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The matter in our Galaxy emits different kinds of radiation.
5/22/2018 The matter in our Galaxy emits different kinds of radiation.
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The ground state in hydrogen is actually two energy levels and if it is left alone long enough…
Called the “spin flip” transition l = 21 cm And there’s lots of Hydrogen out there!!!
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Radio observations help map the galactic disk
Looking for 21-cm wavelengths of light … emitted by interstellar hydrogen as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts this allows the interstellar hydrogen to be mapped
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A Map of the Milky Way Based on 21-cm wavelength light mapping
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Zone of Avoidance
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Spiral Galaxy M83 observed in both visible light and radio wavelengths.
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Don’t galaxies look like they spin?
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We investigate the “Rotation Curve” of our Galaxy by plotting the velocity as a function of radial distance d from the center. d d
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Orbital Velocities in the Disk
5/22/2018 Orbital Velocities in the Disk rotation curve – a plot of rotational (orbital) speed .vs. distance from the center speed radius
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Do galaxies rotate like a Merry-Go-Round ?
(solid body rotation)
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Differential Rotation of the Galaxy
The Sun orbits at 230 km/s or about 500,000 mph
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Do they rotate like planets in our solar system ? (Keplerian falloff)
5/22/2018 Do they rotate like planets in our solar system ? (Keplerian falloff)
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The Galaxy’s Rotation Curve
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Most of the matter in the Galaxy has not yet been identified
According to Kepler’s Third Law, the farther a star is from the center, the slower it should orbit Observations show that speed actually increases with distance from the center This could be due to gravity from extra mass we cannot see - called DARK MATTER.
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Orbital Velocities in the Disk
Stars in the Galactic disk should orbit according to Kepler’s Laws Here is what we observe: The flat rotation curve of our Galaxy implies that: its mass in not concentrated in the center its mass extends far out into the halo But we do not “see” this mass we do not detect light from most of this mass in the halo so we refer to it as dark matter
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Zone of Avoidance Halo
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The Very Large Array (VLA) in New Mexico
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Center of the Galaxy Radio
Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*. X-ray
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Center of the Galaxy in Sagittarius
Infrared Visual
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Center of the Galaxy We measure the orbits of fast-moving stars near the Galactic center. these measurements must be made in the infrared in particular, this star passed within 1 light-day of Sgr A* using Kepler’s Law, we infer a mass of 2.6 million M for Sgr A* What can be so small, yet be so massive?
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X-ray Flare from Sgr A* The rapid flare rise/drop time (< 10 min) implied that the emission region is only 20 times the size of the event horizon of the 2.6 million M black hole. Observations are consistent with the existence of a supermassive black hole at the center of our Galaxy. Energy from flare probably came from a comet-sized lump of matter…torn apart before falling beneath the event horizon! Chandra image of Sgr A*
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The Star–Gas–Star Cycle
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Halo vs. Disk Stars Stars in the disk are relatively young.
fraction of heavy elements same as or greater than the Sun plenty of high- and low-mass stars, blue and red Stars in the halo are old. fraction of heavy elements much less than the Sun mostly low-mass, red stars Stars in the halo must have formed early in the Milky Way Galaxy’s history. they formed at a time when few heavy elements existed there is no ISM in the halo star formation stopped long ago in the halo when all the gas flattened into the disk
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Stellar Orbits in the Galaxy
Stars in the disk all orbit the Galactic center: in the same direction in the same plane (like planets do) they “bobble” up and down this is due to gravitational pull from the disk this gives the disk its thickness Stars in the bulge and halo all orbit the Galactic center: in different directions at various inclinations to the disk they have higher velocities they are not slowed by disk as they plunge through it nearby example: Barnard’s Star
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Mass of the Galaxy We can use Kepler’s Third Law to estimate the mass
Sun’s distance from center: 28,000 l.y. = 1.75 x 109 AU Sun’s orbital period: 230 million years (2.3 x 108 yr) P2 = 42/GM a3 mass within Sun’s orbit is 1011 M Total mass of MW Galaxy : 1012 M Total number of stars in MW Galaxy 2 x 1011
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Galaxies
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Galaxies seem to take one of four different appearances
I. SPIRALS
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SPIRALS
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SPIRALS
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The tightness of a spiral galaxy’s arms is correlated to the size of its nuclear bulge
Type Sa Type Sb Type Sc
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Variety of Spiral Arms Flocculent spirals (fleecy)
Grand-design spirals (highly organized)
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We easily see these spiral arms because they contain numerous bright O and B stars which illuminate dust in the arms. However, stars in total seem to be evenly distributed throughout the disk.
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Spiral Structure Our Galactic disk does not appear solid.
it has spiral arms, much like we see in other galaxies like M51 These arms are not fixed strings of stars which revolve like the fins of a fan. They are caused by compression waves which propagate around the disk. such waves increase the density of matter at their crests we call them density waves they revolve at a different speed than individual star orbit the Galactic center Note how the spiral arms appear bluer compared to the bulge or the gaps between the arms. M 51
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Compression Wave
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Spiral Arms The compression caused by density waves triggers star formation. molecular clouds are concentrated in arms…plenty of source matter for stars short-lived O & B stars delineate the arms and make them blue & bright long-lived low-mass stars pass through several spiral arms in their orbits around the disk
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Galaxies seem to take one of four different appearances
II. BARRED SPIRALS
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BARRED SPIRALS
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BARRED SPIRALS
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Bars of stars run through the nuclear bulges of barred spiral galaxies
Type SBa Type SBb Type SBc
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Galaxies seem to take one of four different appearances
Type E0 Type E3 Type E6 III. ELLIPTICALS
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ELLIPTICALS
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Elliptical galaxies display a variety of sizes and masses
Giant elliptical galaxies can be 20 times larger than the Milky Way Dwarf elliptical galaxies are extremely common and can contain as few as a million stars
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Galaxies seem to take one of four different appearances
IV. IRREGULAR
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Galaxies seem to take one of four different appearances
Spirals Barred Spirals Ellipticals Irregulars
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This classification scheme is known as the Hubble Tuning Fork Scheme
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