Caty Pilachowski Indiana University June 2009 The International Year of Astronomy 2008 – 400th anniversary of the invention of the telescope in Zeeland by.
Download ReportTranscript Caty Pilachowski Indiana University June 2009 The International Year of Astronomy 2008 – 400th anniversary of the invention of the telescope in Zeeland by.
Caty Pilachowski Indiana University June 2009 The International Year of Astronomy 2008 – 400th anniversary of the invention of the telescope in Zeeland by Lipperhey 2009 - The International year of Astronomy – 400th anniversary of the first astronomical use of the telescope by Galileo Telescopes change our view of the Universe The star clusters of the Milky Way reveal secrets about the origins of galaxies Globulars Clusters! What are they? What do we know about them? What can we learn from them? Where did they come from? What is a Globular Cluster??? Lots and lots of stars! Formed together over a relatively short time Held together by the mutual gravity of the stars Gravity pulls the stars into a spherical ball Sizes about 10 light years Ages ~ 13 billion years Closest ~10,000 LY away 100,000 – 1,000,000 stars!! Imagine the Night Sky… Stellar densities as high as 100 stars per cubic light year, many times greater than the density of stars near the Sun Some nearby stars will be red giants! Many stars brighter than magnitude -5 Some brighter than magnitude -10 The disks of some stars could be resolved with binoculars! (but don’t look!!) Discovering Globular Clusters M22 in Sagittarius (Abraham Ihle, 1665 – telescope!) M5 in Serpens (Gottfried Kirch, 1702) M4, M71 in Scorpius & Sagitta (de Cheseaux, ~1745) M15, M2 in Pegasus & Aquarius (Maraldi, 1746) Omega Centauri in Centaurus (Edmund Halley, 1677, resolved into stars with telescope M13 in Hercules (Halley, 1714) Herschel discovered 37 more, calling them “globular clusters” Milky Way Globulars Now 158 known The Milky Way Galaxy probably contains ~200 71 in the constellations Sagittarius, Ophiuchis, and Scorpius (summer-time!) Omega Centauri – The most massive Size – 23 LY Mass – 2 million x solar Distance: 16,000 LY NGC 6366 The least massive Size – 11 LY Mass – 10,000 x solar Distance – 13,000 LY Messier 4 – The Closest Size – 8.5 LY Mass – 63,000 x solar Distance: 6500 LY Palomar 14 - The Most Distant Size – 80 LY Mass – 24,000 times solar Distance – 236,000 LY Pal 14 – The Biggest Size – 80 LY Mass – 24,000( ?) times solar Distance – 236,000 LY NGC 6528 - The Smallest Size – 2.7 LY Mass – 130,000 x solar Distance: 21,500 LY Where do we find Globular Clusters? The Milky Way is surrounded by a halo of globular clusters The Milky Way Harlow Shapley used the Globular Clusters to find the center of the Milky way ColorMagnitude Diagrams Measure the brightness and color of each star Plot brightness on the vertical axis (bright at the top) and color on the horizontal axis (bluer stars on the left, redder stars on the right) color or temperature Brightness Brightness Red Giants Typical Cluster CMDs Red Giants color or temperature Brightness Red Giants metal CMDs tell us a lot!! abundance ages distances color or temperature metals 1/40 of solar Elliptical orbits fill a spheroidal volume slightly bluer color Less metal-poor population metals ¼ of solar less elliptical orbits hug the disk slightly redder color Milky Way: Two cluster groups, distinguished by orbit and color 30 Number of Clusters Metal-poor population Harris 1999 20 10 0 -2.6 -2.2 -1.8 -1.4 -1 -0.6 -0.2 0.2 [Fe/H] M3 M15 Determining the Composition of Globular Clusters Up to 100 stars at a time! Spectroscopy of Cluster Giants Precise measurements of the strengths of spectral lines allow us to determine the abundances of elements in cluster stars Omega Centauri – the most unusual globular cluster CMD Why so different??? Rey et al. AJ 2004 What Do We Learn from Spectra? Stars in a cluster all have the same iron abundance Iron and most other metals form in supernova explosions Clusters formed from thoroughly mixed material, with no new supernovae Other metals (sodium, aluminum, magnesium) and oxygen vary from star to star within a cluster Processes while the cluster formed, and later in the stars themselves, changed the abundance of these metals Omega Cen contains stars with a range of age and metal abundance Formation of stars was episodic, extended over ~4 Gyr Must have formed away from MW disk Rey et al. AJ 2004 Omega Cen Metallicity Distribution Number of Stars 80 Number of Stars 40 Messier 12 60 40 20 0 30 -2.1 -1.7 Caretta et al. -1.3 -0.9 [Fe/H] 20 10 0 -2.2 -1.8 -1.4 [Fe/H] CTIO Hydra data, 180 stars, Johnson et al. 2008 -1 -0.6 -0.5 And Another Surprise! Spectroscopic observations from the Gemini 8-m telescope suggest that Omega Cen may host a black hole – about 104 solar masses! Artist’s conception – Lynette Cook A Globular Cluster – NOT! Modern evidence suggests that Omega Cen is not a globular cluster, but the former nucleus of a small galaxy Similar tidal captures are occurring today in the Milky Way A handful of “globular clusters” share similar properties with Omega Cen (e.g. M54 in Sagittarius) A new class of objects! The Milky Way Is accreting clusters today!! Nearby dwarf galaxy discovered in 1994 in the direction of Sagittarius Distance about 88,000 light years Merging with the Milky Way Sagittarius GCs now part of the Milky Way Sagittarius Tidal Stream Orbits the Milky Way Orbital period about a billion years “Tidal stream” of stars from Sagittarius circles the Milky Way Sagittarius may contain significant dark matter Other Galaxies Contain Globular Clusters, Too! Virgo’s M87 (52M LY) contains thousands of GCs NGC 4660 – Galaxies that form lots of stars fast have more globular clusters NGC 3311 – The Most! NGC 3311 is a giant elliptical galaxy in the core of the Hydra Cluster of Galaxies More than 16,000 globular clusters! (176M LY) Milky Way satellites Masses ~107 solar masses (stars plus dark matter) Fornax has 6! Even Dwarf Galaxies have GCs 500,000 light years Other dwarfs also contain globular clusters Bigger galaxies form MANY MORE GCs CMD for a Galaxy’s Clusters Just as for stars in a single cluster, we can measure the luminosity and color for each cluster in a galaxy Brighter Two Cluster Populations Associated with galaxy halo bluer more metal poor* really old Luminosity Blue population associated with disk/bulge redder less metal poor* not quite as old Fainter Red population Different formation mechanisms? Color or metal abundance* Blue & Metal-poor Red & Less Metal-poor Where do Globular Clusters come from? Clues to the formation of GCs GCs are OLD GCs are metal-poor All galaxies have GCs GCs come in two types – red and blue GCs formed early in the history of the Universe and are connected to the formation of galaxies Globular cluster systems tell us about galaxy formation and evolution Where do Blue Globular Clusters come from? The old blue clusters formed with their host galaxies Old blue clusters trace galaxy formation and dark matter Where do Red Globular Clusters come from? The (slightly) younger red clusters formed when galaxies merged Red clusters trace merger history and the build-up of disks and bulges in galaxies To study the epoch of formation of the first globular clusters will require a new generation of even larger telescopes The giant, segmented-mirror telescope In space and on the ground JWST Beyond 30-meters ESO’s Overwhelmingly Large Telescope Celebrating 400 years of astronomy with the telescope Thanks…