STUDY NOTE! - key notes from the textbook and lectures

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Astronomy & Astrophysics
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Stefan Mochnacki

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Chapter 19 Our Galaxy 19.1 The Milky Way Revealed What does our galaxy look like? Spiral galaxy: a galaxy that looks like a flat, white disk with a yellowish bulge at its centre. The disk is filled with cool gas and dust, interspersed with hotter ionized gas, and usually displays spiral arms. Spiral arms: the bright, prominent arms, usually in a spiral pattern, found in spiral galaxies Disk: the portion of a spiral galaxy that looks like a disk and contains an interstellar medium with cool gas and dust; stars of many ages are found in the disk Bulge: the central portion of a spiral galaxy that is roughly spherical and bulges above and below the plane of the galactic disk Halo: the spherical region surrounding the disk of a spiral galaxy Globular clusters: spherically shaped clusters of up to a million or more stars; globular clusters are found primarily in the halos of galaxies and contain only very old stars the entire galaxy is about 100,000 ly in diameter and the disk is 1000ly thick our Sun is located in the disk about 28,000 ly from the galactic centre How do stars orbit in our galaxy? Stars in the disk orbit in roughly circular paths that all go in the same direction in nearly the same plane Stars in the bulge and halo soar high above and below the disk on randomly oriented orbits Orbits of Disk Stars Individual stars bob up and down through the disk as they orbit The general orbit of a star around the galaxy arises from its gravitational attraction toward the galactic centre, while the bobbing arises from the localized pull of gravity within the disk itself Orbits of Halo and Bulge Stars Individual bulge and halo stars travel around the galactic centre on more or less elliptical paths, but the orientations of these paths are relatively random These swooping orbits explain why the bulge and halo are much puffier than the disk Halo and bulge stars soar to heights above the disk far greater than the up-and-down bobbing of disk stars The great difference between orbits in the disk and orbits in the bulge and halo indicate that disk stars and halo stars have very different origins Stellar Orbits and the Mass of the Galaxy The orbital motion of the Sun and other stars gives us a way to determine the mass of the galaxy Newtons law of gravity determines how quickly objects orbit one another Newtons version of Keplers third law allows us to determine the mass of a relatively large object when we know the period and average distance of a much smaller object in orbit around it For example, we can use the Suns orbital velocity and its distance from the galactic centre to determine the mass of our galaxy lying within the suns orbit 19.2 Galactic Recycling The birth of the Sun and the planets could not have occurred without galactic recycling that takes place within the disk of the galaxy and its interstellar It gradually changes the chemical composition of the interstellar medium The newly created elements mix with other interstellar gas and thereby become incorporated into new generations of stars How is gas recycled in our galaxy? The galactic recycling process proceeds in several stages, making s utpat- e gas-star cycle Molecular clouds star formationnuclear fusion in starsreturning gashot bubbles atomic-hydrogen clouds molecular clouds star formation nuclear fusion in starsetc Stars are born when gravity causes the collapse of molecular clouds They die when theyve exhausted their fuel for fusion They ultimately return much of their material back to the interstellar medium during their lives through stellar winds, and at death through a planetary nebula in the case of low-mass stars or through supernova in the case of high-mass stars Gas from Dying Stars All stars return much of their original mass to interstellar space in two basic 1. Stellar winds that blow throughout their lives 2. Death events such as planetary nebulae and supernovae The high-speed gas ejected into space by supernovae or powerful stellar winds sweeps up surrounding interstellar material, excavating a bubble of hot, ionized gas around the exploding star Superbubbles and Fountains Superbubble: arise when many individual bubbles combine to form a much larger bubble Galactic fountain: when a superbubble grows too large for the Milky Way and spills out, creating a massive blowout The gravity of the galactic disk slows and eventually halts the rise of the gas from a blowout The ejected gas starts to cool and form clouds Gravity then causes these clouds to rain back down into the disk, where their contents mix with the gas in that region of the galaxy Cooling and Cloud Formation Atomic hydrogen gas: cool gas in the galaxy that hydrogen atoms remain neutral rather than being ionized after the gas that makes the bubbles cool, it becomes part of this widespread atomic hydrogen gas in the galaxy it emits a spectral line with a wavelength of 21 centimetres Atomic hydrogen gas tends to be found in two distinct forms: 1. Tenuous clouds of warm atomic hydrogen 2. Smaller, denser clouds of cool atomic hydrogen Gravity slowly draws blobs of this gas together into clouds From Atomic to Molecular Clouds As the temperature drops further in the centre of a cool cloud of atomic hydrogen, the hydrogen atoms combine into molecules, making a molecular cloud Where do stars tend to form in our galaxy? Galactic environments rich in molecular clouds tend to spawn new stars easily, while gas-poor environments do not Star-Forming Regions Anywhere where we can see large, hot stars, is a region of active star formation because they burn out so quickly Spiral Arms The spiral arms are home to both molecular clouds and numerous clusters of young, bright, blue stars surrounded by ionization nebulae Spiral arms are enormous waves of star formation that propagate through the gaseous disk of a spiral galaxy Spiral arms are places in a galaxys disk where stars and gas clouds get more densely packed 19.3 History of the Milky Way Two differences between halo stars and disk stars give us clues to their oriWe dont find any young stars in the halo, while in the disk we see stars of many different ages The spectra of halo stars show that they contain fewer heavy elements than do disk stars Because of these striking differences, astronomers divide the Milky Ways stars into two distinct populations: 1. The disk population contains both young stars and old stars, all of which have heavy-element proportions of about 2% 2. The spheroidal population consists of stars in the halo and bulge, both of which are roughly spherical in shape. Stars in this population are always old and therefore low in mass The halo does not contain the cold, dense molecular clouds required for star formation, containing almost no gas at all, except VERY hot gas How did our galaxy form?
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