Earth Science 25.3 The Universe
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Earth Science 25.3 The Universe . The Universe . Earth Science 25.3 The Universe . On a clear and moonless night, away from city lights, you can see a marvelous sight; our own Milky Way Galaxy. Galaxies are large groups of stars, dust, and gases held together by gravity.

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Earth Science 25.3 The Universe

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Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Universe


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • On a clear and moonless night, away from city lights, you can see a marvelous sight; our own Milky Way Galaxy.

  • Galaxies are large groups of stars, dust, and gases held together by gravity.

  • There may be 400 billion stars in our Milky Way galaxy alone.

  • Our galaxy looks milky because the stars of the solar system are concentrated within a flat disk; the galactic disk.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Milky Way Galaxy:

  • Imagine hiking in a forest. You look around and see trees in every direction and it seems like you are in the center of the forest.

  • When astronomers began to survey stars located along the plane of the Milky Way, it seemed like a forest…..stars in every direction, and it seemed we were at the center of it all.

  • This is not the case however; Earth and our solar system are located on one of many arms of a spinning galaxy.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Size of the Milky Way:

  • It’s hard to study the Milky Way galaxy with optical telescopes because large quantities of interstellar matter block our vision.

  • With radio telescopes, scientists have partly determined the structure of the galaxy.

  • The Milky Way is a large spiral whose disk is about 100,000 light years wide and about 10,000 light years thick at it’s center.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Structure of the Milky Way:

  • Radio telescopes reveal that the Milky Way has at least three distinct spiral arms, with some signs of splintering.

  • Our sun is positioned in one of these arms about two thirds of the way from the center, or about 30,000 light years from the galactic nucleus.

  • The stars on these arms rotate around the galactic nucleus.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • The most outward arms move the slowest, and the ends of the arms appear to trail.

  • Our solar system makes one revolution around the galactic center about once every 230 million years.

  • Surrounding the galactic disk is a nearly round halo made of thin gas and numerous clusters of stars.

  • These star clusters do not rotate as the arms in the galaxy do.

  • Although some of the star clusters are dense, space is so large that the rotating arms pass through these star clusters with no chance of colliding.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Types of Galaxies:

  • In the mid 1700s, German philosopher Immanuel Kant proposed that fuzzy patches of light scattered among the stars were actually distant galaxies like our Milky Way.

  • Today we know that the universe includes hundreds of billions of galaxies, each containing hundreds of billions of stars.

  • From these hundreds of billions of galaxies, scientists have identified several basic types.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Spiral Galaxies:

  • Spiral galaxies are usually disk shaped, with a greater concentration of stars near their centers.

  • There are numerous variations of them however. Viewed broadside, the arms are often seen extending from the center nucleus and sweeping gracefully away.

  • The outermost arms of these stars rotate the slowest, giving the galaxy the appearance of a pinwheel.

Types of spiral galaxies


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • One type of spiral galaxy, however, has it’s arms arranged in the shape of a bar, which rotates as a rigid system.

  • This type is called a barred spiral galaxy.

  • Recent evidence indicates our Milky Way Galaxy may be a barred spiral galaxy.

  • Spiral galaxies are generally quite large.

  • About 10 percent of all galaxies are thought to be barred spirals and another 20 percent are spiral galaxies.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Elliptical Galaxies:

  • About 60 percent of galaxies are classified as elliptical galaxies.

  • Elliptical galaxies range in shape from round to oval.

  • Although most are small, the very largest known galaxies; 200,000 light years in diameter, are elliptical.

  • This type of galaxy does not have spiral arms.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Irregular Galaxies:

  • Only 10 percent of the known galaxies have irregular shapes and are classified as irregular galaxies.

  • The best known irregular galaxies, the large and small Magellanic Clouds, are easily visible from the Southern Hemisphere with the unaided eye.

  • These clouds are our nearest neighbors; only 150,000 light years away.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Irregular Galaxies:

  • In addition to shape and size, one of the major differences among different types of galaxies is the age of their stars.

  • Irregular galaxies are composed mostly of young stars, while elliptical galaxies contain old stars.

  • The Milky way and other spiral galaxies have both young and old stars, with the youngest stars located in the arms.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Galaxy Clusters:

  • Once astronomers discovered that stars were found in groups, they wondered whether galaxies also were grouped or just randomly distributed among the universe.

  • They found that, like stars, galaxies are grouped in clusters.

  • Some clusters may contain thousands of galaxies. Our own galaxy, called the Local Group, contains at least 28 galaxies.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Galaxy Clusters:

  • Of these 28 galaxies located in our cluster; 3 are spiral galaxies, 11 are irregular galaxies, and 14 are elliptical galaxies.

  • Galaxy clusters also make up huge groups called superclusters, which in turn make up vast threadlike structures called filaments.

  • Thesefilamentsare the largest known structures in the universe.

superclusters


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Quasars:

  • In the 1960s, astronomers discovered objects that were very bright and very far away.

  • They called them quasi-stellar objects, or quasars, since they looked like stars.

  • Because it takes their light billions of years to reach Earth, quasars must have existed when the universe was very young.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Quasars:

  • Quasars must emit huge amounts of radiation, or they would be too dim for us to detect.

  • The leading theory at present is that they are massive black holes in the centers of very young galaxies.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

25.3 Part B

The Expanding Universe


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Expanding Universe:

  • Recall the Doppler Effect from our previous chapter.

  • Remember…… when a source is moving away, it’s light appears redder than it actually is because it’s wavelengths appear lengthened.

  • Objects approaching have their light waves shifted toward the blue or shorter wavelengths.

  • Therefore, the Doppler Effect reveals whether a star is moving toward or away from Earth.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Expanding Universe:

  • The amount of light shift allows us to calculate the rate of movement by the object in relation to the Earth.

  • Large Doppler shifts indicate higher speeds, smaller shifts indicate lower speeds.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Red Shifts:

  • One of the most important discoveries of modern astronomy was made in 1929 by Edwin Hubble.

  • Observations completed years before revealed that most galaxies have Doppler shifts toward the red end of the spectrum.

  • This red shift occurs because light waves are “stretched”, which shows that Earth and the source are moving away from each other.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Red Shifts:

  • Hubble set out to explain this red shift. Hubble realized that dimmer galaxies were probably farther away than brighter galaxies.

  • He tried to determine whether a relationship existed between the distances to galaxies and their red shifts.

  • Hubble used estimated distances based on relative brightness and Doppler red shifts to discover that galaxies that exhibit the greatest red shifts are the most distant.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Hubble’s Law:

  • A consequence of the universal red shift is that it predicts that most galaxies; except for a few nearby, are moving away from us.

  • Recall that the amount of Doppler shift depends on the speed at which the object is moving away.

  • Because the more distant galaxies had greater red shifts, Hubble concluded that they must be moving away from us at greater speeds.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Hubble’s Law:

  • This idea is currently known as Hubble’s Law.

  • Hubble’s law states

    • Galaxies are retreating from us at a speed that is proportional to their distance.

  • Hubble was surprised at this discovery because it implied that the more distant galaxies are moving away from us many times faster than those galaxies near us.

  • This means that the red shifts of distant galaxies demonstrate that the universe is expanding around us.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Hubble’s Law:

  • To help visualize the nature of this expanding universe, imagine a loaf of raisin bread that has been set out to rise for a few hours.

  • As the dough doubles in size, so does the distance between all the raisins.

  • However, the raisins that were originally farther apart travelled a greater distance in the same time span than those located closer together.

Raisin Dough

Analogy


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Hubble’s Law:

  • We therefore conclude that in an expanding universe, as in the raisin dough analogy, those objects located farther apart move away from each other at a more rapid rate.

  • Another feature of the expanding universe can be demonstrated by this analogy.

  • No matter which raisin you select, it will move away from all the other raisins.

Raisin Dough

Analogy


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Hubble’s Law:

  • Likewise, no matter where one is located in the universe, every other galaxy (except those located in the same cluster) will be moving away from you.

  • Hubble had indeed advanced our knowledge of the universe with this theory.

  • Because of his great contributions to astronomy, the Hubble Space telescope is named in his honor.

Raisin Dough

Analogy


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Big Bang Theory:

  • How did the universe begin?

  • Any theory about the origin of the universe must account for Hubble’s Law; the fact that all distant galaxies are moving away from us.

  • Although all galaxies appear to be moving away from Earth, it seems unlikely that our planet is the center of the universe.

Raisin Dough

Analogy


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Big Bang Theory:

  • A more probable explanation exists.

  • Imagine a balloon with paper punch dots glued to it’s surface.

  • When the balloon is inflated, each dot spreads apart from every other dot.

  • Similarly, if the universe is expanding, every galaxy would be moving away from every other galaxy.

  • This concept of the expanding universe led to the widely accepted big bang theory.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Big Bang Theory:

  • According to the Big Bang Theory, the universe began as a violent explosion from which the universe continues to expand, evolve and cool.

  • The Big Bang Theory states that at one time, the entire universe was confined to a dense, hot, super-massive ball.

  • Then, about 13.7 billion years ago, a violent explosion occurred, hurling this matter in all directions. The Big Bang marks the beginning of the universe; all matter and space were created in this instant.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Big Bang Theory:

  • Recall that protons, neutrons, and electrons are some of the subatomic particles that make up all matter.

  • Scientists think that these particles formed within a few seconds from the energy of the big bang.

  • Scientists can model conditions in the early universe, using machines called particle accelerators.

  • These devices smash subatomic particles together at high speeds.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • These collisions of subatomic particles produce other particles that may exist for only a few billionths of a second.

  • But these experiments give scientists a brief glimpse of matter in the early universe.

  • After several hundred thousand years, the universe became cool enough for atoms to form.

  • Gases in the universe continue to cool and condense.

  • These gases eventually formed the stars and galaxies.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

Supporting Evidence:

  • Scientists have gathered substantial evidence that supports the big bang theory.

  • For example, the red shift of galaxies we have discussed already indicates that the universe is still expanding.

  • Scientists discovered a type of radiation called cosmic microwave background radiation.

  • This energy was detected as faint radio signals coming from every direction of space. Scientists think that this radiation was produced during the big bang.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Big Crunch:

  • If the universe began with the big bang, how will it end?

  • One view is that the universe will last forever.

  • In this scenario, the stars will slowly burn out, being replaced by an invisible form of matter and black holes that will ravel outward through an endless , dark, cold universe.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

The Big Crunch:

  • The other possibility is that the outward flight of galaxies will slow and eventually stop.

  • Gravitational construction would follow, causing the galaxies to collapse into the high-density, high-energy mass from which the universe began.

  • This scenario, the big bang operating in reverse, is called ”the big crunch”.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • Material called “dark matter” could affect the rate at which the universe expands.

  • Dark matter is matter that can not be directly observed because it does not give off radiation.

  • Galaxies contain most of the visible mass in the universe, but there is evidence that what we see makes up less than 1/10th of the universe’s mass.

  • Measurements of galactic rotation indicates that dark matter accounts for the rest of the universe’s mass.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • Whether the universe will expand forever or collapse upon itself depends on it’s average density.

  • If the average density of the universe is more than it’s critical density; about one atom for every cubic meter, the gravitational field is enough to stop the outward expansion and cause the universe to contract.

  • But, if the density of the universe is less than the critical value, it will expand forever.


Earth science 25 3 the universe

Earth Science 25.3 The Universe

  • Current estimates place the density of the universe below the critical density, which predicts an ever expanding, open universe.

  • However, the universe is expanding faster now than it was in the past.

  • This may be due to a theoretical new force that astronomers have named “dark energy”.

  • The view currently favored by scientists is that we live in an expanding universe with no ending point.


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