EXAM 3 STUDY GUIDE 16.1-16.3 Interstellar Medium- the gas and dust that lies in between the stars in the Milky Way galaxy Nuclear Fusion & Gravitational Contraction generate energy to help a star or gas clouds maintain its internal thermal pressure. Gravitational Contraction- most important before the star finishes forming Nuclear Fusion- most important during the star's lifetime Interstellar clouds (molecular clouds)- cool clouds in which stars form. Only 10-30 K, while the FIRST clouds never cooled below 100 K Called "molecular" clouds because they are cold enough to allow their hydrogen atoms to pair up into hydrogen molecules (H2). Density= 300 molecules/ cubic cm. 1% of the mass of a molecular cloud is in the form of dust Hydrogen (H2) is the most abundant element & molecule in an interstellar molecular cloud. Interstellar dust consists mostly of microscopic particles of carbon & silicon, oxygen, iron. Studying spectra of interstellar gas clouds determines the chemical composition Emission lines when the clouds are glowing Absorption lines when the clouds absorb light from stars behind them Infrared light gives us our best views of stars forming in dusty clouds. It can pass through dusty clouds, unlike visible light Visible light radiated by stars in a dusty cloud is absorbed by dust, which heats the dust grains so that they emit the absorbed energy as infrared light. If you look by eye at a star near the edge of a dusty interstellar cloud. The star will look DIMMER & REDDER than it would if it were outside the cloud. Dimmer because the dust absorbs some of the star's light, & redder because dust scatters blue light more effectively than red light. To determine if Interstellar reddening is taking place or a Doppler shift, notice REDDENING doesn?t change the WAVELENGTH. Most interstellar clouds remain stable in size because THERMAL PRESSURE opposes the force of gravity within the cloud. Thermal pressure - pressure resulting from the thermal motions of the particles in the cloud, proportional to temperature & density. Thermal pressure is increasing when the cloud's temperature and density are both increasing. A cold, dense gas cloud is most likely to give birth to stars. This cloud has lower thermal pressure (due to the low temperature) and stronger gravity (due to the high density), giving gravity the upper hand. Magnetic fields have an effect on star formation in molecular clouds by helping to resist gravity, so more total mass is needed before the cloud can collapse to form star. The magnetic fields that thread clouds inhibit the movement of gas, thereby adding to the resistance to gravity provided by thermal pressure (and turbulent gas motions).The very first stars in the universe were made from ONLY hydrogen & helium. The first stars were much more massive than stars born today. Because there were no heavy elements at that time, these stars formed in clouds that had no dust and no molecules like carbon monoxide. This made it more difficult for the clouds to radiate energy away, so they needed larger mass to form stars Stars born TODAY are made approximately of 98% hydrogen & helium, and 2% of heavier elements. Protostar- clump of gas that will become a new star. A star that is still in the process of forming It generates energy by gravitational contraction, as it is not yet hot enough for nuclear fusion it its core. Protostars are too cool to emit much ultraviolet light. Star formation process includes Strong protostellar winds of particles blowing out into space from a protostar Powerful "jets" shooting out along the rotation axis of a protostar The formation of a spinning disk of material around a protostar Conservation of Angular Momentum- explains why a collapsing cloud usually forms a protostellar disk around a protostar. A cloud must radiate away much of its thermal energy to allow a gravitationally-collapsing gas cloud to continue to collapse. Otherwise, the cloud would heat up, which would increase the thermal pressure and halt the gravitational collapse. Close binary star systems form when gravity pulls two neighboring protostars quite close together, but angular momentum causes them to orbit each other rather than colliding. Protostellar disk forms as a consequence of conservation of angular momentum, which makes the cloud rotate faster as it shrinks in size. Magnetic fields transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away. Therefore slowing the rotation of a protostar. Angular momentum transfer MUST happen for rotation to slow down. The life track of a star on an H-R diagram shows the surface temperature & luminosity the star will have at each stage of its life. A star's particular life track depends ONLY on its mass. A protostar turns into a main-sequence star when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space. This is what we consider to be the birth of a star A core temperature of 10 MILLION K is required before hydrogen fusion can begin in a star. The surface of a protostar radiates energy while its core shrinks & heats. Gravity continues to compress the core, which grows hotter until the onset of fusion. The core of a protostar that will eventually become a brown dwarf shrinks until degeneracy pressure takes place. This halts the contraction of a brown dwarf A main-sequence star is hotter & dimmer than a protostar. A 1 solar mass protostar appears to the right of the main sequence, and higher up than the Sun Protostars are brighter than the sun. Lower mass stars take longer in all phases of life. A 1 solar mass star will take LONGER in the protostellar phase than a 3 solar mass star. The approximate range of masses that newborn main sequence stars can have is 0.1 to 150 solar masses. The vast majority of stars in a newly formed star cluster are LESS massive than the sun. A large molecular cloud that will give birth to a cluster of stars will result in a few massive stars that form, live, and die before the majority of the star's clusters even complete their protostar stage. High massive stars can form in a million years or less, and then live just a few million years. Low-mass stars take tens of millions of years or more just to reach the beginning of their main-sequence lives. In general, the lower the mass, the more common the star. An extremely massive star (well over 100 M) lives a short life because it may blow itself apart because of radiation pressure. Excess radiation pressure is why stars cannot be more massive than 150 solar masses or so. Brown Dwarfs Are supported against gravity by degeneracy pressure depends on DENSITY, NOT depend on the object's temperature. Form like ordinary stars but are too small to sustain nuclear fusion in their cores. Have masses less than about 8% that of our Sun. On an H-R diagram a brown dwarf would be located below and to the right of the lowest part of the main sequence. They are dimmer & cooler than the smallest main-sequence stars If stellar birth masses applied to brown dwarfs they would outnumber all ordinary stars. LIFE TRACK Formation of a protostar, convective contraction, radiative contraction, self-sustaining fusion 17.1-17.4 Stars with less than about 2 times the mass of our Sun are considered low-mass stars. Our sun is considered a low-mass star (1M) The lowest mass stars live the longest Low-mass stars form planetary nebulae, and low-mass stars are far more common than high mass stars. Planetary nebulae are more common than a star blowing up as a supernova. Intermediate-mass Stars- 2-8 solar masses ?High-Mass? stars have masses more than 8 times the size of our Sun. They are massive enough to die in supernova explosions. Low Mass Star Lifetime Protostar, main-sequence star, red giant, planetary nebula, white dwarf High Mass Star Lifetime Protostar, main-sequence star, red giant, supernova, neutron star Decades are NOT enough time to notice any changes in stars. Stars remain in a main sequence stage for longer than any other stage When a main-sequence star exhausts its core hydrogen fuel supply the core shrinks while the rest of the star expands Gravity shrinks the core until hydrogen shell burning begins; the shell burning produces so much energy that the outer layers of the star expand, and the star becomes a subgiant and then a red giant. H-R diagrams of globular clusters help us understand the life tracks of low-mass stars. Globular cluster- comprised of stars that formed at the same time The globular clusters in the Milky Way are full of old low-mass stars in various stages of evolution. The main source of energy for a star as it grows in size to become a red giant is hydrogen fusion in a shell surrounding central core. A 1 solar mass red giant is more luminous than a 1 solar-mass main sequence star because fusion reactions are producing energy at a greater rate in the red giant through hydrogen shell burning. Any star's luminosity depends on how much energy it is generating Helium fusion reaction- 3 helium nuclei fuse to form 1 carbon nucleus. Helium Flash- the sudden onset of helium fusion in the core of a low-mass star ONLY occurs in LOW mass stars Horizontal branch- stars where helium fusion has already begun in the core, while hydrogen fusion continues in a shell around the core. Planetary Nebulae- Gas ejected from a low-mass star in the final stage of its life Has NOTHING to do with planets The fate of our Sun is to become a white dwarf that will slowly cool with time. This will happen in about 5 billion years. A red giant does NOT have fusion occurring in its central core Fusion is only occurring in shells AROUND the core. Low-mass red giant stars manufacture most of the carbon atoms in our bodies. Carbon is produced by helium fusion in the core, then dredged up to the surface by convection and blown out into space by winds and by the ejection of a planetary nebula. Helium & Helium each have two protons (and two neutrons - but neutrons are electrically neutral) therefore each have a charge of +2. Since these two nuclei have the greatest total charge between them, they will feel the strongest electromagnetic REPULSION. High-mass stars ALWAYS die in supernovae. 10 solar mass star will definitely die a supernovae Hydrogen Fusion Proton-proton chain (low- mass stars)- fuse hydrogen into helium CNO Cycle (high-mass stars)- Faster chain of hydrogen fusion. A set of steps by which four hydrogen nuclei fuse into one helium nucleus Each successive stage of nuclear core burning becomes shorter and shorter. Carbon fusion occurs in high-mass stars but NOT in low-mass stars because the cores of low-mass stars never get hot enough for carbon fusion. Heavier elements require higher temperatures for fusion, and in low-mass stars (at the ends of their lives) degeneracy pressure stabilizes the cores before they become hot enough to allow high enough temperatures. MASS determines whether the star's core will ever be compressed enough to fuse oxygen into another element. Iron significant to understanding how a supernova occurs because iron CANNOT release energy by fusion or fission. A star with an iron core has no way to generate the energy needed to counteract the crush of gravity. This is the crisis that, in a fraction of a second, initiates the supernova. After a supernova explosion, the remains of the stellar core may be either a neutron star or a black hole Supernova 1987A was in the Large Magellanic Cloud, about 150,000 light-years away, making it the nearest one detected since Kepler's supernova in 1604. Occurring at a time when we were capable of studying it with a telescope. BURST OF NEUTRINOS! Gold is produced during supernova explosions in high mass stars Fusion in the core produces elements as heavy as iron Heavier elements like gold are produced during the supernova itself. The heavier the element the higher temperature needed for fusion to take place. Helium-capture reactions- reactions where a helium nucleus fuses with another nucleus. Helium-capture can change: carbon oxygen neon magnesium. At higher temperatures the core plasma can fuse heavy nuclei to one another Fusing carbon oxygen = silicon, 2 oxygen sulfur, 2 silicon iron The sudden collapse of an iron core into a compact ball of neutrons marks the beginning of a SUPERNOVA. A star with a hydrogen-burning shell and an inert helium core is a subgiant that grows in luminosity until helium fusion begins in the central core. If hydrogen, RATHER than iron, had the lowest mass per nuclear particle nuclear fusion COULD NOT power stars. Stars are made mostly of hydrogen, so if hydrogen could not be fused into helium, stars could not shine. Observations show that elements with atomic mass numbers divisible by 4 tend to be more abundant than those in between because at the end of a high-mass star's life, new elements form through a series of helium captures reactions. Helium has atomic mass number 4, so each subsequent reaction tends to make an element with an atomic mass number divisible by 4. Supernova Remnant- expanding cloud of debris from a supernova explosion A spinning neutron star has been observed at the center of a supernova remnant. As a high-mass star explodes, the core of the star either becomes a neutron star or a black hole, so it's natural to find neutron stars embedded in supernova remnants. A more massive star should have evolved faster and become a giant first. Once it became a giant, material "spilled over" to its companion. Thus the companion gained mass at the giant's expense, becoming the more massive star. Tidal forces increase in importance as the ratio of the size of the objects to their physical separation increases. Main sequence stars in a system like the Algol system are small compared to their physical separation. This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage is the star's energy supplied by primarily by gravitational contraction? II- Stage II is the protostar stage during which energy comes from gravitational contraction. During which stage does the star have an inert (non-burning) helium core? IV- The core is made of helium after the main sequence stage. It is inert, as the star becomes a red giant with energy supplied by hydrogen shell burning. Which stage lasts the longest? III- Stage III is the hydrogen burning main-sequence stage, which is the longest stage of the star's life. During which stage does the star have an inert (non-burning) carbon core surrounded by shells of helium and hydrogen burning? VIII- Stage VIII is when the star is expanding as a double-shell burning red giant. What will happen to the star after stage VIII? Its outer layers will be ejected as a planetary nebula and its core will become a white dwarf. Stage VIII is the second red giant stage, and for a low-mass star that means the next events are planetary nebula ejection and death as a white dwarf. 18.1-18.4 White Dwarf- what most stars become when they die Most stars are low-mass stars that ultimately become white dwarfs. A typical white dwarf is as massive as the Sun but ONLY about as large in size as Earth White dwarfs= pretty dense The more massive a white dwarf the smaller its radius. If you had something the size of a sugar cube that was made of white dwarf matter, it would WEIGH as much as a TRUCK WHITE DWARF Density= couple of tons/cubic cm The density of a white dwarf is typically a couple of tons per cubic centimeter. The maximum mass of a white dwarf is 1.4 times the mass of our Sun The white dwarf limit, AKA Chandrasekhar limit. This is because Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit. Accretion disk- a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole. The hot gas gradually accretes onto the central object. The gas in the inner parts of the disk is hotter & travels faster than the gas in the outer parts of the disk. Accretion disks are made primarily of hydrogen & helium gas. The primary factor determining IF a white dwarf has an accretion disk is IF the white dwarf is in a close binary system where gas from its companion can spill over, NOT its mass. The accretion disk around a neutron star is much hotter and emits higher-energy radiation than a white dwarf. Because of the much stronger gravity near the surface of the neutron star. Nova- an explosion on the surface of a white dwarf in a close binary system As hydrogen accretes onto a white dwarf, the temperature and pressure increase until fusion can occur. Novae vs. Supernovae Novae occur ONLY in binary star systems, while supernovae can occur both among single stars & among binary star systems. The same star can undergo novae explosions more than once, but can undergo only a SINGLE supernova. Novae are much less luminous than supernovae. Novae & Supernovae eject gas into space, often creating a nova remnant that remains visible for many years Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit? The white dwarf will EXPLODE completely as a white dwarf supernova. WITHOUT any corpse behind. Neutron star- the remains of a star that died in a massive star supernova (if no black hole was created) Neutron stars form only as the result of iron core collapse in massive star supernovae. The Sun will NEVER undergo a white dwarf supernova explosion because it does NOT have another star orbiting it. White dwarf supernovae occur only in close binary star systems. A typical neutron star is more massive than our Sun and about the radius of a small asteroid (10 km) Much higher density than a white dwarf. If you had something the size of a sugar cube that was made of neutron star matter, it would weigh as much as a LARGE MOUNTAIN. Much higher density than a white dwarf Pulsars- rapidly rotating neutron stars. The pulses occur as their magnetic fields sweep by us with each rotation. Kept from collapsing by neutron degeneracy pressure. Must have a very strong magnetic field and rotate quite rapidly. ALL pulsars are neutron stars, but NOT ALL neutron stars are pulsars. Any neutron star may appear to us as a pulsar only if it has beams of radiation sweeping by us with each rotation, regardless of whether it is in a close binary system. An X-Ray burst and a Nova BOTH involve explosions on the surface of stellar corpse. The surface of a white dwarf for a nova and the surface of a neutron star for an X-ray burst. Black Hole- an object with gravity so strong that not even light can escape Although we are NOT 100% certain that black holes exist, we have strong observational evidence in favor of their existence. If YOU fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon. If YOU WATCH someone else fall into a black hole, you will NEVER see him (or her) cross the event horizon; you'll only see him fade from view, as the light he emits or reflects becomes more and more redshifted. X-ray bursts take place on the surface of a neutron star. A black hole has no surface, and hence cannot have X-ray bursts. Chandra X-Ray Observatory is most likely to discover a black hole in a binary system. Accretion disks around black holes emit X-Rays The minimum mass of a black hole that forms during a massive star supernova is roughly 3 solar masses. A collapsing stellar core must contain about 3 solar masses (or more) for the crush of gravity to overcome neutron degeneracy pressure. Event horizon of a black hole is the point beyond which neither light nor anything else can escape Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). NOTHING would happen to Earths orbit. Orbits are determined by the strength of gravity, which depends only on mass. Since the Sun's mass did not change, the orbit would not change. Singularity of a Black hole- It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions. Schwarzschild radius of a black hole depends on ONLY the mass of the black hole. Karl Schwarzchild computed radius from EINSTEIN?S GENERAL THEORY OF RELATIVITY RADIUS= Circumfrence/2? The more massive the black hole, the larger the Schwarzschild radius. Even an object as small as you could become a black hole if there were some way to compress you to a size smaller than your Schwarzschild radius. For black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers. The Schwarzschild radius is a property of the black hole itself and does not depend on whether the black hole has a companion object. Gamma Ray Bursts Based on their distribution in the sky, we can rule out a connection between gamma ray bursts and X-ray binaries in the Milky Way Galaxy. Are among the most luminous events that ever occur in the universe. Were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth. We have detected the sources of gamma ray bursts in other wavelengths NOT just gamma rays The distribution of gamma ray bursts in the sky all but rules out an origin within our own galaxy, and optical afterglows of some bursts have been detected in faint, distant galaxies. Degeneracy Pressure- Black holes form when gravity overcomes neutron degeneracy pressure. Arises from a quantum mechanical effect that we don't notice in our daily lives. Can continue to support an object against gravitational collapse even if the object becomes extremely cold. Neutron degeneracy pressure arises from neutron interactions & electron degeneracy from electron interactions. Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars. Both types of degeneracy pressure arise for different reasons than the thermal pressure that supports the structures of stars that still have ongoing nuclear fusion. The white dwarf that remains when our Sun dies will be mostly made of Carbon In its final stages of life, the Sun will fuse helium into carbon, but it will NEVER become hot enough for carbon fusion. What would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly to appear in your hometown? The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star. The neutron star is far more massive than Earth, so Earth would end up wrapped around it and compressed to extremely high density. As you look at a clock you dropped into a black hole from a high orbit time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted. A 15 MSun star has a total lifetime of only a few million years. If it is already a red giant, it is within a million years of supernova The accretion disk radiates prodigious amounts of X rays, as evidenced from the fact that we see these types of systems as X-ray binaries. They will provide enough radiation to kill him long before he begins to feel effects from tidal forces. 19.1-19.4 Milky Way The diameter is about 100 times as great as the thickness Diameter = 100,000 light-years & Thickness = 1,000 light-years. That is why the disk appears so thin. The overall chemical composition is about 70% hydrogen, 28% helium, and 2% everything else. Elements heavier than hydrogen and helium constitute about 2% of the mass of the interstellar medium. The older the star, the lower its abundance of heavy elements. Stars must manufacture heavy elements. A star that formed early in the history of the disk should have a LESS elements heavier than hydrogen and helium vs. a star that formed recently in the disk of the galaxy to differ from one The stars orbit the galaxy in roughly the same plane and in the same direction Also tend to bob up and down as they orbit, but this motion still keeps them within the disk and hence in roughly the same plane If we could see our own galaxy from 2 million light-years away, it would appear as a flattened disk with a central bulge and spiral arms It would look much like the Andromeda galaxy looks to us. Most stars in the Milky Way's halo are very old No recycling of gas in the halo, so halo stars are quite old. Halo stars orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane. Galaxy Bulge- bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy The central bulge is even visible to the naked eye, since it makes the Milky Way in the night sky wider in the direction of the galactic center (toward Sagittarius). The Suns location within in the Milky Way is in the galactic disk, roughly halfway between the center and the outer edge of the disk. The Sun lies about 28,000 light-years from the center of the galaxy, which is just over half the roughly 50,000 light-year radius of the disk. Galactic fountain model- describes gas cycles between the disk of the galaxy & regions high above the disk Hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo. The gas going upward comes from superbubbles that form from the combined shock waves of many supernovae. As it rises the gas cools and gravity eventually brings it back down to the disk. Interstellar Medium- the gas and dust that lies in between the stars in the Milky Way galaxy Affects our view of most of the galaxy by preventing us from seeing most of the galactic disk because the interstellar dusts ABSORBS visible & ultraviolet light. Milky Way's interstellar medium in 50 billion years will have much less gas than it does today. With each subsequent generation of stars, some material is "locked away" permanently in brown dwarfs & stellar corpses. Thus, the amount of gas available for recycling gradually declines with time. Watching an INTERSTELLAR MEDIUM over hundreds of millions of years, gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles. Magellanic Clouds- two small galaxies that orbit the Milky Way Galaxy Large & Small Magellanic Clouds ORBIT us 150-200,000 light years away, VISIBLE with the NAKED eye from SOUTHERN hemisphere. Protogalactic Cloud- a cloud of hydrogen & helium that contracts to become a galaxy (galaxy in the process of forming) Star formation- the stars that formed first could orbit the center of the galaxy in any direction at any inclination, which is why they now make up the halo. Ionization Nebula- a colorful cloud of gas that glows because it is heated by light from nearby hot stars Ex. Orion Nebula Red & orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found only in near star forming clouds. Hot blue stars tend to outline the spiral arms and are not even distributed elsewhere. The total mass of the Milky Way Galaxy that is contained within the Sun's orbital path by applying Newton's version of Kepler's third law to the orbits of the Sun or other nearby stars around the center of the Galaxy. (Orbital Velocity Law) Most of the mass of the Milky Way is in the form of dark matter because the orbital speeds of stars far from the galactic center are surprisingly high These stars are feeling gravitational effects from unseen matter in the halo Star-Gas-Star Cycle- the continuous recycling of gas in the galactic disk between stars & interstellar medium. Over time, the star-gas-star cycle leads the gas in the Milky Way to have a greater abundance of heavy elements. Globular clusters are distributed throughout the halo, and are found in the disk only if they are currently passing through it on their orbits. Cosmic Rays- subatomic particles that travel close the speed of light, likely to be produced by supernova Sgr A* in the center of our Galaxy is by far the brightest source of visible light lying in the direction of the galactic center. We CANNOT see the center of the galaxy WITH VISIBLE light. The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with radio telescopes observing at a wavelength of 21 centimeters. Radio emission in the 21 cm line is the only significant emission from most atomic hydrogen gas. Spiral Arms- where most star formation occurs because the enhanced density of gas in the spiral arms leads to star formation. Appear bright because they contain more hot young stars than other parts of the disk Have enhanced density that leads to more star formation, and young hot stars don't live long enough to move far from the places where they are born. The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy. Called a spiral density wave. The pattern that we see as the spiral arms remains stationary relative to the galactic disk, while stars and gas clouds orbit through them. The gas and stars slow as they move through the arms (which are regions of enhanced gravity due to a higher gas density) Milky Way Gas- The most common form of gas in the disk of the Milky Way galaxy is ATOMIC HYDROGEN GAS This is gas that is too warm for molecules to form, but cool enough so the hydrogen is not ionized. We believe a 3 to 4 million solar mass black hole lies in the center of the Milky Way, existence is inferred from the orbits of stars near the galactic center X rays from this region presumably come from an accretion disk around the black hole. Dust grains are made of heavier elements, NOT from the hydrogen and helium that make up 98% of the chemical content of the galaxy. In fact, dust represents only about 1% of the mass in molecular clouds. Applying the Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center This method tells us only the mass within that orbit, not the total mass of the entire galaxy. Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful? Chandra X-Ray- the gas is quite hot & emits very little visible light Most nearby stars move relative to the Sun at speeds below about 30 km/s, suppose a star passed by at 300 km/s it is probably a halo star passing through the disk. Halo stars have very different orbits, so when they pass through the disk they have high speeds relative to the nearby disk stars. STAR- GAS- STAR CYCLE Atomic Hydrogen Gas- 21 cm RADIO EMISSION Molecular Clouds- RADIO EMISSION Interstellar Dust- INFRARED (60-100) Interstellar Dust (deeper)- INFRARED (1-4) VISIBLE LIGHT- Emitted by stars SCATTERED & ABSORBED by dust Hot gas bubbles & X-Ray binaries- X-RAY EMISSION Collisions of cosmic rays with atomic nuclei in interstellar clouds- GAMMA RAY 13
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