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• Visited class web page?• Got and READ Syllabus?• Got Lecture-Tutorials?• Got Mastering Astronomy account and intro assignment?• Read the first chapter of Johnson?

A foot print on the moon

Check list

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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 GalaxyEnglish 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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A large, glowing ball of gas that generates heat and light through nuclear fusion

Star

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Planet

A moderately large object that orbits a star; it shines by reflected light. Planets may be rocky,

icy, or gaseous in composition.

Mars Neptune

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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) SystemA 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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GalaxyA 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).

• Thus, we see objects as they were in the past:The farther away we look in distance,

the further back we look in time.

Destination Light travel time

Moon 1 second

Sun 8 minutes

Sirius 8 years

Andromeda Galaxy 2.5 million years

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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?

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How far is a light-year?

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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.)(R.I.P.) = head of a pin, 1/3 mile away

On this scale, where is the nearest star? On this scale, where is the nearest star?

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• On this scale, the nearest stars would be a triple-star system formed by a cantaloupe, an apple and an orange, located a continent away.

• There is essentially nothing in between!!

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BIGBIGNUMBERSNUMBERS

LittleLittle

NUMBERSNUMBERS

kilo-mega- (aka million)

giga- (aka billion)tera- (aka trillion)

centi-

milli-

Scale of the Universe

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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, counter-clockwise 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

Universeexpands