Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom [email protected]/astronomy100OWL assignment (Due Today)• There is be an OWL assignment due on Thursday April 14 at 11:59 pm.• There are 15 questions and a perfect score will give you 2 homework points.Homework Assignment(Due May 3)• Make up a test question for next test• Multiple Choice• A-E possible answers• 1 point for handing it in• 1 point for me using it on test• The question needs to be on material that will be on the 3rd exam• 15 people got extra HW credit for me using their question (or inspiring a question)Astronomy Help Desk• There is an Astronomy Help Desk in Hasbrouck 205. • It is open Monday through Thursday from 7-9 pm.Last Class• We live in Milky Way Galaxy• Milky Way Galaxy is Spiral Galaxy• Flat rotation curve due to Dark MatterIf you are interested in astronomy articles• Go to www.space.com• Show simulationsGlobular Cluster• Cluster of a million or more stars in a area of 60-150 light years• Tend to be found in Halos of Galaxies• Tend to have very old starsM80Galaxies• Usually labeled by an NGC and then a number• NGC is New General CatalogSpiral galaxies• Spheroidal Component – Bulge and Halo• Disk that slices through the Halo and Bulge•Spiral armsNGC 6744NGC 4414NGC 4594 – Sombrero GalaxyNGC 1300Barred SpiralBarred Spiral• Have a straight bar of stars with spiral arms curling away from the bars• Some astronomers think that the Milky Way Galaxy is a barred spiral since our bulge appears to be elongatedLenticular Galaxy• Galaxy with disk but no spiral arms• They look lens-shaped when viewed edge-onNGC 2787Elliptical Galaxies• Do not have significant disk component• Looks like bulge and halo of spiral• Very little star formationM 87Irregular Galaxies• Do not look like Spiral or Elliptical GalaxiesNGC 1313Irregular Galaxies• Distant galaxies are more likely to be irregular than closer ones• Irregular galaxies more common when the universe was youngerDistances• Distances are hard to measure in space• Apparent brightness = Luminosity4π x (distance)2What you can measure• You can measure apparent brightness• If you know the object’s luminosity• You can calculate the distanceStandard Candles• A standard candle is a light source of known luminosity• If you can measure its apparent bright and know its luminosity• You can determine its distanceFor example• If we see a star like the Sun• We measure its apparent brightness• We assume its luminosity is the same as the Sun• We then can calculate its distanceHowever• Sun-like stars are relatively dim• So we can’t use this method for distances greater than 1,000 light yearsMain Sequence Fitting• We identify a star cluster that is close enough to determine its distance by parallax• We plots its H-R diagram• Since we know the distances to the cluster stars• We can determine their luminositiesThen• We can look at stars in other clusters that are very far away• Measure apparent brightnesses• We assume that stars of the same color have the same luminosity• Use that to calculate distancesNearby star clusterMain Sequence Fitting• Main sequence fitting only works for stars in our galaxyFor other Galaxies• We use Cepheid VariablesVariable Star• Variable Stars change in brightnessCepheid Variables• Cepheid Variableschange in brightnessregularlyInterestingly• For Cepheid Variables• The period of the brightness changes is a function of luminositySo• So if you know the period of the brightness changes• You know the luminosityEdwin Hubble (1889-1953)• Hubble used Cepheid Variables to determine the distance to the Andromeda Galaxy• Demonstrated it was a separate galaxyRemember• At the dawn of the 20th century, most astronomers thought that the Milky Way Galaxy was the universe, and it measured only a few thousand light-years across.Hubble• Kept on measuring distances to galaxies• Since you can’t see Cepheid Variables in far-away galaxies, he assumed the “brightest stars” in galaxies had the same luminosity• Made a mistake since the “brightest stars” were actually star clustersRemember• As something moves away from us• The wavelength of light from the source increasesFound out• The more distant a galaxy,• The greater its redshift• The faster it is moving away from usCame up with Hubble’s Law• Velocity = Hubble’s Constant x distance•v = Hox d• Hubble’s Constant is the slope of the lineSo•d = v/Ho• So if you can measure the velocity that a galaxy is moving away from you• You can calculate its distanceAnd• You can calculate the velocity that something is moving away from you from its redshiftDifficulties• Galaxies do not obey Hubble’s Law perfectly because they can velocities due to gravitational interactions• Distances are only as accurate as well as we know Hubble’s ConstantConstant•Ho = 71 km/s/megaparsec• 1 megaparsec = one million parsecsImportance of Hubble’s Constant• Remember: v = d/t•d= vt•d = v/Ho• so t = 1/Ho• so if you know Hubble’s constant, you can determine the age of the