 Star birthLife Cycle of a starThe death of a starLBV'sWhite dwarvesNeutron StarsBrown StarsStar birth Star birth begins in the heart of a nebula. Nebulae are acted upon many forces, but in star birth the most important of those is gravity. The inward pull of gravity slowly pulls the nebula into a tighter and tighter sphere. As it does this, heat begins to build up inside the nebula. This is a result of the increasing density in the nebula. The denser it becomes, the hotter it becomes. Fairly soon, the outward force generated by the heat begins to balance the inward force of gravity. When it becomes dense and hot enough, fusion begins. At first, it isn't anything steady, just a sputter here and there, and at this point you have a protostar. After it becomes a little denser, things will start to become more steady, and after a while, you've got yourself a small red star. However, not all of the gas and dust is sucked up into the star. The rest of it is formed into a disk which forms around the star. The solar wind created by the star soon blows most of the gas out of the inner portion of the disk into the outer portion, leaving only dust near the star. To the top of this page Life Cycle of a Star The life cycle of a star begins with Star birth, which I have detailed in the section above. There is a standard path that all stars follow after star birth. There are two important factors in a star, and they are what make a star do what they do. Gravity, and the outward pressure generated by fusion. Gravity is what holds a star together. The pressure in the center of a star, provides an outward force that prevents a star from falling in on itself. All stars follow the same path, but how they go about it depends mainly on a stars size. Small stars will usually end up living a long, long life, and when they die they will slowly eject their outer layers into planetary nebula, and their core will remain behind as a white dwarf. Large stars go about their lives differently. Although they have much more hydrogen then smaller stars, they burn their hydrogen much quicker. Since they are so much larger, their cores are denser and hotter, so they fuse hydrogen into helium much more readily. When they have lived out their significantly shortened lives, (from 10 to 100 million years, compared to our suns 10 billion) they die in a huge explosion called a supernova, leaving behind either a neutron star or a black hole. Medium size stars, like ours, die the same way a small star does, but don't live as long. Our star has lived out about half of it's 10 billion year life span. As a star makes it's way through it's many years, it slowly passes through a few stages. The attributess of a star are determined mainly by their starting mass. A few stars will eventually reach the stage of LBV, but very few do (there are about 35 known LBV's, if I remember correctly), and no one knows how. Small stars proceed to the red giant and then red supergiant stages at the end of their lives. After those stages, stars that began their lives small will end them in the white dwarf stage. Why this happens is the important part. As small stars begin fusing the helium in their cores to hydrogen, they slowly run out, until there is no more helium left in their core to fuse. When that happens, fusion stops, and the outward pressure provided by it is gone, allowing gravity to contract the size of the entire star. As the star compresses, it's core becomes more dense, it's temperature increases, and as it becomes more dense, it's gravity increases, since denser objects produce more gravity. As the core's gravity increases, it increases the pressure on the layers of hydrogen above it, until those ignite. At this point, you have an inert helium core, with a layer of hydrogen above it that is undergoing fusion and dumping the helium by product onto the core. The helium core doesn't contract further because of electron degeneracy pressure. The atoms in the helium core become so closely pressed together that the repulsion between electrons is all that holds the core's gravity from compressing it any more. At this point, the degenerate core is basically a white dwarf star. It's immense gravitational pull compresses the layers of hydrogen outside the core, causing their rate of fusion to increase dramatically. The increased fusion, in turn causes the outer layers to billow outwards, so the star expands and becomes a red giant. As the hydrogen continues fusing, the helium that is created falls onto the core like ash from a fire. Pretty soon, enough helium has built up so that the core is large enough to start fussing helium. The core's dynamics, however, are different for a degenerate core. In a normal core fusing helium, if the cores temperature increases, it's rate of fusion increases, which causes the outer layers to billow out. The outer layers billowing out reduces pressure on the core, so that the rate of fusion slows down, balancing it out. However, in a degenerate core, because the cores pressure isn't caused by the outer layers, it works differently. When the temperature increases, the fusion rate increases, which increases the temperature, which causes more fusion. There's no control mechanism in a degenerate core, so it creates a helium "flash". However, eventually, the increased fusion does cause the core to lose it's degeneracy, and the rate of fusion levels out. Now the core enters a stage of helium fusion, with an outer layer fusing hydrogen. And, pretty soon, the star builds up a large core, composed of oxygen and carbon. Just like before, the core becomes degenerate. Outside the core, you a double ring structure, with an inner layer fusing helium, and an outer layer fusing hydrogen. Just like before, the immense gravity of the core causes the helium layer to fuse at a very high rate. Eventually, the fusion in the core becomes so strong that the star can no longer hold it's outer layers together, and it eventually blows it's outer layers away forming a planetary nebula, with the inert core remaining as a white dwarf star. Although there is a short stage of relaxation ossilations I skipped for simplicity, that is otherwise how it goes. As large stars age, they go through completely different processes. When they run out of hydrogen to fuse, they don't have much problem. Initially, the star will contract a little bit, but it doesn't take anywhere near enough contraction to start fusing the helium core. Also, because the helium core doesn't go long until fusion starts, it never enters a stage of degeneracy pressure, so when it starts fussing, it's a controlled reaction. When it switches from hydrogen to helium burning, there is very little change in the starts outer appearance. When it finishes burning helium, it switches to oxygen and carbon the same way. In fact, the star continues just like that until it begins to build up an iron core. After iron, the amount of binding energy in atoms decreases to much to benefit from fusion, because when you fuse two iron atoms together, you don't produce energy, some is actually removed from the surrounding area. So, no matter how large a star is, it can't productively fuse anything past iron in it's core. To the top of this page The death of a star In the end, just like everything else, a star dies. How and why a star dies varies on the star's original size, just like everything else. Small stars die a very quiet death. As they expand into the red giant and red super giant phases, their radius increases a lot, so mush so that the star can no longer hold it's outer layers together. The gas in it's outer layers is simply blown away, leaving a planetary nebula. In the end, everything is blown off the star, leaving the core to float throught space on it's own. Also by this time, all fusion has stopped in the core, which is now dubbed a white dwarf. White dwarves still radiate light, despite the fact that they are not generating energy. There is simply so much heat left in a white dwarf that they continue shining for a long time. Large stars die a quite different death. Since they are so much larger and have so much more gravity, in the end, it isn't the outward force that wins. In fact, the outward force gives up. As the star begins to fuse bigger and bigger atoms, it eventually makes iron in it's core. Unfortunately for stars, when you fuse two atoms to make iron, instead of releasing energy, it absorbs energy, so pretty soon you begin to have problems. No matter how dense a star gets, even if it fuses iron, it will get no usable energy out of the reaction. Eventually, the star simply runs out of fuel. That's when gravity wins. Without the outward force generated by fusion present, the star begins to collapse in on itself, generating larger and larger amounts of pressure an heat in the core. This does in part start fusing iron into heavier elements, which is where all the elements more massive than iron are formed, right before a supernova explosion (actually, any non man made element other than helium was formed inside a star's core) Also, there is one important rule of physics that comes into play here. At low temperatures, matter prefers to have more binding energy, in otherwords, prefers to be in large elements. However, at very high temperatures, those only reachable in the core of a large star at this stage in it's life, matter prefers less binding energy, or small elements like hydrogen and helium. Because of this, during this stage, a lot of the heavy elements begin splitting into smaller elements, undoing most of what fusion has done. With such a sudden change, something has to happen, and the sudden change is what brings on the supernova. The resulting explosion can be seen nearly across the universe, and scatters the heavy elements inside it's core across the galaxy. Although the rest of the star explodes, the core continues collapsing. As the core collapses, things get so packed in there that electrons are actually forced into the core of the atom, equalizing the charges and turning what was once a mass of protons and electrons into a mass of neutron. This forms a neutron star, which are basically large balls of neutrons. Neutron stars are even more massive and smaller than white dwarves. Also, if the star is massive enough, and the core shrinks further, a black hole can form, which is so massive even light can't escape. Back to the top of this page LBV's In the previous section, I mentioned LBV's (Also known as Luminescent Blue Variables) as a stage some stars go through near the end of their lives. Very few do though, and not to many LBV's are known to exist. The last count I heard was 35. LBV's are VERY large blue stars, with many many solar masses . LBV's are generally from 90 to even 130 solar masses. They are actually very near the theoretical limit to a stars mass. If a star's mass were a little above 140 solar masses, so much matter would be fusing inside the core every second that it would be to much for gravity to hold together, and the star would literally rip itself apart. LBV's are very peculiar stars. They are constantly undergoing changes in the amounts of matter and energy they give off. One of the most famous of these is Eta Cairne (pictured below), which is located about 8000 light years from Earth. 
It was originally thought to be a variable star, due to it's periodic changes in brightness. Around 1837 it let out a huge explosion. Two huge bubbles of gas extended out from its poles, each composing of at least 1 solar mass. It became the second brightest star in the sky in one night. To understand the significance of this, you need to know that Eta Cairne is 8000 light years away. Most of the brightest stars in our sky are only 20 light years away. Eta Cairne was so bright it nearly out shown stars 400 times closer. That's quite a feat. Most LBV's do something like that during their lives. They also frequently let out smaller explosions. Their normal outputs are also much larger than other stars. The solar wind coming off from most will generally let out a solar mass of gas in a few hundred years. Also, The light they let out is millions of times greater than our sun, and they can be seen easily across the immense galactic voids. One of the farthest away visible by the hubble is 10 million light years away, it is impossible to see normal stars at such a great distance, seeing a normal star that far away, even for the hubble, would be like a person trying to see a candle on the moon with the naked eye. They are generally many, many times bigger than our sun. Also, the energy they give off varies greatly. One, P Cygni, is currently glowing at about the fifth magnitude, bright enough to see with the naked eye. It was first observed at the third magnitude in 1600, then faded in 1626, but began to brighten in 1655. After many fluctuations, it settled to it's current brightness in 1715. It is estimated to be about 6000 light years away, while other stars it's brightness are generally dozens of times closer, making it one of the brightest stars known.Back to the top of this page White dwarves White dwarves are the remain of small stars that died. They have a limit to their mass of about 1.4 solar masses. They generally sit inside their planetary nebula for the rest of their long lives. However, gravity is strong enough to hold the core together, and it stays in a tightly packed ball called a white dwarf. This ball will generally have a lot of mass, anywhere up to 1.4 solar masses, but it is about the size of the Earth, making it very dense. That is part of what contributes to the cores large gravitational field. They are generally very hot, attesting to their previously hot lives. White dwarves never die, they simply cool down. Over the years, the large amounts of energy they have left over from the past simply radiates away into space. The white dwarves stay the same for the rest of eternity, slowly radiating off the last of their heat. Back to the top of this page Neutron Stars Neutron Stars are very interesting stars. They are a small ball of tightly packed neutrons. They are even more dense than a white dwarf. They always have more than the 1.4 solar masses limited to a white dwarf, and are even smaller, packed into an area a little bigger than Mount Everest, making them the densest things around, with the exception of black holes. There are many different types of neutron stars you can encounter. All neutron stars have a few similar characteristics. One is that they all spin very fast and with lots of accuracy. Most will complete a few rotations every few seconds, and they spin with an accuracy that exceeds atomic clocks. Most scientists are looking to use them to keep more accurate time and correct our clocks. Also, all neutron stars have very powerful magnetic fields, much more powerful than our suns. Also, neutron stars travel very fast. They actually travel fast enough to escape a galaxies gravitational pull. During a galaxy's life time, it will release thousands of neutron stars into inter-galatic space. Pulsars differ from normal neutron stars because of the jets of radiation they spew from their poles. Huge jets of radiation, usually x-rays, visible for very large distances, are released in streams and travel outward from the star. Interestingly enough, the first extra-solar planet ever discovered was found orbiting a pulsar. Millisecond pulsars are just like pulsars, but they rotate a lot faster. As their name implies, they will rotate once in a period of time measured by milliseconds. These will sometimes rotate hundreds of times per second. Magnetars are theoretical in existence, although two are believed to exist. These are neutron stars with ultra powerful magnetic fields. These are the hardest of all neutron starts to detect because they let out little to no radiation, so are very dim. The two that were "discovered" (it is still in debate weather or not they are magnetars) were discovered because of one property of neutron stars. Their magnetic spheres are so dense that it occasionally tears a hole in the crust. It just rips a small area apart. In doing so, they let out large amounts of soft (low energy) x-rays, that make them easily visible across the galaxy. Two of these bursts was believed to be seen a while ago. Back to the top of this page Brown Stars First of all, I would like to make it abundantly clear that Jupiter is not a brown star. I remember being told that in my younger years in elementary and middle school, and I'm pretty sure that is still taught today. However, it is not true at all. Brown stars are failed stars. They form by the collapse of a small nebula, which doesn't have the mass required to produce enough heat to start fusion. Jupiter formed in the gaseous disk that formed around the sun during the sun's birth, and swept up gas as it orbited, until it slowly grew to it's present day size. Two completely different origins. Also, there are some chemical differences, and brown stars will be significantly larger than a gas giant like Jupiter. Now then, since that is out in the back to brown stars. For starters, as I said, brown stars are failed stars. There is not much to say about them. Until recently, the proved very hard to find, despite their predicted large numbers. Since they can't generate energy through fusion, they have only the heat generated from the nebula's collapse, which is quickly released, causing them to fade into obscurity, dim enough to make them nearly impossible to detect, even for Hubble. To get around this, two teams of astronomers started looking in the young star forming regions of IC 348 and the Trapezium cluster at the center of the Orion nebula. Since star formation is an ongoing process there, they thought they would find younger, and hence, brighter brown dwarves. They were right. About 30 brown dwarves were found in IC 348, and about 50 in the center of the Orion Nebula. After this counting, brown dwarves have risen in status from dought of their existence to being very numerous. They found mostly free-floating brown dwarves, and noticed that they mainly tend to be low mass, like stars. The studies show that there may be just as many brown dwarves in the nebulae as stars. Despite their large numbers, however, thanks to their low mass brown dwarves can only account for about .1% of the dark matter in the galaxy. Back to the top of this page |