Structure and Composition of the Universe

1. Stars are held together by 2. A star that has various non-star objects orbiting it is part of a 3. Which number most accurately describes t he maximum number of galaxies in the universe? 4. A cloud of dust and gas is a 5. A massive object in space from which nothing can escape is a 6. The arms of a spiral galaxy are areas where 7. The Milky Way is

1. Stars are held together by *gravity 2. A star that has various non-star objects orbiting it is part of a *planetary system 3. Which number most accurately describes the maximum number of galaxies in the universe? *billions 4. A cloud of dust and gas is a *nebula 5. A massive object in space from which nothing can escape is a *black hole 6. The arms of a spiral galaxy are areas where *stars form 7. The Milky Way is *spiral

irregular

In this image, you see another example of an irregular galaxy, one which does not have any particular shape at all

1. Why will the position of the stars today be different than the position of the same stars six months from now? 2. Explain what is meant by parallax in terms of the stars. 3. Besides parallax, how else can we determine the distances in the universe? 4. A star's parallax angle is found to be 0.77. How far away is the star in parsecs? In light-years?

1. The stars will appear to be in a different position because of Earth's movement around the Sun. 2. Parallax is the apparent shift in the position of t he stars due to our change in location as Earth orbits the Sun. 3. We can use star magnitude to measure distances. This is done by comparing the difference between a star's absolute and apparent magnitude. 4. Distance is 1/p, so the star is 1/0.77 = 1.30 parsecs away. This is 4.23 light-years away.

How do scientists know the distances between objects in the universe?

After all, we do not have tools that can measure such incredible distances

irregulars.

All other types of galaxies are collectively called irregulars. They have irregular shapes or peculiar morphologies. In this image, you see a ring galaxy, which has a ring of

Questions 1. True or False: The big bang model says that the universe is still expanding today. 2. True or False: According to the big bang model , the universe was originally cold and dense. 3. True or False: We know the big bang theory makes sense because the distance between galaxies is getting smaller. 4. True or False: We theorize that the universe has been expanding outward for only about one million years. 5. True or False: The big bang model asserts t hat the fabric of space is expanding outward. 6. True or False: The background of space is filled with radiation that does not come from any object. 7. True or False: Light waves become redshifted as they move away from us in space. 8. True or False: As distant galaxies move away from us, the radiation that comes from them approaches the blue end of the electromagnetic spectrum.

Answers 1. True Feedback: The big bang model says that the universe is still expanding today. 2. False Feedback: The big bang model says that the universe was originally hot and dense and has subsequently been expanding and cooling. 3. False Feedback: We observe that the distance between galaxies is expanding. 4. False Feedback: According to the big bang model, the universe has been expanding outward for billions of years. 5. True Feedback: The big bang model asserts that the fabric of space is expanding outward. 6. True Feedback: The background of space is filled with radiation that does not come from any object. 7. True Feedback: Light waves become redshifted as they move away from us in space. 8. False Feedback: As distant galaxies move away from us, the radiation that comes from them approaches the red end of the electromagnetic spectrum.

Types of Galaxies

Biologists classify leaves by their shapes and number of parts. Geologists classify rocks by the way they formed and the types of minerals that make them up. Take a look at these four images. They show different types of galaxies. How might astronomers categorize galaxies? Roll over each image with your mouse to learn about different types of galaxies. pg 14

Elliptical galaxies

Elliptical galaxies have elliptical shapes, but may vary in just how elliptical they are; they may be highly elongated or nearly spherical. These galaxies can grow to

The Composition of the Universe

Everything on Earth is made of ordinary atom s—like atoms of carbon or oxygen or hydrogen. Most things are made of compounds of atoms, when two or more atoms of different types combine. Water, for example, is a compound t hat forms when atoms of hydrogen combine with atoms of oxygen. Anything made up of ordinary atoms is called baryonic material. Your body and all of your surroundings are made of baryonic material.

Slide 2

For many years, scientists believed that t he universe was made of baryonic material. But evidence shows that it is not. What scientists have noticed is that there are gravitational effects on visible matter. Because the matter acts like so mething is pulling it, scientists have come up with theories to explain it. Scientists believe there must be some type of energy and matter that are causing these effects. They call them "dark energy" and "dark matter" because we cannot see them. Dark matter cannot be detected because dark matter does not emit or absorb light. Dark energy is the source of the energy needed to keep the universe expanding. This hypothetical type of energy, called dark energy, makes up 70 percent of the universe. Twenty-three percent of the universe is made up of dark matter. Of the very small percentage of the universe t hat is made up of baryonic material, the most abundant element is hydrogen. Helium is the second most abundant. All of the stars and planets and the gases in outer space are baryonic material. These only account for about three to four percent of the inferred mass of the universe

Structure and Composition of the Universe: Introduction

How do we learn about what the rest of the universe was like millions or even billions of years ago? We study "fossil" light. It is actually light that is left over from the universe's formation. The radiation continues through space even today and can be detected by special NASA tools. Since it originated when the universe began, it is the oldest light ever detected. It is like a fossil or a "baby picture" of the universe. It shows us what the universe was like all those years ago. When we observe this light, we see variations in its brightness and color. These variations have clues about the properties, age, and content of the universe—just like a fossil dinosaur bone contains clues about how a dinosaur looked, what it ate, and where it lived. Much of our current knowledge about the universe comes from observations of background radiation that moves through the universe today. It has been doing so ever since it first formed 13 billion years ago.

Origin of the Universe

Imagine an inflated balloon with some dust particles on the surface. As the balloon expands, the individual dust particles get farther and farther apart. We have observed that objects in the universe are doing the same thing. This has led scientists to believe that they were once much closer together and have been expanding outward for billions of years. This assumption gave rise to our current understanding of the origin of universe. It is outlined in a model called the big bang model. The big bang model is based on evidence and observation. It is based on the idea that, initially, the universe was a hot dense sphere, and that it has expanded, and is still expanding today. This theory states that at some very ancient time—at a moment known as the big bang—the sphere exploded and space itself began expanding. And like the dust on the surface of an inflating balloon, all matter simply rode along on the expanding space. We have also been able to observe background radiation in the universe, which scientists can tell comes from extremely far away. This radiation can only be detected by radio telescopes and does not come from any star, galaxy, or other object. It appears to be radiation that "fills" the background space of the universe. Scientists believe this constant background radiation that comes to earth equally in all directions is left over from the beginning of time. As the universe expanded, this background radiation moved outward too.

15th century BC Ancient Hindus

In the 15th to 8th centuries BC, the ancient Hindus put together some of humanity's first written discussions of the universe. This group of people from ancient India believed that the universe goes through cycles of creation, destruction, and rebirth— with each cycle lasting more than 4 million years. In the 15th to 8th centuries BC, the ancient Hindus put together some of humanity's first written discussions of the universe. This group of people from ancient India believed that the universe goes through cycles of creation, destruction, and rebirth, with each cycle lasting more than four million years. Their ideas are recorded in the Vedas, the sacred texts of ancient Hinduism. In the picture, you can see an example of their written language, called Sanskrit.

Slide 3

In this picture, you see a telescopic image of distant stars and galaxies. Notice arcs of blue light in some parts of the picture. The arcs show pl aces where light is being bent by the influence of gravity from matter that cannot otherwise be detec ted—evidence scientists use to explain their dark matter and dark energy theories.

Measuring Distances in The Universe

Let's start out by doing a fun little experiment. Hold your thumb up close to your nose and focus on an object behind your thumb. Now close your eyes one at a time so that you have just the left eye open and then just the right eye open. Change eyes a few times. What do you notice? You should notice that your thumb appears to shift back and forth. Really it stays in place, but because we have two eyes separated by about an in ch of space, each eye views objects from a slightly different angle. This is called parallax.

NASA's WMAP

NASA's WMAP satellite is an advanced spacecraft on a mission to record data about the background radiation of the universe—radiation that initiated when the universe formed and that still moves through space today.

Slide 2:

Now consider the fact that the Earth is moving around the sun. Because of this movement, we get different views of the stars at different times of the year. It's similar to the thumb experiment on a much larger scale. At one point in time, we see the stars from one angle and at another point in time, we see the same stars from another angle. In the picture above, you can see how a snapshot of the stars taken today will differ from a snapshot of the same stars taken 6 months from now. There is an apparent shift in the position of the stars due to a change in Earth's location. This is referred to as t he parallax of the stars. The amount that the stars appear to move during that 6 month time is called their parallax angle. Scientists have worked out mathematically that the distance from Earth to a star is 1 over the parallax angle, or d = 1 over p. This equation gives us a distance in units of parsecs, which we can convert to light years if we want to. Scientists have been carefully determining the parallax angle values for individual stars for hundreds of years. With this data, they have been able to calculate the distance to those stars from Earth.

Light-Years

One common unit of distance used in astronomy is the light-year. A light-year is defined as the distance that light travels in one Earth year. It is equal to just under 10 trillion kilometers. But scientists do not usually convert light-years to kilometers or miles; they simply speak in terms like "3.6 light-years away," or "12.8 light-years from Earth," etc. In this picture you see that Alpha Centuri, the closest star to Earth besides our Sun, is 4.3 light-years away from us. This is the same as 23,462,794,000,000 miles away.

How many light-years are represented by five parsecs?

One parsec is 3.26 light-years. So five parsces (5 x 3.26) = 16.3 light-years.

Slide 3:

Parallax works well for measuring stellar distances for stars that are no more than a few thousand light years away. For stars that are farther away, parallax values become too small to be of much worth, and we have to use other methods of determining their distance. One of these methods relies on the apparent and the absolute magnitudes of stars. Take a look at this picture. It shows some common stars that we can see here on Earth. The stars are located at different distances from us, and that's why some appear brighter than others. On the left we see their apparent magnitude, which is how bright a star appears to us. On the right you see absolute magnitude, which is how bright the star really is. Absolute magnitude is the brightness we would perceive for the stars if they were all located at the same distance from us. Remember that a very bright star might appear dim to us if it is very far away. Our sun is really not that bright of a star when compared to some other stars in the universe, but it is close to us and thus appears very bright.

Questions 1. Discuss the beliefs of the ancient Hindus about the universe. 2. Describe the differences between Ptolemy's and Copernicus's views about the universe. 3. Describe the contributions of Kepler and Newt on to our understanding of the universe.

Sample Answers 1. The ancient Hindus gave us some of our earliest writings about the universe. This group of people from ancient India believed that the universe goes through cycles of creation, destruction, and rebirth—with each cycle lasting more than four million years. Their ideas are recorded in the Vedas, the sacred texts of ancient Hinduism. 2. Ptolemy believed in a geocentric model. In other words, he believed Earth is the center of the universe. He argued that the Sun moves around Earth. Copernicus, on the other hand, believed in a heliocentric model, or that the Sun is the center of a solar system that includes planets orbiting around it. Today we know the heliocentric model is correct. 3. Kepler used math to show that planets move around the Sun in elliptical orbits. Newton's work on gravity gave more proof to the heliocentric model and explained how the planets stay in their orbits as a result of the force of gravity.

Spiral galaxies

Spiral galaxies consist of a rotating disk of stars, gas, and dust around a central mass of older stars. Spiral galaxies are named for the spiral arms that extend from the center

Slide 4

Take a look at this table. It lists some common stars, including our sun, and shows apparent magnitude in the second column and absolute magnitude in the last column. It is important to realize that magnitude scales are set up so that the more negative a number is, the more brightness it represents. The more positive a number is, the more dimness it represents. Our sun is listed as having an apparent magnitude of -26.8, the star listed on the table with the highest apparent brightness. On the other hand, our sun has an absolute magnitude value of +4.83, or the most positive value on the table and star on the table that has the least absolute magnitude. The sun appears very bright, but in terms of absolute brightness, it is really not that bright. Remember that absolute magnitude is the brightness at which we would perceive each star, if we could somehow place them all at the same distance away from us. We can see here that if we could line up our sun at the same distance as all the other stars on the table, Betelgeuse would give the glow of the brightest and our sun would be the dimmest. Scientists have compiled data tables like this one for thousands and thousands of stars. They know that the brightness of light becomes fainter with given distances and are able to mathematically figure out how far away a star is by looking at the difference between its apparent and absolute magnitude.

How many miles away is the sun from earth?

The Sun, our closest star, is more than 90 million miles (150 kilometers) away from Earth.

Timeline: Beliefs About the Universe

The universe includes everything that exists. Since that is a LOT of stuff, we are still learning much about the universe. But we have also already gained an amazing amount of scientific knowledge about the nature and composition of the universe. This activity will give you a glimpse at how our understanding of the universe has developed and changed over time.

Avatar: The Composition of the Universe

Today we have a solid understanding that our Earth is part of a solar system in which planets orbit a central sun. And we know that beyond our own solar system, there are billions, even trillions, of other stars and celestial objects. We are still learning about the types of matter that make up the vast expanse of space, but we have learned much about the universe. This presentation goes over some of what we do know about what the universe if made of.

Section Warm-Up

We are here—somewhere in the Milky Way Galaxy. The Milky Way is a cluster of billions of stars, one of which is our relatively small Sun. The Milky Way itself is just one of billions of galaxies in the universe.

Astronomical Distances

We usually think of distance in terms of meters and kilometers or feet and miles. But what if you could travel to another planet, or another galaxy, or to a far edge of the universe?

Astronomical

When you talk about astronomical distances, miles are pretty puny. Just inside our galaxy alone, there are billions of miles. In astronomy, we use different units of measurement for distance. Click through the tabs to learn more about measuring astronomical distances.

An Expanding Universe

With new advances in mathematics, science, and telescope technology near the beginning of the twentieth century, scientists observed that the space between distant galaxies is expanding—a consequence of the big bang. In the picture, you see the electromagnetic spectrum, which is a range of all possible types of electromagnetic radiation (radio, microwave, etc). The red squiggly line shows how the different types of radiation waves are stretched out. Notice that the waves are the most stretched out at the red end of the frequency bar. They are much more scrunched together in the blue/purple area. A strong line of evidence in support of an expanding universe is something called the cosmological redshift. This has to do with the way we perceive radiation from other objects in the universe. Light waves are a form of electromagnetic radiation. Because the universe is expanding, the travelling light waves become stretched out. Their color appears to move from the blue end to the red end of the light spectrum. Scientists call this a redshift. The light we see from far away galaxies appears reddish rather than white. The farther away galaxies are, the redder they appear. pg 8

Parsecs

You may also see stellar distances measured in units called parsecs. The galaxy you see in the picture is about 17,000 parsecs in diameter. A parsec is a unit of length used in astronomy and is equal to about 31 trillion kilometers (or 19 trillion miles). So this galaxy is 17,000 x 31 trillion kilometers in diameter. A parsec is also equal to 3.26 light-years. The parsec is an SI (International System of Units) unit of measurement. Like other SI units, we can add prefixes to it to increase its value. Just like we can add the prefix "kilo" to "meter" to mean 1,000 meters or one kilometer, we can add the prefix "mega" to "parsec" to mean one million parsecs or one megaparsec. Parsecs are most appropriate for measuring distances within our galaxy; the megaparsec is best for measuring the much more immense distances between galaxies.

The Structure of the Universe The atoms that make up all of the baryonic material in the universe combine to make several different structural features. Click through the images to learn more about these structures.

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