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Margaret M. Hanson, PhD
Associate Professor of Physics
The Andromeda Galaxy
Our Galaxy’s Globular Clusters
• Reside in Halo
• Perhaps 1000s
originally formed
• 200 exist today
• Contain 100,000
solar like stars
• Variety of orbits
• Ages greater than
10 billion years
M10 -- Globular Cluster
How do Massive Clusters form?
We used to think massive cluster formation to
be a thing of the past (the Globular Clusters).
First, we began to see evidence of very
massive clusters (proto-globular clusters)
forming in other galaxies
Young Globular Cluster
NGC 1850 in the LMC
30 Doradus in the LMC
How do Massive Clusters form?
Massive clusters can form in relatively recent
times. What triggers them to form?
Massive clusters are seen to form when
enormous dynamical stress or collisions
occur within galaxies.
Galaxy
Collisions
The Milky Way is colliding, too!
Where would Young Cluster live?
 Stars form when gas and dust collapse under its own
self gravity.
 Gas and dust reside ONLY in the disk of our Galaxy.
 Young clusters must reside in the disk of our Galaxy.
Studies in our galaxy are not able to see very
far in the disk and sample a very small portion
of our galaxy.
Luminosity of Disk Star Clusters
Massive stars are
rare.
Massive clusters
are rare, too.
 h & chi Per
 Orion
Massive Clusters in our Galaxy
Recent events have forced us to re-evaluate
the notion of young massive clusters within
the Milky Way.
1) HST imaging reveals dense, young,
galactic clusters previously unresolved.
NGC 3603
Massive Clusters in our Galaxy
Recent events have forced us to re-evaluate
the notion of young massive clusters within
the Milky Way.
2) Infrared imaging discovers new, very
massive clusters near the galactic center
Massive Clusters in our Galaxy
Recent events have forced us to re-evaluate
the notion of young massive clusters within
the Milky Way.
3) Deeper imaging of known galactic
clusters has revealed a more substantial
number of stars.
Westerland 1 and its massive stars
Even nearby we are uncertain!
The Cyg OB2
Cluster
Distance = 1.7 kpc
60 massive stars
identified.
Knoedlseder (2000)
found the Infrared
Cluster to be
‘rounder’ and
containing more stars
than the Optical
Cluster.
Contours of stellar counts
Why the difference? Dust!
CO intensity
map
The interstellar matter (gas and dust) forces
us to observe at longer wavelengths.
Are there super massive star clusters
in our Galaxy?
Westerlund 1: D = 3-5 kpc, Av= 11, >100 O stars.
Consistent with being “one of the most massive
young clusters in the Local Group”.
Cygnus OB2: ~100 O stars, D = 1.7 kpc, Av = 5-10
Infrared Surveys: have found > 50 new galactic star
clusters in the inner galaxy.
We will need to move ALL observations
and analyses to the infrared.
Spectra of Hot Stars at 2 microns
45,000 K
O3-O4
C IV
N III
He II
42,000 K
O5-O6
40,000 K
O7-O8
36,000 K
O9-B0
29,000 K
B1-B2
He I
2.058
He I 2.11
Br Gamma
Observing time
granted
8.2m VLT with ISAAC.
2 nights March 2000
2 nights June 2000.
2 nights June 2001
8.2m Subaru with IRCS.
2 nights November 2001
2 nights June 2002.
Future Research Goals
• Develop quantitative methods for the analysis
of massive stars and massive clusters relying
on infrared light only.
• Continue to search for clusters deep within the
Milky Way galaxy using infrared imaging.
• Ultimate goal: uncover the massive clusters
and the hidden structure of the Milky Way.