if a star is 10 times the radius of the sun and half as hot, what will its luminosity be?
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7.257276\n\nhttp://www.go.ednet.ns.ca/~larry/astro/HR_diag.html
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Our sun is a small to mid-range example of a star.
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They can fall anywhere on the H-R diagram. The age and the temperature of the star are located on the axes, but the distance of the star isn't mentioned.\n\nAlso, say the ratio of stars visible to the naked eye and stars visible with a telescope is f/g (g obviously being bigger). This ratio is differenciated by brightness*nearness, where brightness*nearness has to be greater than a certain number to be visible by the naked eye. Since most stars are main sequence stars, the graph of stars can look like this:\n\n....*..............*............*.M\n................*........*.*M..*..\n......*............*..M..*....*...\n..*............*M..*.......*......\n........*..M.*..*.........*.......\n.*..*M.*....*.....*...........*..\n\nAs you can see, most stars are on the diagonal that cuts from the bottom left to the top right M's. If you randomly take away a  number of stars, then the diagonal will most likely be there. Thus, most visible stars are near the graph of (f*brightness/g*nearness) which is the diagonal.
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Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are extremely hot, while others are cooler. You can tell by the color of light that the stars give off. If you look at the coals in a charcoal grill, you know that the red glowing coals are cooler than the white hot ones. The same is true for stars. A blue or white star is hotter than a yellow star, which is hotter than a red star. Temperature of stars range in the high hundreds to thousands degrees F. So to touch the stars that are in the sky is almost impossible, as your body would burn due to high temperatures surrounding them.
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The brightness and apparent size of the Sun would be inversely proportional to the square of one's distance from it.  If you moved to 2000 miles from the Sun, about 1/50,000 the Earth's distance from it, the Sun would appear 2.5 billion times larger (in apparent area, but only 50,000 times wider) and brighter.  Hubble can't be moved close to the Sun because Hubble orbits the Earth at a fixed orbital radius.  In addition, Hubble and most other astronomical observation tools cannot be pointed at the Sun, because the light output would overwhelm their sensors.  Only a very few devices have been designed to observe the Sun.
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There is no fire on the sun.  Our star is converting hydrogen into helium (which is nuclear fusion and thus creates heat) because it is so massive that the atoms its made from are being compressed by its own gravity.\n\nEventually, it will run out of fuel and stop "burning" all together, becoming a white dwarf, then a black one.\n\nLarger stars will explode and disperse their materials (the things you and I are made of) across the universe.  The largest will become black holes and try to fix the mess created by the others.
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The best way to compare other stars to the sun is to talk in terms of\n\nTheir size: how many solar diameters are they?\nTheir mass: how many solar masses are they?\nTheir brightness: how does their luminosity compare to the sun = 1?\n\nMorgan-Keenan spectral classification\nThis stellar classification is the most commonly used. The common classes are normally listed from hottest to coldest (with mass, radius and luminosity compared to the Sun) and are: \n\nClass Temperature Star colour Mass Radius Luminosity \nO 30,000 - 60,000 K Bluish ("blue") 60 15 1,400,000 \nB 10,000 - 30,000 K Bluish-white ("blue-white") 18 7 20,000 \nA 7,500 - 10,000 K White with bluish tinge ("white") 3.2 2.5 80 \nF 6,000 - 7,500 K White ("yellow-white") 1.7 1.3 6 \nG 5,000 - 6,000 K Light yellow ("yellow") 1.1 1.1 1.2 \nK 3,500 - 5,000 K Light orange ("orange") 0.8 0.9 0.4 \nM 2,000 - 3,500 K Reddish orange ("red") 0.3 0.4 0.04 \n\nThese classes are further subdivided by Arabic numerals (0-9). A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The sun is classified as G2.\n\nSpectral types\n\nClass O\nClass O stars are very hot and very luminous, being bluish in colour; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars, constituting as few as 1 in 32,000. O-stars shine with a power over a million times our Sun's output. \n\nExamples: Zeta Puppis, Epsilon Orionis \n\nClass B\nClass B stars are extremely luminous and blue. As O and B stars are so powerful, they live for a very short time. They constitute about 0.13% of main sequence stars -- rare, but much more common than those of class O.(\n\nExamples: Rigel, Spica \n\nClass A\nClass A stars are amongst the more common naked eye stars. As with all class A stars, they are white or bluish-white. They comprise perhaps 0.63% of all main sequence stars.\n\nExamples: Vega, Sirius \n\nClass F\nClass F stars are still quite powerful but they tend to be main sequence stars. Their colour is white with a slight tinge of yellow. These represent 3.1% of all main sequence stars.\n\nExamples: Canopus, Procyon \n\nClass G\nClass G stars are probably the best known, if only for the reason that our Sun is of this class. Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be. These are about 8% of all main sequence stars. \n\nExamples: Sun, Capella \n\nClass K\nClass K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus while others like Alpha Centauri B are main sequence stars.  These make up some 13% of main sequence stars.\n\nExamples: Arcturus, Aldebaran \n\nClass M\nClass M is by far the most common class if we go by the number of stars. All the red dwarfs go in here and they are plentiful; over 78% of stars are red dwarfs, such as Proxima Centauri. M is also host to most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. \n\nThe red color is deceptive; it is because of the dimness of the star. When an equally hot object, a halogen lamp (3000 K) which is white hot is put at a few kilometers distance, it appears like a red star.\n\nExamples: Betelgeuse, Barnard's star \n\nSpectral types for rare stars\nA number of new spectral types have been taken into use for rare types of stars, as they have been discovered:\n\nW: Up to 70,000 K - Wolf-Rayet stars. \nL: 1,500 - 2,000 K - Stars with masses insufficient to run the regular hydrogen fusion process (brown dwarfs). Class L stars contain lithium which is rapidly destroyed in hotter stars. \nT: 1,000 K - Cooler brown dwarfs with methane in the spectrum. \nC: Carbon stars. \nR: Formerly a class on its own representing the carbon star equivalent of Class K stars, e.g. S C
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It's going to be difficult to tell you more than I know, but I'll give it my best shot.\n\nThe sun is big and bright and hot.  It is a giant fusion reactor creating energy by fusing atoms of matter together.  The energy is transmitted out into space as photons of light.\n\nIt is a medium sized yellow star which began making energy about 4 billion years ago and is estimated to have about 5 billion years left before its fuel begins to run out.
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Here is a comprehensive list of the 26 nearest stars compiled from the Dept. of Astronomy, University of Wisconsin at Madison...\n\nThe answer to your question then is:\n\nProxima Centauri - 4.2 light years from the Sun
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The burned out centre of a low mass star is a white dwarf. It's a hot ball of carbon and oxygen, though too cool for thermonuclear reactions to occur. They're about the size of Earth and are too dim to be seen with the naked eye.
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The closest star to the Earth is the actually Sun, at a distance of 8.3 light minutes (93 million miles), but I suppose you weren't asking about that. \n\nThe next closest star is Alpha Centauri, which is 4.3 light years away.
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This should help you:  http://www.le.ac.uk/physics/faulkes/web/stars/o_st_overview.html
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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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Basically, it's the degree of brightness of a star.\n\nThe websites below might be able to help you with your question.
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The brightest stars, the ones you can see with the naked eye, are all B8 and B9.  Some of the dimmer ones are of other classes--B,A,F, and perhaps some cooler classes, too.
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light my son light.\ndarkness means 0% illumination.As light from a star  travels in all direction it covers all of the universe(and never gets destroyed in vaccum so keeps on travelling) And there are so many stars.\nonly a small part of the space is filled with light absorbing object and black holes.So almost 90% of space is occupied by light.\nso light is the king!!!!
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The sun is a star.  All stars eventually burn out.  See facts below:\n\nThe Sun will last about 5 billion years as it is now, a so called Main Sequence Star, and burns 700 billion tonnes a second. This may look like a lot but the Sun actually burns Hydrogen, a gas, and it turns it into Helium, another gas. This happens in the central part of the Sun, its Core, through a process called nuclear fusion. A side effect of fusion is that light and heat are created. So only a small amount of this Hydrogen, only a few Billion tonnes a second, is actually burned off to create light. I would not worry that so much Hydrogen is burnt every second as the sun has a mass of 1.989 times 1030Kg. That is 1,989,000,000,000,000,000,000,000,000,000,000 Kg, or one million nine thousand, eight hundred and nine billion, billion, billion Kg. So the odd few Billion tonnes will not be missed. \n\nWhat is really interesting is what happens after the Hydrogen has been used up in the Suns core. At this point in our Sun's life the central part of the Sun, its core, shrinks and heats up and its atmosphere will expand to cover the Earth, and possibly Mars. The Sun is now called a Red Giant Star. If you can find the constellation Orion, the bright red star in the upper left hand corner is a Red Giant called Betelgeuse. After only a few million years the core will shrink again and the our Suns atmosphere will be blown off into space to become Planetary Nebulae. What will be left is a type of Star called a White Dwarf. This will last for many tens of billions of years, slowly cooling and getting dimmer. So, our star, the Sun, will last for 5 Billion years as it is, a few million years as a Red Giant, and finally tens of Billions of Years as a White Dwarf.