Download The Universe



Knowledge of our celestial surroundings began
with the early studies of the sky by the ancient
Greeks and their careful observations of the stars
and planets
 Accurate star catalog - Hipparchus
Models of the “universe” were simple and included
the Earth, Sun, Moon, planets and stars
 Geocentric – Aristotle, Ptolemy
 Heliocentric – Asistothenes
 Milky Way of stars - Democritus

Next significant advance in understanding
of our surroundings came with the
discovery of the telescope and its use by
Galileo and others in the 16th century



Planets and moons
Heliocentric solar system
Patches of dense stars too numerous to count



Careful observations of the diffuse “clouds” of stars first
studied in the 10th century by Arab astronomers were
made by Charles Messier in the late 18th century
 109 brightest nebula were cataloged (Messier Objects)
 Included the Andromeda Galaxy (M31) – Our Sister
Galaxy
Over 5,000 nebula were later cataloged by William
Herschel
“Island Universes” was coined by Immanuel Kant in the
1750s to describe the distant diffuse star clouds that
appeared to be independent of the Milky way galaxy


The “Great Debate” in 1920 between Harlow Shapley and
Heber Curtis was a discussion on nature of the many
star-rich nebula that could be either:
 Small objects within our Milky Way Galaxy (our Milky
represented the entire universe)
 Distant star clouds were far beyond the boundaries of
our Milky Way Galaxy (the universe would be much
larger than the Milky Way)
The debate was settled later in the 1920s when
spectroscopic measurements of the diffuse nebulae
showed them to be much farther than the stars in the
Milky Way
 The universe was discovered to be a collection of
many galaxies, and the Milky Way was just one of
many


Precise measurements of many of the galaxies by Ernst Opik
and Edwin Hubble in the 1920s led to distance determination
methods that included Doppler shift measurements based on
relative velocities between distant galaxies and an observer on
Earth
Following Hubble’s classification of galaxy shapes and types,
the distance to many galaxies showed that the universe was in
constant expansion
 Distant galaxies were receding faster than nearby galaxies
 Velocity of recession was proportional to distance

Hubble law provided an accurate measurement of
 Universe expansion
 Distance to galaxies and clusters of galaxies (easier to
measure at very large distances)

Variations of and improvements in the Hubble classification of
galaxies are still used today
Hubble’s data of
receding galaxy
clusters (velocity
vs. distance)
Inverse of the Hubble
relation
(velocity/distance)
gives the age of
the universe
Receding galaxy clusters
 Hubble law states that velocity is proportional to
distance
Interpretation (hypothesis)
 As we look back in time, the universe expands
faster
 Highest expansion rate is at the beginning of the
formation of the universe
Big Bang


Initially called the “primeval atom”, George Lemaitre
hypothesized that the universe began as a small entity
that expanded into the present-day universe according
to the Hubble relation
Refinements over several decades transformed the
primeval atom hypothesis of Lemaitre into the Big
Bang theory that is accepted today as the broad
collection of theories that best describes the formation
of the observable universe

Early mythology reflected the primitive
efforts to interpret or understand the
external world represented in the night sky

Our efforts today to understand the
universe contain many theories that tie in
the smallest and largest components of the
physical world



Smallest – quantum physics
Largest – Einstein’s theory of general relativity
Still missing – gravity, dark matter, dark energy
Research into the origin and evolution of the
universe include both observation and
theory
Theory must fit observation to be valid, and
must be validated by scientists in the field
to become a robust model (theory)
Basic observations that must be fit to the theory

Dark night sky (Obler’s paradox)

Receding galaxy clusters (expanding universe)
 Good correlation between distance and recessional velocity

Cosmic background radiation left over from beginning of universe

3:1 H to He ratio

Oldest stars are 13 billion years old

Age of galaxies
 Oldest are 13 billion years
 Some dwarf galaxies are still forming

Evolution of galaxies
 Earliest galaxies are small
 Dwarf galaxies often with irregular shapes
 Later galaxies are larger, better formed, and more complex
Scales used to measure the universe (small to large)
Solar system - near

Distance to planets – measured in months and years in spacecraft
travel time for a spacecraft traveling at 10-20 km/s = 22,500-45,000
mph

Light minutes to light hours to the nearby planets
Light minute is the distance traveled by light in one minute = 18,000,000
km
 Light second is the distance traveled by light in one second = 299,800 km

Solar system – far

Pluto – 6 light hours (45 years at spacecraft speeds in a Hohmann
transfer orbit)
Nearest stars

4-5 light years

Distance of one light year is the distance traveled by light in one
year = 9.461×1012 km = 6.324×104 AU

Approximately 100,000 years at spacecraft speeds

Distance to Galaxy center – 30,000 light years (ly)

Diameter of Milky Way Galaxy – 100,000 ly




Distance to Andromeda galaxy (nearest large galaxy) –
2,000,000 ly
Distance to nearby Virgo cluster of galaxies – 60,000,000 ly
(6x107 ly)
Distance to the Great Attractor – 2.5x108 ly
Distance to the early surface of the expanding universe –
45x109 ly (45 billion light years)

This distance is 3.3 times the age of the universe in light travel
time, but remember the universe is expanding and that mass
curves space
Largest structures
The largest structures in the
universe are the
superclusters that string
together and form web-like
features composed of
galaxies
These filamentary structures
can extend billions of light
years in length with large
collections of
superclusters making up
the dense regions and little
visible mass in the dark
voids between them
Galaxies are large collections of stars, gas, dust, and dark matter

For the typical galaxy, the gas, dust and dark matter are an order
of magnitude more massive than the stars

The atomic composition of a galaxy is predominantly hydrogen
and helium, with a fractional amount of other material
The arrangement of stars and gas within a galaxy produces a
characteristic shape to each galaxy, with several common features
found among the various galaxy shapes

The original classification of the galaxies according to their
basic shape was devised by Edwin Hubble in the 1920s
 Elliptical
 Spiral
 Barred spiral

Galaxies range in size from roughly 107 to 1014
solar masses (Mo)

There are approximately 100 billion galaxies
making up the baryonic (atomic) universe

Formation process is hierarchical
 Galaxy and galaxy cluster formation are in highdensity regions of H-He gas (top-down)
 Galaxies form as small objects first, then evolve
to form larger galaxies (bottom-up)
 Early galaxies are mostly dwarfs
 Galaxies evolve to giants by collisions and
cannibalism
Galaxies, like solar systems, are gas-dominated,
rotating systems that have a large central mass

The rotational patterns in the disk-shaped (spiral)
galaxies follow the same orbital relation as the
planets in our solar system

Rotation speed slows and orbital period increases
with increasing distance from the center

Keplerian rotation is not uniform in most spiral
galaxies

Rather than decrease as
1/r1/2 as the planets do,
the orbital velocities
often exhibit constant
rotational velocity with
increasing distance,
suggesting significant
other mass not visible in
the outer regions of the
visible galaxy

This extra mass is also
not visible in any of the
electromagnetic bands,
and hence is called dark
mass, or dark matter

A diagram of the flat
rotation curve due to
hypothesized dark matter
is shown below for spiral
galaxy NGC 3198

Dark matter is also found surrounding groups,
clusters and superclusters of galaxies

Dark matter is unusual because it does not form
in the same central concentration as the galaxies
or groups (its not cold) , yet it does not disperse
over time (its not hot)

Both "cold" and "hot" dark matter models for
dark matter are problematic since neither
accurately portrays the actual distribution based
on its gravitational influence
Spiral galaxies are easily
identified by their armed
spiral structures that extend
from the center of the
galaxy outward to the edge
of the equatorial disk
Spiral arms that typify spiral
galaxies outline elongated
sites of ongoing star
formation and are brighter
than the surrounding disk
because of the young, hot
OB star
M101 is shown on the right
Barred spiral galaxies
include a straight bar
feature in the central
region of the otherwise
spiral galaxy
Roughly half of all spirals
are observed to have a
bar structure that
extends from the
galaxy center to the
inner reach of the
spiral arms
NGC 1300 is shown on the
right
Elliptical galaxies are generally
egg-shaped with little if any
disk feature and can be
spherical
These elliptical galaxies are
generally the most massive,
and also represent the endpoint in collisions between
large spiral galaxies
M87 is shown on the right
Irregular galaxies have few of the
dominant features of the
larger spirals and elliptical
galaxies
Irregular shapes generally lack an
equatorial disk of the spiral
galaxies and the symmetrical
shape of the ellipsiodal
galaxies
The irregular galaxy morphology
(shape) is generally
associated with smaller dwarf
galaxies, analogous to the
irregularly-shaped
planetesimals compared to
the spherical planets and
large moons
NGC 1569 is shown on the right
Our own Milky Way
Galaxy is barred spiral
type classified as an
SBc type
 SB = barred spiral
 C = diffuse outer
arms



Diameter = 100,000 ly
Mass = 100 billion
solar masses
Sun’s orbit period =
250 My

The Milky Way Galaxy is but one
of nearly 50 galaxies located in
a gravitationally-bound
collection of large and small
galaxies called the Local Group

The two primary galaxies of the
Local Group are the Milky Way
and the Andromeda galaxies

Each has more than a dozen
dwarf satellite galaxies orbiting
their center

Andromeda and the Milky Way
are in orbit around the Local
Group’s center of mass and on
a collision course in about 5 By

Diameter of the Local Group of
galaxies is approximately 10
Million light years (10 Mly)

The Milky Way Galaxy and the
Local Group of neighboring
galaxies are also members of
the huge Virgo supercluster of
galaxies

The Virgo supercluster contains
more than 100 groups and
clusters (including the Virgo
cluster) and spans more than
100 Mly in diameter

The Virgo supercluster is
thought to be just one of
millions of superclusters in the
observable universe