Astronomy chapter summaries for ASTR Chapter 16 PDF

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Astronomy chapter summaries for ASTR Chapter 16...

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Chapter 16 1. Stellar Nurseries Where do stars form? The black patches within the Milky Way are interstellar gas clouds that appear dark because they block our view of stars behind them. These gas clouds provide the raw material for star formation. The Interstellar Medium The gas and dust found in the spaces between stars are the interstellar medium. The gas between the stars is composed mostly of hydrogen and helium. We use spectroscopy to measure the abundances of the new elements that stars have added to the interstellar medium. The most straightforward technique is to observe the spectrum of a star whose light has passed through an intervening cloud of interstellar gas. The cloud absorbs some of the star’s light, leaving absorption lines in the star’s spectrum. The wavelengths of the lines tell us that chemical contents of the cloud, and comparing the amounts of light absorbed by different atoms and molecules allows us to determine the composition of the cloud. The interstellar medium consists (by mass) of 70% hydrogen, 28% helium, and 2% heavier elements. Virtually all the gas between stars in the Milky Way Galaxy has approximately the same chemical composition, but they can look very different from place to place. Some clouds are extremely hot but low in density, some are cold and relatively dense, and others have conditions in between. Star-Forming Clouds Stars are born in interstellar clouds that are particularly cold and dense. These clouds are usually called molecular clouds because they are cold enough and dense enough to allow atoms to combine together into molecules. The temperature of a molecular cloud is typically between 10K and 30K. The average density is typically about 300 molecules per cubic centimeter. Molecular hydrogen (H2) is by far the most abundant molecule in these clouds, because hydrogen and helium are most abundant elements and helium atoms do not combine with other atoms into molecules. Despite its abundance, molecular hydrogen is difficult to detect, because molecular clouds are usually too cold for H2 to produce emission lines that we can study in spectra. Interstellar Dust Not all of the material in a molecular cloud is gaseous. Elements such as carbon, silicon, oxygen and iron are found in tiny, solid grains of interstellar dust. Overall, about half the atoms of elements heavier than helium are found in dust grains. We conclude that interstellar dust grains constitute about 1% of a molecular cloud’s mass. Despite the tiny size of these grains, there are so many of them that they profoundly affect how light travels through a molecular cloud. First, dust grains scatter or absorb virtually all the visible light that enters a molecular cloud, preventing us from seeing stars that lie behind it. Second, stars seen near the edges of a molecular cloud appear redder that the stars outside the cloud, a phenomenon known as interstellar reddening.

Interstellar reddening occurs for much the same reason that our Sun appears redder when viewed through smoke or smog: Dust grains block shorter-wavelength (bluer) photons of visible light more easily than longer-wavelength (redder) photons. Near the edges of a molecular cloud, where stars are only partially obscured, the blocking of blue light causes stars to appear redder than their true colors. We can distinguish interstellar reddening from a Doppler shift because reddening doesn’t change the wavelengths of a star’s spectral lines. As a result, we can determine the amount of reddening by comparing a star’s observed color to the color expected for a star of its spectral type. The amount of reddening tells us how much dusty gas lies between Earth and a distant star. Infrared observations allow us to see directly through molecular clouds to stars lying at much greater distances. More important, infrared observations reveal new stars embedded within the clouds themselves, caught in the very act of birth. Much of the radiation produced by young stars within the molecular cloud cannot escape the cloud directly, because dust grains absorb the visible light (and some of the infrared light) from these stars. The absorbed radiative energy heats the dust grains, which may be further heated by radiative energy from newly formed stars just outside the cloud. The dust grains therefore become warm enough to emit thermal radiation in the infrared and microwave bands of the electromagnetic spectrum. Consequently, clouds that appear dark in visible-light photos often glow when observed in long-wavelength infrared light. Why do stars form? Stars form when gravity causes a molecular cloud to contract and the contraction continues until the central object becomes hot enough to sustain nuclear fusion in its core. Gravity Vs. Pressure Gravity can create stars only if it can overcome the outward push of the pressure within a gas cloud, which depends on both the density and the temperature of the cloud. The type of pressure, which depends on both density and temperature, affects virtually all the gases including the atmospheres of planets and the plasma throughout the interior of the Sun. This temperature-dependent pressure in ordinary gas clouds is thermal pressure. Thermal pressure can resist gravity in most interstellar gas clouds because their low gas densities keep gravity quite weak. Only in molecular clouds, where gas densities are tens to thousands of times greater than average, is gravity strong enough to overcome thermal pressure. Gravity is stronger in molecular clouds because more mass is packed into each cubic centimeter of volume, but their thermal pressure isn’t much greater than in other clouds because of the low temperatures. Gravity has the upper hand in many molecular clouds. Preventing a Pressure Buildup Regions of molecular cloud in which the gravitational attraction is stronger than the thermal pressure are forced to contract (gravitational contraction). As gravity makes a gas cloud shrink, it converts some of the cloud’s gravitational potential energy into thermal energy. If the cloud cannot get rid of that thermal energy as quickly as it is released, then thermal energy builds up inside the cloud. If all thus thermal energy remained within the cloud, it would raise the cloud’s temperature and thermal pressure, eventually bringing the process of star formation to a halt. Molecular clouds avoid this fate because they quickly rid themselves of any thermal energy that builds up. Collisions between gas molecules in the cloud transform the thermal energy

into photons by exciting the rotational and vibrational energy levels of those molecules, which then produce emission lines in the infrared and radio portions of the spectrum. As long as the photons produced by these colliding molecules can escape the cloud, the cloud’s temperature can remain low. With no significant buildup of thermal energy, gravity continues to dominate over thermal pressure, so all parts of the cloud that remain cool can contract to form stars. Clustered Star Formation Most stars are born in large clusters. Stars tend to form in clusters because gravity is stronger in a high-mass gas cloud, making it easier for gravity to overcome the outward force due to thermal pressure. At the temperatures and densities of typical molecular clouds, gravity can begin to overcome thermal pressure only in clouds with masses greater than a few hundred times the mass of the Sun. However, star-forming clouds usually contain thousands of times the mass of the Sun. Question: How do these clouds resist gravity long enough to grow to such large masses before they begin to form stars? First, individual gas clumps within molecular clouds move at substantially different speeds. This indicates that the overall gas motion is turbulent. Stars can form in such a cloud only if gravity is strong enough to overcome the turbulent gas motion, and overcoming that motion can require considerably more mass than is needed to overcome thermal pressure alone. Second, magnetic fields can help the cloud resist gravity. Light from stars usually travels through space with its electric and magnetic fields vibrating in random directions, but starlight that have passed through a molecular cloud often has its electric and magnetic fields aligned in particular directions (polarizations) The friction can slow or even halt the gravitational collapse of a molecular cloud Fragmentation of a Molecular Cloud Gravity in a molecular cloud with a large enough mass is strong enough to surmount all these obstacles, which is why the cloud collapses. Because molecular clouds are turbulent and lumpy, there are plenty of small, dense clumps within a contracting cloud that are soon able to shrink on their own. A large molecular cloud therefore splits into numerous individual cloud fragments (molecular cloud cores). Isolated Star Formation A molecular cloud does not necessarily need to be very massive to form a star, as long as it is unusually dense and cold. Small, isolated molecular clouds with are indeed observed, and they tend to form just one or a few stars at a time. The First Generation of Stars The composition of gas clouds in our galaxy, and of the stars they produce, has not changed much during the last 5 billion years or so. The composition of interstellar gas must have been different, because virtually all elements heavier than helium have been produced through the lives and deaths of stars. Stars in the oldest globular clusters, with ages greater than 12 billion years, contain less than 0.1% of their mass in the form of elements heavier than helium. The first generation of stars must have been born before any heavy elements had been produced, so they must have formed in clouds of gas containing only hydrogen and helium from the Big Bang.

The relatively high temperatures of these first-generation molecular clouds would have made it more difficult for gravity to overcome pressure, requiring stars to form in relatively massive cloud fragments that would have made only high-mass stars. Massive stars produce heavy elements and release those elements into space when they die. The massive first-generation stars would therefore have seeded the interstellar medium with substantial quantities of heavy elements that could be incorporated into all subsequent generations of stars. 2. Stages of Star Birth What slows the contraction of a star-forming cloud? Trapping of Thermal Energy Cloud contraction makes it increasingly difficult for emitted photons to escape. Collisions between the excited molecule and other molecules can then change the absorbed photon’s energy back into thermal energy. A similar effect happens with dust grains. The central region of the cloud fragment eventually grows dense enough to trap almost all the radiation inside it. When that happens, the inner regions of the contracting cloud can no longer radiate away their heat. The central temperature and pressure begin to rise dramatically, and the rising pressure pushes back against the crush of gravity, slowing the contraction. The dense center of the cloud fragment is now a protostar (pre-main-sequence-star) – a clump of gas that will become a new star. Growth of a Protostar by Gas Infall A protostar’s mass grows with time because a molecular cloud fragment contracts in an inside-out fashion. Gravity is stringer near the protostar, where the gas density is greatest. The gas in the outer part of the cloud fragment feels a weaker gravitational pull, so it initially remains behind as the protostar forms. However, because the gas beneath has already contracted to make the protostar, the outer regions of the cloud fragment are left with little pressure support from below. The rain of mass onto the protostar continues until the gas surrounding the protostar is gone. What is the role of rotation in star birth? Law of conservation of angular momentum Protostellar Disks Random motions of gas particles inevitably give a gas cloud some small overall rotation. As the gas contracts in size, the law of conservation of angular momentum ensures that the rotation will become much faster. The rapid rotation prevents gas from raining directly down onto a protostar. Instead, it settles into a protostar disk similar to the spinning disk of gas from which the planets of the solar system formed. The most important feature of the disk around a protostar is that it helps to transfer angular momentum away from the infalling gas, enabling the protostar to grow more massive. Gas in a protostellar disk can gradually spiral inward toward the protostar because of friction. Individual gas particles in the disk obey Kepler’s laws, which means that gas in the inner parts of the disk moves faster than gas in the outer parts.

The rubbing against slower-moving gas with a slightly larger orbit generates friction and heat, slowly causing the orbits of individual gas particles to shrink until the gas particles fall onto the surface of the protostar. Because the process in which material falls onto another body is called accretion, a disk in which friction causes material to spiral inward is often called an accretion disk. The protostellar disk is also important to slowing the rotation of the protostar itself. The protostar’s rapid rotation generates a strong magnetic field. As the magnetic field lines sweep through the protostellar disk, they transfer angular momentum to outlying material, slowing the protostar’s rotation. The strong magnetic field may also help to generate a strong protostellar wind. Protostellar Jets Many young protostars fire high-speed streams of gas (jets) into interstellar space. Sometimes the jets are lined with glowing blobs of gas. The fact that jets are aligned with the disk’s rotation axis indicates that angular momentum plays a large role. Magnetic field lines passing through the protostellar disk get twisted into a ropelike configuration by the disk’s rotation, and this twisted field may help channel jets of charged particles along the rotation axis. The disruptive effects of jets are likely to cause some of the turbulent motions observed in molecular clouds. Single Star or Binary? Angular momentum is also part of the reason so many stars belong to binary star systems. As a molecular cloud contracts, breaks up into fragments, and forms protostars, some of those protostars end up quite close to one another. Gravity can pull two neighboring protostars, and they go into orbit around each other because each pari of protostars has a certain amount of angular momentum. In some cases, gravitational interactions between the binary pair of protostars and other protostars and gas clumps in their vicinity can remove angular momentum from the binary system. If that happens, then their orbit gets smaller, and the two stars can end up quite close to each other (close binary system) How does nuclear fusion begin in a newborn star? From Protostar to the Main Sequence The central temperature of a protostar is typically about 1 million K when its wind and jets blow away the surrounding gas. To ignite fusion, the protostar needs to contract further boost the central temperature. The key factor in allowing the central temperature to rise is radiation of energy from the protostar’s surface into space. The energy going into space comes from thermal energy inside the star. Without this loss of thermal energy, the interior pressure would hold gravity at bay, so the protostar would stop contracting and its central temperature would remain fixed. A contracting protostar retains half the thermal energy released by gravitational contraction. A protostar becomes a true star when its core temperature exceeds 10 million K, making it hot enough for hydrogen fusion to operate efficiently through the proton-proton chain.

The Surface Temperature of a Protostar When a protostar first begins to contract, its surface gradually heats up as the protostar shrinks. However, once the protostar contracts to the point at which its surface temperature reaches 3000K, the temperature remains steady for most of the rest of the contraction process. During most of the protostar’s contraction, the dominant form of energy transport within its interior is convection. The convection gas below a protostar’s surface therefore rises until it reaches a layer at which the photons can escape to space. The temperature of this layer remains 3000K even as the protostar contracts, because at greater temperatures collisions can strip electrons from hydrogen atoms. Photons remain trapped within the convection gas as it rises and cools until it reaches the 3000K layer, at which point the gas becomes fully neutral and transparent, allowing the photons to escape to space. When radiative diffusion begins, the increased flow of energy through the protostar allows both the luminosity and the surface temperature of the protostar to increase until fusion ignites in the core. The ignition of fusion brings the star into energy balance, which stabilizes the luminosity and surface temperature and ends the contraction. Birth Stages on a Life Track Stage 1 – Formation of a Protostar. The stage ends when the surface temperature reaches about 3000K, placing it on the right side of the H-R diagram. Stage 2 – Convective Contraction. The protostar’s surface temperature remains near 3000K as long as convection remains the dominant mechanism for energy transport. The protostar’s life track drops almost straight downward on the H-R diagram. Stage 3 – Radiative Contraction. The protostar’s surface temperature begins to rise when the primary energy transport mechanism switches from convection to radiative diffusion. This rise in temperature brings a slight rise in luminosity, even though the protostar continues to contract. Stage 4 – Self-sustaining Fusion. Fusion becomes self-sustaining when the fusion rate becomes high enough to balance the rate at which radiative energy escapes from the surface. At this point, the star settles into its hydrogen-fusing main sequence life. 3. Masses of Newborn Stars What is the smallest mass a newborn star can have? The Origin of Degeneracy Pressure A quantum mechanical effect known as degeneracy pressure halts the contraction of a protostar with a mass less than 0.08 times that of the Sun before its temperature grows high enough to sustain fusion. The minimum mass for a star is therefore 0.08Msun. Starlike objects with masses below this limit are called brown dwarfs. What is the greatest mass a newborn star can have? The flood of photons coming from an extremely massive star exerts radiation pressure that can drive a star’s outer layers into interstellar space. This form of pressure should blow apart stars with masses somewhat above 100Msun, though the precise upper limit is uncertain. What are the typical masses of newborn stars?

Low-mass stars are far more numerous than high-mass stars. For every star with a mass above 10Msun in a newborn star cluster, there are typically 10 stars with masses between 2 and 10Msun, 50 stars with masses between 0.5 and 2Msun, and a few hundred stars with masses below 0.5Msun....