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They all have the same luminosity. They all have the same radius. How big is the Universe?: How big is the Universe? Spiral nebulae were identified not long after development of the telescope around 1600 In the 1600’s, it was suggested that spiral nebula are separate galaxies so far away that the stars blur together, but most people thought they were clouds of gas The question wasn’t resolved until 1923. Are there different types of objects here?: Are there different types of objects here? Great debate: Great debate Two astronomers held a great debate in 1920 Harlow Shapley argued the Milky Way was the whole Universe Heber Curtis argued the Milky Way was just one of many galaxies – 'island universes' Held in the Smithsonian's Museum of Natural History – the auditorium still looks the same Distance to the Andromeda spiral nebula: Distance to the Andromeda spiral nebula In 1923, Edwin Hubble found Cepheid variables in the Andromeda nebula and showed that the 'nebula' was at a great distance, much larger than the size of the Milky Way. A Cepheid is found with an oscillation period of 30 days. It is 700,000 times dimmer than another Cepheid with a period of 30 days at a known distance of 1000 pc. How far away is the dimmer Cepheid?: A Cepheid is found with an oscillation period of 30 days. It is 700,000 times dimmer than another Cepheid with a period of 30 days at a known distance of 1000 pc. How far away is the dimmer Cepheid? Flux and Luminosity: Flux and Luminosity Flux decreases as we get farther from the star – like 1/distance2 Mathematically, if we have two stars A and B Standard Candles: Standard Candles Standard Candles: Standard Candles Measure the distance to star A to be 1000 pc. Measure the flux of star A. Measure the flux of star B with same spectral type and luminosity class to be lower by a factor of 700000 Find the distance to star B How big is the Universe?: How big is the Universe? Greeks (up about 100 B.C.) Earth at Center Universe extends to ‘sphere of Saturn’, largest measured distance is from Earth to Sun at several million miles Renaissance (1500-1650) Sun at Center Universe extends to `distant stars’ with inferred distance of about 100 billion miles, largest measured distance is from Sun to Saturn at about 1 billion miles How big is the Universe?: How big is the Universe? Parallax to stars First parallax measured in 1838 to star 61 Cygni of 0.3 arcseconds for a distance of 11 ly = 7×1013 miles. Distance to center of Milky Way from star counts 5000-10,000 ly (1785-1810) from globular clusters 50,000 ly (1915) Distance to Andromeda nebula from Cepheids 2,000,000 ly (1923) (really 2,500,000 ly) What evidence do we have that there is hidden mass in the galaxy?: What evidence do we have that there is hidden mass in the galaxy? cool clouds of hydrogen RR Lyrae variable stars in globular clusters flat rotation curve at large radii dusty regions in the plane Galaxies: Galaxies Types of galaxies Elliptical Spiral Irregular M100: M100 NGC 1365: NGC 1365 M87: M87 NGC 3377: NGC 3377 NGC 4449: NGC 4449 Classifying Galaxies: Classifying Galaxies Elliptical galaxies: Elliptical galaxies little interstellar gas and dust very little star formation mainly old stars (billions of years old) few or no young stars (millions of years old) Elliptical galaxies: Elliptical galaxies Often occur in clusters: Often occur in clusters Spiral galaxies: Spiral galaxies Bulge Old stars Disk Gas, dust, Young and old stars Spirals vary in prominence of bulge, tightness of arms, presence of bar: Spirals vary in prominence of bulge, tightness of arms, presence of bar Irregular galaxies have asymmetric shapes and usually lots of young stars: Irregular galaxies have asymmetric shapes and usually lots of young stars They are often found near other galaxies In which type of galaxy are star’s orbits distributed in random directions?: In which type of galaxy are star’s orbits distributed in random directions? elliptical galaxies spiral galaxies barred spiral galaxies blue galaxies Our Galaxy is a member of a small cluster called the Local Group: Our Galaxy is a member of a small cluster called the Local Group MW eating neighbors: MW eating neighbors Review Questions: Review Questions What was the definitive evidence showing that 'spiral nebulae' are actually entire galaxies outside of the the Milky Way? What are the types of galaxies? How do the rotation patterns of stars differ in elliptical versus spiral galaxies? What is the Local Group? Cosmic Distances: Cosmic Distances How to measure distances Primary distance indicators Secondary and tertiary distance indicators Recession of galaxies Expansion of the Universe Stellar Parallax: As the Earth moves from one side of the Sun to the other, a nearby star will seem to change its position relative to the distant background stars. d = 1 / p d = distance to nearby star in parsecs p = parallax angle of that star in arcseconds Stellar Parallax Stellar Parallax: Stellar Parallax Most accurate parallax measurements are from the European Space Agency’s Hipparcos mission. Hipparcos could measure parallax as small as 0.001 arcseconds or distances as large as 1000 pc. How to find distance to objects farther than 1000 pc? Standard Candles: Standard Candles Distances to galaxies: Distances to galaxies Standard candles, such as Cepheid variables, the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances. The Distance Ladder: Each stage in the ladder overlaps the previous and next Cepheid distances are critical Tully-Fisher, fundamental plane apply to whole galaxies Supernova are now the best estimators at large distances The Distance Ladder Doppler effect for light: Doppler effect for light Light from distant galaxies is redshifted: Light from distant galaxies is redshifted Distances and velocities of galaxies: Distances and velocities of galaxies If you measure the distances to a large set of galaxies and also measure the speed of the galaxies using the redshift, what do you find? Hubble expansion v = H0d: Hubble expansion v = H0d Expansion of the Universe: Expansion of the Universe Motion at constant velocity: Motion at constant velocity distance = velocity time velocity = 0.5 cm/s time = distance / velocity = 3 cm/(0.5 cm/s) = 6 s Receding galaxy: Receding galaxy When were galaxies in the same place? time = distance / velocity = 7 Mpc/(0.508 Mpc/Gyr) = 13.8 Gyr ago Velocity = = 500 km/s = 0.508 Mpc/Gyr Hubble expansion v = H0d: Hubble expansion v = H0d Time = distance/velocity = d/H0d = 1/H0 = 1/(71 km/s/Mpc) = 13.8 Gyr An observer at a distance of 3 billion light years from us looking in our general direction would see: An observer at a distance of 3 billion light years from us looking in our general direction would see most of the galaxies approaching her. the same Hubble’s law that we see. about equal numbers of red and blue shifted galaxies. everything rushing away from a point near the Milky Way galaxy. Expansion of the Universe: Expansion of the Universe Blow up the balloon to about a 3 inch diameter. Twist the neck and hold it closed so that no air escapes, but do NOT make a knot because you will need to blow it up some more. Make SIX dots on its surface to represent galaxies and label them A-F. Measure and record the distances from cluster A to each of the other 5 clusters. Measure and record the distances from cluster D to each of the other 5 clusters. Blow up the balloon up more, to a diameter of about 6 inches. Measure the distances between the same clusters again and record them. Expansion of the Universe: Expansion of the Universe Are all the other clusters moving away from cluster A? Are all the other clusters moving away from cluster D? Is there a cluster that could be considered to be at the center of the universe as represented by the surface of the balloon? Formation of Galaxies: Formation of Galaxies Spiral versus elliptical Young Universe Collisions and Interactions Starbursts Elliptical galaxies Formation of a Spiral Galaxy: Formation of a Spiral Galaxy Formation of an Elliptical Galaxy: Formation of an Elliptical Galaxy Stellar Birthrate in Galaxies: Stellar Birthrate in Galaxies Formation of Galaxies: Formation of Galaxies This picture of galaxy formation is incomplete Mergers, collisions, and interactions between galaxies are very important in their formation, particularly in the early stages of the Universe (why?) Expansion of the Universe: Expansion of the Universe The Universe is expanding This means that the Universe used to be smaller In the early stages of the Universe there were more galaxies they were closer together therefore, they interacted more Young Universe: Young Universe Young Universe: Young Universe Young Universe: Young Universe Young Universe: Young Universe Colliding galaxies: Colliding galaxies The Mice: The Mice Cartwheel galaxy: Cartwheel galaxy Seyfert’s Sextet: Seyfert’s Sextet Interacting galaxies: Interacting galaxies Interacting galaxies: Interacting galaxies Starburst galaxy – M82: Starburst galaxy – M82 M82 in X-rays: M82 in X-rays Colliding galaxies: Colliding galaxies Movie Galaxy interactions: Galaxy interactions Interactions can rip stars out of galaxies, producing tidal tails Interactions can disturb gas in and between galaxies, producing starbursts Collisions can randomize stellar orbits leading to the formation of elliptical galaxies Formation of an Elliptical Galaxy: Formation of an Elliptical Galaxy Movie Galaxy growth via interactions: Galaxy growth via interactions Galaxies initially form from mergers of several gas clouds Galaxies then are changed by interactions Galaxies grow gradually by galactic cannibalism Interactions disturb gas leading to starbursts Collisions can randomize stellar orbits leading to the formation of elliptical galaxies Review Questions: Review Questions How are elliptical versus spiral versus irregular galaxies formed? How do the star formation histories of elliptical versus spiral galaxies differ? Why do galaxy interactions tend to cause star formation? Was the population of galaxies different in the past? You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.