Download 8and10Dec_2014

Material from Units 79 -- 86
Our Galaxy, the Milky Way
• A galaxy is a large
collection of billions of
stars
• The galaxy in which the
Sun is located is called
the Milky Way
• From our vantage point
inside the galaxy, the
Milky Way looks like a
band of stars across the
night sky, with dark dust
lanes obscuring the
center of the band.
An Early View of the Milky Way
• It is difficult to know
exactly what the Milky Way
looks like from outside the
galaxy!
– Similar to trying to figure
out what kind of car you are
in, from the inside!
• William Herschel (who
discovered the planet Uranus)
created a “map” of the Milky
Way, based on observations.
• He incorrectly placed the Sun
close to the center of the galaxy
The Shape of the Milky Way
Kapteyn’s Universe
• Jacobus Kapteyn improved on
Herschel’s view of the galaxy
• Using more modern equipment,
Kapteyn attempted to count the
number of stars in the galaxy, and
estimate their distance from the Sun
• The model was called Kapteyn’s
Universe, as the existence of other
galaxies was unknown!
• He revised the size of the galaxy to
around 18,000 parsecs (18
kiloparsecs, or kpc), again with the
Sun near the center
• Both Herschel and Kapteyn were
correct in depicting the shape of the
galaxy as a disk, with most of the
stars lying in more or less the same
plane (the galactic plane)
Moving the Center of the Galaxy
• Harlow Shapley used observations
of globular clusters to correctly
deduce the location of the Sun
within the Milky Way
• He reasoned that if the Sun were at
the center of the galaxy, then
globular clusters would be found in
• He noted that there were more
globular clusters found in the
direction of Sagittarius than
elsewhere
• Therefore, the center of the galaxy
must be in the vicinity of
Sagittarius!
Today’s view of the Milky Way
• Today we know that the
Milky Way is a spiral
galaxy approximately 30
kps across.
• The Sun is located
around 8 kpc from the
center, in one of the
spiral arms.
• Most of the stars are
concentrated in the
galactic plane, or in the
central bulge at the center
of the galaxy
• Inside the bulge is the
nucleus of the galaxy
• Surrounding the disk is a roughly spherical
distribution of stars called the halo.
• Globular clusters are distributed throughout
this halo, surrounding the center of the
galaxy.
The Interstellar Medium
The Sombrero Galaxy
• Space is far from empty!
– Clouds of cold gas
– Clouds of dust
• In a galaxy, gravity pulls the dust
into a disk along and within the
galactic plane
• This dust can obscure visible
light from stars and appear to be
vast tracts of empty space
• Fortunately, it doesn’t hide all
wavelengths of light!
Emission Nebulae
• We frequently see
nebulae (clouds of
interstellar gas and
dust) glowing faintly
with a red or pink
color
• Ultraviolet radiation
from nearby hot stars
heats the nebula,
causing it to emit
photons
• This is an emission
nebula!
Reflection Nebulae
• When the cloud of
gas and dust is simply
illuminated by nearby
stars, the light
reflects, creating a
reflection nebula
• Typically glows blue
Dark Nebulae
• Nebulae that are not
illuminated or heated
by nearby stars are
opaque – they block
most of the visible
light passing through
it.
• This is a dark nebula
Composition of Interstellar Clouds
• Light passing through an
interstellar cloud can hold
clues as to the cloud’s
composition
• Atoms in the cloud
absorb specific
frequencies of starlight
passing through, creating
absorption lines
• Astronomers can analyze
these spectra to determine
what the clouds are made
of.
• Spectra show that interstellar gas clouds are
made of mostly hydrogen and helium, just like
the Sun
• Dust particles do not absorb light the same
way that gas atoms do, but using similar
methods tells us that the dust is made of
Heating and Cooling in the ISM
• Gas in the ISM is heated by
radiation from nearby stars and by
stellar winds
• Gas is cooled by re-radiating away
energy, especially clouds that are
shielded (shadowed) by dust or other
cooler stars
• O and B stars are very good at
heating, as they put out mostly UV
photons
• These UV photons can ionize
neutral hydrogen, with two effects:
– Causes gas to glow a reddish-pink
– Liberated electrons emit radio waves
that can be detected!
– These radio waves penetrate dust well,
allowing us to map the galaxy.
The Tuning Fork
• Edwin Hubble organized
different galaxy types into a
tuning fork shaped diagram
• Ellipticals are labeled E0-E7
– E0 is almost perfectly
spherical, E7 is quite
flattened
• Spirals are labeled Sa – Sd
– Sa galaxies have tightly wound
arms and a large central bulge
– Sd galaxies are loosely wound and
have a small central bulge
• Barred Spirals are labeled SBa –
SBd
– Same flow as the Spirals
Additions to the list…
• Dwarf galaxies (left) are difficult to
detect, and may be the building blocks of
larger galaxies
• Low Surface Brightness galaxies (above
left) are very large, yet very faint galaxies
that have very little new star formation
occurring
Differences in Star and
Gas Content
• Ellipticals:
– Low in gas and dust, so contains
mostly older Pop II stars
– Contain very high temperature,
very low density clouds of gas that
cannot condense into stars.
• Spirals:
– Lots of gas and dust, so have
active regions of star formation
– Have both Pop II and younger Pop
I stars
• Irregulars:
– Many hot, young stars
– Large amounts of interstellar
matter
– Might be young galaxies
A look back in time
• The Hubble Space Telescope was
pointed at a part of the sky that
looked empty, taking a 100-hour
exposure
• Very distant galaxies were
detected, some closer than others
• This technique allows us to see
galaxies at various stages of
formation
• These early galaxies tend to be
smaller than the Milky Way, and
to not fall into Hubble’s
classification scheme
Galactic Collisions
• Galaxies can collide, though not in the sense of a car accident!
• The galaxies pass through one another, and their immense gravitational
pull tears both galaxies apart!
• Eventually, a new elliptical galaxy will form…
Galaxy collision and merger
A Ring Galaxy
• If a smaller galaxy
plows through the
middle of a larger
one, a ring galaxy
can result!
• Stars are not
destroyed, only their
orbits are disturbed,
redistributing them
through the new
galaxy
Galactic Mergers
Young galaxies possibly merging to form a larger system
A Picture of the Universe
• This all-sky image gives the positions of over a
million galaxies, each with billions of stars…
Our Galactic Neighborhood
• The smallest
organization of galaxies
are called galaxy groups
• Our local group is called
the Local Group
• The Local Group
contains 40 known
members, including the
Andromeda Galaxy and
the Large and Small
Magellanic clouds,
dwarf satellite galaxies
of the Milky Way
Members of the Local Group
Rich and Poor Galaxy Clusters
• Rich clusters:
– Contain hundreds to
thousands of member
galaxies
– Are roughly spherical, with
the largest galaxies near the
center
– Contain mostly elliptical and
type S0 galaxies
– Lots of hot gas and dust
• Poor clusters
– Contain only tens of
galaxies
– Have a ragged, irregular
appearance
– More spiral and irregular
galaxies
Superclusters
• Clusters of clusters are called
superclusters
– Contain a few to many dozen
clusters of galaxies
– Can be Mpc across!
– The Local Group is part of the Local
Supercluster, shown at left.
• The Local Supercluster is heading
toward a region of space known as
the Great Attractor, where there are
a large number of massive
superclusters
• There may be super-superclusters!
Missing Mass
• In Unit 73, we calculated the mass
of the Milky Way by measuring the
orbital velocities of dwarf galaxies
in orbit around our galaxy
• We can also count the number of
stars in the galaxy, and estimate the
galactic mass. The two numbers do
not agree!
• Rotation curves do not show the
expected decrease in stars’ orbital
velocities with distance from the
galactic center, so there must be
much more mass present in our
galaxy
• Astronomers cannot find a large
majority of this mass!
• Astronomers call the missing mass
dark matter
Many galaxies have flat rotation curves!
Dark matter is not unique to the Milky Way!
Figure 78.03
• 99 percent of the stars in a galaxy
are within 20 kpc of the center
• Gas extends far out into the disk,
but is not very massive!
• Galaxies are now thought to be
embedded in a dark matter halo that
surrounds the entire galaxy
• Unfortunately, dark matter cannot be
detected directly.
Dark Matter in Clusters of Galaxies
• Missing mass is also a
problem in clusters of
galaxies!
– Not enough visible mass to
hold the clusters together
by gravitation, and to keep
hot gas in their vicinity
– Cluster mass must be 100
times greater than the
visible mass!
– Once again, dark matter
seems to be the solution
Gravitational Lenses
• Dark matter warps space just like ordinary
matter does
• The path of light rays bends in the presence of
mass
• A galaxy or other massive object can bend
and distort the light from objects located
behind it, producing multiple images
• This is called gravitational lensing
Figure 78.06
Radiation, Matter and Antimatter
• In the first second of the early universe,
matter did not really exist; rather, everything
was radiation or energy. Cosmologists call
this time period the early universe.
• When energy is converted into matter,
antimatter is formed as well.
• For a proton-antiproton pair to form, the
temperature must be more than 1013 K!
• Matter and antimatter annihilate on contact,
releasing energy
• There must have been an asymmetry in the
amount of matter and antimatter formed in
order for there to be a predominance of
ordinary matter today.
Olber’s Paradox
• Over very large distances,
galaxies in the universe are
more or less uniformly
distributed (homogeneous)
• If there are galaxies in every
direction, however, why do we
not have a fully-lit sky? We
should see a star in any
direction we look!
– This is called Olber’s Paradox
• If there is an edge to the
universe, we should be able to
see our way “out of the
woods”
Olber’s Paradox
A Solution?
• In a sense, there is an edge
to the universe, an edge in
time
• Light travels at a finite
(though fast) speed
• The size of the visible
universe is defined as the
distance light can travel in
the age of the universe
• Galaxies exist at greater
distances, but light from
them has not reached us
yet.
• The edge is called the
cosmic horizon
• If we wait long enough, the
night sky might become
bright!
The Curvature of the Universe
• Remember that mass and
energy can curve the space
around it.
• As the Universe expands,
the distances between the
galaxies increases, like
galaxies painted on the
surface of an inflating
balloon
• If the universe was like an
expanding balloon (but with
the galaxies distributed in
three dimensions), travel in
any direction would
eventually bring you back to
your starting place (a closed
universe)
Other Possible Curvatures of Space
• In addition to a closed, or positive curvature of space,
there are two other options
– Space could be flat, or have zero curvature
– Space could be curved away from itself, or have negative
curvature
– Geometry behaves differently with each curvature!
Measurements of the Curvature of Space
• If space is closed, distant galaxies
or clumps of mass will appear
larger than they really are
• If space is flat, there will be no
apparent distortion in size
• If space is open, distant objects
will appear smaller than they
really are
• Recent measurements show that
space is very nearly flat!
Density of the Universe
• If we can measure the
density of the universe,
we can predict how much
gravitational energy the
universe has, and
therefore whether it will
collapse or keep
expanding
• The critical density of the
universe, C, is the
density at which the total
energy of the universe is • M = /C, where  is the measured density
of the universe
zero – gravitational
energy balances
3H 2 the other • If M > 1, the universe will recollapse
two.  C 
• If M < 1, the universe will expand forever
8G
• If M = 1, the universe is exactly at the
critical density
Supernova Type Ia Findings
• We also need to know how the
universe is expanding – this can help
us determine the value of M
• We can measure the recession
velocity of distant galaxies using
Type Ia supernovae as standard
candles
• It appears that the expansion rate
at a time when the universe was
half its current size (z=1) was
slower than it is today!
• This shows that the expansion
rate is increasing with time!
Very puzzling!
Life on Earth
• Life formed on Earth
relatively soon after the
planet’s formation
– For ¾ of the Earth’s history,
only algae and single-celled
life forms existed
– Slowly, more complex
lifeforms developed
• By 250 million years before the
present, dinosaurs and early
mammals had evolved.
• Hominids, our distant ancestors,
developed 5.5 million years ago
• Homo Sapiens evolved only
500,000 years ago!
Figure 83.04
• Life tends to draw on the substances that
are most plentiful: Carbon, Nitrogen,
Oxygen and Hydrogen
• Amino acids are organic molecules
containing these substances
• Amino acids form proteins, which provide
structure and energy to cells
• All life contains DNA – this instruction
packet contains all the information needed
to build an organism
The Origin of Life
• So how did amino acids form out
of the substances available on the
early Earth?
• Probably started thanks to complex
chemical reactions in the
atmosphere and surfaces of Earth
• The Miller-Urey experiment
attempted to duplicate the
environment of the early Earth
• A variety of complex organic
molecules formed in their
“atmosphere”
Fossil Eukaryote Algae
The Search for Life on Mars
• It appears that Mars at
some point in its history
was very much wetter
and warmer than it is
today
• Scientists have been
looking for life there
• The Viking landers
(1970’s) tested for the
presence of microbes,
but returned
inconclusive results
• We are still looking!
Fossils of Ancient Martian Life?
SETI
• SETI: Search for Extra-Terrestrial
Intelligence
• Listens for electromagnetic evidence of
intelligence elsewhere in the universe
• To date, evidence has been sparse.
As a star converts most of its hydrogen in its core into
helium, the star gets
a. less luminous and smaller
b. hotter and fainter
c. more luminous and bigger
d. less luminous and red
A hydrogen burning shell is created near the helium core
because
a. helium diffuses into the shell
b. hydrogen diffuses into the core
c. core is hot and dense
d. both a. and b.
If we observe a star cluster which has all the stars
of main sequence present, this cluster is
a. old
b. young
c. was born as a result of supernova explosion
d. both a. and c.