observers to help prevent mistakes.