Module 10 Quiz
Differentiate between the different stages of a star and describe what elements are formed during each stage. f. Beginning of the end for larger stars i. supernova ii. neutron star iii. black hole
Beginning of the End for Massive Stars. A more massive star will have a different theoretical ending than the slow cooling of a white dwarf. A massive star will contract, just as the less massive stars, after blowing off its outer shells. In a more massive star, however, heat from the contraction may reach the critical temperature of 600 million kelvins to begin carbon fusion reactions. Thus, a more massive star may go through a carbon fusing stage and other fusion reaction stages that will continue to produce new elements until the element iron is reached. (See binding energy and figure 11.10 on page 252.) After iron, energy is no longer released by the fusion process, and the star has used up all of its energy sources. Lacking an energy source, the star is no longer able to maintain its internal temperature. The star loses the outward pressure of expansion from the high temperature, which had previously balanced the inward pressure from gravitational attraction. The star thus collapses, then rebounds like a compressed spring into a catastrophic explosion called a supernova. A supernova produces a brilliant light in the sky that may last for months before it begins to dim as the new elements that were created during the life of the star diffuse into space. These include all the elements up to iron that were produced by fusion reactions during the life of the star and heavier elements that were created during the instant of the explosion. All the elements heavier than iron were created as some less massive nuclei disintegrated in the explosion, joining with each other and with lighter nuclei to produce the nuclei of the elements from iron to uranium. As you will see in chapter 13, these newly produced, scattered elements will later become the building blocks for new stars and planets such as the Sun and Earth. The remains of the compressed core after the supernova have yet another fate if the core has a remaining mass greater than 1.4 solar masses. The gravitational forces on the remaining matter, together with the compressional forces of the supernova explosion, are great enough to collapse nuclei, forcing protons and electrons together into neutrons, forming the core of a neutron star. A neutron star is the very small (10 to 20 km diameter), superdense (10^11 kg/cm^3 or greater) remains of a supernova with a center core of pure neutrons. Because it is a superdense form of matter, the neutron star also has an extremely powerful magnetic field, capable of becoming a pulsar. A pulsar is a very strongly magnetized neutron star that emits a uniform series of equally spaced electromagnetic pulses. Evidently, the magnetic field of a rotating neutron star makes it a powerful electric generator, capable of accelerating charged particles to very high energies. These accelerated charges are responsible for emitting a beam of electromagnetic radiation, which sweeps through space with amazing regularity (figure 12.10). The pulsating radio signals from a pulsar were a big mystery when first discovered. For a time, extraterrestrial life was considered as the source of the signals, so they were jokingly identified as LGM (for "little green men"). Over three hundred pulsars have been identified, and most emit radiation in the form of radio waves. Two, however, emit visible light, two emit beams of gamma radiation, and one emits X-ray pulses. Another theoretical limit occurs if the remaining core has a mass of about 3 solar masses or more. At this limit, the force of gravity overwhelms all nucleon forces, including the repulsive forces between like charged particles. If this theoretical limit is reached, nothing can stop the collapse, and the collapsed star will become so dense that even light cannot escape. The star is now a black hole in space. Since nothing can stop the collapsing star, theoretically a black hole would continue to collapse to a pinpoint and then to a zero radius called a singularity. This event seems contrary to anything that can be directly observed in the physical universe, but it does agree with the general theory of relativity and concepts about the curvature of space produced by such massively dense objects. Black holes are theoretical and none has been seen, of course, because a black hole theoretically pulls in radiation of all wavelengths and emits nothing. Evidence for the existence of a black hole is sought by studying X rays that would be given off by matter as it is accelerated into a black hole. Another form of evidence for the existence of a black hole has now been provided by the Hubble Space Telescope. Hubble pictured a disk of gas only about 60 light-years out from the center of a galaxy (M87), moving at more than 1.6 million km/h (about 1 million mi/h). The only known possible explanation for such a massive disk of gas moving with this velocity at the distance observed would require the presence of a 1-2 billion solar-mass black hole. This gas disk could only be resolved by the Hubble Space Telescope, so this telescope has provided the first observational evidence of a black hole.
Describe what elements make up 99% of the known universe a. dark matter
-Most of the stars we see in the universe consist of 93 percent hydrogen, 6 percent helium, and only 1 percent all other elements (this has been determined by analyzing the light from the stars). -Here on earth, things including our bodies, consist mainly of other elements besides hydrogen. But this is a special case; the stars are made mostly of hydrogen. -Visible matter is only 10% of total matter. 90% of the matter of the universe is called dark matter because we don't see it and don't know what it is. We know dark matter is there because it attracts other matter by gravity. Motions of stars and galaxies cannot be explained by the gravitational attraction of the visible, luminous matter. Stars in the neighborhood of galaxies move much faster than they would if they were attracted only by the visible stars. We don't know the nature of this dark matter, so we don't have the slightest idea what 90% of the world is made of.
Describe the nature of the universe at the following timeframes just after The Big Bang. d. a fraction of a second--> a second
A fraction of a second after the Bang At a fraction of a second after the Bang, the temperature was high enough to split the helium and nuclei into neutrons and protons. One Second After the Big Bang (and earlier) Going back in time, we come to a moment at about one second after the Big Bang, when the temperature was about 10 billion degrees, corresponding to an energy concentration of about a few million electron volts. At that point the thermal energy is high enough for creating pairs of electrons and antielectrons (positrons). This is the process of matter-antimatter formation mentioned before. Hence, at one second and earlier, when the temperature was even higher, space was filled by a plasma composed not only of hydrogen and helium nuclei and their electrons but also of a rather dense gas of electrons and positrons.
Describe the nature of the universe at the following timeframes just after The Big Bang. a. "The Bang"-->a millionth of a second after
At this point of our backward journey in time, the universe was filled with hot, dense gases of quarks and antiquarks, electrons and positrons, and a very intense, high frequency thermal light radiation. There was also a hot, dense gas of neutrinos, which survived the whole evolution and should be present even today, though much less hot and dense, together with the cool three-degree light radiation. We stop at this point, which is a millionth of a second after the Big Bang. We are practically at it anyway.
Differentiate between the different stages of a star and describe what elements are formed during each stage.
A star is born in a gigantic cloud of gas and dust in interstellar space, then spends billions of years calmly shining while it fuses hydrogen nuclei in the core. How long a star shines and what happens to it when it uses up the hydrogen in the core depends on the mass of the star. Of course, no one has observed a life cycle of over billions of years. The life cycle of a star is a theoretical outcome based on what is known about nuclear reactions. The predicted outcomes seem to agree with observations of stars today, with different groups of stars that can be plotted on the H-R diagram. Thus, the groups of stars on the diagram—main sequence, red giants, and white dwarfs, for example—are understood to be stars in various stages of their lives.
Differentiate between the different stages of a star and describe what elements are formed during each stage. d. Back toward main sequence
Back Toward Main Sequence After about 500 million years as a red giant, the star now has a surface temperature of about 4,000 kelvins compared to its main sequence surface temperature of 6,000 kelvins. The radius of the red giant is now a thousand times greater, a distance that will engulf Earth when the Sun reaches this stage, assuming Earth is in the same position as it is today. Even though the surface temperature has decreased from the expansion, the helium core is continually heating and eventually reaches a temperature of 100 million kelvins, the critical temperature necessary for the helium nuclei to undergo fusion to produce carbon. The red giant now has helium fusion reactions in the core and hydrogen fusion reactions in a shell around the core. This changes the radius, the surface temperature, and the brightness with the overall result depending on the composition of the star. In general, the radius and brightness decrease when this stage is reached, moving the star back toward the main sequence (figure 12.7).
Differentiate between the different stages of a star and describe what elements are formed during each stage. e. Beginning of the end for smaller stars i. white dwarf ii. black lump of carbon
Beginning of the End for Less Massive Stars. After millions of years of helium fusion reactions, the core is gradually converted to a carbon core and helium fusion begins in the shell surrounding the core. The core reactions decrease as the star now has a helium-fusing shell surrounded by a second, hydrogen-fusing shell. This releases additional energy and the star again expands to a red giant for the second time. A star the size of the Sun or less massive may cool enough at this point that nuclei at the surface become neutral atoms rather than a plasma. As neutral atoms, they can absorb radiant energy coming from within the star, heating the outer layers. Changes in temperature produce changes in pressure, which change the balance between the temperature, pressure, and the internal energy generation rate. The star begins to expand outward from heating. The expanded gases are cooled by the expansion process, however, and are pulled back to the star by gravity, only to be heated and expand outward again. In other words, the outer layers of the star begin to pulsate in and out. Finally, a violent expansion blows off the outer layers of the star, leaving the hot core. Such blown-off outer layers of a star form circular nebulae called planetary nebulae (figure 12.8). The nebulae continue moving away from the core, eventually adding to the dust and gases between the stars. The remaining carbon core and helium-fusing shell begin gravitationally to contract to a small, dense white dwarf star. A star with the original mass of the Sun or less slowly cools from white to red, then to a black lump of carbon in space.
Describe the four steps of the quantum ladder
How did the universe begin about 12 billion years ago? The question concerns the very large—space, galaxies, etc.—but also the very small, namely the innermost structure of matter. The reason is that the early universe was very hot, so that matter was then decomposed into its constituents. These two topics hang together, and this is what makes them so interesting. The quantum ladder is the innermost structure of matter in steps. The innermost structure of matter is made of atoms, with a nucleus in the middle and electrons around it. The nucleus is made of protons and neutrons, called nucleons. The protons and neutrons (nucleons) are made up of quarks. Quantum ladder: -The deeper you go, the stronger the forces become 1) In the piece of metal, the chemical force that keeps the atoms together has the strength of a few electron volts (this is a measure of force strength) 2) In the atom, the electrons are bound to the nucleus by a few tens of electron volts 3) The protons and neutrons are bound within the nucleus by millions of electron volts 4) The forces between the quarks in a nucleus are in the billions of electron volts Nucleons are composed of quarks Electrons have never been shown to be composite Heating a piece of material (temperature) is equivalent to increasing the energy of motion of the constituents of that piece, be they atoms or electrons or other particles. On the last rung of the quantum ladder, where billions of electron volts are available, by means of accelerators or when the universe was very hot, new phenomena appear. This is called the subnuclear realm. Antimatter plays an important role at that stage. There is an antiparticle to every particle. -antielectron called a positron -antiproton -antineutron -antiquark -antiatom -antimolecule -antimatter of all sorts made of anti-electrons and antinuclei They carry the opposite charge of the actual particle We don't find antimatter in our environment because when an antiparticle hits a particle, they annihilate. A small explosion occurs, and the two entities disappear in a burst of light energy. This is in agreement with E= mc2 which says that mass (the masses of the particle and antiparticle) is a form of energy. Also, a high concentration of energy can give rise to the birth of a particle and antiparticle. This is called pair creation. To summarize the quantum ladder, let me quote a prophetic statement by Newton, who wrote three hundred years ago: Now the smallest particles of matter may cohere by the strongest attractions, and compose bigger particles of weaker virtue. And many of these may cohere, and compose bigger particles whose virtue is still weaker. And so on for diverse successions, until the progression ends in the biggest particles on which the operation in chemistry and the colors of natural bodies depend, which by cohering compose bodies of a sensible magnitude... ...like that piece of metal. He foresaw the ideas of the structure of matter that were developed centuries after his time
Describe what evidence is involved with an understanding that the universe is expanding a. Hubble's law
Hubble's law, also known as the Hubble-Lemaître law,[1] is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit to the red end of the spectrum. One of the first measurements of the distance to other galaxies was made by Edwin Hubble at Mount Wilson Observatory in California. When Hubble compared the distance figures with the observed redshifts, he found that the recession speeds were proportional to the distance. Farther-away galaxies were moving away from the Milky Way, but galaxies that are more distant are moving away faster than closer galaxies. This proportional relationship between galactic speed and distances was discovered in 1929 by Hubble and today is known as Hubble's law. The conclusion was that all the galaxies are moving away from one another and an observer on any given galaxy would have the impression that all galaxies were moving away in all directions. In other words, the universe is expanding with component galaxies moving farther and farther apart.
Differentiate between the different stages of a star and describe what elements are formed during each stage. b. main sequence
Main Sequence Stage Where the star is located on the main sequence and what happens to it next depend only on how massive it is. The more massive stars have higher core temperatures and use up their hydrogen more rapidly as they shine at higher surface temperatures (O type stars). Less massive stars shine at lower surface temperatures (M type stars) as they use their fuel at a slower rate. The overall life span on the main sequence ranges from millions of years for O type stars to trillions of years for M type stars. An average one-solar-mass star will last about 10 billion years.
Describe the nature of the universe at the following timeframes just after The Big Bang. c. a microsecond--> a fraction of a second
Microsecond after the Bang- the head and the corresponding energy concentration were high enough not only to decompose protons and neutrons into quarks but also to produce quark-antiquark pairs. A fraction of a second after the Bang-At a fraction of a second after the Bang, the temperature was high enough to split the helium and nuclei into neutrons and protons.
Describe the nature of the universe at the following timeframes just after The Big Bang. b. a millionth of a second--> a microsecond
Millionth of a second after the big bang- At this point of our backward journey in time, the universe was filled with hot, dense gases of quarks and antiquarks, electrons and positrons, and a very intense, high frequency thermal light radiation. There was also a hot, dense gas of neutrinos, which survived the whole evolution and should be present even today, though much less hot and dense, together with the cool three-degree light radiation. Stopping at this point is a millionth of a second after the Big Bang. Microsecond after the Bang- the head and the corresponding energy concentration were high enough not only to decompose protons and neutrons into quarks but also to produce quark-antiquark pairs.
Differentiate between "Main Sequence", red giant, and white dwarf stars
Most of the stars plotted on an H-R diagram fall in or close to a narrow band that runs from the top left to the lower right. This band is made up of main sequence stars. Stars along the main sequence band are normal, mature stars that are using their nuclear fuel at a steady rate. Those stars on the upper left of the main sequence are the brightest, bluest, and most massive stars on the sequence. Those at the lower right are the faintest, reddest, and least massive of the stars on the main sequence. In general, most of the main sequence stars have masses that fall between a range from ten times greater than the mass of the Sun (upper left) to one-tenth the mass of the Sun (lower-right). The extremes, or ends, of the main sequence range from about sixty times more massive than the Sun to one-twenty-fifth of the Sun's mass. It is the mass of a main sequence star that determines its brightness, its temperature, and its location on the H-R diagram. High-mass stars on the main sequence are brighter, hotter, and have shorter lives than low-mass stars. These relationships do not apply to the other types of stars in the H-R diagram. There are groups of stars that have a different set of properties than the main sequence stars. The red giant stars are bright but low-temperature stars. These reddish stars are enormously bright for their temperature because they are very large, with an enormous surface area giving off light. A red giant might be one hundred times larger but have the same mass as the Sun. These low-density red giants are located in the upper right part of the H-R diagram. The white dwarf stars, on the other hand, are located at the lower left because they are faint, white-hot stars. A white dwarf is faint because it is small, perhaps twice the size of the Earth. It is also very dense, with a mass approximately equal to the Sun's. During its lifetime, a star will be found in different places on the H-R diagram as it undergoes changes. Red giants and white dwarfs are believed to be evolutionary stages that aging stars pass through, and the path a star takes across the diagram is called an evolutionary track. During the lifetime of the Sun, it will be a main sequence star, a red giant, and then a white dwarf.
Describe the nature of the universe at the following timeframes just after The Big Bang. e. a second --> 300,000 years
One Second After the Big Bang (and earlier) Going back in time, we come to a moment at about one second after the Big Bang, when the temperature was about 10 billion degrees, corresponding to an energy concentration of about a few million electron volts. At that point the thermal energy is high enough for creating pairs of electrons and antielectrons (positrons). This is the process of matter-antimatter formation mentioned before. Hence, at one second and earlier, when the temperature was even higher, space was filled by a plasma composed not only of hydrogen and helium nuclei and their electrons but also of a rather dense gas of electrons and positrons. 300,000 years --> microsecond Let us tell the story in reverse, going back in time form 300,000 years to a microsecond. In that inverse sense, the universe must be regarded as contracting and getting hotter. When the temperature was hotter than 10000 degrees K, the atoms were decomposed and formed a "plasma," a dense gas of nuclei and electrons. The plasma was bathed in shining light, visible light, during the time when the temperature was between a thousand and a few ten thousand degrees. That light was more and more ultraviolet (that is, of a higher frequency) at earlier times, when the temperature was higher. This radiation should be considered the same as today's 3 degrees K radiation but enormously compressed at the early seconds of the expansion. Compression makes light hotter and of higher frequency.
Describe the relationship between the volume of space and the temperature of space
Our fifth question has to do with the temperature in the universe. How hot is it out there? Let us consider a kiln, such as potters use, to understand the situation. Take a kiln and heat it up. First you can see no light, but the kiln radiates microwaves. When it gets hotter it radiates infrared radiation, which we do not see but can feel as heat radiation. At higher temperatures it becomes red, then yellow and white, then ultraviolet; at millions of degrees it will radiate X-rays. Today, in the immediate surroundings within a few million light years, the temperature is very low in space. It was measured a few decades ago when two Princeton physicists, A. Penzias and R. Wilson, found a very cool microwave radiation in space corresponding to heat radiation of only 5 degrees above absolute zero—the lowest possible temperature, which is minus 460 degrees F. An appropriate measure of very low temperatures is the Kelvin scale. Zero degree Kelvin is absolute zero. The Kelvin scale uses Celsius degrees above absolute zero. Thus, the space temperature in our neighborhood is 3 degrees K. This is the temperature in space between the stars. The stars are much hotter inside, but there is so much space between them that their higher temperature does not count. Was the temperature always 3 degrees K? No, it was much warmer at earlier times, a fact that is related to the expansion of the universe. Let us go back to the kiln again. Imagine a kiln made in such a way that we can expand or contract its volume at will. The laws of physics tell us that the temperature of a kiln drops when it expands and rises when it contracts. Thus, we must conclude that the expansion of the universe lowers the temperature. It must have been hotter at earlier times. For example, about 6 million years ago, the temperature was roughly twice as high—that is, near 6 degrees K. At the very beginning, about 12 billion years ago, when space was extremely contracted, the temperature must have been extremely high. This has interesting consequences.
Differentiate between the different stages of a star and describe what elements are formed during each stage. a. protostar
Protostar Stage The first stage in the theoretical model of the life cycle of a star is the formation of the protostar. As gravity pulls the gas of a protostar together, the density, pressure, and temperature increase from the surface down to the center. Eventually, the conditions are right for nuclear fusion reactions to begin in the core, which requires a temperature of 10 million kelvins. The initial fusion reaction essentially combines four hydrogen nuclei to form a helium nucleus with the release of much energy. This energy heats the core beyond the temperature reached by gravitational contraction, eventually to 16 million kelvins. Since the star is plasma, the increased temperature expands the volume of the star. The outward pressure of expansion balances the inward pressure from gravitational collapse, and the star settles down to a balanced condition of calmly converting hydrogen to helium in the core, radiating the energy released into space (figure 12.6). The theoretical time elapsed from the initial formation and collapse of the protostar to the main sequence is about 50 million years for a star of a solar mass.
Differentiate between the different stages of a star and describe what elements are formed during each stage. c. Red giant
Red Giant Stage The next stage in the theoretical life of a star begins when much of the hydrogen in the core has been fused into helium. With fewer hydrogen fusion reactions, less energy is released and less outward balancing pressure is produced, so the star begins to collapse. The collapse heats the core, which now is composed primarily of helium, and the surrounding shell where hydrogen still exists. The increased temperature causes the hydrogen in the shell to undergo fusion, and the increased release of energy causes the outer layers of the star to expand. With an increased surface area, the amount of radiation emitted per unit area is less, and the star acquires the properties of a brilliant red giant. Its position on the H-R diagram changes since it now has different brightness and temperature properties. (The star has not physically moved. The changing properties move its temperature brightness data point, not the star, to a new position.)
Describe what evidence is involved with an understanding that the universe is expanding b. red shift/Doppler effect
Redshift and Hubble's Law As described in chapter 5, the Doppler effect tells us that the frequency of a wave depends on the relative motion of the source and observer. When the source and observer are moving toward each other, the frequency appears to be higher. If the source and observer are moving apart, the frequency appears to be lower. Light from a star or galaxy is changed by the Doppler effect, and the frequency of the observed spectral lines depends on the relative motion. The Doppler effect changes the frequency from what it would be if the star or galaxy were motionless relative to the observer. If the star or galaxy is moving toward the observer, a shift occurs in the spectra lines toward a higher frequency (blueshift). If the star or galaxy is moving away from the observer, a shift occurs in the spectral lines toward a lower frequency (redshift). Thus, a redshift or blueshift in the spectral lines will tell you if a star or galaxy is moving toward or away from you.
Describe what limits scientists from being able to see the entire known universe (pg 169). a. cosmic horizon
The cosmic horizon is about 12 billion light years. We now approach the fourth point regarding our present universe. How far can be seen into space? Since the universe is about 12 billion years old, we cannot see farther than about 12 billion light years. We call this distance the cosmic horizon of today. As we will see later in more detail, the Big Bang was a tremendous explosion in which space expanded almost infinitely fast, creating matter over a region probably much larger than what is visible today. Light from those farther regions has not had enough time to reach us today but may do so in the future. There is another interesting consequence: the farther we look within the cosmic horizons, the younger are the objects we see. After all, it took time for the light to reach us. The light we see of a galaxy, say, 100 million light years away, was emitted 100 million years ago. A picture of the galaxy shows how it was 100 million years back. The outer circle is the cosmic horizon. The broken circle is about 6 billion light years away, and objects there appear to us only 6 billion years old. What about objects at or very near the horizon? What we see there is matter in its first moments, matter just or almost just born. Thus, if we had very good telescopes, we could see the whole history of matter in the universe, starting far out and ending near us. Beware of the following misunderstanding. One could wrongly argue that, say, the regions that are 6 billion light years away were much nearer to us when they did send out their light, and therefore we should see them earlier than 6 billion years after emission. This conclusion is false, because the light velocity must be understood as relative to the expanding space. Seen from a nonexpanding frame, a light beam running against the expansion—that is, toward us—moves slower than the usual light velocity. As it were, light is dragged along with the expansion.
Describe what evidence is involved with an understanding that the universe is expanding c. background radiation
The current model of how galaxies form is based on the big bang theory of the creation of the universe. The big bang theory considers the universe to have had an explosive beginning. According to this theory, all matter in the universe was located together in an arbitrarily dense state from which it began to expand, an expansion that continues today. Evidence that supports the big bang theory comes from Albert Einstein's theory of general relativity as well as three areas of physical observations: 1) Expansion of the universe. The initial evidence for the big bang theory came from Edwin Hubble and his earlier work with galaxies. Hubble had determined the distances to some of the galaxies that had redshifted spectra. From this expansive redshift, it was known that these galaxies were moving away from the Milky Way. Hubble found a relationship between the distance to a galaxy and the velocity with which it was moving away. He found the velocity to be directly proportional to the distance; that is, the greater the distance to a galaxy, the greater the velocity. This means that a galaxy twice as far from the Milky Way is moving away from the Milky Way at twice the speed. Since this relationship was seen in all directions, it meant that the universe is expanding uniformly. The same effect would be viewed from any particular galaxy; that is, all the other galaxies are moving away with a velocity proportional to the distance to the other galaxies. This points to a common beginning, a time when all matter in the universe was together. 2) Background radiation. The big bang occurred as some unstable form of energy expanded and cooled, eventually creating matter and space. The initial temperature was 10 billion kelvins or so, and began to cool as the universe expanded. The afterglow of the big bang is called cosmic background radiation. A measurement of cosmic background radiation today agrees with the radiation that should be present according to an expanding model of the universe. 3) Abundance of elements. The proportion of helium in the universe should be about 24 percent, based on an expanding model of the universe. A measurement of the abundance of helium verifies the proportion as predicted by the big bang theory. Cosmic Background Explorer (COBE) spacecraft studied diffuse cosmic background radiation to help answer such questions as how matter is distributed in the universe, whether the universe is uniformly expanding, and how and when galaxies first formed. The 2003 results from NASA's orbiting Wilkinson Microwave Anisotropy Probe (WMAP) produced a precision map of the remaining cosmic microwave background from the big bang. WMAP surveyed the entire sky for a whole year with a resolution some 40 times greater than COBE. Analysis of WMAP data revealed that the univers is 13.7 billion years old, with a 1 percent margin of error. The WMAP data found strong support for the big bang and expanding universe theories. It also revealed that the content of the universe includes 4 percent ordinary matter, 23 percent of an unknown type of dark matter, and 73 percent of a mysterious dark energy.
Describe how matter is distributed throughout space
The second fact concerns the distribution of matter in space. We know that is very uneven. We see stars, but nothing in between; we see galaxies and clusters of galaxies. However, if we average over a large part of space containing many stars and galaxies, we find that luminous matter is very thinly distributed, only about one hydrogen atom per cubic meter. To this we must add ten times as much dark matter.
Describe how a star is born a. nebulae b. protostar
Theoretically, stars are born from swirling clouds of hydrogen gas in the deep space between other stars. Such interstellar (between stars) clouds are called nebulae. These clouds consist of random, swirling atoms of gases that have little gravitational attraction for one another because they have little mass. Complex motions of stars, however, can produce a shock wave that causes particles to move closer together, making local compressions. Their mutual gravitational attraction then begins to pull them together into a cluster. The cluster grows as more atoms are pulled in, which increases the mass and thus the gravitational attraction, and still more atoms are pulled in from farther away. Theoretical calculations indicate that on the order of 1 x 10^57 atoms are necessary, all within a distance of 3 trillion km (about 1.9 trillion mi). When these conditions occur, the cloud of gas atoms begins to condense by gravitational attraction to a protostar, an accumulation of gases that will become a star. Gravitational attraction pulls the average protostar from a cloud with a diameter of trillions of kilometers (trillions of miles) down to a dense sphere with a diameter of 2.5 million km (1.6 million mi) or so. As gravitational attraction accelerates the atoms toward the center, they gain kinetic energy, and the interior temperature increases. Over a period of some 10 million years of contracting and heating, the temperature and density conditions at the center of the protostar are sufficient to start nuclear fusion reactions. Pressure from hot gases and energy form increasing fusion reactions begin to balance the gravitational attraction over the next 17 million years, and the newborn average star begins its stable life, which will continue for the next 10 billion years. The interior of an average star, such as the Sun, is modeled after the theoretical pressure, temperature, and density conditions that would be necessary to produce the observed energy and light from the surface. This model describes the interior as a set of three shells: (1) the core, (2) a radiation zone, and (3) the convection zone (figure 12.3). Our model describes the core as a dense, very hot region where nuclear fusion reactions release gamma and X-ray radiation. The density of the core is about twelve times that of solid lead. Because of the exceedingly hot conditions, however, the core remains in a plasma state even at this density. The model describes the radiation zone as less dense than the core, having a density about the same as the density of water. Energy in the form of gamma and X rays from the core is absorbed and reemitted by collisions with atoms in this zone. The radiation slowly diffuses outward because of the countless collisions over a distance comparable to the distance between the Earth and the Moon. It could take millions of years before this radiation finally escapes the radiation zone. In the model, the convection zone begins about seven-tenths of the way to the surface, where the density of the plasma is about 1 percent the density of water. Plasma at the bottom of this zone is heated by radiation from the radiation zone below, expands from the heating, and rises to the surface by convection. At the surface, the plasma emits energy in the form of visible light, ultraviolet radiation, and infrared radiation, which moves out into space. As it loses energy, the plasma contracts in volume and sinks back to the radiation zone to become heated again, continuously carrying energy from the radiation zone to the surface in convection cells. The surface is continuously heated by the convection cells as it gives off energy to space, maintaining a temperature of about 5,800 K (about 5,500 degrees Celsius). As an average star, the Sun converts about 1.4 x 10^17 kg of matter to energy every year as hydrogen nuclei are fused to produce helium. The Sun was born about 5 billion years ago and has sufficient hydrogen in the core to continue shining for another 4 or 5 billion years. Other stars, however, have masses that are much greater or much less than the mass of the Sun so they have different life spans. More massive stars generate higher temperatures in the core because they have a greater gravitational contraction from their greater masses. Higher temperatures mean increased kinetic energy, which results in increased numbers of collisions between hydrogen nuclei with the end result an increased number of fusion reactions. Thus, a more massive star uses up its hydrogen more rapidly than a less massive star. On the other hand, stars that are less massive than the Sun use their hydrogen at a slower rate so they have longer life spans. The life spans of the stars range from a few million years for large, massive stars, to 10 billion years for average stars such as the Sun, to trillions of years for small, less massive stars.
1. Explain why we are made of "star-stuff".
We're made of star stuff," Sagan famously stated in one episode. His statement sums up the fact that the carbon, nitrogen and oxygen atoms in our bodies, as well as atoms of all other heavy elements, were created in previous generations of stars over 4.5 billion years ago.
