Space and time ch4

Space and Time
The Blue Planet: Chapter 4

Outline
• The Sun
• The Solar System
• Other Suns and Planetary Systems
• Time and Change

The Sun
• Each visible point of light in the night
sky, except nearby planets, is actually a
sun or collection of suns
• Or rather, our Sun is an ordinary star
• The is dominated by hydrogen and
helium at 98% of its mass
• Of course, the Sun provides the light
and energy for life to exist on Earth

The Solar System
• Beyond the Sun, there are 8 planets in the solar
system, at least 5 dwarf planets and vast
numbers of asteroids, comets, meteoroids, and
moons
• The innermost planets are small, rocky, metallic,
and dense: terrestrial planets
– Mercury, Venus, Earth and Mars
• The outer planets are much larger, less dense
and gaseous: Jovian planets (gas giants)
– Jupiter, Saturn, Uranus and Neptune

The Solar System
• The early model of our solar system was
geocentric, meaning that people thought all
objects revolved around the Earth
• Today we know it is heliocentric, meaning that all
objects revolve around the Sun
• Any hypothesis for the origin of the solar system
must account for as many of its features as
possible:
– All solar system objects revolve in the same direction,
around the sun, moons around their respective
planets, and all on the same plane, consistently

The Solar System
• The origin of the Sun was probably
similar to the origins of billions of other
stars in the universe, so the prevailing
model for the origin of the solar system
is the nebular hypothesis
• This proposes that a huge swirling
cloud of cosmic gas and dust (a nebula)
formed the sun and planets

The Solar System
• Gravity pulled the slowly swirling cloud
of dust and gas inward, as this
happened the gar became hotter and
denser
• Eventually temperature and pressure
was high enough that nuclear fusion
started and a star was born: the Sun
• Surrounding the new Sun was a
flattened, rotating disc of gas and dust

The Solar System
• By the time the Sun started burning, the cooler
outer portions of the solar nebula had become
so compressed that solid particles and liquid
droplets began to condense from the gas
• These condensates, through accretion,
became the building blocks of the planets,
moons, and other objects in the solar system
• Distance from the Sun and condensation
temperatures explain the distinct materials of
the terrestrial and Jovian planets

The Solar System
• Space missions continue to provide
evidence indicating that all objects in
the solar system formed at the same
time from a single solar nebula
• Beyond the end of the nebular
hypothesis story, five key factors played
determining roles in the subsequent
evolution of the terrestrial planets

The Solar System
• Melting, impacts, and differentiation
– Colliding bodies convert kinetic energy into
heat energy
– As planetary accretion climaxed about 4.56
billion years ago, bigger collisions mean
more kinetic energy and more heat
– Terrestrial planets began to melt, at least
partially, and dense metallic liquids sank
while lighter materials floated
– Planetary differentiation by chemical
segregation

The Solar System
• Volcanism
– After partial melting, the interior of the
planets still remained hot because of
radioactive elements
– All planets are slowly cooling, larger
planets slower than smaller planets
– Volcanism is an indicator of high internal
temperature

The Solar System
• Planetary mass
– Determines the orbit of a planet, and how
many moons it captures
– Determines whether the planet has
sufficient gravitational pull to hold onto its
atmosphere

The Solar System
• Distance from the Sun
– Determines if water can exist as a liquid
• Biosphere
– Presence or absence of a biosphere plays
an essential role in the development of the
biogeochemical cycles that control the
composition of Earth’s atmosphere

The Solar System
• We do not know if any other terrestrial
planets have molten or partially molten
cores, which has provided Earth with a
strong magnetic field
• All terrestrial planets have experienced
volcanic activity, indicating an internal
heat source, and have been through
intense collisions
• Apparently unique to Earth is tectonic
activity

The Solar System
• The outer planets are shrouded by thick
atmospheres that have not escaped the
planets’ enormous gravitational pull
• Their bulk compositions are therefore
about the same as the nebula from which
they formed: Jupiter’s composition is
remarkably similar to that of the Sun
• Huge storm systems are common in the
gas giants’ atmospheres, and all probably
have rocky cores

The Solar System
• Moons
– The 19 largest moons are roughly spherical
in shape, the smaller ones can be extremely
irregular
– Some formed by coalescence from the same
mass as the solar nebula, others by
gravitational capture, and others by collision
– Earth’s moon is 1/4 the size of Earth, making
it the largest natural satellite in comparison
with its parent planet, it likely formed from a
catastrophic collision

The Solar System
• Asteroids and Meteorites
– Subplanetary objects orbiting the sun
– Commonly rocky and/or metallic
• Pluto and the Dwarf Planets
– Minor planets or small bodies that are
orbiting the sun, massive enough to be
spherical, but not massive enough to have
cleared its orbital path
– In addition to Pluto, are Eris, Haumea,
Makemake, and Ceres

The Solar System
• Comets, the Kuiper Belt, and the Oort Cloud
– The dwarf planets belong to a group that
includes thousands of other objects outside of
Neptune’s orbit called the Kuiper Belt
– Similar to the Asteroid belt in appearance, but
consists mainly of icey rather than rocky bodies,
akin to comets
– The Oort Cloud is further out still, and also
appears to be a store of cometary material

Other Suns and Planetary Systems
• Stars are classified by color and
brightness
– Color is an indication of temperature, blue
light comes from short wavelengths and is
hot, while red light comes from long
wavelengths and is cool
– Each color designates the star’s spectral
class, from 9 (hottest) to 0 (coolest)

• A star’s brightness is a function of both the
star’s luminosity (energy emitted) and its
distance from the Earth
– This requires a normalization of star
distances, which is difficult to measure, but
can be done to 300 light-years
• Once temperature and luminosity are
known, they can be compared with values
on the Hertzsprung-Russell diagram
– White dwarfs, main sequence and red giants

Other Suns and Planetary
Systems

• The H-R diagram can be used to
explain the evolution of a star
– The smaller the star, the longer it can live
• For the lifetime of most stars, a balance
is reached between gravitational and
radiation forces, where it maintains the
stable luminosity and temperature of a
main sequence star

• A star the size of our Sun will fuel
nuclear fusion for about 10 billion years
• When the hydrogen fuel is used up,
nuclear fusion ceases, gravity takes
control, and the helium-rich core
contracts
• As the core collapses, it heats up, and a
shell of hydrogen in the inner radiative
layer begins shell fusion, the star
expands, and becomes a red giant

• The core continues to contract,
eventually becoming hot enough for
helium fusion to form carbon, the shell
slowly diminishes in size becoming a
white dwarf
• Eventually it loses its luminosity and
becomes a dead star known as a black
dwarf

• Astronomers believe that 5-10% of the
200-400 billion stars in the Milky Way
have characteristics similar to those of
our Sun, and it is likely that they have
planetary systems like our own
• These planets are called exoplanets,
and as of June 2009, 353 exoplanets
had been found

Time and Change
• Scientists estimate the age of the
universe by looking at the rate at which
objects are moving apart from each
other
• The hypothesis is that everything
originated at one location in an explosion
called the Big Bang
• The universe is 2 to 3 times as old as
the Sun and the solar system

Time and Change
• To deal with the ages of materials within
the Earth system and elsewhere in the
universe, scientists use two concepts of
time and age
– Relative age: refers to the order in which a
sequence of past events occurred
– Numerical age: is the actual time, in years,
when a specific event happened,
calculated using radioactive decay

Time and Change
• Using these tools and worldwide
comparison and correlation of rock
units, geologists have assembled a
geologic column that summarizes in
chronological order the succession of
known rock units
– Major divisions include the Hadean,
Archean, Proterozoic, Paleozoic, Mesozoic
and Cenozoic Eons

Time and Change
• In the 17th and 18th centuries, people
hypothesized that all of Earth’s features
were formed by a few great catastrophic
events - this idea is catastrophism
• In the late 18th century, this idea was
tested with geological evidence
• James Hutton, with the use of the
scientific method, proposed a counter
theory called the principle of
uniformitarianism

Time and Change
• Hutton observed the slow, steady effects of
erosion
• Determined that mountains must slowly
weather away, that new rocks form from
the debris of erosion, and be thrust back up
into mountains
• Couldn’t explain what caused this to
happen, but reasoned that everything
moves slowly in repetitive continuous
cycles

Time and Change
• The principle of uniformitarianism, which
essentially states that “the present is the
key to the past,” indicates that the Earth
is incredibly old
• This concept is important to all branches
of science, but we also know that some
events are so large and damaging that
they can cause catastrophic change

Space and time ch4

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