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 about 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 the material
universe (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 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 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
Astronomers must prove their case “beyond a reasonable
doubt”
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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 powersof-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
(empty space)
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 (= 5.9 x 1012 mi)
Powers of Ten movie
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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
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 astronomical unit
(AU)
Thus 1 AU = 150 million km
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, plus
other smaller objects
The Earth is one of the nine
planets
A planet is a body of
significant size that orbits a
star and does not produce its
own light
The Sun is a star
Its diameter is 1.5 million km
A star is large body which (at
some point during its life)
produces light by nuclear
reactions
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Solar System (2)
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
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 present-day 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 (believed
to be billions of years)
They die or collapse in spectacular ways
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Some 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
NOAO/AURA/NSF
Andromeda
NOAO/AURA/NSF
Local Group of Galaxies
M31, along with our Galaxy, 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
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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