Chapter 10 Notes: Nebulae Astronomy Name: SOLUTION SET Date: 4/11 An introduction to interstellar medium A World of Dust 1. The space between stars is not completely EMPTY, but filled with very dilute GAS and DUST, producing some of the most beautiful objects in the sky. 2. We are interested in the INTERSTELLAR medium because: o o Dense interstellar clouds are the BIRTH PLACES of stars Dark clouds alter and BEND the light from STARS behind them. Use pp 198-199 in your text and the video The Universe Nebulas to answer these questions. II. Three Kinds of Nebulae Name each type of nebula and describe the physical and chemical properties (temp, density, color, etc) and why they appear how they appear. A. EMISSION NEBULAE PINK IN COLOR, EMISSION PRODUCED WHEN A HOT STAR EXCITES THE GAS AROUND IT TO PRODUCE A SPECTRUM. RED, BLUE AND VIOLET BALMER LINES BLEND TO MAKE THE EMISSION SPECTRUM PINK. B. REFLECTION NEBULA DUST IN NEBULA REFLECTS STARLIGHT, ABSORPTION SPECTRUM IS BLUE. GAS PRESENT IN THE NEBULA IS NOT EXCITED TO EMIT PHOTONS SO ONLY DUST IS VISIBLE C. DARK NEBULA DENSE CLOUDS OF GAS AND DUST THAT OBSTRUCT OUR VIEW OF MORE DISTANT STARS. SHAPE OF A DARK NEBULA DEPENDS ON THE PRESENCE OR ABSENCE OF A BREEZE OR CURRENT MOST ARE ROUND, SUGGESTING A LACK OF MOTION, BUT SOME ARE TWISTED AND DISTORTED.
Reading Questions from Chapter 10, pg 212. RQ 1. EVIDENCE FOR INTERSTELLAR MEDIUM INTERSTELLAR EXTINCTION DUST MAKES DISTANT STARS APPEAR FAINTER THAN THEY SHOULD INTERSTELLAR REDDENING SOME O STARS APPEAR RED INSTEAD OF BLUE BECAUSE THE DUST SCATTERS THE LIGHT FROM THOSE STARS WE CAN SEE EMISSION, REFLECTION AND DARK NEBULAE RQ 2. EVIDENCE FOR GAS EMISSION NEBULAE SHOW THE EMISSION SPECTRUM OF EXCITED HYDROGEN EVIDENCE FOR DUST INTERSTELLAR EXTINCTION AND DARK NEBULAE RQ 5. BLUE PHOTONS ARE PREFERENTIALLY SCATTERED IN A DUSTY REFLECTION NEBULA, JUST LIKE BLUE PHOTONS ARE PREFERENTIALLY SCATTERED IN EARTH S ATMOSPHERE SO BOTH REFLECTION NEBULAE AND EARTH S ATMOSPHERE APPEAR BLUE.
M16 -- Star Birth in the Nest of the Eagle Stars are born from the gas of interstellar space. When they eventually burnout and die, they bequeath their legacy back to the interstellar medium from which they formed. The signposts marking this ongoing cycle of birth, death, and renewal would be easily visible to any casual observer who had a bird's-eye view of our pinwheel-shaped galaxy. Spread across our galaxy such an observer would see majestic spiral arms, highlighted by bright young stars and the glowing clouds of gas that those stars illuminate. On a clear, dark summer night earth-based observers can see these glowing clouds, called nebulae, scattered along the track of the Milky Way. Many can be found by looking in the direction of the great star clouds in the summer constellation, Sagittarius. One of the most unique star-birth regions is the Eagle Nebula, (also called M16 because it is in the Messier Catalog of "fuzzy" permanent objects in the sky, that was compiled more than 200 years ago by French astronomer Charles Messier) it is visible in binoculars near the border between the constellations of Sagittarius and Serpens. The nebula is actually a bowl-shaped blister on the side of a dense cloud of cold interstellar gas. Most of this cloud is so dense and cool that its hydrogen atoms are bound as molecules. This "molecular hydrogen" is the raw material for building new stars. The cloud contains microscopic dust particles of carbon (in the form of graphite), silicates and other compounds similar to those found in terrestrial and lunar rocks. Though this trace dust accounts for only a fraction of the nebula's mass, it's enough dust to absorb visible light -- cloaking some of the visual details of star birth. A cluster of about 100 newborn stars glitters inside the open "bowl" of the nebula. A few of these stars are much more massive than our Sun is, and so are tremendously hotter and brighter than the Sun. The brightest of these stars may be 100,000 times brighter than the Sun and have temperatures of nearly 90,000 degrees Fahrenheit (50,000 degrees Kelvin). These young stars emit intense ultraviolet radiation which is so energetic it heats the surrounding gas, causing it to glow like the gas inside a fluorescent light bulb. When this ultraviolet light hits the bowl-shaped surface of the molecular cloud, it heats that gas, causing it to "evaporate" and stream away from the surface. If one could watch the process for more than a million years, they would see the bowl grow increasingly larger as the radiation from the stars eats deeper into the molecular cloud. Unlike other stellar nebula which we see face-on -- like the great Orion Nebula -- M16 presents astronomers with a unique side view of the structure of a typical star-birth region: the cluster of hot, young stars in the center of the cavity, the evaporating surface of the cloud, and finally the great cold mass of the cloud itself. The Eagle Nebula's name comes from its symmetrical appearance which is reminiscent of a bird of prey with outstretched wings and talons bared. The Eagle's "talons" are actually a series of dense columns of gas that protrude into the interior of the nebula. These columns form as a result of the same process that causes the bowl to grow. Because the columns are denser than their surroundings, they are not evaporating as rapidly as the surrounding gas, and so remain. The process is analogous to the formation of towering buttes and spires in the deserts of the American Southwest. These geological features formed when wind and rain eroded away softer ground, but places where the rock was harder resisted erosion and were left behind. Inside these interstellar columns, the gas density can get so high that gravity takes over and causes the gas to start collapsing into ever-smaller clumps. As more and more gas falls onto these growing clumps they get further compressed by their own weight, until finally they trigger nuclear fusion reactions in their cores, and "turn on" as stars. However, in M16 this process may not get a chance to go on to completion. If a forming star and the gas cloud that surrounds it are "uncovered" by photoevaporation before the star finishes growing, the mass of the young star may be "frozen." The star can't grow any more simply because the cloud from which it was drawing material is gone. In M16 Hubble Space Telescope's high resolution seems to have caught about 50 stars in this situation. These are called EGGs "evaporating gaseous globules." The acronym is appropriate because these EGGs are objects within which stars are being born and are now emerging. M16 is where the action is today, but it won't remain so forever. Within another few million years, star formation will have exhausted or dispersed the available raw material, and the massive stars that illuminate the Eagle will have lived out their short lives and died in spectacular supernova explosions. But even though the "birth cloud" nebula will be gone, most of the stars that formed there will remain. The offspring of the Eagle will "take wing" among the rest of the hundreds of billions of stars that make up our galaxy.
Giant Molecular Clouds: Breeding Grounds for Star Birth Space between stars in a galaxy is nearly empty, except for a scattering of hydrogen atoms. The atoms are so far apart that, if an atom were an average- size person, each person would be separated by about 465 million miles, which is the distance between our Sun and Jupiter. These atoms are moving very fast because they are extremely hot, baked by ultraviolet radiation from stars. This makes it difficult for atoms to bond to form molecules. Those that do form don't last for long. If radiation doesn't break these molecules apart, a chance encounter with another atom will. Some parts of space, however, are not wide open frontiers containing a few atoms. These cosmic spaces comprise dense clouds of dust and gas left over from galaxy formation. Since these clouds are cooler than most places, they are perfect breeding grounds for star birth. When the density is 1,000 times greater than what is found in normal interstellar space, many atoms combine into molecules, and the gas cloud becomes a molecular cloud. Like clouds in our sky, these molecular clouds are puffy and lumpy. Molecular clouds in our Milky Way Galaxy have diameters ranging from less than 1 light-year to about 300 light-years and contain enough gas to form from about 10 to 10 million stars like our Sun. Molecular clouds that exceed the mass of 100,000 suns are called Giant Molecular Clouds. A typical full-grown spiral galaxy contains about 1,000 to 2,000 Giant Molecular Clouds and many more smaller ones. Such clouds were first discovered in our Milky Way Galaxy with radio telescopes about 25 years ago. Since the molecules in these clouds do not emit optical light, but do release light at radio wavelengths, radio telescopes are necessary to trace the molecular gas and study its physical properties. Most of this gas is very cold (about -440 degrees Fahrenheit) because it's shielded from ultraviolet light. Since gas is more compact in a colder climate, it is easier for gravity to collapse it to form new stars. Ironically, the same climate that is conducive to star formation also may shut off the star birth process. The problem is heat. Young stars are very hot and can heat the molecular gas to more than 1,000 degrees Fahrenheit, which is an unfavorable climate for star birth. When the temperature exceeds about 3,000 degrees Fahrenheit, the gas molecules break down into atoms. The density of the gas can increase considerably near the centers of some Giant Molecular Clouds: Gas as dense as 1 billion molecules per cubic inch has been observed. (Though dense by astronomical standards, such gas is still 100 billion times thinner than the air we breathe here on Earth at sea level!) In such dense regions, still denser blobs of gas can condense and create new stars. Although the star formation process is not fully understood, there is observational evidence that most stars are born in the densest parts of molecular clouds. What happens when stars begin forming in Giant Molecular Clouds depends on the environment. Under normal conditions in the Milky Way and in most other present-day spiral galaxies, star birth will stop after a relatively small number of stars have been born. That's because the stellar nursery is blown away by some of the newly formed stars. The hottest of these heat the surrounding molecular gas, break up its molecules, and drive the gas away. As the celestial smog of gas and dust clears, the previously hidden young stars become visible, and the molecular cloud and its star-birthing capability cease to exist. Two years ago the Hubble Space Telescope revealed such an emerging stellar nursery in the three gaseous pillars of the Eagle Nebula. Giant Molecular Clouds in colliding galaxies may experience a different fate. As the collision crunches the interstellar gas and stars form at an accelerating rate, the gas pressure around the surviving Giant Molecular Clouds increases one-hundred- to one-thousand-fold. Calculations suggest that the hot surrounding gas can trigger rapid star birth throughout the clouds by driving shock waves into them. The several hundred thousand stars that form from the cold molecular gas in such clouds use up most of the gas before it has time to be heated and dispersed. The result of such violent events is the nearly complete conversion of Giant Molecular Clouds into rich star clusters, each containing up to 1 million stars. Observations by the Hubble telescope suggest that many of these newly born star clusters remain bound by their own gravity and evolve into globular clusters, like those observed in the halo of our Milky Way.
Nebular Questions (hah!...it s funny trust me) Astronomy Name: Period: Use the articles provided and the any additional sources necessary to answer the following questions: From Giant Molecular Clouds article: 1. Is all the space in galaxies totally empty? NEARLY EMPTY EXCEPT FOR A SCATTERING OF HYDROGEN ATOMS 2. If so how can gaseous clouds form. If not, what s the difference between the clouds and the rest of space? CLOUDS ARE LEFT OVER FROM GALAXY FORMATION 3. How do space clouds compare to that of clouds on earth? MOLECULAR CLOUDS ARE PUFFY AND LUMPY LIKE ATMOSPHERIC CLOUDS 4. How large are Giant Molecular Clouds? BETWEEN LESS THAN 1 LIGHT YEAR ACROSS AND 300 LIGHT YEARS ACROSS 5. How many are in a galaxy? 1.000 TO 2,000 6. How are they detected? Explain! MOLECULES DO NOT EMIT OPTICAL LIGHT BUT DO EMIT RADIO WAVES, SO THEY ARE DETECTED BY RADIO TELESCOPES IN PLACES WHERE THEY CANNOT BE DETECTED BY OPTICAL TELESCOPES 7. Explain why cold temperatures are good for star birth and why hot temperatures are bad for star birth. COLD TEMPERATURES ALLOW FOR DENSITIES GREAT ENOUGH FOR MOLECULES TO FORM, HOT TEMPERATURES BREAK THE MOLECULES INTO ATOMS MAKING IT MORE POSSIBLE FOR THEM TO EVAPORATE 8. How much more dense is a giant molecular cloud than our air? 100 BILLION TIMES THINNER THAN OUR ATMOSPHERE 9. Why does star birth in galaxies like the Milky way stop after small numbers of stars have been born? THE INTERSTELLAR MEDIUM IS BLOWN AWAY BY THE NEW STARS 10. According to the Antennae galaxy website, what is another trigger for star formation? COLLIDING GALAXIES 11. Explain why this happens? THE COLLISION OF GALAXIES CAUSES THEIR INTERSTELLAR MEDIUM TO CRUNCH AND FORM NEW STARS.
From M16 article 12. What causes the columns of the Eagle Nebula? THE COLUMNS ARE DENSER REGIONS OF THE NEBULA THAT HAVE BEEN MORE RESISTANT TO THE EVAPORATION OF THE SURROUNDING GAS. 13. In what constellation is the Eagle Nebula? (RA 18:18, Dec. 13O 47 ) BETWEEN SAGITTARIUS AND SERPENS 14. What are EGG s? (include what EGG stands for and what s happening there) EVAPORATING GASEUS GLOBULES A FROZEN PROTO STAR THAT CANNOT DRAW MORE GAS IN TO GROW BECAUSE THE GASSES HAVE EVAPORATED 15. Look at the illuminated portion of the Eagle Nebula image. How many EGGs can you count on that pillar? FROM P 221 5 IDENTIFIED BY AUTHOR, AND APPROXIMATELY 8 MORE. 16. Look at the eagle nebula image to see EGG s on stalks that stick out from pillars. Draw a sketch, labeling the pillar, and EGG. 17. What will stop star formation in the Eagle Nebula? THE NEBULA WILL EXHAUST OR DISPERSETHE AVAILABLE RAW MATERIAL. 18. What is the main element in gas clouds? HYDROGEN 19. What is evaporating the gas around the EGG s? ULTRAVIOLET RADIATION FROM A NEARBY STAR HEATS THE GASSES 20. Why aren t the EGG s evaporating as fast? GRAVITY OF THE DENSE DUST COUNTERACTS THE EVAPORATION EFFECT
Chapter 11 The Formation of Stars Notes/Homework Astronomy Name: Period: Use pages 215-229 to answer the following questions. 11-1 Making Stars from the Interstellar Medium 1. What is the key to star formation? THERE IS A CORELLATION BETWEEN YOUNG STARS AND CLOUDS OF GAS 2. A typical Giant Molecular Cloud: a. has a diameter of 50 PC b. has a mass of 10 5 SOLAR MASSSES c. has a density of 10 20 TIMES LESS DENSE THAN A STAR d. has a temperature of A FEW DEGRESS KELVIN 3. There are four factors that gravity must overcome in order to begin star formation. Briefly describe each. a. The thermal energy of Hydrogen. CAUSES MOTION OF ATOMS AND MOLECULES, DISPERSING THEM b. Interstellar magnetic field IONIZED GASSES CANNOT MOVE FREELY THROUGH A MAGNETIC FIELD c. rotation RAPID ROTATION RESISTS FURTHER CONTRACTION OF THE CLOUD d. Turbulence of the interstellar medium CURRENTS THROUGH THE MEDIUM MAKES IT DIFFICULT FOR A CLOUD TO CONTRACT 4. Explain the role that shock waves are believed to play in the formation of stars. COMPRESSES A CLOUD, CREATING AREAS THAT ARE VERY DENSE AND START TO COLLAPSE GRAVITATIONALLY 5. List the four possible causes of shock waves. SUPERNOVA EXPLOSIONS, IGNITION OF HOT STARS IONIZES GAS WHICH CREATES A SHOCK WAVE WHEN IT IS PUSHED INTO COLDER, DENSER INTERSTELLAR MATTER COLLISION OF MOLECULAR CLOUDS SPIRAL PATTERN OF SOME GALAXIES SPIRAL ARMS MAY BE SHOCK WAVES 6. Explain free fall contraction and how it contributes to the formation of stars. ATOMS FALL TOWARDS THE CENTER OF THE GRAVITATIONAL MASS, COLLIDE WITH EACH OTHER AND PRODUCE THERMAL ENERGY 7. An important principle in astronomy. Whenever a cloud of gas CONTRACTS, gravitational energy is converted into thermal energy and the gas grows HOT. Whenever a cloud of gas ROTATES, thermal energy is converted into gravitational energy and the gas ACCRETES (OR CONTRACTS MORE_). 8. What is a protostar? How long is it s life (see page 218)? Draw a simple sketch of it and it s cocoon. THE LIFE OF THE PROTOSTAR DEPENDS ON ITS MASS BETWEEN 100 MILLION YEARS FOR A LOW MASS STAR, TO 0.16 MILLION YEARS FOR A HIGH MASS STAR.
9. What is a protostellar disk and what are they responsible for? FLATTENED DISK OF GAS AND DUST SURROUNDING A PROTOSTAR, THESE ARE RESPONSIBLE FOR THE FORMATION OF PLANETS. 10. When the protostar becomes HOT enough, it can drive away surrounding GAS and DUST and become VISIBLE. 11. What is the birth line and how does it relate to the main sequence? THE LINE ON THE H-R DIAGRAM THAT IS APPROXIMATELY PARALLEL AND TO THE RIGHT OF THE MAIN SEQUENCE LINE WHERE PROTOSTARS FIRST APPEAR WHEN THEY BECOME VISIBLE. 12. Using the two pages after 219, give at least four pieces of evidence of star formation. LOW MASS STARS IN CLUSTER NGC2264 WHICH ARE NEAR THE BIRTH LINE T TAURI STARS ARE IRREGULAR IN BRIGHNESS SUGGESTING THEY ARE SURROUNDED BY DUST AND GAS BRIGHT HOT STARS HAVE A SHORT LIFE EXPECTANCY SO THEY MUST HAVE BEEN FROMED RECENTLY HERBIG-HARO OBJECTS SMALL NEBULAE THAT FLUCTUATE IN BRIGHTNESS AND PRODUCE FLICKERING JETS THAT EXCITE INTERSTELLAR MEDIUM EVIDENCE OF STAR FORMATION BECAUSE ONLY DISCS COULD PRODUCE A JET EAGLE NEBULA PHOTOS SHOW EVAPORATING DUST DRIVING AWAY THE GAS TO EXPOSE GLOBULES 11-2: The Source of Stellar Energy 13. How is the CNO cycle different from the proton-proton chain version of fusion? VERY SIMILAR BUT USES A CARBON ATOM AS CATALYST AND IS VERY TEMERATURE SENSITIVE. AT LOWER TEMPERATURES CNO CYCLE RELEASES VERY LITTLE ENERGY BUT AT HIGHER TEMPERATURES RELEASES MUCH MORE ENERGY. 11-3: Stellar Structure 14. Stars can exist only as long as energy can move from their CORE to their SURFACE. 15. Summarize the three methods of energy flow THEN explain how it applies to stars. a. Convection HOT REGIONS OF GAS RISE TO THE SURFACE WHERE THE GAS IS COOLED AND SINKS. IN STARS THIS CARRIES ENERGY TO THE SURFACE AND MIXES THE GASSES.
b. Conduction TRANSFER OF HEAT ENERGY BY CONTACT AND ONLY IMPORTANT IN STARS WITH VERY HIGH INTERNAL DENSITIES. c. Radiation HEAT ENERGY FROM THE CORE IS RADIATED OUT TO THE ATMOSPHERE. THIS HEATS A STARS PHOTOSPHERE WHICH RELEASES LIGHT ENERGY AS A RESULT. 16. The sun is balanced between two forces GRAVITY trying to squeeze it tighter and PRESSURE trying to make it expand. 17. Upper main sequence stars fuse Hydrogen on the CNO cycle, and lower main sequence stars fuse hydrogen on the PROTON-PROTON chain. Reading Questions from page 229. RQ 3. A - MANY STARS ARE FORMED FROM CLOUDS OF GAS EJECTED BY A SUPERNOVA EXPLOSION EVIDENCE THAT STAR FORMATION IS SOMETIMES CYCLICAL B BOK GLOBULES, T TAURI STARS AND HERBIG HARO OBJECTS ARE EVIDENCE THAT PROTOSTARS EXIST C- ORION NEBULA HAS T TAURI STARS, AND SUPER HOT STARS THAT MUST BE VERY YOUNG (BECAUSE THEY HAVE A SHORT LIFE EXPECTANCY) DQ 1 (p. 232) ANCIENT ASTRONOMERS LIKELY OBSERVED SUPERNOVAE, IF THEY OBSERVED A SUPERNOVA OF A CHARTED STAR, THEN THEY WOULD HAVE KNOWN THAT STARS CAN UNDERGO TREMENDOUS CHANGES IN LUMINOSITY AND THEN DISAPPEAR.