Transcript Science and the Universe

Science and the Universe
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Nature of Astronomy
• The study of the objects that lie beyond
the atmosphere of our planet Earth
• The study of the processes by which
these objects interact with one another
• Humanity’s attempt to organize the
knowledge our universe into a clear
history of the universe from the instant
of its birth to the present time and
beyond
• And much more…
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Nature of Science
• Science is a method (or process) by which
one attempts to understand nature and
how it behaves
• The ultimate judge in science is the
experiment or observation: what nature
itself reveals
• Science accepts nothing on faith
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Scientific Method Involves…
• Many (repeated) observations over
a period of time
• Analyzing the trends and patterns
• The formulation of models of the
particular phenomena
• Testing the models with further
experiments and observations
• Iteration of the above process
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Concrete Example of the Scientific Process
• Ancient astronomers built a geocentric model
of the universe
• Better observations required the model to add
circles after circles to the movements of the
planets to keep the Earth at the center
• The geocentric model eventually could no
longer explain all the observed facts and was
abandoned in favor of a heliocentric model,
which fit the experimental evidence better
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Some Remarks about the Scientific Process
• New models/ideas are called hypotheses
• Contrary to common beliefs, much is yet to be
learned in astronomy (and physics in general)
• New phenomena are observed constantly, and
new hypotheses needed to explain these
• Some observational facts are very well
understood, but others remain “mysterious”
• Typically, new ideas are difficult to test either
because the amount of data is small, or
because they require observations that are
difficult based on current technology
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Some Remarks about Astronomy
• Astronomy is an observational science
– It’s impossible for an astronomer to experiment
with planets or stars in a laboratory on Earth
• Much of astronomy is also a historical science
– The events being observed have already happened
– This is similar to a detective trying to solve a crime
that occurred before the detective ever got to the
scene
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Laws of Nature
• Over the centuries, from experiments and
observations, scientists have extracted certain
fundamental principles, called scientific laws
• Laws are the same everywhere in the universe
• Astronomy would be difficult or even impossible
without such laws
• Laws (models and theories) are constantly
refined by scientists to follow new, and typically
more comprehensive, observations of nature
• Describing the laws of nature requires a precise
language: mathematical models and equations
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Numbers in Astronomy
• Distances in astronomy are huge
• It is convenient to use the power-of-ten
notation, sometimes called scientific
notation
• Examples
– 5,200,000 = 5.2 millions = 5.2 x 106
– 0.000125 = 1.25 x 10-4
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Light Years
• Astronomers use a convenient unit called
“light-year” to describe distances in the
universe
• One light-year, abbreviated as 1 LY, equals
the distance that light travels in one year
• This unit is useful because light (of any color)
– always travels at the same speed in vacuum
– has the largest speed at which anything can
travel
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Speed of Light (in Vacuum)
• Light travels at the amazing speed
of 300,000 kilometers per second
(1 kilometer = 0.6 mile) or
– 300,000 km/s = 3 x 105 km/s
– 3 x 108 m/s
– 1,080,000 km/hour
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One Light-Year (1 LY)
• 60 seconds in 1 minute
– 3 x 105 km/s x 60 s/min = 1.80 x 107 km/min
• 60 minutes in 1 hour
– 1.80 x 107 km/min x 60 min/hour = 1.08 x 109 km/hour
• 24 hours in a day
– 1.08 x 109 km/hour x 24 hour/day = 2.592 x 1010 km/day
• 365.24 days in a year
– 2.592 x 1014 km/day x 365.24 day/year = 9.46 x 1012
km/year
• 1 LY = 9.46 x 1012 km
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How We Learn about Distant Objects
• Information about distant objects comes
to us almost exclusively via various
forms of light
• All such light travels at the speed of light
(namely, 1 LY every year)
• This leads to two consequences …
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Consequence 1
• The fact that light takes time to travel
sets a limit on how quickly we can learn
about events in the universe
– If a star is 10 LY away, the light we
see from it tonight left that star 10
years ago
– If that star exploded today, we would
learn about it only 10 years from now
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Consequence 2
• Since light takes time to travel, an
observation of distant objects in the sky
is actually an observation of what
happened in the past
– The further away the objects we look
at, the more ancient they are
– This is a potential benefit in disguise,
providing a way to study the history
of the Universe
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An Imaginary Journey from Home
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Earth
Planets
Solar System
Star Clusters
Galaxy
Galaxy Clusters
Galaxy Super-clusters
The Universe!
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Planet Earth
• A nearly spherical planet
about 13,000 km in
diameter
• Distinguished by the
large amount of liquid
water it has, which
covers 2/3 of its surface
• Supports “intelligent life”
• Has one natural satellite:
the Moon
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The Moon
• The Moon’s diameter
is about 3476 km
• The Moon-to-Earth
distance is about 30
times the Earth’s
diameter
• Light travels from
the Moon to the
Earth in about 1.3
seconds
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Photo by John French, Abrams Planetarium
Michigan State University
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More about the Earth
• The Earth revolves around the Sun
– The period of revolution is 3 x 107 s (1 year)
• The average Earth-to-Sun distance is about 150
million km
– This defines a unit of distance called
1 astronomical unit (AU)
• The Earth travels at about 110,000 km/hour in
its orbit around the Sun
• Light takes 8 minutes to travel from the Sun to
the Earth
– Our latest news from the Sun is always 8 minutes old!
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Solar System (1)
• The solar system consists of the
Sun at its center and nine planets
orbiting the Sun
– The Earth is one of the nine
planets
• The Sun is a star
– Its diameter is 1.5 million km
• A planet is a body of significant
size that orbits a star and does not
produce its own light
• A star is large body which (at some
point during its life) produces light
by nuclear reactions
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Solar System (2)
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• The other 8 planets are
visible from Earth because
of the (Sun’s) light they
reflect towards us (albedo)
• The 9 planets are located at
enormous distances from
the Sun as illustrated here
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Jupiter
• Being the largest planet in the solar system,
Jupiter has a diameter of 143,000 km
– 11 times Earth’s size
• Its distance to the Sun is
about 5 AU
• Like many other planets, it
has a number of natural
satellites
or moons
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The Milky Way (1)
• The Sun is our local star
• Other stars are also suns: typically enormous balls
of glowing gas that generate vast amounts of
energy and light via nuclear reactions
• Other stars look small and faint
compared to our sun because they
are extremely distant from us
• The nearest star beyond the Sun is
Proxima Centauri, which is 4.3 LY
away
• The stars we see at night are part of
a gigantic complex of stars called
the Milky Way
• The Milky Way is a Galaxy
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The Milky Way (2)
• The Sun is about 30,000 LY from the center of
our Galaxy
• From our location within the Galaxy, we cannot
see through its far rim because the space
between stars is not empty, but contains (an
extremely sparse distribution of) interstellar dust
or gas which absorbs visible light
• The interstellar gas and dust are believed to be
the raw material for future formation of stars
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The Milky Way (3)
• There is indirect evidence that much of our Galaxy is
made of material that cannot be observed with presentday instruments
– Thus, much of our Galaxy seems to consist of dark
matter
• Stars in a number of places form clusters, each
containing up to hundreds of thousands of stars and
taking up a volume of space hundreds of light years
across
• Stars have a long, but not infinite, life (billions of years)
– They die or collapse in spectacular ways
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Nearby Objects
• Our Galaxy, the Milky Way, is not alone
• Many other galaxies have been discovered and studied, and
most are at enormous distances from ours
– So many galaxies have been found that it would be foolish to give
them all proper names -- they are usually given a catalog number
• The Magellanic Clouds are two of the small galaxies nearest
to ours
• The nearest large galaxy is
Andromeda, or M31
The two Magellanic Clouds
Andromeda
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Local Group of Galaxies
• M31 is part of a small cluster of over 40 galaxies which is
called the Local Group
• At distances of about 10 to 15 million LY, one finds other
small galaxy groups or clusters
• At about 50 million LY, one finds
a very large cluster containing
thousands of galaxies — called
the Virgo Cluster
• Some of the observed clusters
appear to form superclusters
• Our Local Group and the Virgo
Cluster are part of one such
supercluster which stretches
over a diameter of 60 million LY
Center of Virgo Cluster
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And beyond…
• At even greater distances, galaxies are too dim to see,
and one finds spectacular objects called quasars
• Quasars are brilliant centers of galaxies, glowing with
enormous intensities
– One theory suggests that a giant black hole is
swallowing whole neighborhoods of raw material to
produce the observed huge and intense emissions
• Quasars allow us to probe the universe 10 billion or
more LY away and, thus, 10 billion years in the past!
– This may bring us closer to the earliest times of the
universe, and even its beginning!
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