Which galaxy is not a companion of the Milky Way?A)Large Magellanic CloudB)Small Magellanic CloudC)TriangulumQuiz Question 1Which galaxy is not a companion of the Milky Way?A)Large Magellanic CloudB)Small Magellanic CloudC)TriangulumQuiz Question 1vrvrvrvrA.B.C.D.Which rotation curve best describes the disk of the Milky Way?Quiz Question 2vrvrvrvrA.B.C.D.Which rotation curve best describes the disk of the Milky Way?Quiz Question 2•How are galaxies classified?•How are the distances to galaxies measured?•Can we estimate the age of the Universe?!!Questions About GalaxiesWhat is the significance of this classification? It’s not clear.Hubble’s Classification SchemeElliptical SpiralBulge-to-disk ratio decreasesFig. 20-10Galaxy ClassificationNot all galaxies fit neatly into the classification scheme.Tidal TailPolar RingDwarf EllipticalThe SMC—Irregular Galaxy ClassificationDistances to Nearby GalaxiesFig. 20-12•Cepheids are Standard Candles•Cepheid variables are bright and observable out to several Mpc•Cepheids are periodic variable stars•Characteristic timescales are days to months•More luminous Cepheidshave longer periodsThe Cepheid P-L RelationHow do the masses of Cepheid variable stars compare to the Sun?A.Cepheids are less massive than theSunB.Cepheids are about as massive as the SunC.Cepheids are more massive than the SunConcept Question: CepheidsHow do the masses of Cepheid variable stars compare to the Sun?A.Cepheids are less massive than theSunB.Cepheids are about as massive as the SunC.Cepheids are more massive than the SunRemember: L ∝ M3Concept Question: CepheidsHubble’s Lawv (km s-1) = H0(km s-1Mpc-1) × d (Mpc)H0= 70±5 km s-1Mpc-1(The uncertainty in H0is due to statistical and random measurement error.)So, if we can measure the recessional velocity, we can solve for the distance to a galaxy: d = v/H0The Expansion of the UniverseRedshiftFig. 20-15Redshift is denoted by zz = (λobserved-λrest)/λrestAnd for nearby galaxies:v (km s-1) = c (km s-1) × zSo, with c = 3×108m s-1,the distance can be derived:d = cz/H0Rest frameObserved frameThe Expansion of the UniverseWhich is the observed quantity for a galaxy?d = cz/H0A)d, the distanceB)c, the speed of lightC)z, the redshiftD)H0, the Hubble constantConcept QuestionWhich is the observed quantity for a galaxy?d = cz/H0A)d, the distanceB)c, the speed of lightC)z, the redshiftD)H0, the Hubble constantConcept QuestionFig. 20-21Study these two in your bookThese techniques are not perfect, and they build on each other to some extent, but they are some of the tools (there are more) available to us to measure distances to galaxies.The Cosmic Distance Ladderv = H0× dNotice that the units of the Hubble constant are inverse time:H0= 70 km s-1Mpc-1.Notice the similarity of these two equations:d = v × td = v × H0-1⇒ t = H0-1= 14 billion years!(Note that this assumes a constant expansion rate…)A First Estimate of the Age of the
View Full Document
Unlocking...