Galaxies - Weebly



16.GalaxiesHow do we measure the distance to galaxies? Pulsating stars, such as Cepheid variables, cycle with a fixed relation between period and luminosity. By comparing how bright a variable star appears from Earth, to its actual luminosity, we can calculate the distance. Three Major Classification of Galaxies Huubble classified galaxies according to their shape:Spiral Galaxy, Elliptical Galaxy, Irregular GalaxyParts of a Spiral Galaxy , flattened galactic disk, containing spiral arms, central galactic bulge, containing a dense nucleus of many stars, extended spherical halo of faint, old stars Spiral Arms Spiral arms contain a large amount of gas and dust (15% of disk mass), with many hot, young stars (looking blue). New star formation in on-going. The spiral arms spin, as stars orbit around the galactic nucleus. The origin of spiral arms is based on density wave theory. Barred Spirals Similar to Spiral Galaxies are Barred Spirals, having an elongated central region (bar). The Milky Way has a “weak” bar. Elliptical Galaxies Elliptical galaxies are spherical or oblong shaped, with no real structure. Typical stars in elliptical galaxies are old, cool, low-mass stars, making them loo… reddish. There is little gas and dust, and little formation of new stars. Elliptical galaxies range greatly in size, from ten million solar masses to one hundred times the mass of the Milky Way.Irregular Galaxies Unusual-looking galaxies (that resemble neither spiral nor elliptical galaxies) are called irregular galaxies. Irregular galaxies have much gas and dust, where new stars are being formed, but…they don’t have spiral arms. The Milky Way has two nearby satellite galaxies, which are irregular galaxies—the Large and Small Magellanic Clouds Cause of Galaxy Types Rotation affects the shape of galaxies, with spirals rotating faster than ellipticals. Evidence indicates that merging (colliding) spiral galaxies form an elliptical galaxy (when little gas and dust remains), but form a large spiral, when considerable gas and dust does remain. Galaxy Collisions and Mergers. Galaxy collisions are common, but the individual stars are generally unaffected by the collision. Existing gas and dust can cause a burst of star formation—called star burst galaxies. Measuring the Mass of a Galaxy We can determine the velocity of individual stars in a galaxy by measuring the Doppler shift of the light. And since the stars are orbiting the galaxy, we can use this motion to calculate the total mass encircled within the orbit.Dark Matter When measuring the mass of a galaxy, we expect the mass to decrease further from the center (since there are fewer stars), but the high velocity of the stars shows that there is a lot of matter in the galaxy that we can’t see (not stars). This is dark matter. The dark matter comprises about 90% of the galaxy’s mass.Dark matter is made of… Active Galaxies Galaxies that emit great amounts of energy from a small core are known as active galaxies. Active galaxies that emit much energy at radio wavelengths are radio galaxies. Very distant, extremely bright, radio-energy emitters are quasars (quasi-stellar radio source).The power source of active galaxies is likely due to energy interactions with super-massive black holes. Our Galaxy, The Milky Way The Milky Way is a (barred) spiral, 100,000 light years in diameter, containing some 200 billion stars (though composed primarily of dark matter), and is approximately 13 billion years old (based on the age of the oldest stars). The Sun is located near the outer edge, and is orbiting the galactic core at 220 km per second. Milky Way in the Night Sky When we look at the Milky Way at night, it looks like a hazy band of light. The haze is the light from billions of stars. Since the galaxy is flat, like a pizza, with the Sun toward the edge, we see most of the stars arranged in a band of light. Star Clusters within Our Galaxy Open Clusters contain a few hundred stars, characterized by young, hot, blue stars (Population I stars)—located in the galaxy’s spiral arms. Globular Clusters contain a few thousand to several million stars, characterized by old, cool, red stars (Population II stars)—located in the galaxy’s halo.Nebulae A nebula is a visible region of gas. Hot gas glows red—an emission nebula.Cool gas can scatter light, scattering more blue light than other colors—a reflection nebula. Dense gas can block light coming from behind it—a dark nebula.How Will Our Galaxy End??? As stars die, some become white dwarfs and then black dwarfs. Other stars become supernovae, making matter available to form new stars. But eventually, most of the matter will end up as old, cold, dead stars, and black holes. Cosmology Cosmology is the study of the structure and the evolution of the universe as a whole. Cosmological Principle From Earth, we see countless galaxies in all directions. If we accept that neither the Earth nor any other place is unique in the universe—the Cosmological Principle—then it follows that the universe has no boundaries.The universe is infinite. Hubble Law Edwin Hubble measured the distances to galaxies (using Cepheid variables), and he measured the velocity of the galaxies (using Doppler redshift of spectral lines), and found a correlation.The distant galaxies are moving away faster—the Hubble Law. The whole universe is expanding. By analyzing the velocity of galaxies and the distance between them now, we can determine when the Big Bang occurred—13.7 billion years ago. The Expanding Universe With galaxies all moving away from each other, the universe is expanding. This is not actual motion of the galaxies, but rather the space between the galaxies is stretching. If the galaxies are moving apart now, where were the galaxies in the past? The universe must have begun in a very dense state—The Big Bang. The Big Bang The universe is infinitely large. But it is NOT infinitely old—13.7 billion years old. The “explosion” did NOT occur in a small space. It occurred in all space. Space itself “exploded,” everywhere at once. As space expanded, light travelling between galaxies was stretched into longer wavelengths—the cosmological redshiftThe Universe Becomes Transparent The early universe was extremely hot and dense. As time passed, matter formed, and cooled off.At 380,000 years old, there was enough space between atoms that light could travel—the universe became transparent. At a temperature of 3000 K, the matter of this young universe glowed with infrared light. Over the past 13.7 billion years, the infrared glow of the early universe has stretched into microwave wavelengths.Cosmic Microwave Background In 1963, Penzias and Wilson accidentally detected the cosmic microwave background radiation left over from the Big Bang. This proved the validity of the Big Bang model of creation of the universe. How big is the universe? The universe is infinite, but since the universe is just 14 billion years old, we can only see to a maximum distance of 14 billion light years. This sets a limit to the observable universe. The edge is called the cosmic horizon. What happens next? Two possibilities… 1. The universe can keep expanding forever, with energy getting spread out, with stars dying out—the Big Freeze. 2. The expansion can eventually stop, and gravity will pull the universe back into a hot, concentrated mass—the Big CrunchCritical Density If the universe has a lot of mass (per volume), it will fall back on itself—the Big Crunch, a closed universe. If the universe doesn’t have so much mass (per volume), it will expand forever—the Big Freeze, an open universe. The amount of mass (per volume) between the two is the critical density, creating a flat universe. Dark Energy We can measure how the expansion of the universe is changing over time by comparing the velocity of galaxies now (near ones) with the velocity of galaxies from very long ago (far ones).Surprisingly, galaxies billions of years ago, were moving slower than now, meaning the expansion of the universe has been speeding up. The unknown source of this accelerated expansion isWhat is the universe made of? According to Einstein’s, E=mc2, matter can be converted into energy, and vice versa. So, with dark energy providing the accelerated expansion of the universe, and dark matter providing the dominant mass of galaxies, only 4% of the universe is made of ordinary matter—stars, planets, etc. The Shape of Space Einstein’s general theory of relativity predicts a curvature of space, which is related to the universe’s future. Zero curvature is a flat universe (predicted by Big Bang theory). Positive curvature is the closed universe. Negative curvature is the open universe. Measurements based on the CMB (cosmic microwave background) indicate a flat universe. The End of the Universe The current state of knowledge indicates a flat universe, with zero curvature, at critical density, that will expand forever, with energy being spread out over a greater and greater volume, with everything grinding to a halt—the Big Freeze. Questions to be Answered:What is cosmology?What is the cosmological principle, and what are the ramifications?What is the Hubble Law, and what is it based on?What evidence do we have for the expanding universe?How big is the universe, versus the size of the visible universe?What is the Big Bang, and when did it occur?What does the cosmic microwave background tell us?What is dark energy, and what does it do?What are the possibilities for how the universe can end?What is the universe made of?What is implied by the shape of space?According to present understanding, what is the final fate of the universe? ................
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