tag:blogger.com,1999:blog-55237103960307044772024-03-13T13:16:06.560-07:00Guide To AstronomyBlue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.comBlogger224125tag:blogger.com,1999:blog-5523710396030704477.post-12513603955564850702011-05-31T22:51:00.000-07:002011-05-31T22:54:18.316-07:00The Life Expectancy of a Star<img src="http://everytrueword.files.wordpress.com/2009/12/supernova_logo1-28000715.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />A star dies when it consumes its nuclear fuel, its mass. We might be tempted to conclude that the greater the supply of fuel (the more massive the star), the longer it will live; however, a star’s life span is also determined by how rapidly it burns its fuel. The more luminous a star, the more rapid the rate of consumption. Thus stellar lifetime is directly proportional to stellar mass and inversely proportional to stellar luminosity (how fast it burns). An analogy: A car with a large fuel tank (say a new Ford Excursion that gets 4–8 mpg) may have a much smaller range than a car with a small fuel tank (a Saturn which might get 30–40 mpg). The key? The Saturn gets much better mileage, and thus can go farther with the limited fuel it has.<br />Thus, while O- and B-type giants are 10 to 20 times more massive than the our G-type sun, their luminosity is thousands of times greater. Therefore, these most massive stars live much briefer lives (a few million years) than those with less fuel but more modest appetites for it.<br />A B-type star such as Rigel, 10 times more massive than the sun and 44,000 times more luminous, will live 20 106 years, or 20 million years. For comparison, 65 million years ago, dinosaurs roamed the earth! The G-type sun may be expected to burn for 10,000 106 years (ten billion years). Our red dwarf neighbor, Proxima Centauri, an M-type star that is 1⁄10 the mass of the sun (and 1⁄100 that of Rigel), is only 0.00006 times as luminous as the sun, so will consume its modest mass at a much slower rate and may be expected to live more than the current age of the universe. In the next two chapters we will see how stars go through their lives, and how they grow old and die.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-47589942791639388782011-05-31T22:48:00.001-07:002011-05-31T22:50:42.426-07:00Understanding Stellar Mass<img src="http://blackholes.stardate.org/images/smbh_art_imp_BH01_H.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />The overall orderliness of the main sequence suggests that the properties of stars are not random. In fact, a star’s exact position on the main sequence and its evolution are functions of only two properties: composition and mass.<br />Composition can be evaluated if we have a spectrum of the star, its fingerprint. But how can we determine the mass of a star?<br />Fortunately, most stars don’t travel solo, but in pairs known as binaries. (Our sun is an exception to this rule.) Binary stars orbit one another.<br />Some binaries are clearly visible from the earth and are called visual binaries, while others are so distant that, even with powerful telescopes, they cannot be resolved into two distinct visual objects. Nevertheless, these can be observed by noting the Doppler shifts in their spectral lines as they orbit one another. These binary systems are called spectroscopic binaries. Rarely, we are positioned so that the orbit of one star in the binary system periodically brings it in front of its partner. From these eclipsing binaries we can monitor the variations of light emitted from the system, thereby gathering information about orbital motion, mass, and even stellar radii.<br />However we observe the orbital behavior of binaries, the key pieces of information sought are orbital period (how long it takes one star to orbit the other) and the size of the orbit. Once these are known, Kepler’s third law can be used to calculate the combined mass of the binary system.<br />Why is mass so important? Mass determines the fate of the star. It sets the star’s place along the main sequence and it also dictates its life span.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-85074094228085861072011-03-31T23:29:00.000-07:002011-03-31T23:31:41.327-07:00Making the Main Sequence<img src="http://old.orionsarm.com/eg/h/H-R.jpg" style="margin: 0px auto 10px; display: block; width: 550px; text-align: center;" border="0" /><br />Working independently, two astronomers, Ejnar Hertsprung (1873–1967) of Denmark and Henry Norris Russell (1877–1957) of the United States studied the relationship between the luminosity of stars and their surface temperatures. Their work (Hertsprung began about 1911) was built on the classification scheme of another woman from the Harvard College Observatory, Antonia Maury. She first classified stars both by the lines observed and the width or shape of the lines. Her scheme was an important step toward realizing that stars of the same temperature could have different luminosity. Plotting the relationship between temperature and luminosity graphically (in what is now known as a Hertzsprung-Russell diagram or H-R diagram), these two men discovered that most stars fall into a well-defined region of the graph. That is, the hotter stars tend to be the most luminous, while the cooler stars are the least luminous.<br />The region of the temperature luminosity plot where most stars reside is called the main sequence. Most stars are there, because as we will discover, that is where they spend the majority of their lives. Stars that are not on the main sequence are called giants or dwarfs, and we will see how stars leave the main sequence and end up in the far corners of the temperature-luminosity plot.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-33599115474579963842011-03-31T23:27:00.000-07:002011-03-31T23:28:41.664-07:00Sorting the Stars by Size<img src="http://www.kiroastro.com/images/perspective/sun2.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />The radius of a star can be determined from the luminosity of the star (which can be determined if the distance is known) and its surface temperature (from its spectral type). It turns out that stars fall into several distinct classes. In sorting the stars by size, astronomers use a vocabulary that sounds as if it came from a fairytale:<br /><ul><li>A giant is a star whose radius is between 10 and 100 times that of the sun. </li><li>A supergiant is a star whose radius is more than 100 times that of the sun. Stars of up to 1,000 solar radii are known.</li><li>A dwarf star has a radius similar to or smaller than the sun.</li></ul>Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-53442482701237657122011-03-31T23:24:00.000-07:002011-03-31T23:26:57.246-07:00How Hot Is Hot?<img src="http://www.universetoday.com/wp-content/uploads/2010/02/The-Suns-Chromosphere.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Stars are too distant to stick a thermometer under their tongue. We can’t even do that with our own star, the sun. But you can get a pretty good feel for a star’s temperature simply by looking at its color.<br />The temperature of a distant object is generally measured by evaluating its apparent brightness at several frequencies in terms of a blackbody curve. The wavelength of the peak intensity of the radiation emitted by the object can be used to measure the object’s temperature. For example, a hot star (with a surface temperature of about 20,000 K) will peak near the ultraviolet end of the spectrum and will produce a blue visible light. At about 7,000 K, a star will look yellowish-white. A star with a surface temperature of about 6,000 K—such as our sun—appears yellow. At temperatures as low as 4,000 K, orange predominates, and at 3,000 K, red.<br />So simply looking at a star’s color can tell you about its relative temperature. A star that looks blue or white has a much higher surface temperature than a star that looks red or yellow.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-55776186775910099552011-02-28T22:31:00.000-08:002011-02-28T22:33:52.662-08:00Creating a Star Scale of Magnitude<img src="http://sci.esa.int/science-e-media/img/20/apparent-magnitude.jpg" style="margin: 0px auto 10px; display: block; width: 400px; text-align: center;" border="0" /><br />So astronomers have learned to be very careful when classifying stars according to apparent brightness. Classifying stars according to their magnitude seemed a good idea to Hipparchus (in the second century B.C.E.) when he came up with a 6-degree scale, ranging from 1, the brightest stars, to 6, those just barely visible. Unfortunately, this somewhat cumbersome and awkward system (higher magnitudes are fainter, and the brightest objects have negative magnitudes) has persisted to this day.<br />Hipparchus’ scale has been expanded and refined over the years. The intervals between magnitudes have been regularized, so that a difference of 1 in magnitude corresponds to a difference of about 2.5 in brightness. Thus, a magnitude 1 star is 2.5 ×2.5 ×2.5 ×2.5 ×2.5=100 times brighter than a magnitude 6 star. Because we are no longer limited to viewing the sky with our eyes, and larger apertures collect more light, magnitudes greater than (that is, fainter than) 6 appear on the scale. Objects brighter than the brightest stars may also be included, their magnitudes expressed as negative numbers. Thus the full moon has a magnitude of –12.5 and the sun, –26.8. In order to make more useful comparisons between stars at varying distances, astronomers differentiate between apparent magnitude and absolute magnitude, defining the latter, by convention, as an object’s apparent magnitude when it is at a distance of 10 parsecs from the observer. This convention cancels out distance as a factor in brightness and is therefore an intrinsic property of the star.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-32652427267916585502011-02-28T22:30:00.001-08:002011-02-28T22:30:52.015-08:00Luminosity Versus Apparent Brightness<img src="http://www.interlinear.info/brightstar2.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Ask an astronomer this question, and she will respond that the flashlight, a few feet from your eyes, is apparently brighter than the distant headlights, but that the headlights are more luminous. Luminosity is the total energy radiated by a star each second. Luminosity is a quality intrinsic to the star; brightness may or may not be intrinsic. Absolute brightness is another name for luminosity, but apparent brightness is the fraction of energy emitted by a star that eventually strikes some surface or detection device (including our eyes). Apparent brightness varies with distance. The farther away an object is, the lower its apparent brightness.<br />Simply put, a very luminous star that is very far away from the earth can appear much fainter than a less luminous star that is much closer to the earth. Thus, although the Sun is the brightest star in the sky, it is not by any means the most luminous.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-51568562902546311482011-02-28T22:25:00.000-08:002011-02-28T22:29:46.252-08:00Do Stars Move?<img src="http://www.fotosearch.com/bthumb/TBZ/TBZ135/st01p001.jpg" style="margin: 0px auto 10px; display: block; width: 150px; text-align: center;" border="0" />The ancients believed that the stars were embedded in a distant spherical bowl and moved in unison, never changing their relative positions. We know now, of course, that the daily motion of the stars is due to the earth’s rotation. Yet the stars move, too; however, their great distance from us makes that movement difficult to perceive, except over long periods of time. A jet high in the sky, for example, can appear to be moving rather slowly, yet we know that it has to be moving fast just to stay aloft and its apparent slowness is a result of its distance. Astronomers think of stellar movement in three dimensions:<br /><ul><li>The transverse component of motion is perpendicular to our line of sight—that is, movement</li><li>across the sky. This motion can be measured directly.</li><li>The radial component is stellar movement toward or away from us. This motion must be measured from a star’s spectrum.</li><li>The actual motion of a star is calculated by combining the transverse and radial components.</li></ul>The transverse component can be measured by carefully comparing photographs of a given piece of the sky taken at different times and measuring the angle of displacement of one star relative to background stars (in arcseconds).<br />This stellar movement is called proper motion. A star’s distance can be used to translate the angular proper motion thus measured into a transverse velocity in km/s. In our analogy: If you knew how far away that airplane in the sky was, you could turn its apparently slow movement into a true velocity.<br />Determining the radial component of a star’s motion involves an entirely different process. By studying the spectrum of the target star (which shows the light emitted and absorbed by a star at particular frequencies), astronomers can calculate the star’s approaching or receding velocity. Certain elements and molecules show up in a star’s spectrum as absorption lines (see Chapter 7). The frequencies of particular absorption lines are known if the source is at rest, but if the star is moving toward or away from us, the lines will get shifted. A fast-moving star will have its lines shifted more than a slow-moving one. This phenomenon, more familiar with sound waves, is known as the Doppler effect.<br />How fast do stars move? And what is the fixed background against which the movement can be measured? For a car, it’s easy enough to say that it’s moving at 45 miles per hour relative to the road. But there are no freeways in space. Stellar speeds can be given relative to the earth, relative to the sun, or relative to the center of the Milky Way. Astronomers always have to specify which reference frame they are using when they give a velocity. Stars in our neighborhood typically move at tens of kilometers per second relative to the sun.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-75401762781089550662011-01-31T21:30:00.000-08:002011-01-31T21:47:04.249-08:00Nearest and Farthest<img src="http://img.dailymail.co.uk/i/pix/2008/03_02/AlphaCentauri_468x318.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Other than the sun, the star closest to us is Alpha Centauri, which has the largest known stellar parallax of 0.76 arc seconds. In general, the distance to a star in parsecs (abbreviated pc) is equal to 1 divided by the stellar parallax in arcseconds—or conversely, its parallax will be equal to 1 divided by the distance in parsecs. The measured parallax, in any case, will be a very small angle (less than an arcsecond). Recall that the moon takes up about 1,800” on the sky when full, so the parallax measured for Alpha Centauri is about 1⁄2000 the diameter of the full moon! Using the rule above to convert parallax into distance, we find that Alpha Centauri is about 1.3 pc or 4.2 light-years away. On average, stars in our Galaxy are separated by 7 light-years. So Alpha Centauri is even closer than “normal.” If a star were 10 pc away, it would have a parallax of 1⁄10 or 0.1”.<br />The farthest stellar distances that can be measured using parallax are about 100 parsecs (333 light-years). Stars at this distance have a parallax of 1⁄100” or 0.01”. That apparent motion is the smallest that we can measure with our best telescopes. Within our own Galaxy, most stars are even farther away than this. As telescope resolutions improve with the addition of adaptive optics, this outer limit will be pushed farther out.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-50548666128157788172011-01-31T21:27:00.000-08:002011-01-31T21:29:58.423-08:00How Far Away Are the Stars?<img src="http://www.windows2universe.org/images/star_distance.gif" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Like the campsite separated from you by the Grand Canyon, the stars are not directly accessible to measurement. However, if you can establish two view points along a baseline, you can use triangulation to measure the distance to a given star.<br />There is just one problem.<br />Take a piece of paper. Draw a line one inch long. This line is the baseline of your triangle. Measure up from that line, say, one inch, and make a point. Now connect the ends of your baseline to that point. You have a nice, normal looking triangle. But if you place your point several feet from the baseline, then connect the ends of the baseline to it, you will have an extremely long and skinny triangle, with angles that are very difficult to measure accurately, because they will both be close to 90 degrees. If you move your point several miles away, and keep a 1-inch baseline, the difference in the angles at Points A and B of your baseline will be just about impossible to measure. They will both seem like right angles. For practical purposes, a 1-inch baseline is just not long enough to measure distances of a few miles away. Now recall that if our Earth is a golf ball (about 1 inch in diameter), that the nearest star, to scale, would be 50,000 miles away. So the baseline created by, say, the rotation of the earth on its axis—which would give 2 points 1 inch away in our model—is not nearly large enough to use triangulation to measure the distance to the nearest stars. The diameter of the earth is only so wide. How can we extend the baseline to a useful distance?<br />The solution is to use the fact that our planet not only rotates on its axis, but also orbits the sun. Observation of the target star is made, say, on February 1, then is made again on August 1, when the earth has orbited 180 degrees from its position six months earlier. In effect, this motion creates a baseline that is 2 A.U. long—that is, twice the distance from the earth to the sun. Observations made at these two times (and these two places) will show the target star apparently shifted relative to the even more distant stars in the background. This shift is called stellar parallax, and by measuring it, we can determine the angle relative to the baseline and thereby use triangulation to calculate the star’s distance.<br />To get a handle on parallax, hold your index finger in front of you, with your arm extended. Using one eye, line up your finger with some vertical feature, say the edge of the window. Now, keeping you finger where it is, look through the other eye. The change in viewpoint makes your finger appear to move with respect to a background object. In astronomy, your eyes are the position of the earth separated by 6 months, your finger is a nearby star, and the window edge is a distant background star. This method works as long as the star (your finger) is relatively close. If the star is too far away, parallax is no longer effective.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-24173244186914532362011-01-31T21:23:00.000-08:002011-01-31T21:27:03.140-08:00The Parallax Principle<img src="http://www.astro.ucla.edu/%7Ewright/parallax.gif" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />First, how do we know that the nearest stars are so far away? For that matter, how do we know how far away any stars are? We’ve come a long way in this blog, and, on our journey, we have spoken a good deal about distances—by earthly standards, often extraordinary distances. Indeed, the distances astronomers measure are so vast that they use a set of units unique to astronomy. When measuring distances on the earth, meters and kilometers are convenient units. But in the vast spaces between stars and galaxies, such units are inadequate. As we’ll see in this chapter and those that follow, the way astronomers measure distances, and the units they use depend upon how far away the objects are. Distances between a given point on the earth and many objects in the solar system can be measured by radar ranging. Radar, a technology developed shortly before and during World War II, is now quite familiar. Radar can be used to detect and track distant objects by transmitting radio waves, then receiving the echo of the waves the object bounces back (sonar is a similar technique using sound waves). If we multiply the round-trip travel time of the outgoing signal and its incoming echo by the speed of light (which, you’ll recall, is the speed of all electromagnetic radiation, including radio waves), we obtain a figure that is twice the distance to the target object.<br />Radar ranging works well with objects that return (bounce back) radio signals. But stars, including our sun, tend to absorb rather than return electromagnetic radiation transmitted to them. Moreover, even if we could bounce a signal off a star, most are so distant that we would have to wait thousands of years for the signal to make its round trip—even at the speed of light! Even the nearby Alpha Centauri system would take about eight years to detect with radar ranging, were it even possible.<br />Another method is used to determine the distance of the stars, a method that was available long before World War II. In fact, it is at least as old as the Greek geometer Euclid, who lived in the third century B.C.E. The technique is called triangulation—an indirect method of measuring distance derived by geometry using a known baseline and two angles from the baseline to the object. Triangulation does not require a right triangle, but the establishment of one 90-degree angle does make the calculation of distance a bit easier. It works like this. Suppose you are on one rim of the Grand Canyon and want to measure the distance from where you are standing to a campsite located on the other rim. You can’t throw a tape measure across the yawning chasm, so you must measure the distance indirectly. You position yourself directly across from the campsite, mark your position, then turn 90 degrees from the canyon and carefully pace off another point a certain distance from your original position. This distance is called your baseline. From this second position, you sight on the campsite. Whereas the angle formed by the baseline and the line of sight at your original position is 90 degrees (you arranged it to be so), the angle formed by the baseline and the line of sight at the second position will be somewhat less than 90 degrees. If you connect the campsite with Point A (your original viewpoint) and the campsite with Point B (the second viewpoint), both of which are joined by the baseline, you will have a right triangle. Now, you can take this right triangle and, with a little work, calculate the distance across the canyon. If you simply make a drawing of your setup, making sure to draw the angles and lengths that you know to scale, you can measure the distance across the canyon from your drawing. Or if you are good at trigonometry, you can readily use the difference between the angles at Points A and B and the length of the baseline to arrive at the distance to the remote campsite.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-21547412715184062972010-12-31T23:29:00.000-08:002010-12-31T23:30:52.800-08:00Radius, Luminosity, Temperature: A Key Relationship<img src="http://www.symmetrymagazine.org/images/200602/article13_image1.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br /><br />We don’t have to give up on measuring the sizes of stars, however. We just have to be more clever. What astronomers do is determine the temperature and mass of a star, which can be done using a star’s color and spectrum. Then, using numerical models of how stars hold together, they derive the quantity that they are interested in (radius, for example). It is akin to looking out over a parking lot and seeing a Cadillac. Now, you may not know its size, but you know (consulting a chart) that this model of Cadillac is 18.5 feet long. You can see clearly that it is indeed this particular model of Cadillac, so you know its length, even though you didn’t actually measure it with a ruler.<br />Stefan’s law states that a star’s luminosity (its wattage, or the rate at which it emits energy into space) is proportional to the fourth power to the star’s surface temperature. This relationship can be extended further. A star’s luminosity is not only related to its temperature, but to its surface area. Heat the head of a pin to 400 degrees F and a large metal plate to the same temperature. Which will radiate more heat? Obviously, the object with the larger surface area. Given the same surface temperature, a larger body will always radiate more energy than a smaller one.<br />This relationship can be expressed in this way: A star’s luminosity is proportional to the square of its radius (that’s the surface area term) times its surface temperature to the fourth power (luminosity ×radius2 ×temperature4). Thus, if we know a star’s luminosity and temperature (which can be measured by available astronomical instruments), we can calculate its radius. How do we measure a star’s luminosity and temperature? Let’s see.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-46913045321550613552010-12-31T23:26:00.000-08:002010-12-31T23:28:00.060-08:00Your Standard Solar Model<img src="http://www.intuitive.com/blog/images/ssi-sun-model.png" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br /><br />By combining theoretical modeling of the sun’s (unobservable) interior with observations of the energy that the sun produces, astronomers have come to an agreement on what is called a standard solar model, a mathematically-based picture of the structure of the sun. The model seeks to explain the observable properties of the sun and also describe properties of its unobservable interior. With the standard solar model, we can begin to describe some of the interior regions—regions hidden, beneath the photosphere, from direct observation. Below the photosphere is the convection zone, some 124,000 miles (200,000 km) thick. Below this is the radiation zone, 186,000 miles (300,000 km) thick, which surrounds a core with a radius of 124,000 miles (200,000 km).<br />The sun’s core is tremendously dense (150,000 kg/m3) and tremendously hot: some 15,000,000 K. We can’t stick a thermometer in the sun’s core, so how do we know it’s that hot? If we look at the energy emerging from the sun’s surface, we can work backward to the conditions that must prevail at the sun’s core. At this density and temperature, nuclear fusion is continuous, with particles always in violent motion. The sun’s core is a giant nuclear fusion reactor.<br />At the very high temperatures of the core, all matter is completely ionized—stripped of its negatively charged electrons. As a result, photons (packets of electromagnetic energy) move slowly out of the core into the next layer of the sun’s interior, the radiation zone.<br />Here the temperature is lower, and photons emitted from the core of the sun interact continuously with the charged particles located there, being absorbed and re-emitted. While the photons remain in the radiation zone, heating it and losing energy, some of their energy escapes into the convection zone, which in effect, boils like water on a stove so that hot gases rise to the photosphere and cool gases sink back into the convection zone. Convective cells become smaller and smaller, eventually becoming visible as granules at the solar surface. Thus, by convection, huge amounts of energy reach the surface of the sun. At the sun’s surface, a variety of processes give rise to the electromagnetic radiation that we detect from the earth. Atoms and molecules in the sun’s photosphere absorb some of the photons at particular wavelengths, giving rise to the sun’s absorption-line spectrum. Most of the radiation from a star that has the surface temperature of the sun is emitted in the visible part of the spectrum.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-30504090382900376792010-11-30T22:24:00.000-08:002010-11-30T22:26:50.814-08:00Chain Reactions in the SunThe sun generates energy by the converting the hydrogen in its core to helium. The details are complex, but we may content ourselves with an overview. When temperatures and pressures are sufficiently high (temperatures of about 10 million K are required) 4 hydrogen nuclei (which are protons, positively charged particles) can combine to create the nucleus of a helium atom (2 protons and 2 neutrons). Now the mass of the helium nucleus created is slightly less than that of the four protons that were needed to create it. That small difference in mass is converted into energy in the fusion process. One of the simplest fusion reactions involves the production of deuterium (a hydrogen isotope) from a proton and a neutron. When these two particles collide with enough velocity, they create a deuterium nucleus (consisting of a proton and a neutron) and the excess energy is given off as a gamma ray photon. In the sun, this process proceeds on a massive scale, liberating the energy that lights up our daytime skies. That’s a 4 ×1026 watt lightbulb up there, remember.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-1918400216588167372010-11-30T22:20:00.000-08:002010-11-30T22:22:30.618-08:00Fission in the Sun<img src="http://www.irtc.org/ftp/pub/stills/2005-06-30/fission.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />On December 2, 1942, Enrico Fermi, an Italian physicist who had fled his fascistoppressed native land for the United States, withdrew a control rod from an “atomic pile” that had been set up in a squash court beneath the stands of the University of Chicago’s Stagg Field. This action initiated the world’s first self-sustaining atomic chain reaction. Fermi and his team had invented the nuclear reactor, and the world hasn’t been the same since.<br />Nuclear fission is a nuclear reaction in which an atomic nucleus splits into fragments, thereby releasing energy. In a fission reactor, such as the one Fermi was instrumental in creating, the process of fission is controlled and self-sustaining, so that the splitting of one atom leads to the splitting of others, each fission liberating more energy.<br />Nuclear fission is capable of liberating a great deal of energy, whether in the form of a controlled sustained chain reaction or in a single great explosion, like an atomic bomb. Yet even the powerful fission process cannot account for the tremendous amount of energy the sun generates so consistently. We must look to another process: nuclear fusion.<br />Whereas nuclear fission liberates energy by splitting atomic nuclei, nuclear fusion produces energy by joining them, combining light atomic nuclei into heavier ones. In the process, the combined mass of two nuclei in a third nucleus is less than the total mass of the original two nuclei. The mass is not simply lost, but converted into energy. A lot of energy. One of the by-products of nuclear fusion reactions is a tiny neutral particle called the neutrino. The fusion reactions themselves produce high energy gamma ray radiation, but those photons are converted into mostly visible light by the time their energy reaches the surface of the sun. Neutrinos, with no charge to slow them down, come streaming straight out of the sun’s core. The numbers that we detect give us great insight into a region of the sun that is otherwise inaccessible.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-47945742610684517192010-10-31T12:17:00.000-07:002010-10-31T12:19:05.816-07:00Solar flares<img src="http://api.ning.com/files/rShNeomQNkUT9MtukdgLDz8tmpPj8MAipX*cSjVpm5L*lNYLqje8WbvrPsD4V0mgEVFPF7ueaRbocSBQfq4dOmAvg6fqyDLR/SolarFlareandProminence.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" />Most frequently at the peak of the sunspot cycle, violent eruptions of gas are ejected from the sun’s surface. The prominences and flares may rise to some 60,000 miles (100,000 km) and may be visible for weeks. Solar flares are more sudden and violent events than prominences. While they are thought to also be the result of magnetic kinks, they do not show the arcing or looping pattern characteristic of prominences. Flares are explosions of incredible power, bringing local temperatures to 100,000,000 K. Whereas prominences release their energy over days or weeks, flares explode in a flash of energy release that lasts a matter of minutes or, perhaps, hours.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-19357679943543243172010-10-31T12:16:00.001-07:002010-10-31T12:16:50.484-07:00Understanding Sunspot Cycles<img src="http://www.hao.ucar.edu/education/slides/slide17.jpeg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Long before the magnetic nature of sunspots was perceived, astronomer Heinrich Schwabe, in 1843, announced his discovery of a solar cycle, in which the number of spots seen on the sun reaches a maximum about every 11 years (on average). In 1922, the British astronomer Annie Russel Maunder charted the latitude drift of sunspots during each solar cycle. She found that each cycle begins with the appearance of small spots in the middle latitudes of the sun, followed by spots appearing progressively closer to the solar equator until the cycle reaches its maximum level of activity. After this point, the number of spots begins to decline. The most recent maximum occured in early 2001.<br />Actually, the 11-year period is only half of a 22-year cycle that is more fundamental. Recall that the leading spots on one hemisphere exhibit the same polarity; that is, they are all either north magnetic poles or south (and the followers are the opposite of the leaders). At the end of the first 11 years of the cycle, polarities reverse. That is, if the leaders had north poles in the southern hemisphere, they become, as the second half of the cycle begins, south poles.<br />The cyclical nature of sunspot activity is very real, but not exact and inevitable. Studying historical data, Maunder discovered that the cycle had been apparently dormant from 1645 to 1715. At present, there is no explanation for this dormancy and other variations in the solar cycle.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-73721225644819151312010-10-31T12:14:00.000-07:002010-10-31T12:15:39.273-07:00Sunspots: What They Are<img src="http://starchild.gsfc.nasa.gov/Images/StarChild/questions/sunspot_dia.gif" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Sunspots are irregularly shaped dark areas on the face of the sun. They look dark because they are cooler than the surrounding material. The strong local magnetic fields push away some of the hot ionized material rising from lower in the photosphere. A sunspot is not uniformly dark. Its center, called the umbra, is darkest and is surrounded by a lighter penumbra. If you think of them as blemishes on the face of the sun, just remember that one such blemish may easily be the size of the earth or larger.<br />Sunspots may persist for months, and they may appear singly, although, usually, they are found in pairs or groups. Such typical groupings are related to the magnetic nature of the sunspots. Every pair of spots has a leader and a follower (with respect to the direction of the sun’s rotation), and the leader’s magnetic polarity is always the opposite of the follower. That is, if the leader is a north magnetic pole, the follower will be a south magnetic pole.<br />Sunspots are never seen exactly at the equator or near the solar poles, and leaders and followers in one hemisphere of the sun are almost always opposite in polarity from those across the equator. That is, if all the leaders in the northern hemisphere are south magnetic poles, all the leaders in the southern hemisphere will be north magnetic poles.<br />We have said that sunspots are thought to be associated with strong local magnetic fields. But why are the fields strong in certain regions of the photosphere? A meteorologist from Norway, Vilhelm Bjerknes (1862–1951) concluded in 1926 that sunspots are the erupting ends of magnetic field lines, which are distorted by the sun’s differential rotation. That is, like the gas giant jovian worlds, the sun does not rotate as a single, solid unit, but differentially, at different speeds for different latitudes. The sun spins fastest at its equator—the result being that the solar magnetic field becomes distorted. The field lines are most distorted at the equator, so that the north-south magnetic field is turned to an east-west orientation. In places where the field is sufficiently distorted, twisted like a knot, the field becomes locally very strong, powerful enough to escape the sun’s gravitational pull. Where this happens, field lines “pop” out of the photosphere, looping through the lower solar atmosphere and forming a sunspot pair at the two places where the field lines pass into the solar interior.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-37433250550193533172010-09-30T23:28:00.000-07:002010-09-30T23:29:07.649-07:00Sun Trivias<img src="http://www.sciencedaily.com/images/2005/05/050524000538.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />We have described the layers in the sun’s outer atmosphere, but have ignored some of their more interesting aspects, the storms in the atmosphere. The sun’s atmosphere is regularly disturbed by solar weather in the form of sunspots, prominences, and solar flares. With the proper equipment—or an Internet connection (http://sohowww.estec. esa.nl)—you can observe some of the signs of activity on the sun’s surface.<br />A Granulated Surface<br />If we look at the sun, its surface usually appears featureless, except, perhaps, for sunspots, which we’ll discuss in a moment. However, viewed at high-resolution, the surface of the sun actually appears highly granulated. Now, granule is a relative concept when we are talking about a body the size of the sun. Each granule is about the size of an earthly continent, appearing and disappearing as a hot gas bubble rises to the surface of the sun.<br />Galileo Sees Spots Before His Eyes<br />People must have seen sunspots before 1611, when Galileo (and, independently, other astronomers) first reported them. (As recently as March 2001, sunspots easily visible to the unaided eye have appeared.) The largest spots are visible to the naked eye (at least when the sun is seen through clouds). Yet, at the time, the world was reluctant to accept imperfections on the face of the sun.<br />Sunspots were not (as far as we know) studied before Galileo. Galileo drew a profound conclusion from the existence and behavior of sunspots. In 1613, he published three letters on sunspots, explaining that their movement across the face of the sun showed that the sun rotated.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-28932261876833586982010-09-30T23:25:00.001-07:002010-09-30T23:26:38.785-07:00What is Solar Wind<img src="http://astroprofspage.com/wp-content/uploads/2006/11/magneto.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />The sun does not keep its energy to itself. Its energy flows away in the form of electromagnetic radiation and particles. The particles (mostly electrons and protons) do not move nearly as fast as the radiation, which escapes the sun at the speed of light, but they move fast nevertheless—at more than 300 miles per second (500 km/s). It is this swiftly moving particle stream that is called the solar wind.<br />The solar wind is driven by the incredible temperatures in the solar corona. As a result, the gases are sufficiently hot to escape the tremendous gravitational pull of the sun. The surface of the earth is protected from this wind by its magnetosphere, the magnetic “cocoon” generated by the rotation of the earth’s molten core. As with many other planets, the motion of charged molten material in the earth’s core generates a magnetic field around the planet. This magnetic field either deflects or captures charged particles from the solar wind. Some of these particles are trapped in the Van Allen Belts, doughnut-shaped regions around the earth named after their discoverer. Some of the charged particles rain down on the earth’s poles and collide with its atmosphere, giving rise to displays of color and light called aurora (in the Northern Hemisphere the Aurora Borealis, or Northern Lights, and in the Southern Hemisphere, the Aurora Australis, or Southern Lights). The Auroras are especially prominent when the sun reaches its peak of activity every 11 years.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-80433127011115297852010-09-30T23:21:00.000-07:002010-09-30T23:23:18.581-07:00Understanding Solar Eclipse<img src="http://3.bp.blogspot.com/_dr8MGVv2_rI/TDY-PrGoWvI/AAAAAAAADds/12oz8T5EbTg/s1600/Solar%2BEclipse.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />A solar eclipse occurs when the moon moves across the disk of the sun so that the moon’s shadow falls across the face of the earth. In the heart of that shadow, called the umbra, the sun’s disk will appear completely covered by that of the moon: a total solar eclipse. The umbra, however, only falls on a small region of the earth. Thus a total eclipse can be observed only within the zone of totality, a very narrow area of the earth (where this shadow falls as the earth rotates). For this reason, total eclipses are rare events in any given geographical area. Much more common are partial eclipses, in which the moon obscures only part of the sun. Observers located in the much broader outer shadow of the moon (the penumbra) see such an eclipse.<br />Certainly, partial eclipses are interesting, but a total eclipse can be spectacular, not only dramatically darkening the world, but allowing sight of such solar features as feathery prominences, the chromosphere, and, most thrilling of all, the corona. These features are fleeting, since totality lasts only a few minutes at any one observing location. As mentioned elsewhere in this book, observing the sun directly is very dangerous.<br />Looking at the sun through an unfiltered telescope or binoculars will cause irreversible damage to your eyesight. The sun is no more or less dangerous during an eclipse than at any other time; but the point is that looking directly at the sun is always dangerous and harmful.<br />The sun, during an eclipse or at any time, is most safely observed by projecting its image onto a piece of paper or cardboard. You can project a telescope or binocular image onto a white card held at the correct distance from the eyepiece. But you don’t need a telescope or binoculars to project an image. Just make a pinhole in a stiff piece of cardboard and project the pinhole image onto a white card or paper. (By the way: Do not look<br />through the pinhole directly at the sun!)<br />If you want to look at the sun through your telescope during an eclipse or at another time, purchase a solar filter (glass or Mylar) from any of the major telescope manufacturers. This type of filter attaches to the front of your telescope tube, it does not screw onto the eyepiece.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-76004914821328443322010-08-31T23:37:00.000-07:002010-08-31T23:38:05.045-07:00A Luminous Crown<img src="http://solarious.files.wordpress.com/2008/03/677px-sun-corona.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Corona is Latin for “crown,” and it describes the region beyond the transition zone consisting of elements that have been highly ionized (stripped of their electrons) by the tremendous heat in the coronal region. Like the chromosphere, the corona is normally invisible, blotted out by the intense light of the photosphere. It is only during total solar eclipses that the corona becomes visible, at times when the disk of the moon covers the photosphere and the chromosphere. During such eclipse conditions, the significance of the Latin name becomes readily apparent: The corona appears as a luminous crown surrounding the darkened disk of the sun. When the sun is active—a cycle that peaks every 11 years—its surface becomes mottled with sunspots, and great solar flares and prominences send material far above its surface.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-22686393026032991522010-08-31T23:35:00.000-07:002010-08-31T23:36:56.815-07:00Not That Kind of Chrome<img src="http://www.dlr.de/en/Portaldata/1/Resources/portal_news/newsarchiv2009_3/eit-chromosphere_380.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />The sun’s lower atmosphere is called the chromosphere, normally invisible because the photosphere is far brighter. However, during a total solar eclipse, which blots out the photosphere, the chromosphere is visible as a pinkish aura around the solar disk. The strongest emission line in the hydrogen spectrum is red, and the predominance of hydrogen in the chromosphere imparts the pink hue. The chromosphere is a storm-racked region, into which spicules, jets of expelled matter thousands of miles high, intrude.<br />Above the chromosphere is the transition zone. As mentioned earlier, the temperature at the surface of the photosphere is 5,780 K, much cooler than the temperatures in the solar interior, which get hotter the closer one approaches the core. Yet, in the chromosphere, transition zone, and into the corona, the temperature rises sharply the farther one goes from the surface of the sun! At about 6,000 miles (10,000 km) above the photosphere, where the transition zone becomes the corona, temperatures exceed 1,000,000 K. (For detailed real-time views of the solar photosphere, chromosphere, and corona, see http://sohowww.estec.esa.nl.) How do we explain this apparent paradox? It is believed that the interaction between the sun’s strong magnetic field and the charged particles in the corona heat it to these high temperatures.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-34957848635745305712010-08-31T23:33:00.000-07:002010-08-31T23:35:02.758-07:00The Solar Atmosphere<img src="http://regmedia.co.uk/2008/11/04/solar_atmos.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />The sun does not have a surface as such. What we call its surface is just the layer that emits the most light. Let’s begin our journey at the outer layers of the sun (the layers that we can actually see), and work our way in. When you look up at the sun during the day, what you are really looking at is the sun’s photosphere. The layer from which the visible photons that we see arise, the photosphere has a temperature of about 6,000 K. Lower layers are hidden behind the photosphere, and higher layers are so diffuse and faint (though very hot) that we only see them during total solar eclipses or with special satellites. Above the photosphere in the solar atmosphere are the chromosphere, the transition zone, and the corona. As we move higher in the sun’s atmosphere, the temperatures rise dramatically.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0tag:blogger.com,1999:blog-5523710396030704477.post-90535854889581306922010-07-31T21:52:00.000-07:002010-07-31T21:53:06.321-07:00Four Trillion Trillion Light Bulbs<img src="http://www.kinakokids.com/images/letterbox/bulbs.jpg" style="margin: 0px auto 10px; display: block; width: 300px; text-align: center;" border="0" /><br />Next time you are screwing in a light bulb, notice its wattage. A watt is a measure of power, or how much energy is produced or consumed each second. A 100 watt bulb uses 100 joules of energy every second. For comparison, the sun produces 4 1026 watts of power. That’s a lot of light bulbs—four trillion trillion of them, to be exact. This rate of energy production is called the sun’s luminosity. Many stars have luminosities much higher than that of the sun.<br />The source of the sun’s power—and that of all stars, during most of their lifetimes—is the fusing together of nuclei. Stars first convert hydrogen into helium, and heavier elements come later. The only fusion reactions that we have been able to produce on the earth are uncontrolled reactions known as hydrogen bombs. The destructive force of these explosions gives insight into the enormous energies released in the core of the sun. Nuclear fusion could be used as a nearly limitless supply of energy on the earth; however, we are not yet able to create the necessary conditions on Earth for controlled fusion reactions.Blue Hazehttp://www.blogger.com/profile/14705441334413797395noreply@blogger.com0