ASTR100 (Spring 2008) Introduction to Astronomy Discovering the Universe Prof. D.C. Richardson Sections 0101-0106.

ASTR100 (Spring 2008) Introduction to AstronomyDiscovering the Universe

Prof. D.C. RichardsonSections 0101-0106

How can we know what the universe was like in the past?

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

Light travels at a finite speed (c = 300,000 km/s).

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

• At great distances, we see objects as they were

when the universe was much younger.

• Can we see the entire universe?

A. Because no galaxies exist at such a great distance.

B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.

C. 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)

A. Because no galaxies exist at such a great distance.

B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.

C. 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)

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.

New Year’s Day: The Big Bang

Milky Way forms

Sun & planets form

Oldest known life (single-celled)

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.

Spaceship Earth

How is Earth Moving inOur Solar System?

The Earth rotates around its axis once every day.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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 CThe 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.DEFCorrect

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

Universeexpands

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…

A. all belong to the same star cluster.

B. all lie at about the same distance from Earth.

C. 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 Celestial Sphere

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

What is it?Our view into the plane of our galaxy.

The Milky Way

The Milky Way

How do we locate objects in the sky?

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

We measure the sky in angles...

blank

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.A. 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?

A. 60 arcseconds.B. 600 arcseconds.C. 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.

What moves? The Earth or the sky?

Celestial Sphere

Zenith: Point directly overhead

Horizon: Where the sky meets the ground

Celestial SphereNorth 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.

Celestial equator

Your horizon

A circumpolar star never sets

This star never rises

Ended Here 1/31/08

(lots of questions during lecture, plus organizational

stuff)