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Download The formation of the galaxy is believed to be similar
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The formation of the galaxy is believed to be similar to the formation of the solar system. During the time that stars were being formed, our galaxy didn’t have a disk. All that existed was an irregularly shaped galaxy. All the gas & dust collapsed into a disk. Current models show that the abundance of heavy elements should be greater than what it is actually observed. This may be because low heavy metal abundant gas & dust from the halo is diluting the disk. Astronomers are still unsure about the Milky Way’s origins… Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form: The spiral arms cannot rotate at the same speed as the galaxy; they would ―curl up‖. The spiral arms cannot rotate at the same speed as the galaxy; they would ―curl up‖. Rather, they appear to be density waves, with stars moving in and out of them. This is similar to a water wave, the spirals ―seen‖ are patterns from by these density waves. They are not large masses of matter moving. Dust lanes indicate high density gas (star forming regions). Young O & B class stars & open clusters are found just past these lanes. Further ahead we find older K & M class stars & globular clusters. In essence, the density waves are thought to drive star formation. Or perhaps it’s the other way around… The star formation may contribute to propagation of the arms (driving the density waves). The origin of the spiral arms is not yet understood. Much nonplussary… A few theories… Gravitational effects from neighboring galaxies. Instabilities of the gas near the bulge. Asymmetrical bar within the bulge (causing an off-axis rotation). Possible merger with a dwarf galaxy. The position of the Sun in the Milky Way Galaxy is best described as a) in the disk, slightly more than halfway out from the center. b) very close to the center. c) in an open cluster in the disk. d) in a globular cluster in the halo. The position of the Sun in the Milky Way Galaxy is best described as a) in the disk, slightly more than halfway out from the center. b) very close to the center. c) in an open cluster in the disk. d) in a globular cluster in the halo. The thickness of our galaxy's disk is determined by the a) circular speed of stars around the galaxy. b) random motion of stars in the plane of the disk. c) random motion of stars perpendicular to the disk. d) the amount of matter in nucleus of the galaxy. The thickness of our galaxy's disk is determined by the a) circular speed of stars around the galaxy. b) random motion of stars in the plane of the disk. c) random motion of stars perpendicular to the disk. d) the amount of matter in nucleus of the galaxy. When we observe stars near the center of the Milky Way Galaxy, we detect light that was emitted from those stars about a) 200 million years ago. b) 25,000 years ago. c) 8 years ago. d) 8 minutes ago. e) when the galaxy was formed. When we observe stars near the center of the Milky Way Galaxy, we detect light that was emitted from those stars about a) 200 million years ago. b) 25,000 years ago. c) 8 years ago. d) 8 minutes ago. e) when the galaxy was formed. The mass of our galaxy is best found by a) counting the number of stars in the sky. b) counting the star clusters in the sky. c) counting the hot, massive main sequence stars. d) radio measurements of the amount of interstellar hydrogen. e) measuring the rotation of the Galaxy. The mass of our galaxy is best found by a) counting the number of stars in the sky. b) counting the star clusters in the sky. c) counting the hot, massive main sequence stars. d) radio measurements of the amount of interstellar hydrogen. e) measuring the rotation of the Galaxy. You could best map out the overall spiral structure of our Galaxy by finding a) young open clusters and hydrogen gas. b) evolved stars like planetary nebulae, RR Lyra stars. c) smooth, round globular clusters. d) high velocity stars. You could best map out the overall spiral structure of our Galaxy by finding a) young open clusters and hydrogen gas. b) evolved stars like planetary nebulae, RR Lyra stars. c) smooth, round globular clusters. d) high velocity stars. Choose the best evidence that the disk of the Milky Way Galaxy does NOT rotate like a solid wheel. a) Disk stars have Doppler shifts. b) The brightest disk stars form spiral arm shapes. c) Disk stars rotate twice as quickly that are twice as far from the Galactic center. d) The rotation of disk stars around the Sun decreases with distance according to Kepler's laws. Choose the best evidence that the disk of the Milky Way Galaxy does NOT rotate like a solid wheel. a) Disk stars have Doppler shifts. b) The brightest disk stars form spiral arm shapes. c) Disk stars rotate twice as quickly that are twice as far from the Galactic center. d) The rotation of disk stars around the Sun decreases with distance according to Kepler's laws. Compared to stars like the Sun in the disk of the Milky Way, stars that populate the extended halo of the galaxy were born a) earlier, so have had time to accumulate more heavy elements. b) later, so have used up their heavy elements. c) earlier, from more nearly primordial material, so have fewer heavy elements. d) later, so have accumulated more heavy elements from previous generations of stars. Compared to stars like the Sun in the disk of the Milky Way, stars that populate the extended halo of the galaxy were born a) earlier, so have had time to accumulate more heavy elements. b) later, so have used up their heavy elements. c) earlier, from more nearly primordial material, so have fewer heavy elements. d) later, so have accumulated more heavy elements from previous generations of stars. You observe two stars at the same distance. One is in the disk of the Milky Way, the other in a direction perpendicularly out of the disk. Chances are that the disk star will be a) less luminous, have a smaller Doppler shift and be reddened by dust. b) more luminous, have a smaller Doppler shift and be reddened by dust. c) more luminous, have a larger Doppler shift and be reddened by dust. d) more luminous, have a smaller Doppler shift and be less reddened by dust. You observe two stars at the same distance. One is in the disk of the Milky Way, the other in a direction perpendicularly out of the disk. Chances are that the disk star will be a) less luminous, have a smaller Doppler shift and be reddened by dust. b) more luminous, have a smaller Doppler shift and be reddened by dust. c) more luminous, have a larger Doppler shift and be reddened by dust. d) more luminous, have a smaller Doppler shift and be less reddened by dust. A cold thin cloud of interstellar gas embedded in a hotter, thinner surrounding medium would yield 21-cm radio spectral emission lines of hydrogen showing a) a single broadened emission line. b) broad emission with a narrow spike-like absorption core. c) broad absorption with a narrow spike-like emission core. d) a narrow spike-like emission core and low broad shoulders of emission. A cold thin cloud of interstellar gas embedded in a hotter, thinner surrounding medium would yield 21-cm radio spectral emission lines of hydrogen showing a) a single broadened emission line. b) broad emission with a narrow spike-like absorption core. c) broad absorption with a narrow spike-like emission core. d) a narrow spike-like emission core and low broad shoulders of emission. The primary reason that massive O-type stars are not found in the galactic halo is because they are a) too massive to be kicked into the halo from the disk. b) so massive that they settle into the thinner disk. c) too short-lived to have persisted from halo formation until today. d) closer to us in the disk than in the extended halo. The primary reason that massive O-type stars are not found in the galactic halo is because they are a) too massive to be kicked into the halo from the disk. b) so massive that they settle into the thinner disk. c) too short-lived to have persisted from halo formation until today. d) closer to us in the disk than in the extended halo. One model for the formation of the Milky Way can be divided into 2 phases: a spherical gas cloud (halo) collapsed to form the stars in the Milky Way's spheroid, then rapidly rotating gas collapsed into a disk-shaped configuration of stars. Since disk stars have higher metallicity, which is most likely? Gas ejected from the a) spheroid stars enriched the material now in the disk stars. b) spheroid stars decreased their metallicity. c) spheroid decreased its angular momentum. d) disk stars puffed out the spheroid stars into a rounder shape. One model for the formation of the Milky Way can be divided into 2 phases: a spherical gas cloud (halo) collapsed to form the stars in the Milky Way's spheroid, then rapidly rotating gas collapsed into a disk-shaped configuration of stars. Since disk stars have higher metallicity, which is most likely? Gas ejected from the a) spheroid stars enriched the material now in the disk stars. b) spheroid stars decreased their metallicity. c) spheroid decreased its angular momentum. d) disk stars puffed out the spheroid stars into a rounder shape. The orbital speed of an object depends only on the amount of mass between it and the galactic center: The velocity should diminish with distance. It doesn’t… More than twice the mass of the galaxy would have to be outside the visible part to reproduce the observed curve below – a dark halo. What could this ―dark matter‖ be? It is dark at all wavelengths, not just the visible. • Stellar-mass black holes? Probably no way enough of them could have been created • MACHOs – Brown dwarfs, faint white dwarfs, and red dwarfs? Currently the best star-like option • WIMPs – weird subatomic particles? Could be, although no direct evidence so far A Hubble search for red dwarfs turned up too few to account for dark matter. If enough existed, they should have been detected. The bending of spacetime can allow a large mass to act as a gravitational lens. Observation of such events suggests that lowmass white dwarfs could account for as much as 20% of the mass needed. The rest is still a mystery. This is a view toward the galactic center, in visible light: the two arrows in the inset indicate the location of the center; it is entirely obscured by dust. These images—in infrared, radio, and X-ray—offer a different view of the galactic center: The galactic center appears to have: •A ring of molecular gas 400 pc across • Strong magnetic fields • A rotating ring or disk of matter a few parsecs across • A strong X-ray source at the center Apparently, there is an enormous black hole at the center of the galaxy, which is the source of these phenomena. An accretion disk surrounding the black hole emits enormous amounts of radiation. These objects are very close to the galactic center. The orbit on the right is the best fit; it assumes a central black hole of 3.7 million solar masses.