Download Get ready for quiz # 7

Get
ready
for quiz
#7
Weekly Schedule
Today
• Homework # 8
due
• Quiz # 7
• Galaxies and
cepheid variables
• Lab
Next Tuesday (12/3)
• No class
• Take home quiz/lab
• Online HW #9
Thursday (12/5)
• All things due
Thursday
• No class- Turkey
day
• Take home quiz
• Expanding universe
Lab
– The Big Bang and ET!!!
From Earth, we see few stars when looking out of
galaxy (red arrows), many when looking in (blue and
white arrows).
Milky Way is
how our Galaxy
appears in the
night sky (b).
People began to examine the Milky Way
closer, and discovered that it consisted of
billions of stars.
Imagine that you could travel at the speed of light. Starting
from Earth, how long would it take
you to travel to the center of the Milky Way Galaxy?
A) 2 years
B) 20 years
C) 250 years
D) 2500 years
E) 25,000 years
Milky Way Galaxy
One of the first attempts to measure the Milky
Way was done by Herschel using visible stars.
Unfortunately, he was not aware that most of the
galaxy, particularly the center, is blocked from
view by vast clouds of gas and dust.
14.4 The Formation of the Milky Way
The
formation of
the galaxy is
believed to
be similar to
the
formation of
the solar
system, but
on a much
larger scale.
14.5 Galactic Spiral Arms
Measurement of the position and motion of gas
clouds shows that the Milky Way has a spiral
form.
14.1 Our Parent Galaxy
Our Galaxy is a spiral
galaxy. Here are three
similar galaxies.
14.5 Galactic Spiral Arms
The spiral arms cannot rotate along with the
galaxy; they would “curl up.”
14.5 Galactic Spiral Arms
Rather, they appear to
be density waves, with
stars moving in and
out of them
much as
cars move
in and out
of a traffic
jam.
14.5 Galactic Spiral Arms
As clouds of gas and dust move through the spiral
arms, the increased density triggers star
formation. This may contribute to propagation of
the arms. The origin of the spiral arms is not yet
understood.
http://media.pearsoncmg.com/aw/aw_0media_astro/if/if.h
tml?spiral_arm_pattern
14.6 The Mass of the Milky Way Galaxy
The orbital speed of an object depends only on
the amount of mass between it and the galactic
center.
Once all the galaxy is within an orbit, the velocity
should diminish with distance, as the dashed curve
shows.
It doesn’t; more than twice the mass of the galaxy
would have to be outside the visible part to
reproduce the observed curve.
Dark matter
Gravity acting across vast
distances does not seem to
explain what astronomers see
Galaxies, for example, should fly
apart; some other mass must be
there holding them together
Astrophysicists have thus
postulated "dark matter" - invisible
to us but clearly acting on galactic
scales
http://media.pearsoncmg.com/aw/aw_chaisson_astronomytoday_6/videos/ch25/DarkMa
tter.html
Measuring the Milky Way
• We have already encountered variable stars –
novae, supernovae, and related phenomena –
which are called cataclysmic variables.
• There are other stars whose luminosity varies
in a regular way, but much more subtly. These
are called intrinsic variables.
• Two types of intrinsic variables have been
found:RR Lyrae stars and Cepheids.
14.2 Measuring the Milky Way
The upper plot is an RR
Lyrae star. All such stars
have essentially the same
luminosity curve, with
periods from 0.5 to 1 day.
The lower plot is a
Cepheid variable; Cepheid
periods range from about
1 to 100 days.
14.2 Measuring the Milky Way
The variability of these
stars comes from a
dynamic balance
between gravity and
pressure – they have
large oscillations
around stability.
14.2 Measuring the Milky Way
The usefulness of these stars comes from
their period–luminosity relationship.
14.2 Measuring the Milky Way
This allows us to measure the distances to
these stars.
• RR Lyrae stars all have about the same
luminosity; knowing their apparent magnitude
allows us to calculate the distance.
• Cepheids have a luminosity that is strongly
correlated with the period of their oscillations;
once the period is measured, the luminosity is
known and we can proceed as above.
http://media.pearsoncmg.com/aw/aw_chais
son_astronomytoday_6/videos/ch23/Ceph
eidStarInDistGalaxy.html
14.2 Measuring the Milky Way
Many RR Lyrae stars
are found in globular
clusters. These
clusters are not all in
the plane of the
galaxy, so they are
not obscured by dust
and can be measured.
This yields a much
more accurate picture
of the extent of our
Galaxy and our place
within it.
14.2 Measuring the Milky Way
We have now
expanded our
cosmic distance
ladder one more
step.
Spiral galaxies are classified according to the size
of their central bulge.
Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the
smallest.
Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc
progressively less tight, although the correlation is not perfect.
The components of spiral galaxies are the same as in our own Galaxy: disk,
core, halo, bulge, spiral arms.
Similar to the spiral galaxies are the barred
spirals.
Ellipticals are classified according to their
shape, from E0 (almost spherical) to E7 (the
most elongated).
Ellipticals also contain very little, if any, cool gas and dust, and show no
evidence of ongoing star formation.
Many do, however, have large clouds of hot gas, extending far beyond the
visible boundaries of the galaxy.
15.1 Hubble’s Galaxy Classification
S0 (lenticular) and SB0 galaxies have a disk and
bulge, but no spiral arms and no interstellar gas.
15.1 Hubble’s Galaxy Classification
The irregular galaxies have a wide variety of shapes.
These galaxies appear to be undergoing interactions
with other galaxies.
“Tuning Fork” classification
of Galaxies
15.2 The Distribution of Galaxies in Space
Cepheid variables allow measurement of
galaxies to about 25 Mpc away.
However, most galaxies are farther away then
25 Mpc. New distance measures are needed.
• Tully-Fisher relation correlates a galaxy’s
rotation speed (which can be measured using
the Doppler effect) to its luminosity.
• Type I supernovae all have about the same
luminosity, as the process by which they
happen doesn’t allow for much variation.
The rotation of a galaxy results in Doppler
broadening of its spectral lines.
15.2 The Distribution of Galaxies in Space
With these
additions, the
cosmic distance
ladder has been
extended to about
1 Gpc.
15.2 The Distribution of Galaxies in Space
Here is the distribution
of galaxies within about
1 Mpc of the Milky Way.
Clusters and Superclusters
The Milky Way Galaxy and its neighboring galaxies are
known as the Local Group.
15.2 The Distribution of Galaxies in Space
A nearby galaxy cluster
is the Virgo Cluster; it is
much larger than the
Local Group, containing
about 3500 galaxies.
Clusters and Superclusters
Our local group is situated between the Virgo and Eridanus
clusters, which all together make our Local Supercluster.
Clusters and Superclusters
Our Local Supercluster is part of a network of superclusters.
Clusters and Superclusters
As far as we can see, superclusters hold together like a foam
within which there are bubbles of super large voids.
15.3 Hubble’s Law
Universal recession:
All galaxies (with a
couple of nearby
exceptions) seem to
be moving away from
us, with the redshift
of their motion
correlated with their
distance.
These plots show the relation between distance
and recessional velocity for the five galaxies in
the previous figure, and then for a larger sample.
The relationship (slope of the line) is
characterized by Hubble’s constant H0:
recessional velocity = H0  distance
The value of Hubble’s constant is approximately
70 km/s/Mpc.
Measuring distances using Hubble’s law actually
works better the farther away the object is;
random motions are overwhelmed by the
recessional velocity.
Hubble’s Law: the further away a galaxy is, the
faster it recedes
In fact, the more distant
the galaxy, the faster it
is moving away from us
According to Hubble’s Law, what is the
recessional speed of a galaxy at a distance of 100
Mpc?
Important information:
V = Ho * d
Ho = 70 km/s/Mpc
How far away is a galaxy whose
recessional velocity is 2000 km/s?
Important information:
V = Ho * d
Ho = 70 km/s/Mpc
15.3 Hubble’s Law
This puts the
final step on
our distance
ladder.
Evidence for “Big Bang”
1) Receding galaxies (redshifted)
http://media.pearsoncmg.com/aw/aw_0me
dia_astro/if/if.html?raisin_cake
Evidence for “Big Bang”
2) Cosmic Microwave Background
Radiation
Evidence for “Big Bang”
3) Predicts proportions of H and He
Universe
~75% H
~25% He
Not enough stars to make this
much He, so it was
“synethsized” moments after
“Big Bang”
Dark energy
At the greatest distances,
the Universe's expansion
is accelerating
Thus we have also "dark
energy" which acts to
drive the expansion, in
opposition to gravity
Make up of the universe
Fate of the universe
Heat death