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Astronomy I

The Big Bang & Galaxies

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Overview

1. Cosmology

2. Big Bang Theory

3. The Types of Galaxies

4. The Milky Way Galaxy

5. The Properties of Stars

6. The Lives of Stars

7. Formation of our Solar System

8. The Members of our Solar System

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Cosmology

Cosmology is the study of theoverall structure of the universe.And just what is the Universe?Quite simply, it is everything thatexists. However, from Earth wecannot observe everything in theUniverse. Some things are dark(brown dwarf stars, planets, andDark Matter) and we cannot see

them.Additionally there are parts of the universe whose lighthas not yet reached us in this part of the Universe. Theobservable universe is the Universe that reveals itselfthrough electromagnetic radiation that can be detectedon Earth.

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Cosmology (cont)

This is simply because of thefinite speed of light. At short

distances, like from satellites inorbit of Earth, the light traveltime is only a fraction of asecond. However, the Sun isso distant from Earth

(150,000,000 Kilometers) thatits light takes 8 minutes toreach us. So when you look atthe sun in the sky (never lookat it directly, you'll go blind) yousee it as it was 8 minutes ago.

Because that electromagnetic radiation travels at afinite speed we actually look back in time when we look

into the cosmos.

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The Big Bang

Big Bang theory isa theory of the originof the universe.

According to thistheory, the entireuniverse was at onetime confined in a

dense, hot,supermassiveconcentration

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The Big Bang (cont)

About 15 billion yearsago, a cataclysmic

explosion hurled this

material in all directions,

creating all matter and

space.

Eventually the ejected

masses of gas cooled andcondensed, forming the

stellar systems we now

observe fleeing from their

place of origin.

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Evidence for the Big Bang Theory

Accounts for

galaxies moving

away from us

Cosmic

Background

Radiation

(remains ofradiation

produced by the

Big Bang)

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Cosmic Timeline

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Galaxies

Galaxies are collections ofmillions or billions of stars

bound together by gravity.

Galaxies are not randomly

distributed throughout theuniverse. They are

grouped in galactic

clusters, some containing

thousands of galaxies.Our own, called the Local

Group, contains at least

28 galaxies .

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Types of Galaxy

There are three

general types of

galaxy

Spiral galaxies

Elliptical galaxies

Irregular galaxies

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Spiral galaxies

Typically disk-shaped

Have a somewhat

greater concentration ofstars near their centers

Often contain arms of

stars extending from

their central nucleus Youngest stars in the

arms, oldest in the

center

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Elliptical galaxies

The most abundant

type

Have an ellipsoidalshape that ranges to

nearly spherical

Lack spiral arms

Generally made ofolder stars

Very largest and very

smallest galaxies

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Irregular galaxies

Lack symmetry

Account for only 10

percent of theknown galaxies

Contain mostly

young stars

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The Milky Way Galaxy

A large, disk-shaped,spiral galaxy about100,000 light-years wideand about 10,000 light-

years thick at the center.There are three distinctspiral arms of stars,with some showing

splintering. Sun rotates around the

galactic nucleus onceabout every 200 million

years

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The Milky Way Galaxy (cont)

Our Sun(Sol) ispositionedin one of

these armsabout two-thirds of theway from

the galacticcenter, at adistance ofabout30,000light-years