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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
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 timeHeld 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…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 ClustersM22 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 LYMass – 2 million x solarDistance: 16,000 LY
Size – 11 LYMass – 10,000 x solarDistance – 13,000 LY
Messier 4 – The Closest
Palomar 14 - The Most Distant
Size – 8.5 LYMass – 63,000 x solarDistance: 6500 LY
Size – 80 LYMass – 24,000 times solarDistance – 236,000 LY
Pal 14 – The Biggest
NGC 6528 - The Smallest
Size – 2.7 LYMass – 130,000 x solarDistance: 21,500 LY
Size – 80 LYMass – 24,000( ?) times solarDistance – 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)
TypicalCluster CMDs
color or temperature
Red Giants
Bri
gh
tness
color or temperature
Bri
gh
tness Red Giants
metal abundance
agesdistances
CMDs tell us a lot!!
Red Giants
color or temperature
Bri
gh
tness
Milky Way: Two cluster groups, distinguished by orbit and color
Metal-poor populationmetals 1/40 of solarElliptical orbitsfill a spheroidal
volumeslightly bluer color
Less metal-poor populationmetals ¼ of solarless elliptical orbitshug the diskslightly redder
color
0
10
20
30
Num
ber o
f Clu
ster
s
0.2-0.2-0.6-1-1.4-1.8-2.2-2.6
[Fe/H]
Harris 1999
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 abundanceIron and most other metals form in
supernova explosionsClusters formed from thoroughly mixed
material, with no new supernovaeOther metals (sodium, aluminum,
magnesium) and oxygen vary from star to star within a clusterProcesses 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
abundanceFormation
of stars was episodic, extended over ~4 Gyr
Must have formed away from MW disk
Rey et al. AJ 2004
Omega Cen Metallicity Distribution
0
10
20
30
40
-2.2 -1.8 -1.4 -1 -0.6
[Fe/H]
Num
ber o
f Sta
rs
CTIO Hydra data, 180 stars, Johnson et al. 2008
Messier 12
0
20
40
60
80
-2.1 -1.7 -1.3 -0.9 -0.5
[Fe/H]
Nu
mb
er
of
Sta
rs
Caretta et al.
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
Orbits the Milky WayOrbital period about a billion years“Tidal stream” of stars from Sagittarius circles
the Milky WaySagittarius 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 ~107 solar masses (stars plus dark matter)
Fornax has 6!500,000 light
yearsOther 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
Bri
gh
ter
Fain
ter
Lu
min
osit
y
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-
mirrortelescope
To study the epoch
of formation of the first globular
clusters will require a new
generation of even larger telescopes
JWST
In space and on the ground
Beyond 30-meters
ESO’s Overwhelmingly Large Telescope
Celebrating 400 years of astronomy with the telescope
Thanks…