Transcript 1. Our Place in the Universe - Department of Physics & Astronomy

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Our Place in the Universe
“Space is big. Really big. You just won't believe how
vastly, hugely, mind-bogglingly big it is. I mean, you may
think it's a long way down the road to the chemist's, but
that's just peanuts to space.”
Douglas Adams, The Hitchhiker's Guide to the Galaxy
English humorist & science fiction novelist (1952 - 2001)
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Our Place in the Universe
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What is our place in the universe?
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Star
A large, glowing ball of gas that generates heat
and light through nuclear fusion
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Planet
Mars
Neptune
A moderately large object that orbits a star; it
shines by reflected light. Planets may be rocky,
icy, or gaseous in composition.
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Moon (or Satellite)
An object that orbits
a planet
Ganymede (orbits Jupiter)
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Asteroid
A relatively small and rocky object that
orbits a star
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Comet
A relatively
small and icy
object that
orbits a star
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Pluto
A relatively small
yellow dog from
Disney. “If Pluto
is a dog, then
what’s Goofy?”
Continuing…
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Solar (Star) System
A star and all the material that orbits it, including
its planets and moons
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Nebula
An interstellar
cloud of gas
and/or dust
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Galaxy
A great island of stars in space, all held together
by gravity and orbiting a common center
M31, the great galaxy
in Andromeda
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Universe
The sum total of all matter and energy;
that is, everything within and between all
galaxies
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How did we come to be?
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How can we know what the universe was
like in the past?
• Light travels at a finite speed (300,000 km/s).
Destination
Light travel time
Moon
1 second
Sun
8 minutes
Sirius
8 years
Andromeda Galaxy
2.5 million years
• Thus, we see objects as they were in the past:
The farther away we look in distance,
the further back we look in time.
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Example:
We see the
Orion Nebula as
it looked 1500
years ago.
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Example:
This photo shows the Andromeda Galaxy as it looked
about 2 1/2 million years ago.
Question: When will
we be able to see what
it looks like now?
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Light-year
• The distance light can travel in 1 year
• About 10 trillion kilometers (6 trillion
miles)
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How far is a light-year?
1 light-year = (speed of light) ´ (1 year)
km ö æ 365 days 24 hr 60 min 60 s ö
æ
= ç 300,000
´
´
´
÷´ç
÷
s
1
yr
1
day
1
hr
1
min
è
ø è
ø
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How far is a light-year?
1 light-year = (speed of light) ´ (1 year)
km ö æ 365 days 24 hr 60 min 60 s ö
æ
= ç 300,000
´
´
´
÷´ç
÷
s
1
yr
1
day
1
hr
1
min
è
ø è
ø
=9,460,000,000,000 km
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Can we see the entire universe?
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Navigating the Universe:
Sizes and Scales
“I don’t pretend to understand the Universe. It’s a great deal
bigger than I am”
- Thomas Carlyle (1795-1881)
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Scale models of the Universe
• Scale Sun as a grapefruit (1:10,000,000,000)
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Where are the planets?
• Mercury = grain of sand, 6 meters
away (19 feet)
• Venus = ball point pen tip, 11
meters (36 feet) away
• Earth = tip of ball point pen, 15
meters (49 feet) away
– Moon = 4 cm away from Earth
• Mars = large grain of sand, 23
meters (75 feet) away
• Jupiter = marble, 78 meters (256
feet!)
• Pluto (R.I.P.) = head of a pin, 1/3
mile away
On this scale, where is the nearest star?
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• On this scale, the nearest
stars would be a triplestar system formed by a
cantaloupe, an apple and
an orange, located a
continent away.
• There is essentially
nothing in between!!
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BIG
NUMBERS
kilo-
Scale of the
Universe
mega- (aka million)
giga- (aka billion)
tera- (aka trillion)
Little
NUMBERS
centimilli27
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How is Earth moving in our solar system?
• Contrary to our perception, we are not “sitting still.”
• We are moving with the Earth in several ways, and at
surprisingly fast speeds…
The Earth rotates
around its axis once
every day.
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Earth orbits the Sun (revolves) once every year:
• at an average distance of 1 AU ≈ 150 million km.
• with Earth’s axis tilted by 23.5º (pointing to Polaris)
• and rotating in the same direction it orbits, counterclockwise as viewed from above the North Pole.
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Our Sun moves randomly relative to the other stars
in the local Solar neighborhood…
• typical relative speeds of more than 70,000 km/hr
• but stars are so far away that we cannot easily notice
their motion
… And orbits the galaxy every 230 million years.
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How do galaxies move within the universe?
Galaxies are carried along with the expansion of the Universe.
But how did Hubble figure out that the universe is expanding?
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Hubble discovered that:
• All galaxies outside our Local Group are
moving away from us.
• The more distant the galaxy, the faster it is
racing away.
Conclusion: We live in an expanding universe.
We will revisit this topic again at the end of the course.
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Are we ever sitting still?
Earth rotates on axis: > 1,000 km/hr
Earth orbits Sun: > 100,000 km/hr
Solar system moves among stars: ~ 70,000 km/hr
Milky Way rotates: ~ 800,000 km/hr
Milky Way moves
in Local Group
Universe
expands
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