Chapter 8

Astronomy

LESSON 5 GALAXIES AND BEYOND

OBJECTIVES • Classify galaxies according to their

properties.

• Explain the big bang and the way in which Earth and its atmosphere were formed.

MAIN IDEA The Milky Way is one of billions of galaxies that are moving away from each other in an expanding universe.

VOCABULARY • galaxy

• Milky Way

• spectrum

• expansion redshift

• big bang

• background radiation

WHAT ARE GALAXIES? • A galaxy is a group of star clusters held

together by gravity.

• Stars move around the center of their galaxy in the same way that planets orbit a star.

• Galaxies differ in size, age, and structure.

• Astronomers place them in three main groups based on shape.

• spiral • elliptical • irregular

• Spiral galaxies look like whirlpools.

• Some are barred spiral galaxies.

• Have bars of stars, gas, and dust throughout its center.

• Spiral arms emerge from this bar.

SPIRAL GALAXY

• An elliptical galaxy is shaped a bit like a football.

• It has no spiral arms and little or no dust.

ELLIPTICAL GALAXY

IRREGULAR GALAXY • Has no recognizable shape.

• The amount of dust varies.

• Collisions with other galaxies may have caused the irregular shape.

THE MILKY WAY GALAXY • During the summer at night, overhead, you

will see a broad band of light stretching across the sky.

• You are looking at part of the Milky Way.

• The Milky Way is our home galaxy.

• The Milky Way is a spiral galaxy.

• The stars are grouped in a bulge around a core.

• All of the stars in the Milky Way including our sun orbit this core.

• The closer a star is to the core the faster its orbit.

• Several spiral arms extend from the core.

• Our solar system is located on one of these spiral arms.

• We can not see the center of the Milky Way.

• However, we can see more stars in the direction of the core.

• To find the center of the galaxy, look in the direction of the constellation Sagittarius, the archer.

QUICK CHECK • Fact and Opinion

• “The Milky Way is a spiral galaxy.”

• Is this statement a fact or an opinion? Why?

• The Milky Way is in fact a spiral galaxy. It is based on observations; it contains dust, and spiral arms.

• Critical Thinking

• How are the three types of galaxies similar? • How are they different?

• All contain stars that move around a core.

• A spiral galaxy has a whirlpool type shape with arms and lots of dust.

• An elliptical galaxy has a globe or football shape and little or no dust.

• An irregular galaxy has no recognizable shape and the amounts of dust and gas vary.

WHAT WAS THE BIG BANG?

• When light passes through water droplets it turns into a band of colors.

• White light is typically all the colors of the rainbow and it’s called a spectrum.

• The heated gases of stars produce light.

• As light passes through a star’s outer atmosphere, some of the light is absorbed by the stars atmosphere.

• When scientists look at a spectrum of this starlight, they see that the absorbed light has “dropped out” of the spectrum, forming dark lines called absorption lines.

• When we look at a spectrum from a galaxy, the absorption line patterns don’t appear at the same point in the spectrum as they were formed here on Earth.

• The position of the pattern shifted. • This is because the galaxies are all

moving away from each other as the space between them expands.

• If the absorption lines of a spectrum shift to the blue end, a blueshift occurs.

• This means the galaxy is moving toward us.

• If the line shifts to the red end, a redshift occurs.

• Most lines are redshifted.

• Space expands. • Lines show an expansion redshift. • There is no center to this expansion.

• Observers can consider themselves at the

center and would see the galaxies moving away.

• Astronomers think the galaxies must have been closer to each other in the past.

• The early universe was very dense and its temperature was high.

• At the beginning moment, the universe was extremely tiny, hot, and dense.

• From this it grew rapidly. • This expansion, called the big bang, sent matter

out in all directions.

• According to the big bang theory, the universe is expanding and its density and temperature are decreasing.

• Gravity has caused matter to collect into clumps forming stars and galaxies.

• The galaxies continue to move outward.

• Evidence for the big bang theory comes from background radiation, radiation that is left over from the beginning moments of the universe and is coming from all directions in space.

FORMATION OF THE SOLAR SYSTEM

• Scientists believe that the big bang created our universe.

• This formed protostars and protoplanets.

QUICK CHECK • Fact or Opinion

• “Scientists theorize that the universe was very hot and dense in its first moments.”

• Is this a fact or an opinion?

• Scientists theorize that the universe was very hot and dense is a fact; the idea itself is a theory, not a fact.

• Critical Thinking • What do astronomers think caused the

background radiation found in space?

• The background radiation was caused by the events that occurred at the beginning of the universe.

HOW DID EARTH FORM • Scientists think that Earth is about 4.6 billion

years old.

• Astronomers think that Earth and its atmosphere developed in a series of stages.

• The process began in the nebula that formed the Sun.

• Dust and ice occasionally collided and stuck together.

• Clumps of particles grew until they became the young Earth or proto-Earth.

• The protoplanet’s larger mass and gravity attracted smaller bodies to it.

• Collisions increased.

• Over time, proto-Earth became large enough that its gravity could hold an atmosphere.

• Earth’s original atmosphere was mostly hydrogen and helium.

• The heat of the molten planet and impacts of space objects blew away much of that atmosphere

• leaving water vapor, sulfur, carbon dioxide, and nitrogen released by volcanic eruptions.

• Atmospheric oxygen developed as a waste product of photosynthesis.

QUICK CHECK • Fact and Opinion

• “Plants did not exist on Earth in its first years, because the atmosphere lacked oxygen.”

• Is this statement a fact or an opinion? Why?

• Fact. Oxygen was not added to Earth’s atmosphere until plants began producing it through photosynthesis.

• Critical Thinking • How did Earth’s original atmosphere evolve into

its present one?

• The original hydrogen and helium atmosphere was lost and replaced by volcanic gases of water vapor, carbon dioxide, and nitrogen. Then, oxygen was added after plants appeared and produced the oxygen as a waste product of photosynthesis.