astro1010-lee.com [email protected] uvu survey of astronomy chapter 21 our milky way
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Chapter 20
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The picture below is a 360o panoramic view of our home Galaxy the Milky Way. In as much as we are on the inside of the Galaxy , our view is incomplete.
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From Earth, see relatively few stars when looking out of galaxy (red arrows), many when looking into the disk (blue arrows) of the galaxy. When looking toward the bulge at the center, most our view is blocked by dust and dense gas.
Chapter 20
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One of the first attempts to measure the size and shape of the Milky Way was done by Herschel using visible stars. Unfortunately, he was not aware that most of the Galaxy, particularly the center, is blocked from view by vast clouds of gas and dust.
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Giant stars, after the Helium Flash, pass into the Instability Strip. The variability of these stars comes from a dynamic imbalance between gravity and pressure – they have oscillations around stability.
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The upper plot is an RR Lyrae star. All such stars have about the same luminosity curve, with periods from 0.5 to 1 day.
The lower plot is a Cepheid variable; Cepheid periods range from about 1 to 100 days.
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The usefulness of these stars comes from their period–luminosity relation
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The Period-Luminosity Relation allows us to measure the distances to these bright giant stars.
• RR Lyrae stars all have about the same luminosity; knowing their apparent magnitude and using the inverse square law allows us to calculate the distance.
• Cepheids have a luminosity that is strongly correlated with the period of their oscillations; once the period is measured, the luminosity is known and we can proceed as above.
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We have now expanded our cosmic distance ladder one more step .. this time a giant step
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Many RR Lyrae stars are found in globular clusters. These clusters are not all in the plane of the Galaxy, so they are not obscured by dust and can be measured.
This yields a much more accurate picture of the extent of our Galaxy and our place within it.
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Once we could measure distances in the Milky Way, we could determine our position in it. This artist’s conception shows the various parts of our Galaxy,and location of our Sun. Harlow Shapely and his students were instrumental in determining the size and shape of the Galaxy.
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The Galactic Halo is made of globular clusters and formed very early. The halo is essentially spherical. All the stars in the halo are very old, and there is no gas and dust in the clusters.
The Galactic disk is where the youngest stars are, as well as star forming regions (emission nebulae), and large clouds of gas and dust.
Surrounding the Galactic center is the Galactic bulge, which contains a mix of older and younger stars.
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Stellar orbits in the disk are in a plane and in the same direction; orbits in the halo and bulge are much more random.
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This infrared view of our Galaxy shows much more detail of the Galactic center than the visible-light view does, as infrared seems to penetrate the gas and dust.
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Any theory of galaxy formation must be able to account for all the properties listed below
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y Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form
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One method that astronomers have used to calculate the Mass of the Milky Way galaxy is to use Kepler's 3rd law. We take the radius of our orbit and the period. Plugging those numbers into Kepler's 3rd law we can estimate the mass inside our orbit. This gives just shy of 100 billion solar masses. Other mass estimates of the Milky Way are up to 1 trillion solar masses.
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Once all the Galaxy is within an orbit, the Orbital Velocity should diminish with distance, as the dashed curve shows. It doesn’t; more than twice the mass of the Galaxy would have to be outside the visible part to reproduce the observed curve. This mass is called Dark Matter
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Matter is space matter we cannot see because, unlike stars and galaxies, it does not give off light.
There is much more dark matter in the Universe than bright. Some scientists think 90 percent of matter is dark.
Astronomers know about dark matter because its gravity pulls on stars and galaxies, changing their orbits and the way they rotate.
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This is a view towards the Galactic Center, in visible light. The two arrows in the inset indicate the location of the center; it is entirely obscured by dust.
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Because of interstellar dust along the line of sight, the available information about the Galactic Center comes from observations at gamma ray, hard X-ray, infrared and radio wavelengths. Coordinates of the Galactic Center were first found by Harlow Shapely in his 1918 study of the distribution of the globular clusters.
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Direct study of a black hole such as the one widely suspected to exist at the center of our galaxy is tricky, as black holes swallow up nearby light, rendering themselves virtually invisible. But researchers can infer properties of a black hole from its hearty gravitational influence on nearby stars. A large star S2 in that region was found to orbit a dark concentration of mass, estimated at 3.7 million times the mass of our Sun The laws of physics have ruled out any explanation but one--that this is indeed an enormous black hole.
Chapter 20
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Direct study of a black hole such as the one widely suspected to exist at the center of our galaxy is tricky, as black holes swallow up nearby light, rendering themselves virtually invisible. But researchers can infer properties of a black hole from its hearty gravitational influence on nearby stars. A large star S2 in that region was found to orbit a dark concentration of mass, estimated at 3.7 million times the mass of our Sun The laws of physics have ruled out any explanation but one—that this is indeed an enormous black hole.
