ASTRO 105 1st Edition Exam #2 Study GuideTopics (extensive look; not exhaustive, but covers the most important material)Chandrasekhar Limit- Also known as white dwarf limit- Maximum possible mass for a white dwarf is about 1.4 Msun - Electron speeds are higher in more massive white dwarfs Fundamental limit onmax. mass Theoretical calculations show that the electron speeds would reachthe speed of light in a white tour with a mass about 1.4 times the mass of the sun Because neither electrons nor anything else can travel faster than the speed of light, no white dwarf can have a mass greater than 1.4Msunknown as the Chandrasekhar limit- 1940-as long as mass of white dwarf less than 1.4 Msun, electron degeneracy pressure could balance gravity - Our sun will end up as a white dwarfDark matter- Matter that we infer to exist from its gravitational effects but from which we havenot detected any light- Apparently dominates the total mass of the universe- “Cant see it”- Gives off no detectable radiation or signal, yet we believe it is there due to the effects we see/observe- About 90% of the galaxies mass must be dark matter Could be part neutrinos Other possible candidates include smaller black holes and brown dwarfs Would mostly have to be made up of exotic particlesDwarf stars Brown Dwarf- Very small pre main sequence stars- Mass = 10% Msun- Here, gravity is not sufficient enough to start fusion (no fusion occurring)- Jovian’s are brown dwarfs (Jupiter and Saturn)Red Dwarfs- Low mass (mass about 10% Msun)- Main sequence stars- Slowly fuse hydrogen- Surface temps about 3000 K- Can live up to 500 Billion years- Most common in universe- About 1/3 size of our sun White Dwarfs - As outer layers are busted away, the high temp carbon-oxygen core is exposed this forms a white dwarf- Are stable: gravity balanced by electron degeneracy pressure - Stars that have burned up all hydrogen previously used as fuel - Very dense- 1 teaspoon of white dwarf = about 5 tons - This is the last observable stage of evolution for low and medium mass stars dim, left over star corpses- max. possible mass = 1.4 Msun (Chandrasekhar Limit)Edwin Hubble-1923 photographed M31 (another galaxy called Andromeda)- While examining photo plates, noticed Cepheid’s existing w/in M31 Determined absolute magnitude by observing the periods from bright to dim back to bright, and also the apparent magnitude Cepheid’s used to estimate distances (pulsations)- Determined M31 was 2.2 million Light years away; another galaxy -Noticedthat in spiral galaxies, the larger the nuclear bulge, the tighter the spiral arms.Galaxies- In our visible universe, there are about 1011galaxies (about 100 billion)- Before 1923, it was thought that there was just one galaxy, ours.- Our solar system is in the Milky Way Galaxy- Galactic center: nuclear bulge in the center of a galaxy A planet in the bulge will never experience night.- Masses of an average galaxy range from 109 Msun to 1013 Msun - Types of Galaxies Spiral (Flocculent, Grand Design, Barred) Elliptical Lenticular IrregularGalaxy shapes -Spiral Galaxies- Have nuclear bulge ( larger the bulge, tighter the arms)- Have spiral arms- Rich gas discs- Average spiral galaxies have about 1011 stars (100 billion) and are about 100,000 light years in diameter- Types of Spiralo Flocculent: spiral not well defined Self propagating star formation New, young stars form near center differential rotation drags inner younger regions ahead of the outer older regions stars die and dim before spiral arms fully develop Arms fuzzy and cloudy o Grand Design Spiral: spirals well defined Formation explained by spiral density waves Medium undergoing rotationperturbation in the centerwave which propagates radially outward will produce a spiral density wave o Barred Spiral: 2 prominent spiral arms Current data says Milky Way is Barred Spiral When amount of dark matter is rather small, we see formation of barred spirals Straight bar of stars cut across center Spiral arms cut away from ends of bar -Elliptical Galaxies- Have no spiral arms- Range from being circular up to highly elliptical - Have almost no inter-stellar medium (no ingredients to form new stars) “Stars are there, we just don’t see them form”- Vary greatly in size Some are 20x size of Milky Way -Lenticular Galaxies- Similar to spiral but have no arms- Has nuclear bulge -Irregulars- Have no structures- Cannot be categorized into other shapes/typesGiant and Supergiant stars- Stars in the upper right of an HR diagram - More luminous than main sequence stars of the same surface temp.- Very large in radius - Giants Main sequence stars with masses greater than 40% the mass of our sun become giants when core hydrogen has been converted to helium - Super Giants Arise from main sequence stars (w/ 8Msun < Mass < 25 Msun) form neutron stars after supernova Globular clusters- Spherically shaped clusters of up to a million or more stars- Found primarily in the halos of galaxies Contain only very old starsInterstellar medium - Gas and dust that fills space between stars in a galaxy - Gas composed of mostly hydrogen and helium Basically ingredients to form new stars- Stars form due to gravitational attraction that binds particles that make up the inter-stellar medium- About 10% of galaxy’s mass is inter-stellar medium - Composition 74% hydrogen, 25% helium, 1% others - Very cold and emits almost no visible light Interstellar reddening- Change in color of star light as it passes through dusty gas - Light appears redder as dust grains absorb and scatter blue light more effectively than red light - Blue/purples have large scattering angles (get bent the most, easiest to scatter) Purple scatters the most, but sun produces very little; blue is dominant This is why our sky is blue and our sunsets are red - The day time sky (on Earth) is blue because blue light is a color that easily scattersMilky Way - 100,000 LY’s across- About 1011 stars - Disc about 1,000 Ly’s thick - Rotating disc of matter Spiral structure barred spiral (2 prominent spiral arms)- Center is galactic nucleus/bulge- About 27,000 LY’s away from galactic center- Able to locate center via globular clusters Clusters orbit
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