Astronomy Ch 20 The Astronomy Ch 20 The UniverseUniverse What is the universe?What is the universe?

Sum of all space, matter, and energy Sum of all space, matter, and energy

past, present, futurepast, present, future http://hubblesite.org/explore_astro

nomy/hubbles_universe_unfiltered/

Where do we live in the universe:Where do we live in the universe:

Our Solar System, Milky Way Galaxy, Our Solar System, Milky Way Galaxy, Local Group Cluster, Virgo Super Local Group Cluster, Virgo Super ClusterCluster

Astronomy Ch 20 The Astronomy Ch 20 The UniverseUniverse Section 1:Section 1: The Life and Death of The Life and Death of

StarsStars

What Are Stars?What Are Stars? Stars are huge Stars are huge spheres of very hot gas that emit light spheres of very hot gas that emit light and other radiation. Stars are formed and other radiation. Stars are formed from clouds of dust and gas, or from clouds of dust and gas, or nebulas, and go through different nebulas, and go through different stages as they age. (p. 693)stages as they age. (p. 693)

Stars have influenced cultures for Stars have influenced cultures for thousands of years by helping thousands of years by helping people mark the passage of time people mark the passage of time and by providing markers for and by providing markers for navigation in the night sky.navigation in the night sky.

Stars are born in a nebula (left). Stars are born in a nebula (left). After billions of years, most stars After billions of years, most stars become old and lose their outer become old and lose their outer layers of gas. As a star begins to layers of gas. As a star begins to die, it may become a planetary die, it may become a planetary nebula (right).nebula (right).

light-yearlight-year

Stars are located at various Stars are located at various distances from Earth. We use the distances from Earth. We use the light-year (ly) to describe the light-year (ly) to describe the distance from Earth to far-away distance from Earth to far-away objects such as stars and galaxies. A objects such as stars and galaxies. A light-yearlight-year is the distance that light is the distance that light travels in one year, or about 9.5 × travels in one year, or about 9.5 × 1012 km.1012 km.

Stars are powered by Stars are powered by nuclear fusion reactionsnuclear fusion reactions .. A star is a huge sphere of very hot A star is a huge sphere of very hot

hydrogen and helium gas that emits hydrogen and helium gas that emits light. A star is held together by the light. A star is held together by the enormous gravitational forces that enormous gravitational forces that result from its own mass. Inside the core result from its own mass. Inside the core of a star, the pressure is more than a of a star, the pressure is more than a billion times the atmospheric pressure billion times the atmospheric pressure on Earth. The temperature in this on Earth. The temperature in this incredibly dense core is hotter than 15 incredibly dense core is hotter than 15 million kelvins.million kelvins.

Energy from a nuclear fusion reaction in the core Energy from a nuclear fusion reaction in the core may take tens of thousands of years to reach a may take tens of thousands of years to reach a star’s surface. When the energy reaches the star’s surface. When the energy reaches the surface, the energy is released into space as surface, the energy is released into space as

electromagnetic radiationelectromagnetic radiation

Studying StarsStudying Stars

The telescope allowedThe telescope allowed astronomers to astronomers to study stars in more detail for the first study stars in more detail for the first time.time.

Greeks noticed that stars had color and divided Greeks noticed that stars had color and divided stars by their stars by their apparent brightness brightness

Astronomers did not learn about the nature of Astronomers did not learn about the nature of stars until the optical telescope was invented. stars until the optical telescope was invented. 

As science and technology have improved, As science and technology have improved, telescopes have become more powerful and telescopes have become more powerful and have thus allowed us to see more. have thus allowed us to see more. 

Some stars appear brighter than others.Some stars appear brighter than others. The brightness of a star depends on the star’s The brightness of a star depends on the star’s

temperature, size, and distance from Earth. temperature, size, and distance from Earth. The brightest star in the night sky is Sirius in The brightest star in the night sky is Sirius in

the constellation Canis Major. Sirius appears the constellation Canis Major. Sirius appears so bright because it is close to Earth, only so bright because it is close to Earth, only about 8.7 light-years away. The surface about 8.7 light-years away. The surface temperature of Sirius is about 10,000 K. temperature of Sirius is about 10,000 K.

The sun’s surface is only 6,000 K, but the sun The sun’s surface is only 6,000 K, but the sun is so close to Earth that it prevents us from is so close to Earth that it prevents us from seeing other stars during the day.seeing other stars during the day.

We learn about stars by studying energy.Stars produce various types of electromagnetic radiation: from visible light to X rays to radio waves.

Scientists use optical telescopes to study visible light and radio telescopes to study radio waves emitted from astronomical objects.

Earth’s atmosphere blocks other wavelengths, so telescopes in space are used to study a wider range of the electromagnetic spectrum than telescopes on the ground can detect.

 

Section 1:Section 1: The Life and The Life and Death of StarsDeath of Stars

The Life Cycle of StarsThe Life Cycle of Stars In a way In a way that is similar to other natural that is similar to other natural cycles, stars are born, go through cycles, stars are born, go through stages of development, and stages of development, and eventually die. (p. 698)eventually die. (p. 698)

This graph shows the This graph shows the intensity of light at different intensity of light at different wavelengths for the sun and wavelengths for the sun and for two other stars. for two other stars.

The color spectra of most stars contain dark lines. Dark lines are caused by gases in the outer layers of the stars that absorb the light at these wavelengths.

Astronomers match the dark lines in starlight to the known lines of elements found on Earth to determine what elements make up a star.. 

Stars change in many ways over their life cycle. Small and medium stars are born in giant nebulas of dust and gas and often end up as white dwarfs billions of years later. The images in this life cycle are not shown to scale.

Massive starsMassive stars supernova -If the core that remains after such a supernova supernova -If the core that remains after such a supernova

has occurred has enough mass, the remnant can become a has occurred has enough mass, the remnant can become a

neutron starneutron star. Neutron stars are small in diameter, but they . Neutron stars are small in diameter, but they are very massive. Just a teaspoon of matter from a neutron are very massive. Just a teaspoon of matter from a neutron star would weigh more than 100 million tons on Earth. star would weigh more than 100 million tons on Earth. Neutron stars can be detected asNeutron stars can be detected as

pulsarspulsars, or spinning neutron stars that pulsate radio , or spinning neutron stars that pulsate radio waves.If the leftover core is great enough, it will collapse to waves.If the leftover core is great enough, it will collapse to form something else—aform something else—a

black hole,black hole, which consists of matter so massive and which consists of matter so massive and compressed that nothing, not even light, can escape its compressed that nothing, not even light, can escape its gravitational pull. Because no light can escape a black hole, gravitational pull. Because no light can escape a black hole, black holes cannot be seen directly. Black holes can, black holes cannot be seen directly. Black holes can, however, be detected indirectly by observing the radiation however, be detected indirectly by observing the radiation of light and X rays from objects that revolve rapidly around of light and X rays from objects that revolve rapidly around them.them.

Section 2: The Milky Way and Other Galaxies Galaxies While the nearest stars are a few light-years away

from Earth, the nearest galaxy to our own is millions of light-years from Earth.   A galaxy is a collection of millions to billions of stars. The deeper scientists look into space, the more galaxies they find. There may be more than 100 billion galaxies. If you counted 1,000 galaxies per night, it would take 275,000 years to count all of them. 

Gravity holds galaxies together. Without gravity, everything in space might be a

veil of gas spread out through space. But because of gravity, clouds of gas come together and collapse to form stars. As the first stars in a galaxy age, they throw off gas and dust or become supernovas. New stars then form. The gas, dust, and stars form galaxies because of gravity.

Just as Earth revolves around the sun because of gravity, the solar system revolves around the center of the galaxy because of gravity. It takes our solar system about 226 million years to complete one orbit of our galaxy.

Galaxies are often found together in clusters. Galaxies are not spread out evenly through space. Galaxies

are grouped together and bound by gravity in clusters like the cluster shown in Figure 3. The Milky Way galaxy and the Andromeda galaxy are two of the largest members of the Local Group, a cluster of more than 30 galaxies. New members of the Local Group, are being discovered as more telescopes, such as the Hubble Space Telescope that is shown in Figure 2, become available.

Clusters of galaxies can form even larger groups called superclusters. A typical supercluster contains thousands of galaxies that contain trillions of stars in individual clusters. Superclusters can be as large as 100 million light-years across. They are the largest known structures in the universe.

Section 2: The Milky Way and Other Galaxies Types of Galaxies Galaxies can be divided into three major types: spiral, elliptical, and

irregular.   The three types of galaxies have many stars, but differ in structure.

We live in the Milky Way galaxy.

The Milky Way is a spiral galaxy. Our galaxy is a huge spiraling disk of stars,

gas, and dust. This gas and dust is called interstellar matter

The Milky Way has a huge bulge in the center that contains primarily old red stars

The spiral arms contain young blue stars

Section 2: The Milky Way and Other Galaxies

How Galaxies Change Over Time By studying closer galaxies that might be similar to ancient ones, scientists can slowly piece together the puzzle of how galaxies evolve. (p. 706)

Section 3:Section 3: Origin of the Origin of the UniverseUniverse

What Is the Universe?What Is the Universe? The The universe consists of all space, universe consists of all space, matter, and energy that exists—now, matter, and energy that exists—now, in the past, or in the future. (p. 708)in the past, or in the future. (p. 708)

We see the universe now as it was in the past.We see the universe now as it was in the past. Astronomers need large units of measure to express Astronomers need large units of measure to express

distances. A light-year, which is approximately 9.5 distances. A light-year, which is approximately 9.5 × 10× 101212 km, Remember that while a year is a unit of km, Remember that while a year is a unit of time, a light-year is a unit of distance.time, a light-year is a unit of distance.

It takes time for light to travel in space. When we It takes time for light to travel in space. When we say the sun is 8 light-minutes away, we are say the sun is 8 light-minutes away, we are expressing not only its distance from Earth, but also expressing not only its distance from Earth, but also that we see the sun as it was 8 min ago. We never that we see the sun as it was 8 min ago. We never see it as it is in the present. see it as it is in the present.

When we see distant objects, we see them as they When we see distant objects, we see them as they were when they were younger. were when they were younger.

Astronomers can compare how galaxies age by Astronomers can compare how galaxies age by looking at many galaxies at various distances and looking at many galaxies at various distances and thus at different ages.thus at different ages.

Section 3:Section 3: Origin of the Origin of the UniverseUniverse

What Happened at the What Happened at the Beginning?Beginning? Scientists theorize that Scientists theorize that the universe formed during a the universe formed during a cataclysmic event known as the big cataclysmic event known as the big bang. (p. 710)bang. (p. 710)

scientists estimate that the universe is about 13.7 billion years old.

In 1965, scientists Arno Penzias and In 1965, scientists Arno Penzias and Robert Wilson were making Robert Wilson were making adjustments to a new radio antenna adjustments to a new radio antenna that they had built. that they had built.

They could not explain a steady but They could not explain a steady but very dim signal from all over the sky very dim signal from all over the sky in the form of radiation at microwave in the form of radiation at microwave wavelengths. They finally realized wavelengths. They finally realized that the signal was the that the signal was the cosmiccosmic background radiationbackground radiation predicted predicted by the big bang theory.by the big bang theory.

Radiation dominated the early universe.Radiation dominated the early universe. According to the big bang theory, According to the big bang theory,

immediately after the big bang, the universe immediately after the big bang, the universe was extremely hot and made up of pure was extremely hot and made up of pure energy. There was a period of rapid energy. There was a period of rapid expansion that caused the energy to cool expansion that caused the energy to cool and allowed sub-atomic particles, such as and allowed sub-atomic particles, such as protons, electrons, and neutrons, to form. protons, electrons, and neutrons, to form. The first stars were born about 400 million The first stars were born about 400 million years after the big bang. years after the big bang.

Expansion implies that the Expansion implies that the universe was once smaller.universe was once smaller.

Most galaxies are moving away from Most galaxies are moving away from each other.each other.

Long ago, the entire universe might Long ago, the entire universe might have been contained in an extremely have been contained in an extremely small space.small space.

During the past 100 years, other During the past 100 years, other theories for the origin of the universe theories for the origin of the universe have been proposed. A few are still have been proposed. A few are still being studied, but the big bang being studied, but the big bang theory is the one that is best theory is the one that is best supported by the current evidence supported by the current evidence including the cosmic background including the cosmic background radiation and observations of the radiation and observations of the movement of distant galaxies movement of distant galaxies

Section 3:Section 3: Origin of the Origin of the UniverseUniverse

Predicting the Future of the Predicting the Future of the UniverseUniverse Scientists use their Scientists use their increasing knowledge of the universe increasing knowledge of the universe to hypothesize what might happen to to hypothesize what might happen to the universe in the future. (p. 715)the universe in the future. (p. 715)

They depend on a mixture of theories and They depend on a mixture of theories and precise observations of very faint and precise observations of very faint and distant objects. distant objects.

These observations depend on technology, These observations depend on technology, such as telescopessuch as telescopes

New space telescopes that collect infrared New space telescopes that collect infrared radiation and X rays are being built and radiation and X rays are being built and launched. launched.

Data in these regions of the Data in these regions of the electromagnetic spectrum may provide electromagnetic spectrum may provide important clues about the beginning and important clues about the beginning and future of the universe.future of the universe.

One example of new, more sensitive technology One example of new, more sensitive technology is the is the Chandra X-Ray Observatory,Chandra X-Ray Observatory, which was which was launched into orbit around Earth in 1999. launched into orbit around Earth in 1999.

This observatory can take photographs in the X-This observatory can take photographs in the X-ray part of the electromagnetic spectrum. The ray part of the electromagnetic spectrum. The presence of X rays indicates matter at presence of X rays indicates matter at temperatures of more than one million degrees. temperatures of more than one million degrees.

According to Einstein’s theory of relativity, According to Einstein’s theory of relativity, mass curves space, much in the same way mass curves space, much in the same way that your body curves a mattress when you that your body curves a mattress when you sit on it. sit on it.

In 1919, observations of a total solar eclipse In 1919, observations of a total solar eclipse showed that Einstein was correct. In the showed that Einstein was correct. In the direction of the sun, stars that could be direction of the sun, stars that could be seen only during the eclipse were in slightly seen only during the eclipse were in slightly different positions than expected. different positions than expected.

The future of the universe is uncertain.The future of the universe is uncertain. The universe is still expanding, but it may The universe is still expanding, but it may

not do so forever. The combined gravity of not do so forever. The combined gravity of all of the mass in the universe is also pulling all of the mass in the universe is also pulling the universe inward, in the direction the universe inward, in the direction opposite to the expansion. The competition opposite to the expansion. The competition between these two forces leaves three between these two forces leaves three possible outcomes for the universe:possible outcomes for the universe:

1.1.The universe will keep expanding forever. The universe will keep expanding forever. 2.2.The expansion of the universe will gradually The expansion of the universe will gradually

slow down, and the universe will approach a slow down, and the universe will approach a limit in size. limit in size.

3.3.The universe will stop expanding and start The universe will stop expanding and start to fall back on itself.to fall back on itself.