ASTR100 (Spring 2008) Introduction to Astronomy Discovering the Universe

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ASTR100 (Spring 2008) Introduction to Astronomy Discovering the Universe. Prof. D.C. Richardson Sections 0101-0106. How can we know what the universe was like in the past?. Light travels at a finite speed ( c = 300,000 km/s). Thus, we see objects as they were in the past:

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ASTR100 (Spring 2008) Introduction to AstronomyDiscovering the Universe

Prof. D.C. Richardson

Sections 0101-0106

How can we know what the universe was like in the past?
• Light travels at a finite speed (c = 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.

Example:

This photo shows the Andromeda Galaxy as it looked about 2½ million years ago.

Question: When will be able to see what it looks like now?

M31, The Great Galaxy

in Andromeda

Definition: A light-year
• The distance light can travel in one year.
• About 10 trillion km (6 trillion miles).

Distance = Speed x Time

= (300,000 km/s) x (1 yr) x (31,557,600 s/yr)

= 9,500,000,000,000 km!

= 9.5 x 1012 km

Why can’t we see a galaxy 15 billion light-years away?(Assume the universe is 14 billion years old)
• Because no galaxies exist at such a great distance.
• Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.
• Because looking 15 billion light-years away means looking to a time before the universe existed.
Why can’t we see a galaxy 15 billion light-years away?(Assume the universe is 14 billion years old)
• Because no galaxies exist at such a great distance.
• Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.
• Because looking 15 billion light-years away means looking to a time before the universe existed.

Milky Way forms

Oldest known life (single-celled)

New Year’s Day: The Big Bang

Sun & planets form

First multi-cellular organisms

How do our lifetimes compare to the age of the universe?
• The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year.
How do our lifetimes compare to the age of the universe?
• The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year.

Spaceship Earth

How is Earth Moving inOur Solar System?

The Earth rotates around its axis once every day.

How is Earth Moving inOur Solar System?

The Earth orbits the Sun (revolves) once every year.

How is Earth Moving inOur Galaxy?

The Sun moves randomly relative to other nearby stars, and orbits the galaxy once every 230 million years.

More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy…dark matter!

Most of Milky Way’s light comes from disk and bulge …

…. but most of the mass is in its halo

How do Galaxies Move Within the Universe?

Galaxies are carried along with the expansion of the universe.

Part C

The following statements describe ways in which the analogy might apply to the real universe. Which statements are correct?

A. Both the raisin cake and the universe have a well-defined inside and outside.

B. Raisin 1 is near the center of the cake, just as our galaxy is near the center of the universe.

C. The temperature starts low and ends high in both the raisin cake and the universe.

D. The raisins stay roughly the same size as the cake expands, just as galaxies stay roughly the same size as the universe expands.

E. The average distance increases with time both between raisins in the cake and between galaxies in the universe.

F¡. An observer at any raisin sees more distant raisins moving away faster, just as an observer in any galaxy sees more distant galaxies moving away faster.

Enter the letters of all correct statements in alphabetical order (without spaces). For example, if statements C and E are correct, enter CE.

DEF

Correct

Like any scientific model, the raisin cake analogy has limitations, but it gives us a good overall picture of how the universe is expanding.

Are we ever sitting still? No!

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

Patterns in the Night Sky

What are constellations?
• A constellation is a region of the sky.

88 constellations fill the entire sky (North & South).

Thought Question

The brightest stars in a constellation…

• all belong to the same star cluster.
• all lie at about the same distance from Earth.
• may actually be quite far away from each other.
Thought Question

The brightest stars in a constellation…

• all belong to the same star cluster.
• all lie at about the same distance from Earth.
• may actually be quite far away from each other.
The Celestial Sphere

Stars at different distances all appear to lie on the celestial sphere.

The ecliptic is the Sun’s apparent path through the celestial sphere.

The Milky Way

A band of light making a circle around the celestial sphere.

What is it?

Our view into the plane of our galaxy.

How do we locate objects in the sky?

An object’saltitude (above horizon) and direction(along horizon) specify its location in your local sky.

Angular Measurements
• Full circle = 360º
• 1º = 60 (arcminutes)
• 1 = 60 (arcseconds)
Thought Question

The angular size of your finger at arm’s length is about 1. How many arcseconds is this?

• 60 arcseconds.
• 600 arcseconds.
• 60  60 = 3,600 arcseconds.
Thought Question

The angular size of your finger at arm’s length is about 1. How many arcseconds is this?

• 60 arcseconds.
• 600 arcseconds.
• 60  60 = 3,600 arcseconds.
Why do stars rise and set?

Earth rotates west to east, so stars appear to circle from east to west.

Celestial Sphere

Zenith: Point

directly

Horizon:

Where

the sky

meets the

ground

Celestial Sphere

North

Celestial Pole:

Point on

celestial sphere

above North

Pole

Celestial

Equator:

Line on

celestial

sphere

above Equator

Our view from Earth
• Stars near the north celestial pole are circumpolar and never set.
• We cannot see stars near the south celestial pole.
• All other stars (and Sun, Moon, planets) rise in east and set in west.

A circumpolar star never sets

Celestial equator

This star never rises