Caty Pilachowski Indiana University June 2009 The International Year of Astronomy  2008 – 400th anniversary of the invention of the telescope in Zeeland by.

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Transcript 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…