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…