How do the astronomers measure the chemical composition of stars?
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By using spectra. Each element has characteristic frequencies of light that it emits or absorbs. If light from the star is spread out into a spectrum (like a rainbow), there will be dark lines for absorbtion and bright lines for emission. By identifying the chemicals that correspond to those lines, we deduce the composition of the star.
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Optical Spectrum Analyzer (OSA) is probably what you're looking for...nice for discovering chemical composition of light source and probably a zillion other different things.
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Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects, as well as the formation and development of the universe.\n\nhttp://en.wikipedia.org/wiki/Astronomy\n\n----------------------------------------------------------------------------\n\nGeology (from Greek γη- (ge-, "the earth") and λογος (logos, "word", "reason"))is the science and study of the solid matter of a celestial body, its composition, structure, physical properties, history and the processes that shape it. It is one of the Earth sciences.\n\nhttp://en.wikipedia.org/wiki/Geology\n\n---------------------------------------------------------------------------\n\nChemistry (from Greek χημεία khemeia[1] meaning "alchemy") is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as molecules, crystals, and metals. Chemistry deals with the composition and statistical properties of such structures, as well as their transformations and interactions to become materials encountered in everyday life. Chemistry also deals with understanding the properties and interactions of individual atoms with the purpose of applying that knowledge at the macroscopic level. According to modern chemistry, the physical properties of materials are generally determined by their structure at the atomic scale which is determined by the properties and energies of the interactions.\n\nhttp://en.wikipedia.org/wiki/Chemistry\n\n-----------------------------------------------------------------------------\n\nPhysics (from the Greek, φύσις (phúsis), "nature" and φυσική (phusiké), "knowledge of nature") is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space and time. That is, physics deals with the elementary constituents of the Universe and their interactions, as well as the analysis of systems which are best understood in terms of these fundamental principles. Physics is a study of the inorganic, physical world, as opposed to the organic world of biology, physiology, etc. Physics overlaps with chemistry concerning the electro-chemical interactions of substances.\n\nhttp://en.wikipedia.org/wiki/Physics\n\n--------------------------------------------------------------------------------\n\nMeteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. Meteorological phenomena are observable weather events which illuminate and are explained by the science of meteorology. Those events are bound by the variables that exist in Earth's atmosphere. They are temperature, pressure, water vapor, and the gradients and interactions of each variable, and how they change in time. The majority of Earth's observed weather is located in the troposphere.\n\nMeteorology, climatology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology comprise the interdiscplinary field of hydrometeorology.\n\nhttp://en.wikipedia.org/wiki/Meteorology
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In lay man's terms:\n\nA star: a huge ball of fire, emitting heat and light, mostly composed of gasses and the source of the heat is the nuclear reactions happening isnide it due to the massive pressure in its core. An example is the sun, the temperture of its surface if more than 5000 degress centigrade, while the temperature of its core is several million degrees. A star is 'usualy' much larger than planets(thouthands to millions of times larger) . Some stars have planets orbiting around them (e,g, our sun)\n\nPlanets: Intert masses that do not emit light or radiate heat. Planets revolve around stars and are far much smaller than stars(e.g. earth, mars, saturn, jupiter,etc). Some planets have moons(much smaller planets) revolving around them.Mostly planets are solid masses, but sometimes they are made of gases. The temperature of planets is far cooler than stars.
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Very simply: Every element when heated gives off a radiation frequency that is specific to itself. When the entire spectrum of light from a star is analyzed, those frequencies representing specific elements are absorbed and show up as dark lines in specific areas. Each star has a unique pattern. After correcting for any red or blue shift ( discuss that another time), the same elements will appear in the same spectral region.
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The only way that I know to do this with a normal telescope is to compare the star with others nearby of known magnitude and try to decide if it's brighter or dimmer until you find the same.\n\nOtherwise you'd need to measure the amount of light that comes from it, and I think you need computers to do that.\n\nThe magnitude scale is exponential, so deciding a magnitude ain't that simple. (ie a star of magnitude +2 is ten times brighter than a star of magnitude +3)
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You're right, spectroscopy is the proper method...Below you'll find a link to a quick and simple explanation.
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stars are very far away and that light u see has been traveling through space for maybe decades now and its passed through clouds of dust and a bunch of stuff.  That makes the star twinkle and gives it colors.  Also, stars are not white(unless they are white dwarfs, but those arnt really stars or they are really really big and hot).  Almost all stars have a color to them.  These can be blue, red, yellow, orange, or any of those colors inbetween.
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primarily of hydrogen and helium gases, however, in the nucleus of the that star, higher elements are produced
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atomic-absorption spectrometer
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planets, stars, and quasars
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STARS AND NEBULAE.\n\nMost of the things you can see in the night sky are stars: a few thousand are visible to the unaided eye. A star is a hot ball of mostly hydrogen gas; the Sun is an example of a typical, ordinary star. Gravity keeps the gas from evaporating into space, and pressure due to the star's high temperature and density keeps the ball from shrinking. In the core of the star, the temperature and densities are high enough to sustain nuclear fusion reactions, and the energy produced by these reactions works its way to the surface and radiates into space as heat and light. When the fuel for the fusion reactions is depleted, the structure of the star changes. The process of building up heavier elements from lighter ones by nuclear reactions, and adjusting the internal structure to balance gravity and pressure, is called stellar evolution. \nLooking at a star through a telescope can tell us many of its important properties. The color of a star tells us its temperature, and the temperature depends on some combination of the star's mass and evolutionary phase. Usually, observations also allow us to find the luminosity of the star, or the rate at which it radiates energy as heat and light. \nAll stars visible to the unaided eye are in our galaxy, the Milky Way. The Milky Way is system of roughly 100 billion stars, along with a great deal of interstellar material. The galaxy is shaped like a flattened disk embedded in a round, faint halo. Gravity keeps the stars from drifting away, and the motions of the stars keep the system from falling in on itself. The Milky Way has no distinct edge - the distribution of stars tapers off gradually with increasing distance from the center. The SDSS detects stars more than a million times fainter than can be seen with the unaided eye, far enough to see the outer structure of the Milky Way. \nStar Colors and Luminosities: The H-R Diagram\nAstronomers often make plots of the luminosities of stars compared to their colors. The first such charts were made in the early twentieth century, when astronomers took spectra of thousands of stars, then placed the spectra into a sequence based on the appearance of several different features. The classes of stars were given different letters as names; in order of decreasing temperature, they are OBAFGKMLT. The hottest stars are class O, while the coolest are class T. Two astronomers, Ejnar Hertzsprung and Henry Norris Russell, independently started to wonder what would happen if they compared stars' luminosities to their spectral classifications (or, alternatively, to their temperatures). They knew that some stars were hotter and more luminous than the sun, and that others were cooler and less luminous. Hertzsprung and Russell found that 90% of the stars fell into a narrow band they termed the "main sequence." Today, we call this type of plot a Hertzsprung-Russell (or H-R) diagram. \nAstronomers further divide each letter into ten categories from 0 to 9, with 0 being the hottest star in that spectral class and 9 being the coolest. So, a B1 star is hotter than a B2 star, and a B9 star is hotter than an A0 star. Our own sun is at the "hot" end of the G range with a surface temperature of 5,770° K; therefore, it's called a G2 star. \nSpectral classes give us a convenient way to refer to most types of stars - from O0, the hottest at over 25,000° K, down to T9, the coolest failed stars at 1,000° K. Each spectral type of star is associated with a specific color: all G-type stars like the sun are yellow, all M-type stars appear red, and all B-type stars are bluish. Thus, this system of letters and numbers tells us where a star is on the horizontal (color or temperature) axis of the H-R diagram. \nTwo alternative systems tell us approximately where on the vertical (luminosity) axis a star can be found. One system uses absolute magnitude, a measure of luminosity derived from the apparent magnitude, or brightness of the star
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They are crazy about stars because they want to be astronomers.
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milky way is our own galaxy.sThe compositions of stars are determined through spectroscopy. Spectroscopy is the study of something using spectra. Recall from the electromagnetic radiation chapter that a spectrum is what results when you spread starlight out into its individual colors. By noting what absorption lines (or sometimes, emission lines) are present and their strengths, you can find out a tremendous amount of information. Stars have absorption lines patterns similar to the Sun. This means that they are composed mostly of hydrogen and helium with traces of other elements. \n\nFrom these absorption lines you learn some important things beside the star's composition: \n\nStructure of stars: From the simple fact that you see absorption lines in most stellar spectra, you know that the stars must have a hot dense part that produces a continuous spectrum and an outer layer, or atmosphere, made of cooler, low density gas. The general trend is density and temperature of stars decreases as the distance from the star's center increases. The hot dense part is also gaseous because of the extreme temperatures. Stars have no molten rock in them like the interiors of some of the planets. \nmass of stars:\nTo determine the masses of stars, Kepler's third law is applied to the motions of binary stars---two stars orbiting a common point. The greater the combined mass of the two stars, the greater the gravity acceleration is, and, therefore, the smaller their orbital period. A majority of the several hundred billion stars in the Galaxy are in a system with two or more stars orbiting each other. Usually the binary stars are spectroscopic binary stars. A spectroscopic binary system is two stars orbiting a common point at too great a distance away from us to resolve the two stars individually, but whose binary nature is indicated in the periodic shift of their spectral lines as they orbit around each other. Spectroscopic binary stars are used because (a) there are a lot more far away stars than nearby ones and (b) more importantly, you can easily measure their speeds from the doppler shifted lines. \ncolour of star:\nHot stars appear bluer than cooler stars. Cooler stars are redder than hotter stars. The ``B-V color index'' is a way of quantifying this using two different filters; one a blue (B) filter that only lets a narrow range of colors or wavelengths through centered on the blue colors, and a ``visual'' (V) filter that only lets the wavelengths close to the green-yellow band through. \nA hot star has a B-V color index close to 0 or negative, while a cool star has a B-V color index close to 2.0. Other stars are somewhere in between. Here are the steps to determine the B-V color index: \n\nMeasure the apparent brightness (flux) with two different filters (B, V). \nThe flux of energy passing through the filter tells you the magnitude (brightness) at the wavelength of the filter. \nCompute the magnitude difference of the two filters, B - V. \n\nok thats all,
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check this site...\nhttp://www.bbc.co.uk/science/space/stars/index.shtml
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The stars do move. The motion of a star in space is measured by astronomers in two ways. They use a spectrograph to measure the Doppler shift of the Star's light to determine the redial velocity, which is how fast the star is moving toward or away from the Earth. And they take careful position measurements with telescopes to determine the proper motion, which is the sideways motion in the sky. The star with the fastest proper motion is Barnard's Star, which is only moving at 10.3 arc seconds per year. That is only one degree every 350 years. Yet the star's speed though space is 750,000 miles per hour! It just looks slow from Earth since it is so far away.
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A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. The most familiar and closest star to the Earth is the Sun. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Stellar astronomy is the study of stars.