Chapter 1: Introduction to Astronomy
Hello, adventurer! You are about to start a journey that will take you to the farthest reaches of space, the innermost depths of matter, from the earliest beginning of time, and to the future billions of years from now. The objects and events you will encounter may sometimes blow your mind!
As important as what we think we know about the universe is how we come to know these things. This textbook emphasizes the techniques and process astronomers use to find out about the universe around us and the unifying principles operating “behind the scenes”. Facts will be given as examples of what is found when those techniques are used or as examples of a particular effect of a physical principle in operation, but are not as important as the overall concepts
We will start by taking you on a grand tour of the universe, setting the stage for things to come. You will also be introduced to unimaginable distance and time scales associated with the universe. As you procede, try to form a picture of your place in the universe.
- Section 1-1: A Deep Connection
- We are Star Stuff
- Section 1-2: A Sense of Scale
- Powers of 10
- A Scaled Model of the Solar System
- Light-Year and Look-Back Time
- Moving Outward from Our Solar System
- Section 1-3: A Universe in Motion
- Section 1-4: The Dimension of Time
- Astronomy Cast
Section 1-1: A Deep Connection
The Big Bang, the formation of the universe, gave rise to all the matter and energy in the universe. Minutes after the Big Bang (exactly what the “Bang” is about is an ongoing mystery), hydrogen and helium formed, with trace amounts of deuterium, helium-3 and Lithium. None of the heavier elements on the periodic table existed! How did we, and all the moons, planets, stars and galaxies come to be?
The universe as a whole has continued to expand ever since the Big Bang, but on smaller size scales the force of gravity has drawn matter together. Structures such as galaxies and clusters of galaxies occupy regions where gravity has won out against the overall expansion That is, while the universe as a whole continues to expand, individual galaxies and their contents do not expand. Most galaxies, including our own Milky Way, probably formed within a few billion years after the Big Bang.
We are Star Stuff
Star Stuff Mini-lecture (Click picture to view video.)
Within galaxies, gravity drives the collapse of clouds of gas and dust to form stars and planets. Stars are not living organisms, but they nonetheless go through “life cycles.” After their birth in giant clouds of gas and dust, stars shine for millions or billions of years. The energy that makes stars shine comes from nuclear fusion, the process in which lightweight atomic nuclei smash together and stick (or fuse) to make heavier nuclei. Nuclear fusion occurs deep in a star’s core throughout its life. A star “dies” when it finally exhausts all its usable fuel for fusion.
In its final death throes, a star blows much of its content back out into space. In particular, massive (but short-lived) stars die in titanic explosions called supernovae. The returned matter mixes with other matter floating between the stars in the galaxy, eventually becoming part of new clouds of gas and dust from which new generations of stars can be born. Thus, galaxies function as cosmic recycling plants, recycling material expelled from dying stars into new generations of stars and planets. Our own solar system is a product of many generations of such recycling. An illustration of the stellar cycle is shown below.
The recycling of stellar material has another, even more important, connection to our own existence. By studying stars of different ages, we have learned that the early universe contained only the simplest chemical elements: hydrogen and helium (and a trace amount of lithium). We and Earth are made primarily of “other” elements, such as carbon, nitrogen, oxygen, and iron. Where did these other elements come from? Astronomers have discovered that all these elements were manufactured by massive stars, either through the nuclear fusion that makes them shine or through nuclear reactions accompanying the explosions that end their lives.
The processes of heavy-element production and cosmic recycling had already been taking place for several billion years by the time our solar system formed, about 4.6 billion years ago. The cloud that gave birth to our solar system was about 98% hydrogen and helium. The other 2% contained all the other chemical elements. The small rocky planets of our solar system, including Earth, were made from a small part of this 2%. We do not know exactly how the elements on the Earth’s surface developed into the first forms of life, but it appears that microbial life was already flourishing on Earth more than 3.5 billion years ago. Biological evolution took over once life arose, leading to the great diversity of life on Earth today.
In summary, most of the material from which we and our planet are made was created inside stars that died before the birth of our Sun. We are intimately connected to the stars because we are products of stars. In the words of astronomer Carl Sagan (1934–1996), we are “star stuff.” In the word of Allen Sandage, “The universe is the machine that created you!” As the universe expanded and cool since the big bang, more and more complex structures have arisen. In at least one instance, this complexity has taken on consciousness. It is sentient and curious about its origins. We are the universe wondering about itself! The matter that is in our bodies, was transformed by stars, but the raw materials were created in the big bang. In a very real sense, we were there! The history of the universe is truly our story as well!
Section 1-2: A Sense of Scale
Powers of 10
Distance scales video clip above goes from the unimaginably large to the inconceivably small.
I have provided a clip from Cosmic Voyage to illustrate size scales. The clip starts with acrobats holding a 1-meter ring in St. Marks Square in Venice, Italy and then expands the field of view by ten times every successive ring until it reaches the bounds of the observable universe. Then, the viewer is taken to a drop of water and the field of view is reduced by ten times every ring until the viewer is smaller than a quark (quarks make up protons and neutrons). The clip is longer than one might first expect because of all of those powers of ten that must be counted to include all of the things astronomy covers.
Let’s review powers of ten mathematically for a moment.
For numbers larger than 10, the power of 10 is a positive value and negative for numbers less than 1. For numbers between 0 and 10, the power is a positive fraction. In the examples that follow, notice what happens to the decimal point:
|100 = 1.||=||1. with the decimal point moved 0 places|
|101 = 10.||=||1. with the decimal point moved 1 place to the right|
|102 = 10 x 10 = 100.||=||1. with the decimal point moved 2 places to the right|
|103 = 10 x 10 x 10 = 1000.||=||1. with the decimal point moved 3 places to the right|
|10-1 = 0.1||=||1. with the decimal point moved 1 place to the left|
|10-2 = 0.01||=||1. with the decimal point moved 2 places to the left|
|10-3 = 0.001||=||1. with the decimal point moved 3 places to the left.|
The exponent of 10 tells you how many places to move the decimal point to the right for positive exponents or left for negative exponents. These rules come in especially handy for writing very large or very small numbers. For a more-complete discussion of exponents and powers, go to the Math Review
A Scaled Model of the Solar System
Another way to give you a sense of the distances between things is to use a proportional scaled model. In such a model, everything is reduced by the same amount, so all parts of the model relative to each other are of the same proportional size, much like a road map. To create a scale model, divide all of the actual distances or sizes by the same scale factor (in the example below the scale factor is 8,431,254,000), so the scaled distance = (actual distance)/(scale factor).
The metric system will be used in this text. Readers in the U.S. can multiply the kilometer numbers by 0.6 to get the number of miles and multiply the centimeter numbers by 0.4 to get the number of inches.
For our scale model, let us use a yellow mini-basketball about 16.51 centimeters (6.5 inches) across to represent the Sun and then pace out how far the tiny planets would be in this scale model. Since the real Sun is 1,392,000 kilometers (865,000 miles) across, the scale model has all of the planets and distances reduced by an amount equal to (139,200,000,000/16.51) = 8,431,254,000 times. The largest planet, Jupiter, would be only 1.7 centimeters across (a dime) and about 92.3 meters away. Our little Earth (a grain of sand) would be closer: only 17.7 meters (about 18 big steps) away. Our Sun is much larger than the planets, and, yet, it is just a typical star! Here is a scaled model of our solar system:
Scaled Model of the Solar System
|Object||Real Diameter (km)||Real Distance (million km)||Scaled Size (cm)||Scaled Distance (m)|
|Mercury||4880||57.910||0.058 (tiny! grain of sand)||6.9 (7 big steps)|
|Venus||12,104||108.16||0.14 (grain of sand)||12.8 (13 big steps)|
|Earth||12,742||149.6||0.15 (grain of sand)||17.7 (18 big steps)|
|Mars||6780||228.0||0.08 (almost 1 mm)||27.0 (27 big steps)|
|Jupiter||139,822||778.4||1.7 (a dime)||92.3 (92 big steps)|
|Saturn||116,464||1,427.0||1.4 (a button)||169.3 (169 big steps)|
|Uranus||50,724||2,869.6||0.6 (button snap)||340.4 (340 big steps)|
|Neptune||49,248||4,496.6||0.6 (button snap)||533.3 (533 big steps)|
|Pluto||2274||5,913.5||0.03 (small piece of dust)||701.4 (701 big steps)|
|Oort Cloud||11,200,000||1,328,400 (1,328 km)|
|Proxima Centauri||375,840||40,493,000||4.5 (handball)||4,802,700 (4,803 km)|
The Kuiper Belt is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It contains icy material that orbits in roughly the same plane as the rest of the planets. The Oort Cloud is a huge spherical cloud of trillions of comets surrounding the Sun that is about 7.5 to 15 trillion kilometers across. In our scale model, the middle of the Oort Cloud would be about the distance between Los Angeles and Denver. Proxima Centauri is the closest star to us outside of the solar system (remember that the Sun is a star too!). Proxima Centauri would be from Los Angeles to beyond the tip of the state of Maine on this scale model (from Los Angeles to New Glasgow, Nova Scotia to be more precise!). In our fastest rocket ships (neglecting the Sun’s gravity) it would take almost 70,000 years to reach Proxima Centauri!
Light-Year and Look-Back Time
Light-Year Mini-lecture (Click picture to view video.)
Instead of using ridiculously small units like kilometers, astronomers use much larger distance units like a light year to describe distances between the stars. A light year is how far light will travel in one year. The distance D something travels in a given time interval t is found by multiplying the speed v by the time interval. In compact math notation this is: D = v × t. You can find out how many kilometers a light year is by multiplying the speed of light by a time interval of one year:
1 light year = (299,800 kilometers/second) × (31,560,000 seconds/year) = 9,461,000,000,000 kilometers (9.461 trillion kilometers or 6 trillion miles). The speed of light is the fastest speed possible for anything in the universe to travel despite what you may see in science fiction movies or books.
The light-year as a distance unit works nicely when astronomers discuss what is class look-back time. The farther an object is out in space, the longer it takes for the light to get to us here on Earth. Light from an object that is X light-years away is X years old and one is seeing that object as it was X years ago, when that light was emitted.
The nearest star is about 4.3 light years away which means that it takes light 4.3 years to travel from Proxima Centauri to Earth. The rest of the stars are further away than that! It is because of the H-U-G-E distances and l-o-n-g times it would take extraterrestrial spacecraft to travel to the Earth that many astronomers are skeptical about extraterrestrial beings abducting humans. Patterns of stars, which will be discussed more in Chapter 2, appear to remain unchanged over the centuries, not because they don’t actually move, but because they are so far away their motion isn’t noticeable to the naked eye.
Moving Outward from Our Solar System
The Sun is one star among over 200 billion stars gravitationally bound together to make the Milky Way Galaxy. A galaxy is a very large cluster of billions of stars held together by the force of their mutual gravity on each other. There are about 100 billion galaxies in the visible universe. Given that, on average, there are 100 billion stars in each galaxy, that makes 100,000,000,000 x 100,000,000,000 = 1022 or 10 billion-trillion stars in the visible universe! That definition is a loaded one that will be unpacked and examined in more detail in later chapter but for now let us continue on our brief tour of the universe.
The Milky Way is a flat galaxy shaped like a pancake with a bulge in the center. Stars and gas are clumped in spiral arms in the flat disk part of the Galaxy. Many stars are also found in between the spiral arms. Our solar system is in one of the spiral arms of the Milky Way and is about 26,000 light years from the center of the galaxy. The entire Milky Way is about 100,000 light years across. In our scaled model with the Sun 16.51 centimeters across, the Milky Way would be about 112 million kilometers across or about 38% of the size of the Earth’s orbit around the Sun. Recall that Pluto’s orbit is only 1.4 kilometers across on this scale—the Galaxy is MUCH larger than our solar system! Here is an artist’s view of our galaxy with the Sun’s position marked (note that our entire solar system would be smaller than the smallest dot visible in the picture!):
Let’s reduce our scale model even more so that our galaxy is the size of the mini-basketball. The closest other galaxy is a small irregularly-shaped one about 13 centimeters away from the Sun toward the direction of the Milky Way’s center. It is about the size of a cooked, fat breakfast sausage link in our scale model. Appropriately, the Milky Way is in the process of gobbling up this galaxy. Two famous satellite galaxies of the Milky Way called the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are about 30 centimeters and 35 centimeters away, respectively. The Large Magellanic Cloud is about the size of a tennis ball and the Small Magellanic Cloud is about the size of a ping pong ball. The Andromeda Galaxy is the closest large galaxy to the Milky Way: a ball 19 centimeters in diameter (a volleyball) about 4.8 meters away. The Milky Way and the Andromeda Galaxy are at either end of a group of about 30 galaxies gravitationally bound together in the Local Group. The Local Group can be roughly divided into two clumps with each clump having a large spiral in it: the Milky Way and the Andromeda Galaxy.
The closest large cluster of galaxies is called the Virgo cluster (toward the direction of the Virgo constellation). The Virgo cluster has over 1000 galaxies in it and is roughly 50 meters away in our scale model. Notice that compared to their size, the galaxies are relatively close to one another. Stars inside a galaxy are relatively very far apart from one another compared to the sizes of the stars. You will see that the relative closeness of the galaxies to each other has a significant effect on the development of galaxies.
Groups and clusters can form still larger structures called superclusters. The Local Group and Virgo cluster are part of a the Local Group, also known as the Virgo Supercluster since the Virgo cluster is close to the middle . The Local Group is close to one edge of the Local Supercluster. In our scale model with the Milky Way the size of a mini-basketball, the Local Supercluster is about 190 meters long and the entire observable universe is about 49.5 kilometers in diameter.
Section 1-3: A Universe in Motion
Everything in the universe moves, but since the universe is so vast, and the distances between stars and galaxies so large, stars and galaxies move imperceptibly in our lifetime (at least to the naked-eye). The universe and the objects in it are constantly evolving, but this evolution takes place over long time scales. Most objects seem to change very little in our lifetime.
We live on a rotating Earth, rotating once per day. In the continental United States we move about 1000 mph due to this motion. The Earth revolves around our Sun at about 70,000 mph. The Sun orbits the center of our home galaxy, the Milky Way, once every 230 million years. Galaxies also move amongst themselves, sometimes orbiting or colliding. On the largest scales (larger than 100 million light years) the average distance between galaxies is getting larger: the universe is expanding.
Section 1.4: The Dimension of Time
Time Scales video clip above gives a sense of how old the universe is, how long it took for humans to appear, and some of our history. (Click picture to view video.)
Now let’s try to get a feel for the time scales. I will use another scale model, but instead of reducing distances, I will shrink down time. The scale model is called the “cosmic calendar” in which every second in the “cosmic calendar” corresponds to 430 real years (so 27 cosmic calendar days represents about 1 billion real years). We have evidence that the universe is about 13.7 billion years old. We can then squeeze the universe’s entire history into one cosmic calendar year. The universe starts in the early morning of January 1 at midnight in the cosmic calendar and our present time is at December 31 at 11:59:59.99999 PM. The calendar below outlines events on the cosmic calendar relevant to we humans.
It is rather surprising that we have been able to discover so much about the long term evolution of the universe and the things in it, especially when you consider that we have only been seriously observing the universe for about 100 years, which is only a very slight fraction of the universe’s lifetime. About 100 years ago is when photography was first used in astronomy, making truly systematic observation programs possible.
Astronomy Cast takes a fact based journey through the cosmos as it offers listeners weekly discussions on astronomical topics ranging from planets to cosmology. Hosted by Fraser Cain (Universe Today) and Dr. Pamela L. Gay (SIUE), this show brings the questions of an avid astronomy lover direct to an astronomer. Together Fraser and Pamela explore what is known and being discovered about the universe around us. Transcripts for many shows are available at the Astronomy Cast site for the hearing-impaired and anyone else who is interested.
Getting Started in Amateur Astronomy: Got your eye on that $40 telescope at Wal-Mart? Wait, hear us out first! Discussed, are strategies for getting into amateur astronomy – one of the most worthwhile hobbies out there. We discuss what gear to get, where to look, and how to meet up with other astronomy enthusiasts.
Astronomy in Science Fiction: We work through physics and astronomy in popular science fiction. What they get right, and what they get wrong… so very wrong.