AST201H1 Lecture Notes - Local Group, King Huiwen Of Qin, Veganism
DepartmentAstronomy & Astrophysics
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Chapter 1 ± Our Place in the Universe
Our Place in the Universe
x Earth is a planet in out solar system, which consists of the Sun and all the objects that
orbit it, which include the planets and their moons, chunky rocks of asteroids, icy balls
of comets, and particles of interplanetary dust.
x Our Sun is a star; the Milky Way Galaxy is a huge disk-shaped collection of stars
(containing more than 100 billion stars).
x A galaxy is a great island of stars in space, containing from a few hundred million to a
trillion or more stars.
x Our solar system is located a little over halfway from the galactic center to the edge of
the galactic disk.
x Local Group is groups of galaxies.
x SuperClusters are galaxies and galaxy clusters that are tightly packed.
x Our local group belongs to the local Supercluster.
x The universe is the sum total of all matter and energy, encompassing the superclusters
and voids and everything within them.
Our Cosmic Address:
The Big Bang and the Expanding Universe
x Telescopic observations of distant galaxies show that the entire universe is expanding
Æ average distances between galaxies are increasing with time.
x Estimation that the expansion started about 14 billion years ago Æ called the Big
x While the universe as a whole continues to expand, individual galaxies and their
contents do not expand.
x As the universe grows larger, the matter within it tends to clump into galaxies and
galaxy clusters, forming our own Milky Way a few billion years after the Big Bang.
Stellar Lives and Galactic Recycling
x Gravity drives the collapse of clouds of gas and dust to form stars and planets.
clouds of gas and dust and then shine for millions or billions of years.
x The energy that induced the shining is nuclear fusion LQWKHVWDU¶VFRUHÆ the
process where lightweight atomic nuclei smash together and stick/fuse to make
heavier nuclei. Finally, a star ³dies´ when it exhausts all its usable fuel for fusion.
x :KHQDVWDU³GLHV´LWH[SORGHVsupernovae), blowing out its content back into space.
The returned matter mixes with other matter floating between the stars in the galaxy,
eventually becoming part of new clouds of gas and dust from which new generations
of stars can be born.
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Stars Manufacture the Elements of Earth and Life
x The early universe contained only simple chemical elements: hydrogen and helium
(and a trace of lithium).
x We and Earth are primarily made of other elements: carbon, nitrogen, oxygen, and
x These other elements were manufactured by stars, either through nuclear fusion that
makes stars shine or through nuclear reactions accompanying the explosions that end
x The recycling of matter had already begun several billion years by the time our solar
system formed, about 4.6 billion years ago.
x The cloud that gave birth to our solar system was made of about 98% hydrogen and
helium and 2% of everything else.
x Most of the material from which we and our planet are made was created inside stars
that died before the birth of our Sun.
The Past Universe
x Speed of light is 300,000km/s (8 times around Earth and Earth from Moon in one
second and 8 minutes to reach Earth from the Sun).
x Light from Stars to Earth is measured in distances to the stars in units called light-
years (a measure of distance NOT time) Æ 1 light-year is the distance that light can
travel in 1 year (10 trillion km).
x Because light takes time to travel through space, *the farther we look in distance, the
further back we look in time.
o ie, Sirius is about 8 light-years away, which means it takes 8 years to reach us.
Thus, when we look at Sirius, we are seeing it as it was 8 years ago.
o ie, The Orion Nebula is a giant cloud located about 1,500 light-years from
Earth. Thus, we see the Orion Nebula as it looked about 1,500 years ago. Any
events since, we do not know because the light has not yet reached us.
o ie, the Great Galaxy in Andromeda lies about 2.5 million light-years from
Earth. A picture of it shows both space and time since its diameter is 100,000
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