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Expansion of the Universe
Lab 10
Some Facts of Our Milky Way
Galaxy
• Distance from Sun to galactic center = 8
kpc ± 1 kpc
• Disk of our galaxy = 50 kpc diameter, 0.6
kpc thick, with a central bar-shaped bulge
• Central bulge is 2 kpc in diameter(has
both Pop I and Pop II stars)
• Sun orbits center of galaxy at 790,000
km/hr, takes 220 million years to complete
1 orbit
Facts about H
• Hydrogen most abundant element in the
universe but cannot emit visible light in the
depths of cold interstellar space
• However, DOES emit radio waves
• This H is neutral not ionized, so called H I
Detection of Spin-flips
• Protons and electrons have mass, charge
• Also have angular momentum (spin) which
creates tiny magnetic field
• So energy of H atom is different
depending on orientation of spins (if same
↓, if opposite directions ↑, but energy
difference = 10-6 x electron orbits)
• photon emitted in a spin-flip transition has
λ = 21 cm (long, radio)
Dark Matter!!!!
• Most of mass of galaxy (90%) does not emit
anything that we can detect, so it is called dark
matter
• 90 million solar masses of matter exists inside
the solar orbit
• But orbital speeds do NOT decrease with
distance from galactic center
• So a large amount of mass must exist OUTSIDE
solar orbit
• Therefore, total mass of our galaxy = 1012 M‫סּ‬
MACHOs and WIMPs
• Half of dark matter halo is composed of
MACHOs
• Massive Compact Halo Objects are ~ 0.5
M‫סּ‬
• Some part of dark matter are neutrinos
• Also, some other subatomic particles
called WIMPs (weakly interacting massive
particles) which have a mass of 1010,000> proton
Winding Dilemma
• Stars, dust, gas all orbit the galactic center
with ~ same speed
• But this creates a winding dilemma
• So density waves must sweep around
galaxy, which move more slowly around
the galaxy than the matter inside
• This crowding promotes stellar birth and
recycling of ISM
Just so you know……
• At center of our galaxy is a black hole
millions of times more massive than Sun
• Starlight warms dust grains to 10-90˚K,
which then emits radiation at λ = 30-300
μm (far infra red, Wien’s law)
• At near infra red λ, see cool stars (red
giants) deep within Milky Way
Types of Galaxies
• Galaxies can be
–
–
–
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spiral (Sa fat central bulge, Sc tiny central bulge)
barred spiral (SB)
elliptical (E7 flattest, E0 roundest, old red Pop II)
irregular (both old and young stars, lots of ISM, Irr I
have a hint of structure eg Large Magellanic Cloud,
SMC; Irr II have distorted shapes resulting mostly
from collisions with other galaxies)
• Giant ellipticals are rare, dwarf ellipiticals are
common, ellipticals have no overall rotation, do
not rotate
• Spirals have a lot of overall rotation
Distance ladder – ways to measure
distances
Masers – molecular clouds
• Technique independent of distance ladder
• Luminosity of star stimulates water
molecules in a maser to emit intensely at
microwave λ
• Maser – microwave amplification by
stimulated emission of radiation, like
lasers which are stimulated by electric
current to emit an intense beam of visible
light
Hubble Law
• Most galaxies show a redshift spectrum
• That is, nearby galaxies are moving away
from us slowly, but distant galaxies are
rushing away from us
• This recessional movement is called the
Hubble flow
redshift
• Redshift (z) is found by subtracting
observed wavelength (λ) of a spectral line
from ordinary wavelength (λo) to get
difference (Δλ); divide result by λo
• z = λ- λo = Δλ
λo
λo
Hubble Law
• V = Hod
where v = recessional velocity of galaxy,
Ho= Hubble constant (slope of line),
d = distance to galaxy
• Ho = 71km/s/Mpc
• Or a galaxy 100 million parsecs away from
us is racing away from us at a speed of
7100 km/s
Galaxies collide!
• However, all galaxies are NOT moving
away from us!
• In fact, some are approaching us, like the
Andromeda galaxy
• Collision scheduled for 6 billion years from
today
• Not to worry, our solar system is only ~4.5
billion years old!
Review of terms
• Keplers 3rd law
• Relates size of orbit and time taken to go
around the sun
• P2 = a3
Review of terms
•
•
Newton’s form of Keplers 3rd law
P2 = [(4π2)/(G(m1+m2))]a3
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P = sidereal period (secs)
a = semi major axis (m)
m1 = mass of 1st object (kg)
m2 = mass of 2nd object (kg)
G = universal constant of gravitation or
(6.67x10-11)
Review of terms
• Parsec = A parsec is defined as the distance from the
Sun which would result in a parallax of 1 second of arc
as seen from Earth or 3.26 ly or 3.086x1016 m
• Ly = distance traveled by light in one year or 9.461x1015
m
• Declination – analogous to latitude
• Right ascension – analogous to longitude
• Apparent magnitude - doesn't measure how bright
objects actually are; it measures how bright they appear
to us, which also depends on how close they are eg Sun
has m = -26.74
• Absolute magnitude - measures how bright objects
actually are -- it is defined as the apparent magnitude
that an object would have if it were located at a distance
of 10 parsecs from us or Sun has abs m = 4.83
Review of Terms
• Nanometer – 10-9 m
• Angstrom – 10-10 m, visible light ranges from 4000-8000
angstroms
• Population I stars - young metal-rich, found in spiral
galaxy arms
• Population II stars – old, metal-poor, found in globular
clusters in galactic nucleus
• Semi major axis – ½ major axis
• Cepheids – metal-rich more luminous shorter periods
Type I or metal-poor dimmer Type II, apparent brightness
and luminosity used with inverse square law to calculate
distance to star
• Universe – has billions of galaxies
• Redshift – moving away
• Blueshift – moving to