Main Sequence Stars, 8 November!The process of star formation is longer for smaller stars and shorter for 1.larger stars.!It takes red dwarfs more than 10 million years to move from a condensing A.nebula to a shining star. !It takes stars the mass of the sun several million years.!B.It takes the brightest blue giants about 100,000 years. !C.For 98% of a star's life, it is fusing hydrogen to form helium plus energy in its 2.core. !This is the definition of being on the main sequence. !A.This process can happen elsewhere, but it has to be in an objects core to B.make it a star on the main sequence. !Core fusion gives the vast bulk of conventional stars OBAFGKM. !C.Open Clusters: concentrated scattering of stars with a nebulosity in the 3.background!Usually, a lot of stars are born together in a vast area, often in clusters. !A.A lot of the gas and dust of a nebula is not merged into a star or blown a.away by stellar winds.!This gives you an open cluster. !b.These clusters are labeled by their nebula or by their star cluster, B.depending on which was first noticed by astronomers (which was brightest). !Some of these clusters are very attractive: !C.The Hodge 301 cluster in tarantula nebula.!a.The rosette nebula has a hot star cluster in the middle with brilliant b.nebulosity around it.!The stars cause parts of the nebula to emit light. !1.There is a string of dark nebulosity all across it, where dust is so 2.concentrated that light can't get through it. !This is the place where stars are forming. !A.The Pleiades star cluster ("The Seven Sisters") is the the most famous c.star cluster. !It is easy to see at least the brightest members of this cluster with an 1.unaided eye, even in Berkley. !In more remote locations with better telescopes, you can see many A.more than seven stars in this cluster. !The brightest members of this star cluster have names, but you do B.not need to know them. !There are about 800-3,000 stars in the Pleiades star cluster.!2.Astronomers don't agree on the exact number, because it is hard A.to distinguish the stars in the cluster from those in the background.!All of these stars were born together in a pretty tight bunch a few 3.million years ago. !They are about 8-10 dozen light year across and twice the size of A.the moon in the sky. !They are about 415-430 lightyears from the Earth, depending on 4.which data you consult. !These stars are enshrouded with dust, but not gas, which is a puzzle.!5.This means the cluster is somewhat older, not in its original state. !A.The dust is probably not part of the original cluster, but rather a B.cloud of dust the cluster is running into.!The dust of the Pleiades seems to be following magnetic lines of force 6.around each star, but there is no way to measure this.!The dust looks streaky and follows a different tilt around every A.star. !Why a patch of dust would orient itself along magnetic fields lines B.is a mystery, although there are some ideas why.!If the dust was magnetized, ionized or made of iron this would a.be understandable. !Given a million years, even a large dust particle might be able to b.follow a magnetic line of force. !Until there is better data, this is the story. !C.There are a famous two clusters which appeared to be a double cluster, d.but are actually 900 light years away from each other (one was 8000 light years away, and one was 7100 light years away). !One star cluster is the remnant of a star that died in the last 50000 e.years ago. ! Open star clusters can have as few as a hundred stars and as many as D.thousands of stars. !Open star cluster don't have a whole lot of density, so they do not have a.a lot of gravity. This means that over time, stars tend to drift away from them.!Globular star clusters have the gravity to hold on to most of their stars.!4.Some of these clusters are 10 billion years old. !A.The bright stars in these clusters are red giants, and and the faint B.background stars are main sequence stars. !In a picture of a globular cluster in class, it was possible to discern the a.mottled glow of fainter stars behind prominent bright stars. !All that astronomers know about the inside of stars, they learned from 5.experimenting in labs and computing models:!Experiments in labs can tell astronomers how hydrogen and helium behave A.at different temperatures, densities and pressures. !A great deal of this research was for the military to design hydrogen a.bombs. !These experiments are how astronomers figured out how stars could b.do nuclear fusion. !The idea that stars are doing nuclear fusion in the core, radiating, then B.convecting, then radiating again is only proved by scenarios generated in a computer. !The way astronomers use a computer to determine the structure of a star C.is to approximate everything they know about how hydrogen and helium behave and compare it to other information about stars. !There is a problem with this: the actual situation in the sun is far beyond a.current computing capabilities. Astronomers must make erroneous assumptions to be able to calculate anything. !For example, they must assume that the structure of the sun is in 1.concentric shells without much turbulence. !Astronomers calculate in the inward force of gravity in layers of the sun b.to find out how much outward radiation and gas pressure is needed to put the sun in equilibrium !Assuming the star is in equilibrium (most are) astronomers can use this c.data with the mass of stars in double star systems, their surface temperatures, and lab data about how hydrogen behaves to construct a model of the sun. !They know how heat transfers through hydrogen at various states, so 1.they can figure out how much energy radiates and how much energy transforms in what ways and at what levels. !There are some good points and some bad points to this method: !D.The good points are that this is the only way to do this sort of work, a.and technology is getting better at doing it. !You cannot send a probe or a person to the sun. !1.New generations of computers are able to make smaller shells and 2.segments of shells – which allows more precision. !Though they continue to make assumptions, the numbers A.generated in today's models feel more right than they did in the past. !Maybe newer technology will give such good approximations that B.the aggregate answer will be right. !The bad points are the wrong assumptions made in almost all computer b.models.!Turbulence: Computers cannot compute turbulence, so
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