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Explore the structure and components of our galaxy, including stars, gas, and observations through different wavelengths. Discover the true center of the Milky Way through infrared observations and learn about the census of our galaxy's stars and gas. Observe neutral and molecular hydrogen, ionized gas, and the various processes that produce radiation. Gain insights into star formation, heavy elements, and captivating nebulas.
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Lecture 30: The Milky Way
topics: • structure of our Galaxy • components of our Galaxy (stars and gas) • how we observe different components
The structure of our Galaxy • not so easy to determine the structure of our Galaxy from the inside because of the effects of dust in our disk • initially thought that the Sun was at the center of the Galaxy
The real center of the Galaxy • globular clusters can be seen above the “smog” of the disk • period-luminosity relation for RR-Lyrae stars used to find distances to globular clusters • GC’s in a huge sphere, but the center is located several kpc away from the Sun – this is the true center of the Galaxy
Verified by Infra-red Observations • dust extinction inversely proportional to wavelength – longer wavelengths less obscured • dust emits light at far infra-red wavelengths (30-300 microns) • infra-red observations give us a direct view of the structure of our Galaxy
A Census of Our Galaxy: stars • stars – about 50 billion solar masses • disk stars (about 80 percent) • young, metal-rich, blue • bulge stars (about 20 percent) • mixture of young and old, red and blue • halo stars (less than 1 percent) • uniformly old and red, metal-poor
A Census of Our Galaxy: gas • molecular hydrogen (H2) • about 5 billion solar masses • atomic (neutral) hydrogen (HI) • about 5 billion solar masses • ionized hydrogen (HII) • very small amount of mass, but fills most of the volume of the disk density
Gas in the interstellar medium density (atoms/cm3) temperature
Chemical Composition • 70 percent hydrogen • 28 percent helium • 2 percent “heavy elements” (metals)
Observing neutral Hydrogen • neutral hydrogen emits radiation in the radio due to magnetic spin-flip transitions • often called ’21-cm’ transition
observing molecular hydrogen • molecular hydrogen is too cold to produce emission lines • we can use Carbon Monoxide (CO) or other molecules as a tracer • these molecules produce emission lines in the radio
Observing hot ionized gas • supernovae and massive stars produce “bubbles” of hot gas • as the bubbles expand outwards, they produce shock waves, which cause X-ray radiation
Observing hot ionized gas • because of the high temperatures, the gas gets ionized. • when electrons fall back from their excited state, they produce emission lines at optical wavelengths
Observing hot ionized gas • shock waves from SN also act as subatomic particle accelerators • charged particles (electrons) moving in a magnetic field produce radio-wavelength synchrotron radiation
Cassiopeia A Radio X-ray
Radio CO FIR NIR vis. X-ray gamma ray
Summary • structure of our Galaxy • disk, bulge, halo • components of our Galaxy • stars, atomic gas, molecular gas, ionized gas • how we can observe these components • different physics producing radiation • characteristic wavelengths of each process
atomic gas molecular clouds hot bubbles supernovae and stellar winds star formation heavy elements
