Caty PilachowskiIndiana UniversityJune 2009

The International Year of Astronomy  2008 – 400 th anniversary of the invention of the telescope in Zeeland by Lipperhey  The International year of Astronomy – 400 th anniversary of the first astronomical use of the telescope by Galileo

The star clusters of the Milky Way reveal secrets about the origins of galaxies Telescopes change our view of the Universe

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

100,000 – 1,000,000 stars!! Sizes about 10 light years Closest ~10,000 LY away Ages ~ 13 billion years Wow!

Imagine the Night Sky… S tellar densities as high as 100 stars per cubic light year, many times greater than the density of stars near the Sun S ome nearby stars will be red giants! M any stars brighter than magnitude -5 S ome brighter than magnitude -10 T he disks of some stars could be resolved with binoculars! (but don’t look!!)

Discovering Globular Clusters M22 in Sagittarius (Abraham Ihle, 1665 – telescope!) Omega Centauri in Centaurus (Edmund Halley, 1677, resolved into stars with telescope M5 in Serpens (Gottfried Kirch, 1702) M13 in Hercules (Halley, 1714) M4, M71 in Scorpius & Sagitta (de Cheseaux, ~1745) M15, M2 in Pegasus & Aquarius (Maraldi, 1746) Herschel discovered 37 more, calling them “globular clusters”

 Now 158 known  The Milky Way Galaxy probably contains ~200  71 in the constellations Sagittarius, Ophiuchis, and Scorpius (summer-time!) Milky Way Globulars

Omega Centauri – The most massive NGC 6366 The least massive Size – 23 LY Mass – 2 million x solar Distance: 16,000 LY Size – 11 LY Mass – 10,000 x solar Distance – 13,000 LY

Messier 4 – The Closest Palomar 14 - The Most Distant Size – 8.5 LY Mass – 63,000 x solar Distance: 6500 LY Size – 80 LY Mass – 24,000 times solar Distance – 236,000 LY

Pal 14 – The Biggest NGC The Smallest Size – 2.7 LY Mass – 130,000 x solar Distance: 21,500 LY Size – 80 LY Mass – 24,000( ?) times solar Distance – 236,000 LY

Where do we find Globular Clusters? The Milky Way The Milky Way is surrounded by a halo of globular clusters Harlow Shapley used the Globular Clusters to find the center of the Milky way

Color- Magnitude 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)

Typical Cluster CMDs color or temperature Red Giants Brightness color or temperature Brightness Red Giants

 metal abundance  ages  distances CMDs tell us a lot!! Red Giants color or temperature Brightness

Milky Way: Two cluster groups, distinguished by orbit and color  Metal-poor population  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

M15 M3

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 Rey et al. AJ 2004 Why so different???

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 CTIO Hydra data, 180 stars, Johnson et al Caretta et al.

And Another Surprise! Spectroscopic observations from the Gemini 8-m telescope suggest that Omega Cen may host a black hole – about 10 4 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

 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 Sagittarius Tidal Stream

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)

Even Dwarf Galaxies have GCs  Milky Way satellites  Masses ~10 7 solar masses (stars plus dark matter)  Fornax has 6!  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

Two Cluster Populations  Blue population  Associated with galaxy halo  bluer  more metal poor*  really old  Red population  associated with disk/bulge  redder  less metal poor*  not quite as old Blue & Metal-poor Red & Less Metal-poor Brighter Fainter Luminosity Color or metal abundance* Different formation mechanisms?

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

The giant, segmented-mirror telescope To study the epoch of formation of the first globular clusters will require a new generation of even larger telescopes Bigger than a football field! JWST In space and on the ground

Beyond 30-meters ESO’s Overwhelmingly Large Telescope

Celebrating 400 years of astronomy with the telescope

Thanks…