The Life of a Star

(Apologies for the blurriness - long exposure photos require a tri-pod, which I didn't have on hand that night.)

Orion, the warrior. This is probably one of the most well known constellations, particularly Orion's belt. If you're not familiar with constellations, here's how to pick out Orion. The three stars that make a vertical line at center are Orion's belt. Just to the right of Orion's belt, there is a near horizontal chain of stars. This is Orion's sheathed sword hanging from his belt. The four brightest stars that form a box around the belt and sword are Orion's feet and shoulders. You may also notice a sub-horizontal arc of bright stars between the top of the frame and Orion's (highest) shoulder. This is the bow that he is using to hunt Taurus the bull, or so the legend goes.

If you look closely, you might notice that the lower of Orion's shoulders appears to have a reddish color as opposed to the more usual blueish white color of stars. This star is called Betelgeuse (generally pronounced "Beetle-juice") and is a type of star known as a red supergiant. Similarly to people, stars are born, change as they age, and eventually die. The very largest of the stars die young, sometimes after only a few tens of thousand years. The smaller stars, however, can look forward to a long life, often reaching the ripe old age of billions (our sun) or possibly even trillions of years. Astronomers use the Hertzsprung-Russell (or H-R) diagram to help find out what stage of life a star is in.

Stars spend most of their lifetimes as part of the main sequence. Our sun is roughly an average main sequence star - not a big, bright blue giant nor a small, dim red dwarf. The hotter the star is, the more blue it appears. Therefore, the red Betelgeuse is now cooler than it was as a youngster. As stars age, most move off of the main sequence and into the realms of the red giants and supergiants. (The smallest stars may never quite reach the "giant" phase.) When stars become red giants or supergiants, it is a sign that they are nearing the end of their lifetime. The smaller red giant stars, such as what our sun will become, release their outer atmospheres when they die. The "stardust" expands into space over time, forming a planetary nebula and leaving just the star's core, now called a white dwarf, at the center to slowly cool and fade away. The biggest giants and supergiants, however, meet a much more exciting end. They die in massive stellar explosions, supernovae, which send much of the star's matter flying through space. The remaining core cannot support its own weight anymore and collapses into a very dense neutron star or, in the case of the biggest of the big stars, a black hole.

For stars to continue living, they require energy just as we do. People and animals get their energy from breaking down the food that they eat. Stars, on the other, are more like plants, in the sense that they both make their own energy. However, stars do not require energy from an outside source like plants do when they take in sunlight to help produce their energy. Stars are actually made of their food - in a sense, they live by eating themselves. The most common element in the universe, by far, is hydrogen followed by helium, lithium, and so on down the periodic table. Stars get energy by fusing these small "light" elements into big "heavy" ones. Inside the core of a star, temperatures and pressures are so extreme that the electrons are ripped off the atoms to form a plasma of electrons and ions, the atomic nuclei that is made of protons and neutrons. This hot, dense plasma is the perfect environment for fusion to occur.

Within the hot, charged plasma soup at the star's center, collisions between nuclei are unavoidable. If the collision has enough energy involved, the two nuclei will fuse together into a single larger one. For example, two hydrogen nuclei can combine to create a helium nuclei. This is the process which builds about half of the elements on the periodic table. However, fusion will only release energy when small nuclei are involved. The larger the nuclei, the more energy it takes to fuse them and the less energy is released in the process. As the star turns more and more of its light nuclei into heavier ones, it becomes harder and harder for the star to continue fusing nuclei to get the energy it needs to survive. This is the stage of life that Betelgeuse and other red giant stars have reached. After reaching the red giant stage, the star will continue fusing nuclei until it reaches the point when fusion requires more energy than it produces. For smaller stars like the sun, this limit is reached when carbon is produced. The giant stars, however, can continue fusing nuclei until the entire core has been changed into iron.

Iron is the most stable of all elements. The elements that come before it on the periodic table can produce energy when they are fused together, and those after iron produce energy when they are split (fission). Fission is the process in nuclear bombs that use uranium or plutonium. Because a star cannot create nuclei heavier than iron with fusion, these nuclei can only form when there is enough energy for fusion without needing any energy to be produced - during the star's death. The extreme forces involved in supernovae are capable of creating the heavy nuclei. Since supernovae are basically giant explosions, both the heavy nuclei and the lighter nuclei that were never fused are thrown out into space and end up as part of the next generation of stars and planets. Eventually this stardust, created in extreme environments with fusion and scattered throughout the galaxy by explosions, is brought together in the perfect way to create beings capable of understanding this process - us. As it turns out, we all have an explosive past.


References
Orion - by the Peoria Astronomical Society
The Hertzsprung Russell Diagram - by Richard Powell
Supernovae - by C.R. Nave at Georgia State University
Periodic Table of the Elements - by Mark Winter at The University of Sheffield
Basics: What are Plasmas? - by Dr. Timothy Eastman at Plasmas International
Fission and Fusion - by Yoseph Murtanu on the ChemWiki by University of California, Davis