Transcript Document

Physics 360/Geol 360
Star Clusters and The Milky Way
A typical globular cluster
Spiral Galaxy NGC5236 much like
The Milky Way Appears as a Band
HNSKY view of the sky tonight. Can you identify the Milky Way?
Early Estimates of the Milky Way
Size estimated in early 1900’s
Kapteyn (Dutch)
About 20 kiloparsecs (kpc)
Shapley (US)
About 100 kpc
Both were partly right and partly wrong—they misinterpreted the
dimming effects of interstellar dust. Shapley placed the Sun’s
position about right; Kapteyn incorrectly placed the Sun in the
Shapley measured the galaxy size with RR Lyrae stars.
Sketch of the Milky Way
Population I
Population I and II
Mass ~ 1011 mass of Sun
Population II
Population I
About 1012 stars
Early Estimates of Milky Way (MW) Size
• Review of Distances
– 1 parsec (pc) = 3.26 light years (ly) = 3 x 1013 km
– 1 kiloparsec (kpc) = 1000 parsecs = 3260 ly
• Early Distance Measurements of open and globular clusters were
inaccurate due to dark matter and Hydrogen gas affecting the
Luminosity Law. This gas and dust makes up to 15% of the MW mass
• The Milky Way consists of
– A disk of about 30 kpc (100,000 ly)
– A surrounding halo—dark matter
– A flattened bulge of stars surrounding the core
• Our Solar System lies 8.5 kpc from the center (28,000 ly)
with orbits tilted 60o w/r to the galaxy disk (the Milky Way
band is tilted w/r to the ecliptic)
The Milky Way – Our Galaxy
Many ancients observed a milky band of stars in the night sky running
through the constellations of Cassiopeia, Perseus, Taurus, Monoceros, Vela,
Crux, Norma, Sagittarius, Scutum, Aguila, Cygnus, and Lacerta.
Although spaces between the stars consist of a vacuum as good as can be
gotten on earth (about 10-11 Torr or mm of mercury) there is still plenty
of gas and dust called interstellar medium. About 99% of this medium
is gas and the rest is dust.As to the composition of this dust “The dust
is made of thin, highly flattened flakes or needles of graphite (carbon)
and silicates (rock-like minerals) coated with water ice. Each dust flake
is roughly the size of the wavelength of blue light or smaller. The dust
is probably formed in the cool outer layers of red giant stars and
dispersed in the red giant winds and planetary nebulae. “ from from
Nick Strobel's Astronomy Notes at
The Effect of Interstellar Dust
Light passing through interstellar dust suffers extinction
(dimming of all wavelength of light) and scattering. The
amount of the latter depends on the color of the light.
This page was
copied from Nick
Strobel's Astronomy
Notes. Go to his site
www.astronomynote for the
updated and
corrected version.
Extinction and Scattering continued and early Milky
Way estimates
In 1930 R.J. Trumpler discovered dust as he plotted the
angular diameter of star clusters versus the distance to the
clusters. He derived the latter from the inverse square law of
brightness. After analyzing the data he concluded that more
distant clusters simply have more “stuff” between us and the
clusters hence they appear fainter than they really are.
Trumpler showed that there is dust material between the
stars and that the extinction of starlight is caused by the
scattering of light out of the line of sight. This caused early
observers to (although they recognized the disk shape of the
milky way by observing star clusters) overestimate the
diameter of the milky way. From the law of scattering bluer
wavelengths are scattered more than redder wavelengths.
“The presence of interstellar gas can be seen when you look at the spectral
lines of a binary star system. Among the broad lines that shift as the two
stars orbit each other, you see narrow lines that do not move. The narrow
lines are from much colder gas in the interstellar medium between us and
the binary system. “ from Nick Strobel’s Astronomy Notes.
As was mentioned previously 99% of the material between the stars is gas
and about 90% of the gas is atomic or molecular form of gas with the
remaining 10% Helium plus trace elements.
Remember the dust has more of an effect on the visible wavelengths.
Ionized hydrogen emits light in the visible band but neutral hydrogen and
molecular hydrogen emit in the radio band.
The Anglo-Australian Observatory at is worth looking at!
Nick’s Astronomy Notes offers the following explanation for H II
“H II regions are regions of hot (several thousand K), thin hydrogen
emission nebulae that glow from the fluorescence of hydrogen atoms. The
roman numeral ``II'' of H II means that hydrogen is missing one electron.
A He III nebula is made of helium gas with two missing electrons. A H I
nebula is made of neutral atomic hydrogen. Ultraviolet light from hot O
and B stars ionizes the surrounding hydrogen gas. When the electrons
recombine with the protons, they emit light mostly at visible wavelengths,
and primarily at a wavelength of 656.3 nanometers (giving the hydrogen
emission nebulae their characteristic red color). In this conversion of the
ultraviolet energy, each ultraviolet photon produces a visible photon. The
temperature of the stars causing the nebula to fluoresce can be estimated
from this even though the O and B stars are hidden inside the nebula.
Fluorescent light bulbs operate on the same basic principle except they use
mercury vapor to produce ultraviolet light. The ultraviolet light is then
converted to visible light by the phosphor layer on the inside of the glass
The Orion Nebula -- It is the fuzzy patch you can see in the
sword part of the Orion constellation
Explanation of previous slide from the Anglo-Australian
Stellar Population Types in our Milky Way
• All stars are mainly Hydrogen and Helium
• Population I Stars
– 2% elements other than Hydrogen and Helium
– All spectral types
• Population II Stars
– < 0.1% elements other than H and He
– Only cooler spectral types (G, K, M)
Age of our Milky Way
• Our galaxy’s most ancient stars ~ 15 Billion
Years Old
• (however another guess is 11 – 12 Billion)
• Blue Stars (Pop I) in disk and spiral arms
• Red Stars (Pop II) in bulge and halo
• This is true for other galaxies as well
• Pop II stars may have elongated tilted orbits
whereas Pop I stars orbits are in the disk
The Forces of Gravity Shape our Milky Way
• The center of our MW is probably a dense swarm of gas and stars and
a massive black hole which probably grows more massive by drawing
in interstellar gas. Black holes such as these can grow even as large as
106 in a billion years.
• The flattened shape of a spiral galaxy such as ours implies that it
rotates. Our Sun and nearby stars move around the MW with speeds of
220 km/sec and the disk and the MW disk (near us) makes one
complete rotation in approx. 240 million years.
• What force keeps the galaxy together? – The collective gravitational
attraction of the stars and gas within the MW draws them toward the
• The density of stars in the MW galaxy varies. Near the sun it is about
0.003 stars per cubic ly but near the core about 10 million stars per
cubic ly or ly3.
• Near the edge stars are even spread more thinly than around our Sun.
• On the far edge exists a Halo of Dark Matter.
How we determine the mass of the Milky Way
From the “modified” form of Kepler’s Third Law
Where m = mass of sun, M = mass of galaxy, a = radius of
sun’s orbit around galactic mass, P = years to complete one
Define a = 1.8 x 109 AU, p = 240 million years
M = 1011 M
From Thomas T. Arny,
Explorations an introduction
to astronomy
Finding distance to the center
of the milky way using
globular clusters
“Finding the orbital speed of the sun around the
Milky way. Astronomers take spectra to obtain
the Doppler shift of galaxies in the local group,
the small cluster of galaxies to which the Milky
Way belongs. The Doppler shift of the galaxies
is created by their own motion and that of the
Sun….It turns out that the galaxies move slowly
compared with the Sun’s rotation around the
Milky way so almost all the Doppler shift is
attributable to the Sun’s motion.” Reference
Arny, page 469
Star Clusters
• Open Clusters
– Few hundred stars
– All spectral types
– Population I Stars
7-20 ly across
• Globular Clusters
– Few million stars
– G stars and cooler
– Population II stars
~100 ly across
Star Clusters
Globular Cluster HR Diagrams
• Assume globular cluster
stars formed together
• High-mass stars die first
• Age of cluster comes from
mass of largest star left
• What is the age of this
Distant galaxies avoid the Milky Way disk –why?
Globular clusters show a distant galactic center
IR Picture of the Milky Way
IR travels
through dust
This ‘all-sky’
clearly shows
that we are
at an edge of
the galaxy
Improving View of the Milky
Improving View of the Milky
• Stars appear to occupy a disk centered on the Earth
• Globular clusters occupy a sphere centered 20,000
ly away
• We assume globular clusters are centered on the
• Dust in the disk prevents us from seeing the whole
• IR and radio observations confirm this view
Mapping Spiral Arms
Spiral arms are
mapped by
region markers
Radio waves
travel through
dust and allow
maps over the
whole disk
Spiral Arms
Areas of Star Formation
• Higher density -> gas collapse -> star
• Long-life stars
move on
• High mass stars
die in arm
• Spiral Arms are
– Gaseous nebula
– Hot stars
– Supernova
Inner Galaxy / Core
• The inner galaxy is a bar of stars and gas
• The inner core contains a black hole
Dark Matter
• Star’s near the edge of
the galaxy should orbit
• They orbit quickly
• Since they aren’t flying
into deep space, there
must be extra gravity
• We don’t SEE a source
Dark Matter!
Dark Matter
• Mass about 10 times the “luminous mass”
(stars and nebula)
• What might is be?
• The nature of dark matter is one of the great
problems in astronomy