Chapter 13 Exploring Our Galaxy
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Transcript of Chapter 13 Exploring Our Galaxy
![Page 1: Chapter 13 Exploring Our Galaxy](https://reader036.fdocuments.us/reader036/viewer/2022062304/56813715550346895d9e9f19/html5/thumbnails/1.jpg)
Chapter 13Exploring Our Galaxy
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Our Location in the Milky Way
The Milky Way Galaxy is a disk-shaped collection of stars.
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Views of the Milky Way
• When we look out at the night sky in the plane of the disk, the stars appear as a band of light that stretches all the way around the sky.
• When we look perpendicular to the plane of the Galaxy, we see only those relatively few stars that lie between us and the “top” or “bottom” of the disk.
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Obscuring Dust
Clouds
• Interstellar dust can absorb or scatter light from distant stars, causing them to appear dimmer than they otherwise would
• Therefore they seem farther or even fewer than they actually are.
• This interstellar dust is more concentrated in the disk of our Galaxy, obscuring our view, making objects in the disk appear dimmer than they really are.
• This makes it seem that the objects are farther away than they really are.
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• We determine our location in the Galaxy by observing globular clusters.
• The globular clusters form a spherical halo centered on the center of the Galaxy.
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Cepheid Variables
• Henrietta Leavitt discovered the period-luminosity relation for Cepheid variables.
• The longer a Cepheid’s period, the greater its luminosity. • The period-luminosity law can be used to determine
distances.• Knowing the star’s periodocity, you can find out how far
away the star must be in order to give the observed brightness.
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Cepheid and RR Lyrae
Variables
• The more luminous the Cepheid, the longer its pulsation period.
• RR Lyrae variables are horizontal-branch stars that all have roughly the same average luminosity of about 100 L.
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RR Lyrae Variables in
Globular Clusters
• RR Lyrae variables are commonly found in globular clusters.
• By using the period-luminosity relationship for these stars, we can determine the distances to globular clusters.
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The Infrared Milky Way
• In infrared wavelengths interstellar dust radiates more strongly than stars.
• A far-infrared view of the sky is principally a view of where the dust is.
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The Structure
of Our Galaxy
• There are three major components of our Galaxy: a disk, a central bulge, and a halo.
• The disk contains gas and dust along with metal-rich (Population I) stars.
• The halo is composed almost exclusively of old, metal-poor (Population II) stars.
• The central bulge is a mixture of Population I and Population II stars.
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Measurements of our galaxy match images of other spiral
galaxies.
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Star Orbits in the Milky
Way
• The different populations of stars in our Galaxy travel along different sorts of orbits.
The galaxy in this visible-light image is the Milky Way’s near-twin NGC 7331.
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Stellar Populations: Disk Versus
Central Bulge
• The disk and central bulge of the Milky Way contain different populations of stars. The same is true for this galaxy, NGC 1309, which has a similar structure to the Milky Way Galaxy and happens to be oriented face-on to us.
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Magnetic Interactions in the
Hydrogen Atom
• Electrons and protons are both tiny magnets.
• When the electron flips from the higher-energy to the lower-energy configuration, the atom loses a tiny amount of energy and emits a radio photon with a wavelength of 21 cm.
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The Sky at 21 Centimeters
This image was made by mapping the sky with radio telescopes tuned to the 21-cm wavelength emitted by neutral interstellar hydrogen (H I). Black and blue represent the weakest emission, and red and white the strongest.
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Detecting Our Galaxy’s Spiral
Arms
• If we look within the plane of our Galaxy from our position at S, hydrogen clouds at different locations are moving at slightly different speeds relative to us.
• Radio waves from these various gas clouds are subjected to slightly different Doppler shifts.
• Radio astronomers sort out the gas clouds and map the Galaxy.
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Neutral Hydrogen in Our Galaxy
• Surveys of 21-cm radiation show the distribution of hydrogen gas in a face-on view of our Galaxy. • The distribution suggests a spiral structure.
Details in the blank, wedge-shaped region at the bottom of the map are unknown. Gas in this part of the Galaxy is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift.
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A Spiral Galaxy in Multiple Wavelengths
• Visible-light clearly shows the spiral arms. The presence of young stars and H II regions indicate that star formation takes place in spiral arms.
• Radio shows the emission from neutral interstellar hydrogen gas. The same pattern of spiral arms is traced out in this image as in the visible-light photograph.
• There is a much smoother appearance in this near-infrared view. This shows that cooler stars, which emit strongly in the infrared, are spread more uniformly across the galaxy’s disk.
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The Spiral Arms
• Visible light allows us to see enough OB associations and HII regions to plot the spiral arms in our vicinity.
• Carbon monoxide in molecular clouds emits radio waves that are relatively unaffected by interstellar dust. They have been observed even in remote corners of the Galaxy.
• SO, we believe that the Milky Way has at least four major arms.
• We are located on a minor arm segment called the Orion Arm.
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The Rotation of the Milky Way
• All stars and gas orbit in the same direction.• All matter seems to orbit at about the same speed.• Therefore, the Milky Way is not rotating as a solid disk,
and• The objects orbiting the center of the Milky Way do not
appear to obey Kepler’s third law!
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The Sun’s Orbit and the Mass of the
Galaxy
• In our Solar System all objects orbit one large mass, the Sun, and obey Kepler’s third law.
• In the Milky Way, a star’s orbit is determined by all of the mass inside of its orbit (stars, gas, and dust)
• Our Sun’s orbit gives us clues as to the mass of the entire Milky Way.
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Rotation Curves and the
Mystery of Dark Matter
• The dashed red curve indicates how this orbital speed should decline beyond the confines of most of the Galaxy’s visible mass. • Because there is no such decline, there must be an
abundance of invisible dark matter that extends to great distances from the galactic center.
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The Galaxy and Its Dark Matter Halo
• The dark matter in our Galaxy forms a spherical halo whose center is at the center of the visible Galaxy. • The extent of the dark matter halo is unknown, but its
diameter is at least 100 kiloparsecs. • The total mass of the dark matter halo is at least 10 times the
combined mass of all of the stars, dust, gas, and planets in the Milky Way.
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Dark Matter Speculations
• MACHOS: Massive Compact Halo Objects• Brown dwarfs, white dwarfs, black
holes• Can only account for about half of
the dark matter halo• Searched for via gravitational
lensing• WIMPS: Weakly Interacting Massive
Particles• Theoretical, and not-yet-detected
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The Density-Wave Model
Perhaps, the spiral arms of the galaxy are a pattern that moves through the Galaxy, like ripples in water.
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Star Formation in the Density-Wave Model
• A spiral arm is a region where the density of material is higher than in the surrounding parts of a galaxy.
• Interstellar matter moves around the galactic center rapidly and is compressed as it passes through the slow-moving spiral arms.
• This compression triggers star formation in the interstellar matter, so that new stars appear on the “downstream” side of the densest part of the spiral arms.
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Infrared and Radio Observations of the Galactic Nucleus
• In an infrared image, the reddish band is dust in the plane of the Galaxy and the fainter bluish blobs are interstellar clouds heated by young O and B stars.
• Adaptive optics reveals stars densely packed around the galactic center.
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X rays from around a Supermassive Black Hole
• Looking toward the center of the Galaxy, toward Sagittarius A, we see one of the brightest radio sources in the sky.
• Magnetic fields shape nearby interstellar gas into immense, graceful arches. • X-ray wavelengths show lobes of gas on either side of Sagittarius A.
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http://astro.uchicago.edu/cosmus/projects/UCLA_GCG/uclastars_cinepak75.mp4