Download Chapter 2 Basic Chemistry

Chapter 20
The Universe
The Life and Death of Stars
• What are Stars?
– Are huge spheres of very hot gas that emit blight and
other radiation
– Our Sun is actually a Star
– Are formed from clouds of dust and gas, or nebulas,
and go through different stages as they age
– Located at various distances from Earth
• We use the light-year to describe this distance
• A light-year (ly) is the distance that light travels in one year
– 9.5 x 1012m
The Life and Death of Stars
• Stars:
– Huge sphere of very hot hydrogen and helium gas that emits light
– Held together by the enormous gravitational forces that result from its
own mass
– Pressure in the core is more than a billion times the atmospheric pressure
on Earth with a temperature hotter than 15 million kelvins
– Get their energy from nuclear fusion
• Combines the nuclei of hydrogen atoms into helium
• When hydrogen nuclei collide, they fuse to form new nuclei called deuterons
(one proton and one neutron)
• Another proton collides with deuteron to form a helium isotope, helium-3
• Next, two mhelium-3 isotopes collide to form the nucleus of a helium-4 atom
• Each time that two particles fuse, energy is released
– Atoms near the surface radiate energy into space as radiation and light
– Energy from the core moves very slowly to the surface
• It may take tens of thousands of years
The Life and Death of Stars
• Some stars appear brighter than others
– Depends on the star’s temperature, size , and
distance from Earth
– Can be studied because of the wavelengths of
electromagnetic radiation produced
– Color is related to temperature
• Hot Stars: most energy in the blue wavelength
– Spectral lines reveal the composition
The Life and Death of Stars
• Sun forms from a cloud of gas and dust
– Collapses inward pulled by the force of its own
gravity
– As it collapses it begins to spin
– The center reaches a temperature of 15 million
kelvins after approximately 30 million years
– The onset of fusion marks the birth of a Star
The Life and Death of Stars
• Fusion reactions in the Sun’s core generate
energy that produces an outward pressure
that balances the inward force due to gravity
– Because of these balanced force, the sun will
maintain a stable size for 5 billion years
The Life and Death of Stars
• The Sun becomes red giant before it dies
• Fusion reactions decrease
• Pressure in the core drops and the core will
contract under the weight of its outer layers
causing the temperature in the core to
increase (red giant)
The Life and Death of Stars
• When the core of a red giant sun depletes most of
its helium, it will contract further, which will
cause the outer layers to expand again
– The temperature at the core is not high enough to fuse
heavier elements
– The outer layers will expand out from the core and will
eventually leave the star
– The remnants will become a white dwarf (small, dim,
and very dense star about the size of Earth)
The Life and Death of Stars
• Massive Stars evolve faster than smaller Stars
– The formation of an iron core signals the beginning of a supergiant
Star’s death because, unlike fusing lighter elements, fusing iron
atoms to make heavier elements requires adding energy rather than
releasing it
– When fusion requires more energy than it can produce, there is no
longer any outward pressure to balance the gravitational force
– The core collapses because of its own gravity and then rebounds
with a shock wave that violently blow’s the star’s outer layers
away from the core
– The resulting huge, bright explosion is called a supernova
The Life and Death of Stars
• Some supernovas form a neutron star or a
black hole
• Neutron Star: only a few dozen kilometers
in diameter but are very massive
– A teaspoon of matter would be about 100
million tons on Earth
– Pulsar: spinning neutron star
• Source of pulsating radio waves
The Life and Death of Stars
• If the leftover core has a mass that is greater than
3 solar masses, it will collapse to form a black
hole
– Consist of matter so massive and compressed that
nothing, no even light can escape its gravitational pull
– Because no light can escape a black hole, black holes
cannot be seen directly
• Detected indirectly by observing the radiation of light and X
rays from objects that revolve rapidly around them
The Milky way and Other Galaxies
• Galaxies: is a collection of millions to billions of
stars
– Stars bound together by gravity
• Just as Earth revolves around the sun because of
gravity, the solar system revolves around the
center of the galaxy because of gravity
– It takes our Solar System about 226 million years to
complete one orbit of our galaxy
The Milky way and Other Galaxies
• Galaxies are not spread out evenly through
space
– They are grouped together and bound by
gravity in clusters
• The Milky Way galaxy and the Andromeda galaxy
are two of the largest members of the Local Group
(a cluster of more than 30 galaxies
Our galaxy is a spiral galaxy, with arms consisting of billions
of stars, orbiting, encircling, and winding outward. These
arms have names – we are in the Orion Arm
The Milky way and Other Galaxies
• Types of Galaxies:
– Spiral: spiral arms that are made of gas, dust,
and stars
– Elliptical: have little gas or dust and have a
round shape
– Irregular: do not have a particular shape
The Milky way and Other Galaxies
• Our Solar System is in the spiral arm
(Orion) of the Milky Way Galaxy
– 26,000 light-years from the center
• Many astronomers believe that the center is
a black hole and the gas and dust in the
spiral arms provide materials that allow new
stars to form.
The Milky way and Other Galaxies
• Quasars may be infant galaxies
– quasi-stellar radio sources: or quasars
– We now know that all galaxies have
supermassive black holes at their centers; some
billions of times the mass of the Sun. When
material gets too close, it forms an accretion
disk around the black hole. It heats up to
millions of degrees, blasting out an enormous
amount of radiation.
Origin of the Universe
• The Universe is everything physical that
exists in space and time
– The Universe consist of all space, matter, and
energy that exists-now, in the past, or in the
future
– Between all the Planets and Stars is empty
space
• Space is a vacuum with no air and no pressure
Origin of the Universe
• Astronomers need large units of
measurement to express distances
– A light-year, which is approximately 9.5 x
1012km, is so long that driving a light-year in a
car moving at highway speeds would take more
than 10 million years
– A year is a unit of time
– A light-year is a unit of distance
Origin of the Universe
• It takes time for light to travel in space
– The farther an object is, the older the light that we get
from the object is
– When we say the Sun is 8 light-minutes away, we are
expressing not only its distance from Earth, but also
that we see the Sun as it was 8 minutes ago
– We never see in the present
• When we see objects, we see them as they were when they
were younger
Origin of the Universe
• Scientists who are interested in the early
history of our Universe use powerful
telescopes to study the most distant objects,
whose light was emitted billions of years
ago.
– By studying this ancient light and looking at
other evidence, scientists have been able to
theorize that the Universe formed during a
cataclysmic event known as the big bang
Origin of the Universe
• The Universe is expanding: can be
explained by the Doppler Effect
– When objects move away, the wavelengths are
stretched out (get longer)
• Move towards the red end of the spectrum
• Distant Galaxies have red shifts
– Moving away
• Expansion implies that the universe was smaller in
the past and is getting larger
– Like exploding out!
Origin of the Universe
• Big Bang Theory: states that the Universe
began with a gigantic explosion 13 billion
to 15 billion years ago
– Nothing existed before the Big Bang
• No time, no space
– The explosion released all of the matter and
energy that still exist in the Universe
Origin of the Universe
• According to the Big Bang Theory, immediately after the
big bang, the Universe was extremely hot and made up of
pure energy. There was a period of rapid expansion that
caused the energy to cool and allowed sub-atomic
particles, such as protons, electrons, and neutrons, to form.
Hydrogen nuclei began forming about one second after the
big bang, but the temperature was still too high for atoms
to form and remain stable. Electrons did not combine with
atomic nuclei to form atoms until about 380,000 years after
the Big Bang. The first Stars were born about 400 million
years after the Big Bang.
– The study of cosmic radiation has supported this theory
Origin of the Universe
• The future of the Universe is uncertain: the Universe is still
expanding, but it may not do so forever. The combined
gravity of all of the mass in the universe is also pulling the
universe inward, in the direction opposite to the expansion.
The competition between these two forces leaves three
possible outcomes for the universe
– 1.The universe will keep expanding forever.
– 2.The expansion of the universe will gradually slow down, and the
universe will approach a limit in size.
– 3.The universe will stop expanding and start to fall back on itself.
Origin of the Universe
• A contracting universe could collapse back
to a single point in what is called the Big
Crunch. As things come closer to each
other, galaxies and stars would collide. The
universe would become extremely hot and
very small. At this point, the universe may
end, or another Big Bang could start the
cycle again.
Origin of the Universe
• Dark Matter:
– There is more matter than is visible
– May consist of planets, black holes, brown
dwarfs (starlike objects that lack enough mass
to begin fusion and thus do not emit light)
– May consist of strange atomic particles that no
one knows how to observe
– Still a mystery