|
| :: Previous topic :: Next topic |
| Author |
Message |
DrewTerry
Guest
|
 Posted: Sun Jan 28, 2007 1:46 pm Post subject: Stargazing |
  |
|
| Quote: |  Absorption lines in stellar spectra
The deeper you go into a star, the hotter and denser the gas. The lower layers tend to radiate all the colors rather like a hot solid, while the upper layers act something like the low density gas of the last paragraph through which the radiation passes. Stars are made of the same stuff as found in the Earth (though not in the same proportions), and contain all of nature's chemical elements. As a result, the spectrum of a star displays an extraordinary mixture of absorption lines. Over 100,000 absorption lines are visible in the Sun's spectrum. |
| Quote: |  Emission lines
What goes up must come down. Electrons, like anything else, will attempt to seek their lowest energies. If the electrons gain energy by the absorption of photons, or perhaps by collisions, they must eventually lose it again. They can lose it in collisions or they can, instead of absorbing photons, radiate them. Since the absorption wavelengths are tightly defined, so are the emission wavelengths. If we look at a heated low-density gas WITHOUT looking at a bright source behind it, we will see BRIGHT lines of color at the same spectral wavelengths at which we before saw dark absorptions. For any given atom or ion, the emission spectrum is a simple reversal of the absorption spectrum. Emission lines are easy to produce in the laboratory simply by heating a low-density gas, allowing collisions to kick the electrons to higher energies. The emissions are produced when the electrons drop back down to lower energies. Emission lines are radiated by street lamps (the orange ones radiating sodium lines, the blue ones mercury lines), neon signs, and fluorescent bulbs. They are also radiated by clouds of interstellar gas that are heated and ionized by nearby hot stars. Examples are the great Orion Nebula, a cloud of interstellar gas involved with star birth, planetary nebulae, and supernova remnants. Under some circumstances, stars can radiate emission lines too. For example, Mira variables have hydrogen emission lines that are excited by powerful shock waves -- sonic booms -- made by the stars' pulsations. |
| Quote: | Analysis of absorption lines
The Sun displays an enormous number of spectrum lines, over three dozen appearing here in a 20 A-wide stretch in the yellow part of the spectrum. Roman numeral "I" stands for the neutral ion of an element, "II" for the once- ionized version. Different lines have different strengths. The ionized iron lines are nearly black, whereas those produced by the much rarer elements yttrium (Y), neodymium (Nd), and lanthanum (La) are very weak. E. C. Olson, Mt. Wilson Observatory.
The absorption lines in the Sun and stars can be identified with individual chemical elements or molecular compounds by comparing their positions in the spectrum (their wavelengths) with those observed from pure sources in the laboratory. Some absorptions are very weak, just shallow dips in the spectrum, whereas others are completely black. The "strength" of an absorption line -- the amount of energy removed from the spectrum -- depends on the amount of the particular chemical element in the star causing the line and on the efficiency of absorption. The efficiency is crucial. Hydrogen dominates the Sun, yet absorption lines of ionized calcium dominate the solar spectrum even though there is 440,000 times as much hydrogen as calcium. Hydrogen has a low efficiency of absorption, whereas that of ionized calcium is very high. The efficiency depends on the availability of electrons to move to higher energies and on atomic factors, namely the likelihood of absorption in the presence of a passing photon. The efficiencies depend critically on temperature and can be calculated from theory or measured in the laboratory. Once they are known, we can calculate the abundances of the atoms from the strengths of the absorption lines and therefore calculate the chemical composition of the outer part of a star. Relative absorption line strengths can also be used to find temperatures and densities. Similar rules can be developed to analyze the emission lines radiated by interstellar gas clouds, from which we learn the compositions of the nebulae, including those of the planetary nebulae. |
| Quote: | The spectral sequence
• The classic spectral sequence is illustrated by the spectra of real stars in a historic image published in 1901.
• The strong lines in class A (here, the star Sirius) are hydrogen.
• Neutral helium appears along with hydrogen in class B (Alnilam, Epsilon Orionis), while ionized helium is strong in class O (Naos, Zeta Puppis), the hydrogen lines nearly gone.
• Hydrogen weakens downward too, toward lower temperature, nearly disappearing by class M2 (Betelgeuse).
• The strong lines to the left in classes F (Canopus), G (Capella), and K (Arcturus) are those of ionized calcium.
• The other lines in these cooler classes are those of other metals.
• At the bottom, in class M7 (the long-period variable star Mira), we see bands of absorption produced by the titanium oxide molecule.
Annals of the Harvard College Observatory, vol. 23, 1901.
Because the efficiencies of absorption depend on temperature, so do the appearances of the spectra of the stars. Stellar spectra were first observed in the middle of the 19th century. To the great confusion of the astronomers of the time, most spectra looked nothing like the solar spectrum. Some, like that of Vega, had powerful hydrogen lines, whereas others had none at all and displayed what were later shown to be molecular lines of titanium oxide. It looked as though different stars were made of different elements. As an aid to understanding, astronomers began classifying the spectra, the schemes culminating about 1890 in the one still used today when E.C. Pickering lettered the stars according to the strengths of their hydrogen lines, his assistants Annie Cannon, Antonia Maury, and Williamina Fleming aiding in development and observation. As observation improved, they dropped some letters, rearranged others according to different spectral criteria, and added decimalization. The result was the classic seven-group sequence OBAFGKM. A bit over a century later, as a result of new tehcnologies, astronomers added another two classes whose spectra contained molecules, L and T. About the first thing any astronomer wants to know about a star is its class. The Sun is class G. |
| Quote: | [url=http://www.astro.uiuc.edu/~kaler/sow/spectra.html/]This site is linked to Spectra and The Hertzsprung-Russell (H Diagram[/url
It is the purpose of this site to provide a deep, non-technical review of stars and their natures for the beginner. This page presents facts about stars as we know them without delving into the details of discovery. Parallel sites that explore the spectra of the stars and the HR Diagram examine how we have learned so much of what is presented here. The three sites are linked, allowing you to go back and forth among them to see how stars are born, live their lives, and die, in the process creating other stars, perhaps other earths, and all that is around us.
Brightness and distance
In the second century BC, the Greek astronomer Hipparchus divided the stars into six brightness groups called magnitudes (now apparent visual magnitudes (m or V), first magnitude the brightest, sixth the faintest. The system is still used today, though with a mathematical definition (a star of one magnitude is 2.512... times brighter than the next fainter) that takes the very brightest stars and planets through magnitude zero and into negative numbers. Through the telescope we see much fainter, to near 30th magnitude (4 billion times fainter than the human eye can see alone). Though stars bear some resemblance to the Sun, they appear as points in the sky because they are so far away, the nearest, Alpha Centauri, four light years away. The light year is the distance a ray of light will travel in a year at 300,000 kilometers (186,000 miles) per second, so one light year is about 10 trillion kilometers (63,000 Astronomical Units, where the AU is the average distance between the Earth and the Sun). The stars are so far that distances were not measured until 1846, by means of parallax (viewing the star from opposite sides of the Earth's orbit). The most distant stars the unaided eye can see are over 1000 light years away, which is about the practical limit of parallax measures. The apparent visual magnitude of a star depends on true visual luminosity (in watts) and distance. To compare true visual luminosities, astronomers calculate the absolute visual magnitude (M), the apparent magnitude the star would have were it at a distance of 32.6 light years (10 parsecs, where the parsec is the professional unit of distance, equal to 3.26 light years). The absolute visual magnitude of the Sun is +4.83. Absolute visual magnitudes range from around -10 (a million times more luminous than the Sun) to below +20 (a million times fainter).
The Galaxy
We need to set the stars in context. All those you see at night are part of our local collection of stars, all part of our Galaxy. Trillions of other galaxies flock the Universe, ours one of the larger ones. The principal part of our Galaxy (our own with a capitol "G") is in the shape of a flat disk about 100,000 light years across that contains some 200 billion stars. Our Sun is toward the edge, about 25,000 light years from the center, the whole structure at the Sun's distance rotating with a period of 200 million years. A large portion of the disk's stars are set within pinwheel-like spiral arms that over millions of years come and go, the stars moving in and out of them as they orbit the Galaxy's center. Since we are in the disk, we see the combined light of its billions of stars around our head as the famed "Milky Way," the center of the Galaxy located behind the thick star clouds of Sagittarius. The age of the disk is about 10 billion years. Surrounding the disk is a thinly populated rather spherical halo that seems to date to about 14 billion years.
Defining a star
A star is a body that at some time in its life generates its light and heat by nuclear reactions, specifically by the fusion of hydrogen into helium under conditions of enormous temperature and density. When hydrogen atoms merge to create the next heavier element, helium, mass is lost, the mass (M) converted to energy (E) through Einstein's famous equation E = mc squared, where "c" is the speed of light. The Sun is powered by hydrogen fusion, as are many of the other stars you see at night. The fusion does not take place throughout the star, but only in its deep interior, in its core, where it is hot enough. The temperature at the center of the Sun is 15.6 million degrees Kelvin (K = centigrade degrees above absolute zero, -273 C), and the density is 14 times that of lead. About 40% of the mass of the Sun, occupying about 30% of the radius, is capable of fusing hydrogen. Even under these extreme conditions, the Sun is still a gas throughout.
Masses of stars
To create the conditions for such "thermonuclear fusion," stars must be massive. The Sun has the mass of 333,000 Earths. Stars can range up to about 100 times the mass of the Sun (at which point nature stops making them) down to around 8% that of the Sun, at which point the internal temperature is not high enough to run the full range of nuclear reactions (which requires at least 7 million degrees Kelvin). "Substars" below the 8% limit, called "brown dwarfs," do exist in significant numbers however, and down to around 1/80 the solar mass (13 Jupiter-masses) can fuse their natural deuterium (heavy hydrogen). The lower limit to brown dwarf (substellar) masses is not known.
The compositions of the stars
Stars are made of the same chemical elements as found in the Earth, though not in the same proportions, the chemical compositions found from the stars' spectra. Most stars are made almost entirely of hydrogen (about 90% by number of atoms) and helium (about 10%), elements that are relatively rare on our planet. About a tenth of a percent is left over, that tenth containing all the other elements found in nature. Of these, oxygen usually dominates, followed by carbon, neon, and nitrogen. Of the metals, iron usually dominates. Nevertheless, there is only one atom of oxygen in the Sun for every 1200 hydrogen atoms and only one of iron for every 32 oxygen atoms. However, within this tenth of a percent, the proportions of the numbers of atoms in the Sun is rather similar to what we find here, in the Earth's crust. Other stars can deviate considerably, depending on their states of aging or upon where they are in the Galaxy.
The birth of stars
The space between the stars is filled with dusty gas. Thick dust clouds can even be seen with the naked eye within the Milky Way blocking the light of distant stars and providing much of the Milky Way's structure. Interstellar matter is compressed by the Galaxy's winding spiral arms. The clouds can be further compressed through collisions or by blast waves from exploding high-mass stars. Lumps of matter therefore form within the interstellar clouds. If their gravity is great enough, they can condense into one or more stars. Contraction causes more rapid spin, which creates a disk around the birthing star, from which it can draw matter. Further condensation within the disk can create planets (or even stellar companions). The contraction of forming stars raises the internal temperature, finally to the point of ignition of hydrogen fusion. Gravity would like to make the star as small as possible, but the fusion reactions stabilize it and keep it from contracting any further. The whole life story of a star from here on out is told by the battle between gravity and nuclear fusion, first one, then the other getting the upper hand.
Planets
As a new star condenses from a gaseous lump in interstellar space, it spins faster, the outer parts of the contracting cloud spinning out into a dusty disk. The dust particles, in orbit about the new star, accumulate, building themselves into planets. Here at home, the planets that formed close to the Sun (Mercury through Mars) were in an environment too hot to incorporate much water or light atoms like hydrogen, so they are made of heavy stuff like iron, silicon, and oxygen. In the outer System, the planets contain huge amounts of hydrogen and helium and could grow large, their satellites made largely of water ice. Other stars should grow planets too, planets that could be quite different from our own and that are now being discovered.
Main sequence stars and stellar classes
There are many kinds and classes of stars. Those that are actively fusing hydrogen into helium in the middle, that is, in their cores, are called "main sequence" stars. (For historical reasons, main sequence stars are also commonly referred to as "dwarfs"). The main sequence is the first stage following birth. In general, main sequence stars have chemical compositions similar to that of the Sun. The higher the mass of the main sequence star, the greater its diameter and the higher its surface temperature. Dimensions range from about 10% the size of the Sun (which is 1.5 million kilometers -- 109 Earths -- across) to just over ten times solar, and surface temperatures from under 2000 degrees Kelvin to about 49,000 K (the Sun's surface is at 5780 K). Around the beginning of the 20th century, astronomers divided the stars into seven basic lettered groups that they later learned were related to surface temperature, which for the main sequence are: O (above 31,500 K), B (10,000 - 31,500 K), A (7500 - 10,000 K), F(6000 - 7500 K), G(5300 - 6000 K), K(3800 - 5300 K), and M (2100 - 3800 K). A century later, two more classes were added to account for faint red stars turned up by new technologies: class L (1200 - 2100 K) and T (below 1200 K), the whole set now OBAFGKMLT. The Sun is a G star. The system is decimalized, making the Sun class G2. Examples of main sequence stars are Acrux, Vega, Sirius, Porrima, Chara, Alpha Centauri A and B, and Proxima Centauri. The classes are actually derived from the stars' spectra. The stellar astronomer's greatest tool is the HR diagram, a plot of absolute visual magnitude against spectral class, in which we can see nearly all of the stages of stellar life and death. On it, the main sequence is a band that runs from the highest-mass hydrogen-fusing stars at the upper left to the lowest masses at the lower right.
Star colors
Since the color of a heated body depends on temperature, the different classes take on different, though subtle, colors, from slightly reddish for class M to orange for K, through yellow- white to bluish for classes B and O. Star colors can be noted rather easily even with the unaided eye, especially when those close together contrast against each other. Stars of classes L and T, none of which are visible to the naked eye, range from red through deep red to "infrared" (these optically invisible under any circumstances).
Lifetimes of stars
Main sequence stars have only a certain amount of internal fuel available within their hot cores. When the hydrogen fuel has all turned to helium, the stars begin to die and to produce a number of other different kinds. Because higher mass stars use their hydrogen fuel much more quickly than lower mass stars, those of higher mass live shorter lives. The Sun has a 10 billion year main sequence lifetime (of which half is gone). The most massive stars live only a couple million years, the least massive for trillions, so long that no star with a mass less than about 0.8 solar masses has ever died in the history of the Galaxy. From theory, we calculate that such a 0.8 solar mass star should live for about 13 billion years. The Galaxy should be about as old as its oldest stars, and is thus about 13 billion years old.
Giant stars
Begin with stars more or less like the Sun, those with masses from about 0.8 times that of the Sun to about 10 times the solar mass. When the fuel in a solar-type star's core runs out, the helium core contracts under the effect of gravity and heats up. Hydrogen fusion then expands into a shell around the old burnt- out core, and so much energy is produced that the star temporarily brightens and expands by many times over, the expansion cooling the surface, turning the star into a class M "red giant." When the temperature hits around 100 million degrees Kelvin, the helium is hot enough to fuse into carbon and even a bit further, into oxygen. This new power source stops the core's contraction and the star stabilizes for a time, dimming and heating somewhat at the surface. We commonly see these helium-fusing stars as yellow-orange type K giants. On the HR diagram, the giants run roughly from the middle toward upper right (higher luminosity). Among classes G and K, temperatures are up to a few hundred degrees cooler than they are for main sequence dwarfs. Good examples are Aldebaran and Arcturus. Such stars have diameters tens of times that of the Sun. The giant and subsequent stages up to the actual death of the star (the end of nuclear fusion) takes roughly 10% of the main sequence lifetime.
Cepheid Variable Stars
When more massive stars (2 to 8 times that of the Sun) pass through mid-temperatures either on their way to fusing helium or during various stages of helium fusion, they can become unstable and pulsate in size, temperature, and luminosity. The first of these discovered, Delta Cephei, gave the name "Cepheid" variable to the group. Cepheids, usually classed as F and G supergiants (though not as massive as true supergiants), vary by a couple to a few magnitudes over periods of one to 100 days. A strict relation between absolute magnitude and pulsation period allows us to determine their distances (period gives absolute brightness, and comparison to apparent brightness yields distance.) Cepheids are the major keys to learning distances to other galaxies. The brightest Cepheid in the sky is Polaris , though the variations are too small to be seen by eye. Cepheids occupy the upper range of the HR diagram's, "instability strip."
Bigger red giants and Miras
When the helium in the core has turned to carbon and oxygen, the core shrinks again, and the helium begins to fuse to carbon and oxygen in a shell around the old core, this shell surrounded by another one fusing hydrogen into helium, the two turning on and off in sequence. The star now brightens again, expands even more, and becomes cooler and even redder than before. As the star brightens it becomes unstable and begins to pulsate, the pulsations making it vary, or change in brightness. The star become so huge, near or greater than the orbit of the Earth, that the pulsations can take a year or more. The first of these found, Mira in Cetus, changes from second or third magnitude to tenth, becoming quite invisible to the naked eye. Such stars are now called "long-period variables" (LPVs) or "Mira variables." Thousands, all cool class M giants, are known. On the HR diagram, such advanced giants are at the cool end of the "giant branch," the Miras occupying the coolest and brightest portion.
Creation of elements
The gases of red giants can circulate upward to the tops of the stars, carrying the by-products of nuclear fusion with them. Oxygen is normally more abundant than carbon. If conditions are right, the surfaces of some stars can change their chemical compositions, some becoming very rich in the carbon that was made below by helium fusion, resulting in the reversal of the normal ratio. Mira variables and other old red giants thus divide into oxygen-rich stars and "carbon stars." Raised up along with the carbon are elements such as zirconium and many others that have been made in a huge variety of nuclear reactions that go on at the same time as helium fusion. Other stars' surfaces are enriched in helium and nitrogen.
Winds and mass loss
Such huge giant stars have low gravities and lose mass through powerful winds that blow from their surfaces. Some of the gas condenses into molecules and dust. There may be so much that the star can be buried in it and become invisible to the eye, the glow of the heated dust seen only by its infrared (heat) radiation. Oxygen-rich giant stars make silicate dust, while carbon stars make carbon-dust similar to graphite and soot. Most of the dust that inhabits interstellar space began this way, though since inception it has been highly modified in the freezer of interstellar space. These stars therefore play a powerful role in later star formation. The winds are so strong during the giant stage of a star's life that it can lose half or more of its mass back into space, whittling itself down to little more than the parts that underwent nuclear fusion. |
| Quote: | Planetary nebulae
As a giant star loses almost all of its remaining outer hydrogen envelope, it comes close to revealing its intensely hot core. A fast wind from the core first compresses the inner edge of the old expanding wind. High-energy radiation from the hot core then lights up this inner compressed portion, which is now many times the size of the whole Solar System. These illuminated clouds, which can be quite beautiful, were discovered by William Herschel around 1790, who termed them"planetary nebulae" for their disk-like appearances (they have nothing else to do with planets). Their complex appearances depend to a degree on how matter is lost from the giant stars that make them. Expanding at rates of tens of kilometers per second, they last no more than a few tens of thousands of years. |
| Quote: | White dwarfs
As the planetary nebula dissipates into the gases of interstellar space, it leaves behind the spent, old core (that now includes the dead nuclear fusing shells). These stars, compressed under their gravity, have shrunk to only about the size of Earth. The first ones found (Sirius-B, Procyon-B, and 40 Eridani B) were fairly hot and white, so the class acquired the name "white dwarf" to discriminate it from the main sequence of stars (which were originally called "ordinary dwarfs" to distinguish them from the giants). Though small, white dwarfs still contain near the mass of the Sun, giving them astonishing average densities of a metric ton per cubic centimeter. The tremendous outward pressure exerted under the great density prevents gravity from shrinking them any further. White dwarfs, the remains of stars that began their lives between 0.8 and 10 solar masses, no longer have any source of energy generation and are destined only to cool. The cooling time is so long, however, that all white dwarfs ever created are still visible, though the oldest are becoming cool, dim, and reddish. (There is no such thing as an invisible, cold "black dwarf.") The age of the Galaxy calculated (with the aid of theory) from the oldest white dwarfs roughly agrees with that derived from the coolest (lowest mass) evolved main sequence star. On the HR diagram, they fall in a line rather parallel to, but far fainter than, the main sequence dwarfs. |
| Quote: | High mass stars and supergiants
As they start to die, higher mass stars (those with masses over about 10 times that of the Sun) initially develop the same way as giants, but then their course of evolution becomes very different. High mass stars are already large and luminous. As their dead helium cores contract, heating and firing to fuse the helium to carbon and oxygen, the stars expand to approach the sizes of the orbits of the outer planets, becoming distended red "supergiants." Excellent examples are first magnitude Betelgeuse in Orion and Antares in Scorpius. Supergiants are so massive, in spite of great mass loss through huge winds, that nuclear fusion can proceed farther than it can in ordinary giants. When the helium runs out, the carbon and oxygen mixture compresses and heats, causing it to fuse to a mixture of neon, magnesium and oxygen. Hydrogen and helium fusion had already moved outward into nested shells around the core. When carbon fusion dies out in the core, leaving a mix of neon, magnesium, and oxygen, it too moves outward into a shell. The neon-magnesium-oxygen mixture now in the core then heats and fuses into a mix of silicon and sulfur, each fusion stage taking a shorter period of time. During the course of their evolution, red supergiants can also contract some and heat to make blue supergiants. The great mass-loss suffered by supergiants can strip some of them of their outer envelopes to the point that we see huge surface enrichments of helium, nitrogen, and carbon that have been made by nuclear fusion. Look for them scattered across the top of the HR diagram. |
| Quote: | Supernovae
Finally, the silicon and sulfur fuse to iron, an element that is incapable of energy-generating fusion reactions. Gravity now wins the war that has been going on for the star's lifetime, and since the iron refuses to support itself, the core catastrophically collapses. The iron breaks down into its component particles, protons, neutrons, and electrons (the constituents of atoms), and the whole mass gets compressed into a tight ball of neutrons only a few tens of kilometers across. The collapse produces a shocking blast wave that rips through the surrounding nuclear fusing shells and the remaining outer envelope, and rips the rest of the star apart. On Earth we see the star explode in a grand " supernova," an event so powerful it is easily visible even in another galaxy a huge distance away. The part of the star that is exploded outward is so hot that nuclear reactions produce all the chemical elements, including a tenth of a solar mass of iron, which then blend with the gasses of interstellar space, out of which new stars are formed. |
| Quote: | Frequency and candidates
There are ways of making supernovae other than through core collapse. Nevertheless, supernovae are still rare, taking place in our Galaxy only two or three times a century. Most are hidden from us by the vast clouds of dust that birth the stars. On Earth we observe about five supernovae per millennium, and have not seen one since Kepler's Star of 1604 (probably created in the collapse of a white dwarf, as described later). The great supernovae of 1006 , 1054 (the "Chinese Guest Star"), and 1572 (Tycho's Star) were visible in daylight. Our knowledge of supernovae comes almost entirely from observing them in other galaxies, the best of these exploding in 1987 (SN 1987a)in the Large Magellanic Cloud, a companion to our Galaxy some 165,000 light years away. But keep your eye on Betelgeuse or Antares, which are quite good candidates for core collapse. An even better candidate is the southern hemisphere's Eta Carinae, which underwent a huge eruption in the 19th century and produced a surrounding nebula, a vast cloud of dusty gas. The star should go off within the next million years or so. At their current distances, the explosions of such stars would rival the brightness of a crescent Moon. The blast is so powerful that it if occurred within 30 or so light years, it would probably damage the Earth. Fortunately, no candidate is nearly that close (though such nearby events have almost certainly happened in the past). |
| Quote: | Supernova remnants
As the debris of a supernova clears, we see a gaseous expanding shell around the old star, the "supernova remnant," the debris rich in the by-products of myriad nuclear reactions. We believe all the iron in the Universe has come from such (and related) explosions. Indeed, between ordinary giants, planetary nebulae, and supernovae, all the elements other than hydrogen and helium were created in stars. The most famous supernova remnant is the Crab Nebula in Taurus, the remains of the great supernova of 1054, which was well observed by Chinese astronomers. Tens of thousands of years after the explosion we can still see the mighty blast waves sweeping through the gases of interstellar space, compressing them and perhaps making new stars. |
| Quote: |
Neutron stars and pulsars
At the center of the expanding cloud is a lone neutron star spinning many times per second, with a mass greater than the Sun, a diameter the size of a small town, and an amazing density of 100 million tons per cubic centimeter.The magnetic fields of such collapsed stars are magnified along with the density to strengths millions of millions of times that of Earth. The magnetism is so strong that radiation is beamed out the magnetic axis. The axis is tilted relative to the rotation axis (like that of the Earth), and wobbles around as the little star spins, the beamed energy spraying into space. From a distance, the star looks like a lighthouse: if the Earth is in the way, we get a blast of radiation, and from here see the neutron star as a "pulsar. " Young pulsars emit from low-energy radio waves through high- energy X-rays and gamma rays. As the pulsar ages, it slows, and finally emits only radio waves, which is the case for most of the 600 or so pulsars known. When the rotation period is about 4 seconds there is insufficient energy for the pulsar to be seen at all, and it disappears from view. Not fusing anything, the neutron star is held up forever against gravity by pressure exerted its own extreme density. |
| Quote: | Black holes
The collapsing star of a supernova will turn into a neutron star only if its mass is less than about two or three times that of the Sun. If the mass is greater, then even the star's huge density cannot hold gravity back, and instead of a neutron star the supernova creates a "star" that nothing can support against gravity, and the body contracts forever. At a small enough radius, the gravitational force becomes so great that light can not escape, and the star disappears forever into a collapsing "black hole." What we refer to as the black hole is actually a kind of "surface" at which the velocity required for escape equals light-speed. What goes on inside is unknown. |
| Quote: | Double stars
Most of stars you see at night have companions, a great many obviously double even through a modest telescope. The components of some double stars are nearly equal in mass and brightness. More commonly, one dominates the other, sometimes to the point where a little companion is not really visible at all, and detectable only with the most sophisticated techniques. At the lowest end, we have stars with low-mass brown dwarfs for companions. The stars of some doubles are so far apart that they take thousands of years to orbit; others are so close that they revolve around each other in only days or even hours. Gravitational theory allows us to measure the masses of the stars from the orbits' characters; indeed such measurements are the only way in which we can find stellar masses. Examples of visually-seen double stars are Alpha Centauri, Acrux, Almach, Albireo, and Mizar. |
| Quote: |
Formation of double stars
Formation is still contended. The oldest idea involves simple fission. When a new star condenses from the interstellar gases, it spins faster. If the contracting blob is spinning rapidly enough, it can separate or otherwise develop into a pair or stars rather than a single star. Each of these contracting components can further separate into a double, producing a "double-double" star, the most famous of which is fourth magnitude Epsilon Lyrae. Even more complicated multiples exist. More likely scenarios involve capture with a dense stellar environment, fragmentation of the collapsing birthcloud, and condensation of a companion from a the circumstellar disk that surrounds a new-born star. |
| Quote: | Eclipsing double stars
If the two stars of a pair are fairly close together, and if the plane of the orbit is close to the line of sight, each star can get in the way of the other every orbital turn, and we see a pair of eclipses, one of which is usually of much greater visibility than the other. Eclipsing systems are very important in stellar astronomy, and are used to help determine masses, to find the stars' diameters, temperatures, and even to assess shapes in the cases that the stars' mutual gravities distort each other. Eclipsing doubles are quite common, the most famous second magnitude Algol in Perseus. |
| Quote: | Evolution of wide double stars
In a double star system in which the two have significantly different masses (by far the most common), the higher mass star will use its internal hydrogen fuel the fastest and become a giant first. We then see a red giant, or maybe a helium-fusing, orange class K giant coupled with a main sequence star, also very common. Eventually, the giant produces its planetary nebula and dies as a white dwarf. Good examples of such systems are Sirius and Procyon, each of which are orbited by the tiny dead stars. For each of these systems, and for many others, the white dwarf is by far the LESS massive of the pair, proving that stars really do lose a great deal of their mass back into interstellar space. |
| Quote: | Evolution of close double stars
If the two stars of a double are close together, they can interact. When the more massive becomes a giant, its surface significantly approaches that of the other star. The lower-mass main sequence star can then raise tides in the giant, distorting it. If the two are close enough, matter can flow from the giant to the main sequence star. Good examples that display such behavior are Algol and Sheliak. In more extreme cases, the lost matter can encompass both stars, creating a "common envelope." Friction will then bring the stars even closer together, making the process go yet faster. The stirring of the lost mass can create unusually distorted planetary nebulae. At the end, the white dwarf created from the giant finds itself very close to the remaining main sequence star. In high mass double stars, the higher-mass component can explode and produce a nearby neutron star or even a black hole companion. |
| Quote: | Contamination
Some giant stars have the masses and internal constructions that allow them to bring by-products of deep nuclear fusion to the stars' surfaces, in the most extreme examples creating carbon stars. Mass lost from one of these enriched giants to a close companion can contaminate the companion with the giant's newly- formed chemical elements. When the giant becomes a white dwarf we are left with a seemingly single star (main sequence or evolved giant) with an odd chemical composition. Only with determined observation can we tell that a dim white dwarf is present. Among the most prominent examples are "barium stars" (Alphard an example), giants that have very strong absorptions -- and great overabundances -- of the heavy element barium among several others. All seem to be companions of what were once mightier stars that had become carbon stars and that are now reduced to white dwarfs. |
| Quote: | Novae
If the white dwarf and main sequence remnant of a close double are close enough, the white dwarf can raise tides in the main sequence star, and mass will flow the other way, from the main sequence star to the white dwarf. Theory and observation both show that the flowing matter first enters a disk around the white dwarf from which it falls onto the white dwarf's surface. Instabilities in the disk can make such a star "flicker" over periods of days and weeks, even producing sudden outbursts of light. The star that became the white dwarf had lost almost all of its hydrogen envelope during its own evolution. When enough fresh hydrogen from the main sequence star has fallen onto the white dwarf, it can, in the nuclear sense, ignite, fusing suddenly and explosively to helium. The surface of the white dwarf blasts into space, the star becoming temporarily vastly brighter. On Earth we see a "new" star or " nova" (meaning "new in Latin) erupt into the nighttime sky, not a new star at all but an old one undergoing eruption. Novae are common, 25 or so going off in the Galaxy every year, once a generation one close enough to reach first magnitude. Nova Cygni in 1975 rivalled Deneb, giving the celestial Swan two tails. |
| Quote: | X-rays, neutron stars, and black holes
In a massive double star system, the more massive of the pair may develop an iron core and explode as a supernova, becoming either a neutron star or a black hole. Either of these stellar remains in turn may raise tides in the more-normal companion, causing matter to flow into a disk around the collapsed body, from which it falls into an immense gravitational field. Matter in the disk is so hot it can radiate X-rays. From the motion of the normal star, we can calculate information on the mass of the collapsed one. If the mass of the dark orbiting companion is low, it is a neutron star. But if the mass is great enough, we can infer the existence of an orbiting black hole, the best actual proof we have. Fresh hydrogen falling from the disk onto a neutron star can produce great variability, become compressed and fuse to helium, and then explode violently as the helium fuses to carbon. The result is an X-ray burst similar in nature to a nova. |
| Quote: | White dwarf supernovae
The term "supernova" is derived from "nova" in that the supernova is vastly brighter, no matter that the mechanism of the core collapse of a supergiant is completely different from the mechanism of nova production. White dwarfs, however, can also produce supernovae. No white dwarf can exceed a mass of 1.4 times that of the Sun, a limit discovered in the 1930s by Subramanyan Chandrasekhar when he applied relativity theory to the gases in white dwarfs. If the limit is exceeded, even the white dwarf's enormous pressure cannot hold gravity back and the white dwarf must collapse into a neutron star or a black hole or perhaps even annihilate itself. There are two alternative theories for such an event. A massive white dwarf may accept enough mass from a close main sequence companion and be pushed over the edge before a nova eruption can take place. The white dwarf then collapses, creating a supernova that is grander even than one produced by the collapse of a supergiant's iron core. The main sequence star of a double that contains a white dwarf can also evolve through the giant stage to become a white dwarf, creating a DOUBLE white dwarf system. If the two have been drawn close enough together by interaction during a common envelope phase, they can spiral together by the radiation of gravitational waves predicted by relativity theory. The white dwarfs then merge, again producing a spectacular supernova. In either case, the collapse and resulting explosion makes nuclear reactions that again create all the chemical elements and even more iron than in the type of supernova produced by the collapse of the iron core of a massive star. Kepler's supernova of 1604, the last seen in this Galaxy, was probably of this kind. |
| Quote: | Stars and cosmic recycling
Stars can range in size, depending on mass and age, from only a few kilometers across to the diameter of the orbit of perhaps Saturn. They can range in temperature from near "cold" at only 2000 K for an extreme red giant through far over 100,000 K for the star inside a planetary nebula to over a million K for a neutron star. All the stars you see in the sky will eventually expire, some soon, some not for aeons. Lower mass stars create planetary nebulae and white dwarfs, while higher mass stars make supernovae that result in neutron stars or black holes. Double stars add spice to the product, making novae and a different kind of supernova. All these endings send newly made chemical elements into the interstellar stew, out of which new stars are made. As a result, the heavy element content of the Galaxy increases with time. Ancient main sequence stars, the "subdwarfs," and their giant star progeny have a low abundance of metals, whereas younger stars like the Sun have higher metal contents, allowing us to track the oldest and youngest stars and to determine the age of the Galaxy. New stars therefore contain the by-products of the old, our Earth a distillate of earlier generations. Our Sun will someday make its own contribution, however modest it may be, to generations of stars and planets yet unborn. |
|
|
| Back to top |
|
 |
DrewTerry
Guest
|
| Posted: Sun Jan 28, 2007 2:14 pm Post subject: Gamma Ray Burst |
|
|
| Quote: |  Explanation: It is still not known why the Sun's light is missing some colors. Shown above are all the visible colors of the Sun, produced by passing the Sun's light through a prism-like device. The above spectrum was created at the McMath-Pierce Solar Observatory and shows, first off, that although our yellow-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light. The dark patches in the above spectrum arise from gas at or above the Sun's surface absorbing sunlight emitted below. Since different types of gas absorb different colors of light, it is possible to determine what gasses compose the Sun. Helium, for example, was first discovered in 1870 on a solar spectrum and only later found here on Earth. Today, the majority of spectral absorption lines have been identified - but not all. |
| Quote: | Here's another example - the below pictures show the planet Uranus in true-color (on the left) and false-color (on the right).
The true-color has been processed to show Uranus as human eyes would see it from the vantage point of the Voyager 2 spacecraft, and is a composite of images taken through blue, green and orange filters. The false color and extreme contrast enhancement in the image on the right, brings out subtle details in the polar region of Uranus. The very slight contrasts visible in true color are greatly exaggerated here, making it easier to studying Uranus' cloud structure. Here, Uranus reveals a dark polar hood surrounded by a series of progressively lighter concentric bands. One possible explanation is that a brownish haze or smog, concentrated over the pole, is arranged into bands by zonal motions of the upper atmosphere.
What does Visible Light show us?
It is true that we are blind to many wavelengths of light. This makes it important to use instruments that can detect different wavelengths of light to help us to study the Earth and the Universe. However, since visible light is the part of the electromagnetic spectrum that our eyes can see, our whole world is oriented around it. And many instruments that detect visible light can see father and more clearly than our eyes could alone. That is why we use satellites to look at the Earth, and telescopes to look at the Sky! |
|
|
| Back to top |
|
|
DrewTerry
Guest
|
| Posted: Wed Feb 07, 2007 8:22 am Post subject: |
|
|
This is from NASA:
| Quote: | What does Visible Light show us?
It is true that we are blind to many wavelengths of light. This makes it important to use instruments that can detect different wavelengths of light to help us to study the Earth and the Universe. However, since visible light is the part of the electromagnetic spectrum that our eyes can see, our whole world is oriented around it. And many instruments that detect visible light can see father and more clearly than our eyes could alone. That is why we use satellites to look at the Earth, and telescopes to look at the Sky!  
|
| Quote: | The below pictures show the planet Uranus in true-color (on the left) and false-color (on the right).
The true-color has been processed to show Uranus as human eyes would see it from the vantage point of the Voyager 2 spacecraft, and is a composite of images taken through blue, green and orange filters. The false color and extreme contrast enhancement in the image on the right, brings out subtle details in the polar region of Uranus. The very slight contrasts visible in true color are greatly exaggerated here, making it easier to studying Uranus' cloud structure.
Here, Uranus reveals a dark polar hood surrounded by a series of progressively lighter concentric bands. One possible explanation is that a brownish haze or smog, concentrated over the pole, is arranged into bands by zonal motions of the upper atmosphere. |
| Quote: | | This is 'us' to the computer (in wavelengths of 10/1,000,000,000 hertz): Since the primary source of infrared radiation is heat or thermal radiation, any object which has a temperature radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared. |
| Quote: | The image shows a cat in the infrared. The orange areas are the warmest and the white-blue areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture.  |
| Quote: |  When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. The warmer the object, the more infrared radiation it emits. This image shows a man holding up a lighted match! Which parts of this image do you think have the warmest temperature? How does the temperature of this man's glasses compare to the temperature of his hand?
Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns. (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter). Thats small enough for it to feel like flesh, right? |
| Quote: |  Instruments on board satellites can also take pictures of things in space. The image below of the center region of our galaxy was taken by IRAS. The hazy, horizontal S-shaped feature that crosses the image is faint heat emitted by dust in the plane of the Solar System |
| Quote: |  Many things besides people and animals emit infrared light - the Earth, the Sun, and far away things like stars and galaxies do also!
For a view from Earth orbit, whether we are looking out into space or down at Earth, we can use instruments on board satellites. |
Humans may not be able to see infrared light, but did you know that snakes in the pit viper family, like rattlesnakes, have sensory "pits", which are used to image infrared light?
This allows the snake to detect warm blooded animals, even in dark burrows! Snakes with 2 sensory pits are even thought to have some depth perception in the infrared!
| Quote: | What do gamma-rays "show" us?
Gamma-rays have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum.
These waves are generated by radioactive atoms and in nuclear explosions. Gamma-rays can kill living cells, a fact which medicine uses to its advantage, using gamma-rays to kill cancerous cells. |
| Quote: | How do we "see" using gamma-ray light?  Gamma-rays travel to us across vast distances of the universe, only to be absorbed by the Earth's atmosphere.
Different wavelengths of light penetrate the Earth's atmosphere to different depths. Instruments aboard high-altitude balloons and satellites like the Compton Observatory provide our only view of the gamma-ray sky.
Gamma-rays are the most energetic form of light and are produced by the hottest regions of the universe.
They are also produced by such violent events as supernova explosions or the destruction of atoms, and by less dramatic events, such as the decay of radioactive material in space. Things like supernova explosions (the way massive stars die), neutron stars and pulsars, and black holes are all sources of celestial gamma-rays.
Gamma-ray astronomy did not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope, carried into orbit on the Explorer XI satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons!
Unlike optical light and X-rays, gamma rays cannot be captured and reflected in mirrors. The high-energy photons would pass right through such a device.
Gamma-ray telescopes use a process called Compton scattering,  where a gamma-ray strikes an electron and loses energy,
similar to a cue ball striking an eight ball.  If you could see gamma-rays, the night sky would look strange and unfamiliar.
The gamma-ray moon just looks like a round blob - lunar features are not visible.
In high-energy gamma rays, the Moon is actually brighter than the quiet Sun.[/color] |
| Quote: | Satellites like GOES 6 and Landsat 7 look at the Earth. Special sensors, like those aboard the Landsat 7 satellite, record data about the amount of infrared light reflected or emitted from the Earth's surface.
[color=blue] Except for all these images that always looked to me like they could have come out of a petri dish with an electron microscope.
We have to 'bombard electrons' with 'neutrons as bullets' to create a pattern we recognize (chaos?) we then assume are what gamma rays "look like" ?
Is that not a little bit like trying to figure out where all the balls on the pool table are for your shot by randomly pitching a ball in from an unknown origin off one of the sides and then noting where they end up' but we have to guess where they would have stopped because on this table, as in space through the known universe,
"It is a table universe full of planetary balls in constant, never-resting motion of endless variety angles, inclination and in the matter of the balls hitting, moving, breaking, hitting' that never stops, even when it may appear to not be moving, there is the rotation of the table itself as it turns and angles up and down on the horizontal axis, turning as it rotates on the x axis thru the imaginary center but tipped off-center 23.5º - one turn and one rotation (x,y) per day; and the balls are filled with a core of iron ore of various densities that results in a univeral magnetation and levitation off the surface when upright, or as clinging to the bottom when upside down (and hanging on nothing when it is turned sideways). |
|
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
|
