how do astronomers have an idea of how many stars there are in the universe?
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It's the same way that scientists look at all massive measures: density*volume.  The density (ie the number of stars per square lightyear) is assessed by counting the frequency of galaxies and the size of galaxies.  The mass of galaxies can be inferred by the speeds of rotaion, and the mass will give you an idea as to how many stars there are in the galaxy by looking at the average mass of a star.  Then you get a sense of how many galaxies are in a given volume of space.  They use this, and extrapolate from the size of space (set by the tenets of general relativity, the Big Bang theory, the red-shift observed speed of expansion, etc), and they can thereby infer how many stars there are in the universe.\n\nIt's like the mass of the earth.  We can't figure it out with 10 kilograms, or even 100 kilograms, but we can get an idea as to the magnitude of the mass.  With magnitude, i mean the number of digits to the left of the decimal place.  We call the difference between 10 and 100 "one order of magnitude," and the difference between 100 and 100,000 "three orders of magnitude."  Such scales are commonly used in science for things where exact numbers are impossible to get, but the range of magnitude is important.  Some such commonly known scales are the measure of sound in decibles, the Richter scale for earthquake power, and the pH scale for acidity.  With a pH of seven, one can infer that a liter of water has 3,344,000,000,000,000,000 hydronium ions per liter, whereas it only has 3,344,000,000,000,000 hydronium ions per liter at a pH of 10.  In these cases, the magnitude, rather than the exact number, is the matter of interest.\n\nIt's the same in counting stars.  We can't know the exact number of stars, especially since they constantly form and die--but we can get a pretty good idea as to the magnitude of the number by looking at stellar density and extrapolating the size of the universe.
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Since you've extended the question to ALL possible sand grains in the universe and not just on Earth, there are implied answers.\n\nSince we can only estimate the number of both, it's also a moot point.\n\nFor the record, the further we are able to see, the more stars we see - in every direction, so the potential for the number of stars is infinite. Check out the Hubble Space telescope deep space field..\nhttp://hubblesite.org/newscenter/newsdesk/archive/releases/1996/01/\n\nSand has so far been verified on only 3 planets (Earth, Venus and Mars) . The Outer planets cannot be verified as having sand, since they have extremely dense atmospheres. There are, however, several moons and asteroids which have sand sized material.\n\nHaving said that, there exists the potential for there to be more sand grains than stars, since there exists the possibility of planets (and other objects which may have sand) around every star.\n\nWe don't know how many stars have planets. If only 1 out of 1 million stars have a planet with sand as is found on Earth, sand would win.\n\nIf you limit your question to sand on Earth as opposed to stars in the universe, stars win big.
65484
To clarify more, cepheid stars are one the most common way to determine the distance to a distant galaxy. These stars are extremely uniform in their brightness, so by measuring their brightness as seen from the Earth, scientists can determing how far away that star would have to be to be that faint.
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I read in Time a few years back that in an article, scientists explained a theory that universes connect as a chain, and our Universe- is among many. The idea was quite something to read and of course the diagrams and images they used to portray this thing out was all very fascinating to say the least. It was a rational idea even to me and considering what we found with a pin tip portion of the night sky magnified 1,300 times....hundreds of thousands of Galaxy's all clustered in an array that almost seemed like you are looking at it all through a cylinder. \nSo there is no real way to know for certain, but there could be millions of them...and a chain of universes makes rational sense.
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Stars are not scattered randomly through space, they are gathered together into vast groups known as galaxies. The Sun belongs to a galaxy called the Milky Way. Astronomers estimate there are about 100 thousand million stars in the Milky Way alone. Outside that, there are millions upon millions of other galaxies also!  \n \n  \n \n \n Hipparcos mapped millions of stars in our galaxy, but how many more are there? \nIt has been said that counting the stars in the Universe is like trying to count the number of sand grains on a beach on Earth. We might do that by measuring the surface area of the beach, and determining the average depth of the sand layer. \n\nIf we count the number of grains in a small representative volume of sand, by multiplication we can estimate the number of grains on the whole beach. \n\nFor the Universe, the galaxies are our small representative volumes, and there are something like 1011 to 1012 stars in our galaxy, and there are perhaps something like 1011 or 1012 galaxies. \n\nWith this simple calculation you get something like 1022 to 1024 stars in the Universe. This is only a rough number, as obviously not all galaxies are the same, just like on a beach the depth of sand will not be the same in different places. \n\nNo one would try to count stars individually, instead we measure integrated quantitites like the number and luminosity of galaxies. ESA's infrared space observatory Herschel, to be launched in 2007, will make an important contribution by 'counting' galaxies in the infrared, and measuring their luminosity in this range - something never before attempted.
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There are three basic methods:\n\n1. Parallax. The angle at which the object appears in the sky is very precisely measured at one point in the Earth's orbit, then measured again six months later. Since the Earth is in a different position at these times (186 million miles between the two measurments), the star will appear at slightly different angles, and its distance can then be worked out from these angles using simple trigonometry.\n\n2. Standard candles. Certain types of stars (Cepheid variables, RR lyrae variables), and certain types of stellar events (type Ia supernovae), have luminosities that can be worked out fairly precisely from their intrinsic properties (the variable stars for instance, have a period of variablility that is closely correlated with their luminosity, and the type Ia's luminosity can be worked out by examining its light curve). Since percieved luminosity varies inversely with the square of the distance, and since we konw how much light they put out at the source, we can work out how far away these stars are simply by measuring how bright they are.\n\n3. Redshift. The universe is expanding, and that means that from our perspective, distant galaxies are all moving away from us. This means that the light we recieve from them will be at a lower frequency (that is, redder) than the light they emitted, due to the doppler effect. Edwin hubble discovered that at large scales, the observed redshift is directly proportional to the distance of the object from us. This means that if the object is too far away to measure using parallax and lacks a good standard candle, we can still determine its distance simply by measuring its velocity. Actually doing this is somewhat difficult, and involves looking for patterns in the absorption lines in the object's spectrum to figure out what elements created those lines, and then one you know that you can figure out what the frequency of the light should be. After that, you have to work to filter out as many extraneous sources of redshift as possible, such as the object's gravity, its velocity due to gravitational interactions with other nearby objects (what is called the peculiar velocity), and so forth, to leave just the velocity due to the expansion of the universe - from which the distance is estimated using hubble's law.
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Actually, they know it isn't infinite.\n\n\nThe findings are mostly thanks to hubbles big idea and discovery, but they can now confirm that indeed the visible universe is finite thanks to WMAP.\n\n\n\nAs for the possiblity of space outside of the visible universe...we have no idea.
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Are you including the stars that are being created, as well as those that do not actually exist anymore but have died out/ supernova'd/ etc. that we are just still seeing since the light has to travel so far?  \n\nI would be interested in how you determined this number ^_^
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There's no way of checking (as of yet) to see if there are four billion stars.  They'll just have to take your word for it.  Its probably not that important to them anyways.\n\nBut you can check to see if the paint is wet.  You should never believe anyone 100% anyways.  That's how you become a fool.
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When performing star counts, astronomers consider many different categories which have created been to classify a few stars that have been well studied. One of the hopes of studying the results of star counts is to discover new categories. Different counts typically seek to categorize stars for only a few of the qualities listed below, and determine how common each considered quality is and how stars of that kind are distributed.\n\nTemperature. In astronomy, temperature is usually shown using the letter codes O B A F G K M L T running from blue (type O) through white (type F) to red (type M). Types L and T are used for brown dwarfs, whose 'colors' are in the infrared. \nSize. Size is usually designated by Roman numerals I (supergiants) through V (dwarfs). \nAge. Stars are usually grouped into Population I (young) and Population II (old). \nLocation. In the Milky Way Galaxy the groups are described as thin disk, thick disk, central bulge, and halo. \nMultiplicity. Most stars appear to be members of double star or triple star, or even double-double star systems. Our own sun appears to be rare for not having a companion star. \nThere are many finer subdivisions in all of the above categories.\nIn which category you want me to count the Stars.
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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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Several different ways depending on the distances involved.\n\nFor close stars, we look at where the star appears to be in the sky at times about half a year apart. Think of looking at your outstretched thumb through your left eye, and then your right eye. The thumb appears to be in a different position with respect to a distant building. In the same way, a star will appear to be in a slightly different position when we look at it from different ends of the earth's orbit. The amount of shift depends on the distance to the star and that allows us to figure out the distance. This method is called paralax.\n\nFor slightly more distant stars, we look at clusters of stars and measure their apparent motion across the sky over a course of *years*. Just as train tracks appear to converge at the horizon, the motions of the star will appear to converge. The rate of convergence is related to how far away the cluster is.\n\nWith these methods, we get enough stars to do statistical analyses of the brightness of the stars versus their chemical composition and other aspects of their spectrum. With this knowledge, we can estimate the distances to further stars by just comparing their spectrum to that of already measured stars. Scientific models of stars are used at this stage.\n\nFarther away than this, we look for 'standard candles', that is, stars of known brightness. By comparing the brightness they appear to be versus their actual brightness, we can estimate their distances. A particular type of 'standard candles' are the Cepheid variables. Their actual brightness depends on how fast they vibrate. Since we can see these stars in other galaxies, we can use them to determine the distances to those galaxies.\n\nFinally, after we know the ditances to some galaxies, we can measure the rate at which they move apart as a function of distance. By measure red-shift (hence speed) of more distant galaxies, we can determine their distances.
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Analyzing light can tell you almost anything about the universe.  You can calculate just how excited every photon is, what color or what "brand" of light is emitted from any given star in the entire visible universe.  Photons behave certain ways when they interact with certain substances, so how light bounces or reflects or refracts or in other ways interacts with planets and their atmospheres can give us an idea as to what makes it up.  By combining what we can determine from gravitational influence and the amount of energy and type of light emitted from a star, we can find out how massive a star is and what makes it up.  Light is really the only data we have about the universe--luckily, light is qualitative and quantitative and we can learn a heck of a lot from it.  Every single reaction that takes place on an astronomical scale has a tag associated with it--something it gives off 999,999 times out of a million, where if astronomers see it, they know what's going on.  By seeing light--which is more than visible light, but radio waves, UV rays, and gamma rays--they can postulate anything that's going on, even billion of light years away.
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You can try to count the stars, but there are as many stars in the Universe as there are grains of sand on the beach.  It would be a very difficult task and take millions of lifetimes to complete.  Sorry.
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Unfortunately we do not have any concrete evidence.  Here is a good quote by the late Carl Sagan:\n\n"There are some hundred billion (1011) galaxies, each with, on the average, a hundred billion stars. In all the galaxies, there are perhaps as many planets as stars, 1011 x 1011 = 1022, ten billion trillion. In the face of such overpowering numbers, what is the likelihood that only one ordinary star, the Sun, is accompanied by an inhabited planet? Why should we, tucked away in some forgotten corner of the Cosmos, be so fortunate? To me, it seems far more likely that the universe is \nbrimming over with life. But we humans do not yet know. We are just beginning our explorations. The only planet we are sure is inhabited is a tiny speck of rock and metal, shining feebly by reflected sunlight, and at this distance utterly lost."
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In our Solar System, there are eight official planets, nine if you count Pluto (whose status is being debated).  \n\nAs for the rest of the universe, it is difficult to say, but it is reasonable to assume that planet formation from leftover stellar material is relatively common.  Astronomers usually detect large gaseous planets (like Jupiter) by the slight wobble of the star's motion due to the gravity of the planet, so currently it is almost impossible to detect small planets.\n\nAs of November 2005, there were 164 confirmed extrasolar planets.
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All stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.\n\nStars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars. The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist.\n\nAstronomers estimate that there are at least 70 sextillion (7×1022) stars in the known universe [1]. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way.\n\nThe nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.\n\nCheck the wikipedia link below.