How bright is the Sun? - The Sun's vital statistics - The Sun as a Star - Sun|trek the Sun's vital statistics how bright is the Sun? From Earth the Sun looks very bright. It is the brightest object in the sky! start exploring here . We can’t do your home work for you, but if you have a question about the Sun you can always Early morning photo showing Venus with Jupiter Photo credit: Dr Russell Cockman Sun - SOHO / EIT Don’t be misled though. How bright an object appears depends not only on its true brightness but also on how far away it is. The Sun is the brightest star in our sky, but that’s because it’s also the closest star to us. Astronomers compare the true brightness of stars by calculating how they would look if viewed from a standard distance of 32.6 light years away. If the Sun were viewed from this distance it would appear over 300 times fainter than Sirius (the brightest star in our nighttime sky) appears to us on Earth. The Sun creates daylight on Earth. If you want to know how daylight compares to other forms of light on Earth, check out our Factory entry on illumination .

The Earth's atmosphere scatters the Sun's light and creates a bright sky, so it is difficult to see the stars in the daytime. Even so, planets can often be seen in the early morning or evening sky. Venus, the brightest of the planets, can often be seen in broad daylight. It is very bright not because of its size - it is slightly smaller than Earth - but because it is close to us and has clouds that reflect almost 75% of the sunlight that falls on it. We can't usually see stars in the daytime unless we use a telescope. Even then only the very brightest stars can be observed. However, if we went out in space, we would be able to see the stars all the time, even close to the direction of the Sun. Here is a picture taken with SOHO-LASCO. In the centre is the Sun but it is blocked out by a disk so that we can see the light shining from the material streaming out from the Sun. The position and size of the Sun are marked by a white circle . The white dots sprinkled across the picture are stars . There’s a comet heading towards the Sun .

SOHO can see the stars and the Sun at the same time because it is out in space and so the Earth's atmosphere does not create the bright background which drowns the light of the stars. Sirius: Brightest Star in Earth's Night Sky Sirius: Brightest Star in Earth's Night Sky By Elizabeth Howell, Space.com Contributor | June 24, 2013 06:07pm ET MORE Follow the belt of Orion to find Sirius. Credit: Starry Night Software Sirius, also known as the Dog Star or Sirius A, is the brightest star in Earth's night sky. The name means "glowing" in Greek. With a visual magnitude of -1.46, the star is outshone only by several planets as well as the International Space Station. Because Sirius is so bright, it was well-known to the ancients. What came as a surprise to astronomers was the discovery of a companion star, Sirius B, in 1862. The companion is so much dimmer than Sirius — 10,000 times, in fact — that it wasn't until 2005 that astronomers were able to estimate its mass.

Locating Sirius Sirius is highly visible in the Northern Hemisphere winter night sky because it has a high relative luminosity to other stars, and it's relatively close to Earth (8.6 light-years). If the star were placed next to Earth's sun, Sirius would outshine it more than 20 times over. To find Sirius, use the belt of Orion as a pointer. The three stars point downward toward Sirius to the left. To be more precise, the position of Sirius is: Right Ascension: 06h 45m 08.9s Declination: -16 degrees 42 minutes 58 seconds Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz. Photograph of Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B. Credit: NASA, ESA, H. Bond (STScI), and M.

Barstow (University of Leicester) Sirius in history Today, Sirius is nicknamed the "Dog Star" because it is part of the constellation Canis Major — Latin for "the greater dog." The expression "dog days" refers to the period from July 3 through Aug. 11 when Sirius rises in conjunction with the sun. The ancients felt that the combination of the sun during the day and the star at night was responsible for the extreme heat that is experienced during mid-summer. The star is present in ancient astronomical records of the Greeks, Polynesians and several other cultures. Perhaps most notably, the Egyptians based their calendar on when Sirius was first visible in the eastern sky, shortly before sunrise. They called it the "Nile Star" because it always returned just before the river rose, and so announced the coming of floodwaters, which would renourish their lands. In 1718, Edmond Halley — when studying Sirius and other stars — discovered that stars have "proper motion" relative to one another . Then in 1862, Alvan Graham Clark discovered Sirius had a faint companion, now known as Sirius B .

Recent observations In 2005, astronomers using the Hubble Space Telescope precisely measured the mass of Sirius B for the first time — a mass that is 98 percent that of the sun , NASA stated. The scientists used a spectrometer aboard the orbiting telescope to measure how its light is affected by gravity on the star. Previous attempts with ground-based observatories were foiled because Sirius itself is so bright, and along with the Earth's atmosphere spreading its light, it washed out its fainter companion (which is a white dwarf ). Astronomers have struggled to confirm a few older measurements of Sirius' system. One 1994 paper in Astronomy and Astrophysics suggested the binary system could actually have a third, tiny companion that rotates around Sirius every six years. Observations in 2008 with a European Southern Observatory 3.6 meter telescope, however, revealed nothing . Some have suggested that Sirius was much redder in the past, based on ancient accounts that described the star as red. Among them was Greek astronomy Claudius Ptolemy in A.D. 150. "There is no doubt about what Ptolemy means.

He bluntly says "reddish, and other ancient sources corroborate him," wrote the University of Houston's R.C. Ceragioli in 1995. However, since the matter first received serious attention at the Royal Society in 1760, astronomers have been unable to explain the discrepancy. The stars would not have evolved so quickly in a few thousand years, but there could be effects from the theoretical third companion. Or, some say, observers could have been describing how Sirius appears when it is close to the horizon and the most affected by Earth's atmosphere. — Elizabeth Howell, SPACE.com Contributor Top 10 Brightest Stars By Nick Greene Updated October 03, 2016. Stars are massive shining spheres of hot gas that exist in all galaxies across the universe. They were among the first objects to form in the infant universe, and they continue to be born in many galaxies, including our Milky Way. The star closest to us is the Sun. The next closest star (at a distance of 4.2 light-years) is Proxima Centauri. All stars are made primarily of hydrogen, smaller amounts of helium, and traces of other elements.

The stars you see with your naked eye in the night sky all belong to the Milky Way galaxy , the huge system of stars that contains our solar system. It contains hundreds of billions of stars, star clusters, and clouds of gas and dust (called nebulae) where stars are born. Here are the 10 brightest stars as seen from Earth. These make excellent stargazing targets from all but the most light-polluted cities. The Stars as Seen from Earth The Stars as Seen from Earth Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay First-time Visitors: Please visit Site Map and Disclaimer . Use "Back" to return here. The Stars as seen from Earth Constellations Every culture has told stories about the stars, and grouped the stars into Constellations. These differ radically from one society to the next; traditional Chinese constellations bear almost no resemblance to Western constellations. Most of the constellations now accepted in astronomy are based on traditional Western and Near Eastern star lore, with a few 17th and 18th-century additions.

At one time, a few stars were parts of two constellations, and about a quarter of the stars in the sky were "unformed"; they belonged to no constellation. Now the constellations have fixed boundaries that preserve, as much as possible, the traditional constellation figures. There are 89 constellations that cover the entire sky. Every star belongs to one, and only one, constellation. The constellation names are Latin. The constellations we see from Earth are patterns of stars more or less in the same direction as we see them from Earth. Most of the objects in a constellation are at widely different distances, and not physically connected with each other. For example, "in" the constellation Virgo, we might find the Moon (240,000 miles away), Jupiter (480 million miles), the bright star Spica (220 light years) and the Virgo Cluster of galaxies (40 million light years). If we were to travel to a distant star, none of the constellations would be recognizable any more. Names of the Stars Only a few of the brightest stars have proper names. Each culture has its own star traditions, and most of our star names were invented by the Arabs.

They came into English when Arabic learning came to Europe in the Middle Ages, often with interesting misspellings. For example, the Arabs called a bright red star in Orion Yad-al-Jawza, the Shoulder of Orion, but the Arabic characters for Y and B look much alike. A medieval scholar mistook the Y for a B when transcribing the name into Roman letters, and Betelgeuse it has been to this day. A few star names originated in other ways. Regulus means "little king" in Latin. Cor Caroli, Latin for "heart of Charles" was named in honor of the deposed and executed Charles I of England. Star Catalogs Most stars are designated in accordance with some star catalog. A common system is that of Johannes Bayer, who in 1603 used Greek letters for the stars in each constellation, usually, but not always, starting with alpha for the brightest star and proceeding roughly in order of decreasing brightness. Since Bayer estimated brightness by eye, his system is only approximate. When he ran out of Greek letters, he used lower-case Roman letters and then capitals.

In Bayer's system, Sirius, the brightest star besides the Sun, is Alpha Canis Majoris, the brightest star in the constellation Canis Major. Later on, in 1725, John Flamsteed employed a system that numbered stars in the order in which they crossed the meridian. In Flamsteed's system Sirius is 9 Canis Majoris, the ninth star in Canis Major. Fainter stars are listed in great catalogues like the Bonn Durchmusterung (Sirius is BD -16 1591 in this catalog) or the Palomar Sky Survey. Usually each star designation has an abbreviation for the catalog and then the catalog number of the star. The overwhelming majority of very faint stars have no designation, unless they happen to be of unusual interest. At times various organizations have offered to name stars after people, in return for a fee. Some such enterprises have been run in jest, to raise money for charity; others are merely aimed at the gullible, but none have any official standing. Colors of the Stars When we look at the night sky, the stars vary in color and brightness. They vary in color from blue-white through white, yellow, and orange to red.

The colors are actually very pale; what we call a red star is actually a pale pinkish-orange. Color in stars is due mostly to differences in temperature: blue-white stars are hottest and red stars are coolest. Brightness of the Stars Stars vary in brightness for two reasons. First, they are at varying distances from us, so that distant stars appear fainter. Second, stars actually do give off different amounts of light. Two bright stars in the summer sky, Altair (16 light years) and Deneb (about 1,600 light years) appear about equally bright, even though Deneb is 100 times farther away. Light intensity drops off in proportion to the square of the distance, so Deneb must be about 100 x 100 = 10,000 times as luminous as Altair. Deneb is a stupendous star; its light left before the fall of Rome, yet it is still a bright star in our sky. It is about 70 million kilometers in diameter (half as big as the orbit of Mercury) and about 50,000 times as luminous as the Sun.

Stars that big don't last long despite their huge mass - Deneb did not exist in the days of the dinosaurs and probably won't last more than a few million years longer. Magnitude Astronomers describe the brightness of objects in the sky in terms of Magnitude. The 20 or so brightest stars in the sky (like Deneb and Altair) are Magnitude 1. Stars just visible to the unaided eye are Magnitude 6. This scale was originally devised by the Greek astronomer Hipparchus over 2,000 years ago, and the modern version is largely an extended and more accurate version of the ancient scale. Faint objects have large magnitude numbers, bright objects have small numbers. Each step of one magnitude corresponds to an increase or decrease of about 2.5 times in brightness; a five-magnitude step means a 100-fold increase or decrease. Unfortunately, very bright objects can have zero or even negative magnitudes, and scales with zero and negative numbers are often very confusing to beginners. The nearest star beyond the Sun, Alpha Centauri, has a magnitude of zero. Sirius, the brightest star besides the sun, has a magnitude of -1.4.

Venus at its brightest has a magnitude of -4, the full Moon about -12, the Sun about -27. Apparent and Absolute Magnitude The Sun is about 25 magnitudes brighter than Sirius, and since each step of five magnitudes corresponds to a brightness step of 100 times, the Sun appears 100 x 100 x 100 x 100 x 100 or 10 billion times brighter than Sirius. If Sirius were a star like the Sun, it would have to be 100,000 times farther away to appear as faint as it does (100,000 x 100,000 = 10 billion). That distance is about 1.5 light years. Sirius is actually quite a bit more luminous than the Sun and is really 8.6 light years away. The brightness of stars as we see them from Earth is called Apparent Magnitude. If we can determine the distances to the stars we can determine their true brightness compared to each other.

Astronomers define the Absolute Magnitude of a star as the brightness it would have at a distance of 10 parsecs or 32.6 light years. The absolute magnitude of the Sun is 4.8; it would be barely visible to the unaided eye 32.6 light years away and entirely invisible 50 light years away, a stone's throw in interstellar terms. This humbling fact suggests that interstellar navigation might just be a bit more challenging than many science-fiction epics lead us to believe. Astronomers use the term Luminosity to refer to the total energy output of stars. Variable Stars Most stars are constant in brightness, but there are many Variable Stars whose brightness varies. There are many types of variable stars. Eclipsing Binaries consist of a faint star orbiting a bright one. Normally we see the combined brightness of both stars, but when the faint star hides the bright one, the brightness drops dramatically. By timing the disappearances of eclipsing binaries, astronomers obtain valuable information about the diameters of the stars. There are many types of stars that pulsate and vary in brightness, some regularly and rhythmically, others spasmodically and unpredictably.

Other stars, called Novae, experience explosive outbursts that may increase the brightness of the star by hundreds of times. Finally there are stars that experience catastrophic explosions that may destroy the star. These stars, called Supernovae, flare briefly to billions of times their normal brightness and are perhaps the most violent events ever witnessed. Alpha Centauri, closest to our sun | Brightest Stars | EarthSky Alpha Centauri, closest to our sun By Larry Sessions in Brightest Stars | May 19, 2016 Third star in the system, a red dwarf called Proxima Centauri, is our sun’s closest neighbor at about 4.22 light-years. The Alpha Centauri system is the closest star system to our sun. On our sky’s dome, we see this multiple system as a single star – the third-brightest star visible from Earth. Follow the links below to learn more about Alpha Centauri. Read more: Proxima Centauri’s awesome new planet Hubble Space Telescope image of Proxima Centauri, the closest known star to the sun. Read more about this image. A comparison of the sizes and colors of the stars in the Alpha Centauri system with our sun.

Via Wikimedia Commons. Alpha Centauri A and B are a double-star system, and a third star Proxima – whose location with respect to the other two is indicated here by arrow – might or might not be gravitationally bound to them. Proxima is the closest star to Earth. Image via the European Southern Observatory. Enjoying EarthSky so far? Sign up for our free daily newsletter today! The Alpha Centauri system probably consists of three stars. Alpha Centauri is part of a double, or triple, star system. The two main components are Alpha Centauri A and Alpha Centauri B. The third star, a red dwarf called Proxima Centauri, is thought to be about 4.22 light-years distant and is actually our sun’s closest neighbor among the stars. Is it part of the Alpha Centauri system? The actual status of Proxima as a system member is unclear. It might simply be passing nearby but not part of the system, or it might be gravitationally bound. Still, we say – and others say – that Alpha Centauri is the closest star to our solar system, with the assumption that Proxima is a true part of the Alpha Centauri system.

If you looked through a small telescope at the Alpha Centauri system, you’d see the two main stars, but you wouldn’t see Proxima Centauri. It’s too faint and appears too far (4 diameters of the full moon) away to be easily recognized at part of the system. Taken by itself, Alpha Centauri A is the fourth-brightest star seen from Earth, just slightly outshone by Arcturus. However, the combined light of Alpha Centauri A and B is slightly greater than Arcturus, so in that sense it is the third-brightest. These stars are an average of 4.3 light-years away. Yellow Alpha Centauri A is the same stellar type as the Sun (G2), although a bit larger. It looks bright in our sky because of its nearness to Earth. Just a few degrees away, the star Hadar (a separate star sometimes called Beta Centauri, not to be confused with Alpha Centauri B) appears dimmer in our sky than Alpha Centauri. But in fact, Hadar is much farther away at 525 light-years.

So you see that Alpha Centauri is not a fabulously brilliant star, as stars go. Alpha Centauri’s surface temperature is a few degrees Kelvin less than our sun (that is, about 5770 k), but its greater diameter (about 25% more than the sun) and the overall larger surface area gives it a luminosity nearly 1.6 times that of our star. The smaller member of the system – orangish Alpha Centauri B – is slightly smaller than our sun, with a spectral type of K2. With lower temperature (about 5,300 K) and only half the luminosity of the sun, B would shine as the 21st brightest star in all the heavens by itself. These two brighter components of the system orbit a common center of gravity once every 80 years. The orbit is notably elliptical, with average distance between the two stars of about 11 A.U., with one A.U. being one Earth-sun distance. Proxima is the closest star to Earth.

Faint red Proxima Centauri – at only 3,100 K and 500 times less bright than our sun – is nearly a fifth of a light year from Alpha Centauri A and B. This great distance is what calls into question its status as part of a triple star system. Proxima Centauri is a pipsqueak of a star. It’s red dwarf star with only about an eighth of the mass of the sun. If Proxima replace the sun in our solar system, it would shine only as brightly as 45 full moons. Proxima is a flare star subject to sudden changes in brightness. But its flares are really feeble. On a brighter star, they might not have been noticed. Proxima’s orbit around the two primary stars in the Alpha Centauri system is thought to take as long as half a billion years. It’s the closest star to our Earth and sun, about a trillion kilometers closer than the other two star in the Alpha Centauri system. Hence its name. If you are in the Southern Hemisphere, you can identify Alpha Centauri using the Southern Cross as a guide.

A line drawn through the crossbar of the Cross, eastward, first comes to Hadar (Beta Centauri), then Alpha Centauri. Image via AstroBob You must be south of 29 degrees N. latitude even to glimpse Alpha Centauri. This chart shows the view from Earth’s Southern Hemisphere. From there, Alpha Centauri’s constellation – Centaurus the Centaur – is circumpolar. It circles around and around the sky’s south pole. How to see Alpha Centauri. Unluckily for us in the Northern Hemisphere, Alpha Centauri is located very far to the south on the sky’s dome. Most North Americans never see it. The cut-off latitude is about 29 degrees north, and anyone north of that is out of luck. In the U.S. the line passes near Houston and Orlando, but even from the Florida Keys, the star never rises more than a few degrees above the southern horizon. Things are a little better in Hawaii and Puerto Rico, where it can get 10 or 11 degrees high. Northern Hemisphere dwellers might glimpse Alpha Centauri at roughly 1 a.m. (local Daylight savings time) in early May.

That is when the star system would be highest above the southern horizon. By early July, the system reaches its highest point to the south at nightfall. Meanwhile, in Australia and much of the southern hemisphere, Alpha Centauri is circumpolar, meaning that it never sets. Also called Rigel Kentaurus or Rigel Kent, this neighboring star is probably the most famous star that almost no one in the northern hemisphere has ever seen. For northern observers, there really are no good pointer stars to Alpha Centauri. When the bright star Arcturus is high overhead, Alpha Centauri might low in your southern sky, assuming you are south of 29 degrees N. latitude. Observers in the tropical and subtropical regions of the Northern Hemisphere can find Alpha Centauri by first identifying the distinctive Southern Cross . A short line drawn through the crossbar (Delta and Beta Crucis) eastward first comes to Hadar (Beta Centauri), then Alpha Centauri. Centaurus the Centaur via ConstellationofWords.com History and mythology of Alpha Centauri. The Alpha Centauri system appears to the eye as a single bright star, the brightest star in the southern constellation Centaurus the Centaur.

Two alternative names for this star, Toliman and Bungula, are rarely used any more. The derivations are somewhat questionable, but Toliman may be from the Arabic for ostriches and Bungula apparently derives from Latin meaning hoof. Thousands of years ago, a motion of Earth called precession – which causes the identity of the Pole Star to change over time – caused Alpha Centauri to appear higher in the sky as seen from the Northern Hemisphere than it does now. But it was still far to the south and often difficult to see. Classical myth-makers didn’t spend much time with this constellation, although it was thought to represent an uncharacteristically wise centaur that figured in the mythology of Heracles and Jason. The centaur was accidentally wounded by Heracles, and placed into the sky after death by Zeus. Alpha Centauri itself marked the right front hoof of the Centaur, although little is known of its mythological significance, if any. Ancient Egyptians revered it, and may have built temples aligned to its rising point. In southern China it was part of a star group known as the South Gate.

Astronomers determined the distance to the Alpha Centauri system for the first time in 1839, only a few months after they made the first-ever distance determination to a star (61 Cygni). Bottom line: The Alpha Centauri system is the closest star system to our sun. On our sky’s dome, we see this multiple system as a single star – the third-brightest star visible from Earth. Charts, photos and info about this fascinating star system. Alpha Centauri’s position is RA: 14h 39m 41s, dec: -60° 50′ 07″ What is the brightest star? Audio recording of Dr. Marc reading this page. Click play to hear me read this to you! Our friends at the North Museum Planetarium in Lancaster, Pennsylvania, wonder, "What is the brightest star?" Of course, the star that appears the brightest to all of us on Earth is the Sun. Although it is a rather typical star, not all that different from many of the ones you see at night, we live so close to it that it outshines everything else.

Even the next closest star is more than a quarter of a million times farther from Earth, so it is not surprising that the light from the Sun overwhelms that from other stars. It might be more fun to think about how bright the stars are themselves, without regard to how they look from Earth. After all, as much as we cherish Earth, our planet is not in a special place in the universe, so it isn't really fair to rank the Sun as the brightest only because it is nearest Earth. Suppose we could put all stars at the same distance from us. Then which one would be the brightest? Imagine you are in a boat on the ocean at night, and you see a light. How would you know how bright the light really is? Is it a weak light on a nearby boat or a brilliant one on a distant island? If you know how far it is, then you have an idea of how bright it really is. But now suppose the night is foggy. In that case, it is much harder to judge the true brightness of the light, because you don't know how much of the light is blocked by the fog. To find out the true brightness of a star, scientists need to know how far it is.

Although there are some very clever ways of gauging the distances to stars, they generally work well only for stars that are in the Sun's neighborhood of the Milky Way galaxy. The more distant stars are just so fantastically far from us, that measuring their distances accurately is too difficult. Making it still harder to know how bright a star really is, there is a kind of patchy fog between the stars - space is not truly empty. Although it is not exactly the same as the fog on Earth, gas and dust in space can dim the light of stars. Without a good way to know how much of this interstellar fog is blocking the light, there is no reliable way to discover the true brightness of a star. Although we don't know which star truly is the brightest, we know some are remarkably bright. You can see one of them any clear night this summer. Deneb is the northeastern of the three stars that form a large and easily seen grouping called the Summer Triangle. While Deneb shines the brightest in the constellation Cygnus, 17 other stars glow brighter in our night skies.

But Deneb is much farther from Earth than most of the other stars you see, and this giant is around 100,000 times brighter than the Sun. If Deneb were the same distance from Earth as Vega, another star in the Summer Triangle, not only would it outshine all the stars and planets visible at night, but it would even be bright enough to see in the daytime! Deneb pays a precious price for shining so brilliantly. It is using up its stellar fuel at a furious rate, and it will burn itself out after a relatively brief appearance as a star of our galaxy. While the Sun (and Earth) are more than 4.5 billion years old, Deneb is a youngster at only a few million years. And it will last only a few million years more before it dies a spectacular death. In the meantime, less showy but more typical stars like the Sun will continue their more tranquil lives, ultimately shining for perhaps 1000 times as long as a powerhouse like Deneb. You can find Deneb and other wonders of the night sky with a Star Finder. Make yours for this month here.

Remember in the winter, when you are gazing at the constellation Orion, to think about the two brightest stars there: reddish Betelgeuse and white Rigel. Like Deneb, they could outshine the Sun like a searchlight next to a match. How far is the closest star? | The Sun's Nearest Neighbor How far is the closest star? Hubble image of Proxima Centauri, the closest star to the Sun. NASA The Sun is the closest star to Earth, about 93 million miles away. The Sun’s nearest neighbor, Alpha Centauri, is actually a triple-star system — three stars bound together by gravity. Alpha Centauri A and B are two bright, closely orbiting stars with a distant, dim companion, Proxima Centauri. The inner binary appears to the unaided eye as a single star, the third brightest in the night sky , but it lies 4.37 light years from the Sun. Proxima Centauri alone claims the honor of being our true nearest stellar neighbor at only 4.24 light years away. It’s difficult to conceptualize such vast distances, but a popular analogy sets the Sun at the size of a grapefruit.

If you wanted to get from your grapefruit-sized Sun to a grapefruit-sized Alpha Centauri system, you would have to travel about 2,500 miles, which is about the distance from coast to coast on the continental United States. And that’s just to the Sun’s closest neighbor! But Proxima Centauri is only currently the closest star. The Sun, the Alpha Centauri system, and other nearby stars all move around the Milky Way over time, and they approach and pass each other as they travel. In another 10,000 or so, the closest star will be something else. Brightest Stars: Luminosity & Magnitude Brightest Stars: Luminosity & Magnitude By Elizabeth Howell, Space.com Contributor | June 19, 2013 07:26pm ET MORE This wide-field view of the sky around the bright star Alpha Centauri was created from photographic images forming part of the Digitized Sky Survey 2. The star appears so big just because of the scattering of light by the telescope's optics as well as in the photographic emulsion. Alpha Centauri is the closest star system to the Solar System. Image released Oct.

17, 2012. Credit: ESO/Digitized Sky Survey 2 A glance at the night sky above Earth shows that some stars are much brighter than others. However, the brightness of a star depends on its composition and how far it is from the planet. Astronomers define star brightness in terms of apparent magnitude (how bright the star appears from Earth) and absolute magnitude (how bright the star appears at a standard distance of 32.6 light years, or 10 parsecs). Astronomers also measure luminosity — the amount of energy (light) that a star emits from its surface. Measuring star brightness is an ancient idea, but today astronomers use more precise tools to obtain the calculation. Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz. 0 of 10 questions complete Star Quiz: Test Your Stellar Smarts Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.

0 of Start Over | More Quizzes From Greek to modern times More than 2,000 years ago, the Greek astronomer Hipparchus was the first to make a catalog of stars according to their brightness, according to Dave Rothstein, who participated in Cornell University's " Ask An Astronomer " website in 2003. "Basically, he looked at the stars in the sky and classified them by how bright they appear — the brightest stars were 'magnitude 1', the next brightest were 'magnitude 2', etc., down to 'magnitude 6', which were the faintest stars he could see," Rothstein wrote. Human eyes, however are not very discerning. Large differences in brightness actually appear much smaller using this scale, Rothstein said. Light-sensitive charged-coupled devices (CCDs) inside digital cameras measure the amount of light coming from stars, and can provide a more precise definition of brightness. Using this scale, astronomers now define five magnitudes' difference as having a brightness ratio of 100. Vega was used as the reference star for the scale. Initially it had a magnitude of 0, but more precise instrumentation changed that to 0.3.

Orion is the brightest and most beautiful of the winter constellations. Some of its stars, including Betelgeuse and Rigel, are among the brightest stars. Credit: Starry Night Software Apparent magnitude vs. absolute magnitude When taking Earth as a reference point, however, the scale of magnitude fails to account for the true differences in brightness between stars. The apparent brightness, or apparent magnitude, depends on the location of the observer. Different observers will come up with a different measurement, depending on their locations and distance from the star. Stars that are closer to Earth, but fainter, could appear brighter than far more luminous ones that are far away. "It is the 'true' brightness — with the distance dependence factored out — that is of most interest to us as astronomers," stated an online course on astronomy from the University of Tennessee. "Therefore, it is useful to establish a convention whereby we can compare two stars on the same footing, without variations in brightness due to differing distances complicating the issue." This wide-field view of the sky around the bright star Alpha Centauri was created from photographic images forming part of the Digitized Sky Survey 2.

The star appears so big just because of the scattering of light by the telescope's optics as well as in the photographic emulsion. Alpha Centauri is the closest star system to the Solar System. Image released Oct. 17, 2012. Credit: ESO/Digitized Sky Survey 2 The solution was to implement an absolute magnitude scale to provide a reference between stars. To do so, astronomers calculate the brightness of stars as they would appear if it were 32.6 light-years, or 10 parsecs from Earth. Another measure of brightness is luminosity, which is the power of a star — the amount of energy (light) that a star emits from its surface. It is usually expressed in watts and measured in terms of the luminosity of the sun. For example, the sun's luminosity is 400 trillion trillion watts. One of the closest stars to Earth, Alpha Centauri A , is about 1.3 times as luminous as the sun.

To figure out luminosity from absolute magnitude, one must calculate that a difference of five on the absolute magnitude scale is equivalent to a factor of 100 on the luminosity scale — for instance, a star with an absolute magnitude of 1 is 100 times as luminous as a star with an absolute magnitude of 6. Limitations of absolute magnitude While the absolute magnitude scale is astronomers' best effort to compare the brightness of stars, there are a couple of main limitations hat have to do with the instruments that are used to measure it. First, astronomers must define which wavelength of light they are using to make the measurement. Stars can emit radiation in forms ranging from high-energy X-rays to low-energy infrared radiation. Depending on the type of star, they could be bright in some of these wavelengths and dimmer in others. To address this, scientists must specify which wavelength they are using to make the absolute magnitude measurements. Another key limitation is the sensitivity of the instrument used to make the measurement. In general, as computers have advanced and telescope mirror technology has improved over the years, measurements that are made in recent years have more weight among scientists than those that are made long ago.

Paradoxically, the brightest stars are among the least studied by astronomers, but there is at least one recent effort to catalog their luminosity. A constellation of satellites called BRITE (BRight Target Explorer) will measure the variability of brightness between stars. Participants in the six-satellite project include Austria, Canada and Poland. The first two satellites launched successfully in 2013. Top 26 brightest stars, as seen from Earth Common How Far, How Bright HOW FAR, HOW BRIGHT * Total energy radiated per second ("wattage") * Luminosities of a few stars 3. Brightness * Flux: light energy flowing per second per sq meter * Inverse-square law of light: * Flux (F) & distance (d) ==> luminosity * Sun' flux at Earth (1370 W/m2) ==> solar luminosity 4. Distance 1. Ques. #3, pg. 268. 2. What does a star's flux measure? 3. a) A star with the Sun's luminosity, but located 2 AU from Earth instead of 1 AU, would appear how bright relative to the Sun?

b) A star with the Sun's luminosity, but located 20 AU from Earth instead of 1 AU, would appear how bright relative to the Sun? 4. Jupiter's moon Europa, which effectively lies at the same distance from the Sun as Jupiter (5.2 AU) receives how much sunlight per square meter, relative to the amount received by Earth? 5. a) In the distant future, the sun will likely become 100 times more luminous than at present (when it expands to become a 'giant star'). What will be the solar flux (in watts/m2) at Earth at that time? b) How far from the sun would we have to move so that the sun's flux would be equal to its present value? 6. A certain star is known to be equal in luminosity to the Sun, but its measured flux is only 1/10,000 (one ten-thousandth) the Sun's flux. What would be the distance to this star, in AU? 7. Two stars, A and B, are known to be equal in luminosity, but A appears 16 times brighter (as viewed from Earth) than B.

Which one is more distant, and how much farther away is it than the other? 8. Tripling the luminosity of the star Sirius while also tripling its distance from Earth would have what effect on its apparent brightness, as viewed from Earth? 9. Ques. #7, pg. 268. 10. Prob. #1, pg. 268. 11. a) What is the parallax of Betelgeuse, which lies at a distance of 130 pc from the Sun? b) Would Betelgeuse's parallax change significantly were we to measure its distance from Earth, instead of "from the Sun," as we do in part a) of this question? Explain your answer. 12. Why do stellar parallax measurements work only with nearby stars? 13. a) How would the parallax of a star measured from Earth compare with the parallax of the same star measured from Mars? (E.g., Would the parallaxes be identical, or different?) Explain your answer. b) Suppose the smallest parallax we can measure accurately is 0.01 arcsec (whether we measure from Earth, or Jupiter, or wherever).

Smallest parallax corresponds to greatest distance (since distance is inversely proporational to parallax). Could we measure greater distances accurately from Mars (using stellar parallax) than we can from Earth? Explain your answer. Answers 1. Luminosity measures the total energy radiated by the star per second, at all wavelengths. The standard luminosity unit is the watt. The luminosity of a star in units of Lsun tells us how luminous the star is relative to the Sun. For example: a star with luminosity 0.25 Lsun would be one-quarter the Sun's luminosity. 2. Flux is a measure of the amount of energy flowing per second through a unit area (e.g., one square meter) due to a star (or any other luminous source). 3. a) This star would appear 1/4 as bright as the sun; i.e., its flux would be 1/4 the sun's flux: F a 1/d2 = 1/22 = 1/4. Note that 2 is the factor by which the distance changed, not the absolute value of the distance.

b) This star would appear 1/400 as bright as the Sun: 1/400 = 1/(202) 4. 1/5.22 = 1/27. So, Europa receives about 1/27 as much solar energy per square meter as does Earth. 5. a) The solar flux will be 100 times its present value: F a L = 100 (no change in distance). To convert this into watt/m2, multiply by 1370 watt/m2: f = 100 x 1370 = 137,000 watt/m2. b) We'd have to move to a distance of 10 AU from Sun (10 times Earth's current distance). The inverse-square law predicts that an increase in distance from a source by a factor of 10 will reduce the flux by a factor of 100.

As the future Sun's flux is 100 times the present sun's flux, we need to move to a point 10 times farther away to reduce the flux by a factor of 100, thus reducing it to its present value: F a L/d2 ==> d2 a L//F = 100/1 = 100 ==> d = 10 (AU). 6. This star would lie at a distance of 100 AU from Earth. Increasing the distance by a factor of 100 decreases the flux by a factor of 10,000: F a 1/d2 = 1/(1002) = 1/10,000. 7. Because B appears 16 time dimmer than A (its flux is 1/16 A's flux), B must lie 4 times farther from us than A: F a 1/d2 = 1/(42) = 1/16. 8. The apparent brightness of Sirius would be 1/3 its current value: F a L/d2 = 3/32 = 3/9 = 1/3. 9.

Here's a diagram showing the relationship between the distance to a star (d) and the star's parallax angle (p): . The angle is measured by observing a star twice, the observations occurring six months apart. The target star (here the red star) appears to shift against the background of more distant stars (the green ones). One-half of this apparent shift (the "parallax") is the parallax angle. Parallax angle and distance are simply related: d = 1/p. In this equation, p must be measured in arcseconds (1 arcsec = 1/3600 degree). By measuring distances in this way, we establish a new distance unit: when p = 1 arcsec, the distance is 1 parsec (pc). 10. a) d = 1/p = 1/0.742 = 1.35 pc. b) d = 1/p = 1/0.286 = 3.50 pc. 11.

a) p = 1/d = 1/130 = 0.0077 arcsec b) Betelgeuse's parallax would change by an amount that is so small as to be unmeasurable, because the distance between Earth and the Sun is miniscule compared to 130 parsecs. 12. Getting a distance from stellar parallax requires the measurement of a very small angle; as distance increases, the size of this angle decreases. Eventually, we arrive at angles that are too small to measure. This occurs at a distance of 100 - 200 parsecs. 13. a) The parallax measured from Mars would be larger because Mars' orbit is larger than Earth's orbit (thus the baseline for the parallax triangle measured from Mars would be larger). b) We should be able to measure greater distances more accurately from Mars than from Earth. As the baseline of the parallax triangle is larger from Mars, the smallest parallax angle naturally corresponds to a greater distance.

The 26 Nearest Stars 9.6 M1.3Vc If directions to these stars were included, you could make a map and see that they are distributed around us more or less randomly. What can we learn from this information? Quite a lot. We live very close (500 lightseconds) to a star. This is probably a necessary condition for the originination and maintenance of life. Stars are very far apart (average about 8 lightyears for the closest dozen), compared to their size (about 2 lightseconds for the Sun); by a factor of 250 million or so. Many stars occur in multiple systems, shown here by the suffixes A, B and C from brightest to dimmest. In fact, about 55% of stars in this list are in multiple systems. The nearest star is a triple. And we may be missing many dim stars. Most of the nearby stars are dimmer (higher numbers for absolute magnitude) than our Sun, by factors of 100 to 10,000.

Question: Besides the sun, which star appears brightest from Earth?
Answer: Sirius