Download Other Galaxies, their Distances, and the Expansion of the Universe

Other Galaxies, their
Distances, and the
Expansion of the Universe
Announcements
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Homework # 7 starts today, and is due
Thursday, December 1st.
Homework # 6 is due today (11 pm).
Quiz # 6 will take place on Tuesday, November
29th;
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See website for information on lectures and textbook
units it is based on.
Reading Assignments
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Units 63, 65, 73.2, 73.3, 74, 75, 76
M51
Sombrero
M81
Galaxies show a huge
variety of shapes and
characteristics
These are images obtained by my team with the Spitzer Space Telescope
Some galaxies even show evidence for very violent ejection
of material into the intergalactic space
And some are merging with
each other
The merging Antennae Galaxy (NGC4038/4039)
How do we `Organize all of
this variety?
Galaxies seem to take one of four
different appearances
•  Spirals
•  Barred Spirals
•  Ellipticals
•  Irregulars
This classification scheme is known as
the Hubble Tuning Fork Scheme
SPIRALS
Spirals have a disk
with dust and gas;
They also show, in
general, prominent
spiral arms where the
star formation is
concentrated.
We easily see the spiral arms
because they contain numerous
bright O and B (massive) stars which
illuminate dust in the arms.
However, stars
in total seem to
be evenly
distributed
throughout the
disk.
SPIRALS
Progression from Sa to Sb, Sc, Sd, Sm depends on how
tight the spiral arms are wound and how big is the bulge
The tightness of a spiral
galaxy’s arms is correlated to
the size of its nuclear bulge
Type Sa
Type Sb
Type Sc
Variety of Spiral Arms
Flocculent spirals
(fleecy)
Grand-design spirals
(highly organized)
BARRED SPIRALS
Similar to spirals in many
characteristics.
The bar is not a spiral
arm.
It is made of old stars,
like those in the bulge
Bars of stars run through the
bulges of barred spiral galaxies
Type SBa
Type SBb
Type SBc
Same progression as regular spirals: SBa, SBb, etc.
ELLIPTICALS
Ellipticals have no dust
No cold gas
No star formation
Made mostly of old stars
Question
Given their color, do you expect elliptical galaxies to
have more or less new star formation than
spirals? Why?
1) ellipticals have more new star formation
2) ellipticals have less new star formation
ELLIPTICALS
Type E0
Type E3
Type E7
Progression from E0 (perfectly round) to E7 (shaped like a cigar)
Elliptical galaxies display a huge
variety of sizes and masses
Giant elliptical
galaxies can be 20
times larger than the
Milky Way
n  Dwarf elliptical
galaxies are extremely
common and can
contain as few as a
million stars
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IRREGULARS
Irregulars do not have a specific
shape;
they have plenty of gas, dust and
star formation
Galaxies like to
cluster:
eg. M81 group
…They like company…
Galaxies also like to merge:
Billions of stars all tug on each other instead of just one
planet tugged by the gravity of the Sun.
Galaxies fill the Universe, and are visible at great
distances. Each contains hundreds of millions of stars.
They can be used to trace and derive properties of the
Universe itself, such as whether it is a changing or
unchanging structure, the speed of change, etc.
The Detail is Amazing
Hubble Ultra Deep Field
Determining Distances on Small
Scales
There is a well-tested method for measuring
distances over short length scales:
u  Parallax
- good for measuring distances to a
few hundred light years
Stellar Parallax (Review from a few classes ago)
The baseline is the diameter of the Earth s orbit.
The measurements are taken six months apart.
½ of the angle
between the now
location and the 6month location is
called the stellar
parallax. = P
But what do we do about
objects too far away to use the
parallax method?
Standard Candles
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If we know an
object s true
Luminosity
luminosity, we can
Brightness
=
measure its distance
4 " # distance 2
by measuring its
apparent brightness.
An object that!has a known luminosity
is called a standard candle.
Survey Question
You see a car on the other side of the road coming
towards you with their lights on. When the car is
twice as close (that is, at half the original
distance), how much brighter are the headlights?
1)
2)
3)
4)
5)
the same brightness
twice as bright
four times as bright
sixteen times as bright
½ as bright
So measuring distance with
standard candles is easy!
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Unless you worry about reality getting in
the way:
Dust extinction
n  Atmospheric extinction
n  Making mistakes because objects are getting
dim and hard to measure
n  Finding a standard candle in the first place.
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Reliable standard candles must be calibrated.
Standard Candle #1 - Cepheid
Variable Stars
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Cepheid variable stars
have variable
brightness that is very
regular.
The period of the
variation can be from
days to weeks – and
it seems to be a
reliable indication of
the star’s luminosity!
Cepheid Variable Stars
Henrietta Leavitt
(1868-1921).
Luminosity=4πD2B
Standard Candle #2 - Type Ia
Supernova
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Always the result of a white dwarf with a mass
equal to 1.4 Msun.
These are some of the most energetic events in
the universe – so they are very bright.
Wait a moment…
How can a white dwarf (the stellar
remnant of a low mass star) become a
supernova?
n  It can, if it has a companion (that is, it is
in a binary star).
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When the companion becomes a red giant, some of its mass flows to the white
dwarf, increasing its mass. If the mass increases sufficiently (up to 1.4
Msun), the WD will explode as supernova!
Type Ia (white Dwarf) SNa
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The peak
luminosity of
a Type Ia
supernova is
very high
They are
visible from
the deepest
cosmological
reaches
Type II Snae
are
explosions of
massive stars
Standard Candle #2 - Type Ia
Supernova
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All type Ia supernovae have the same luminosity
(more or less), so this is our best standard
candle yet since they are so bright we can
measure the distances to things that are very far
away!
Supernova
1998ba
2011 Nobel Prizes:
Saul Perlmutter
Brian Schmidt
Adam Riess
Type 1a Supernovae
show that the
Universe is
accelerating!
Survey Question
For an object to be a true standard candle, all
occurrences of the object must have
the same _______?
1)
2)
3)
4)
5)
brightness
distance
age
luminosity
mass
Hubble s Discovery
In the 1920s, Edwin Hubble used the
distance ladder (based on Cepheid
variable stars) to measure the distances of
about 50 galaxies.
n  This work proved that the Andromeda
galaxy was not part of the Milky Way.
n  Hubble and his staff then measured the
redshifts (remember the Doppler Shift) of
the galaxies. They were shocked with
what they found.
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Velocity
Hubble’s distance/velocity data
Distance
The further away is a galaxy, the faster it s moving away from us.
Hubble's Law"
V = H0D
Hubble's Data
Modern Data
The further away is a galaxy, the faster it s moving away from us.
Hubble s Law
v = H0d
A constant:
~70 km/s/Mpc
Velocity that the
galaxy is moving away
from us.
The distance to
the galaxy
Distance to a galaxy
V = H0D
Ho = 73 km/s / Mpc.
The receding velocity causes
the redshift of spectral lines,
which can be used to estimate
distances to distant galaxies
that follow the Hubble's law:
D = V/H0.
e.g., for a galaxy with V =7300
km/s, D=100 Mpc
The Universe is expanding….
Isn t this at odds with our notion
that Earth isn t special?
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Not exactly. Try the balloon analogy or
the loaf of bread with raisins…
The Age of the Universe
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The reciprocal of H0 is the age of the universe (if the
universe has been expanding at the same rate since it
was born).
1/H0 ~ 13.7 billion years
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This is in remarkably good agreement with the age
inferred from measuring the age of the oldest globular
clusters.
This age has been confirmed by other, recent
experiments that we ll talk about in the next few
lectures.
How Big is the Universe?
The Universe we can
observe is as big as the
age of the Universe
multiplied by the speed
of light:
Runiverse = c x age =
4.2 x 109 pc = 4200 Mpc