We invite you to return along on a series of voyages to explore the universe as astronomers know it today. Beyond Earth are vast and luxurious realms filled with objects that haven’t any counterpart on our home planet. Nevertheless, we hope to point out you that the evolution of the universe has been directly liable for your presence on Earth today.
THE NATURE OF ASTRONOMY
Astronomy space science is characterized in light of the fact that the investigation of the articles that lie past our planet Earth and hence the cycles by which these items cooperate with one another. we’ll see, though, that it’s far more . it’s also humanity’s plan to organize what we learn into a transparent history of the universe, from the moment of its birth within the explosion to this moment. Throughout this book, we emphasize that science may be a progress report—one that changes constantly as new techniques and instruments allow us to probe the universe more deeply. In considering the history of the universe, we’ll see again and again that the cosmos evolves; it changes in profound ways over long periods of your time . for instance , the universe made the carbon, the calcium, and therefore the oxygen necessary to construct something as interesting and sophisticated as you. Today, many billions of years later, the universe has evolved into a more hospitable place for all times . Tracing the evolutionary processes that still shape the universe is one among the foremost important (and satisfying) parts of recent astronomy.
THE NATURE OF SCIENCE
The ultimate judge in science is usually what nature itself reveals supported observations, experiments, models, and testing. Science isn’t merely a body of data , but a way by which we plan to understand nature and the way it behaves. This method begins with many observations over a period of your time . From the trends found through observations, scientists can model the actual phenomena we would like to know . Such models are consistently approximations of nature, subject to additionally testing.
As a concrete astronomical example, ancient astronomers constructed a model (partly from observations and partly from philosophical beliefs) that Earth was the middle of the universe and everything moved around it in circular orbits. From the start, our accessible perceptions of the Sun, Moon, and planets fit this model; notwithstanding, after further perceptions, the model must be refreshed by including circle after circle to represent the movements of the planets around Earth at the middle . since the hundreds of years passed and improved instruments were produced for monitoring objects inside the sky, the old model (even with a huge number of circles) proved unable explain all the observed facts. As we’ll see within the chapter on Observing the Sky: The Birth of Astronomy, a replacement model, with the Sun at the middle , fit the experimental evidence better. After a time of philosophical battle, it got acknowledged as our perspective on the universe.
THE LAWS OF NATURE
Over centuries scientists have extracted various scientific laws from countless observations, hypotheses, and experiments. These scientific laws are, during a sense, the “rules” of the sport that nature plays. One remarkable discovery about nature—one that underlies everything you’ll examine during this text—is that an equivalent laws apply everywhere within the universe. the principles that determine the motion of stars thus far away that your eye cannot see them are an equivalent laws that determine the arc of a baseball after a batter has hit it out of the park.
Note that without the existence of such universal laws, we couldn’t make much headway in astronomy. If each pocket of the universe had different rules, we might have little chance of interpreting what happened in other “neighborhoods.” But, the consistency of the laws of nature gives us enormous power to know distant objects without traveling to them and learning the local laws. within the same way, if every region of a rustic had completely different laws, it might be very difficult to hold out commerce or maybe to know the behavior of individuals in those different regions. a uniform set of laws, though, allows us to use what we learn or practice in one state to the other state.
This is to not say that our current scientific models and laws cannot change. New experiments and observations can cause new, more sophisticated models—models which will include new phenomena and laws about their behavior. the overall theory of relativity proposed by Einstein may be a perfect example of such a change that happened a few century ago; it led us to predict, and eventually to watch , a wierd new class of objects that astronomers call black holes.Just the patient cycle of watching nature perpetually cautiously and absolutely can show the legitimacy of such new logical models.
NUMBERS IN ASTRONOMY
In astronomy we affect distances on a scale you’ll never have considered before, with numbers larger than any you’ll have encountered. We adopt two approaches that make handling astronomical numbers a touch bit easier. First, we use a system for writing large and little numbers called scientific notation (or sometimes powers-of-ten notation). this technique is extremely appealing because it eliminates the various zeros which will seem overwhelming to the reader. In scientific notation, if you would like to write down variety like 500,000,000, you express it as 5 × 108 . the tiny raised number after the ten , called a lover , keeps track of the amount of places we had to maneuver the percentage point to the left to convert 500,000,000 to 5.
CONSEQUENCES OF LIGHT TRAVEL TIME
There is one more reason the speed of sunshine is such a natural unit of distance for astronomers. Information about the universe involves us almost exclusively through various sorts of light, and every one such light travels at the speed of light—that is, 1 light-year per annum . This sets a limit on how quickly we will study events within the universe. If a star is 100 light-years away, the sunshine we see from it tonight left that star 100 years ago and is simply now arriving in our neighborhood. The soonest we will study any changes therein star is 100 years after the very fact . For a star 500 light-years away, the sunshine we detect tonight left 500 years ago and is carrying 500-yearold news.
Because many folks are familiar with instant news from the web , some might find this frustrating. “You mean, once I see that star up there,” you ask, “I won’t know what’s actually happening there for an additional 500 years?” But this isn’t the foremost helpful thanks to believe things . For astronomers, now’s when the sunshine reaches us here on Earth. there’s no way for us to understand anything that star (or other object) until its light reaches us.
A TOUR OF THE UNIVERSE
We can now take a quick introductory tour of the universe as astronomers know it today to urge familiar with the kinds of objects and distances you’ll encounter throughout the text. we start reception with Earth, an almost spherical planet about 13,000 kilometers in diameter. an area traveler entering our planetary system would easily distinguish Earth from the opposite planets in our system by the massive amount of liquid water that covers some two thirds of its crust. If the traveler had equipment to receive radio or television signals, or came close enough to ascertain the lights of our cities in the dark , she would soon find signs that this watery planet has sentient life.
THE UNIVERSE ON THE LARGE SCALE
In a very rough sense, you’ll consider the system as your house or apartment and therefore the Galaxy as your town, made from many homes and buildings. within the twentieth century, astronomers were ready to show that, even as our world is formed from many, many towns, therefore the universe is formed from enormous numbers of galaxies. (We define the universe to be everything that exists that’s accessible to our observations.) Galaxies stretch as far into space as our telescopes can see, many billions of them within the reach of recent instruments. once they were first discovered, some astronomers called galaxies island universes, and therefore the term is aptly descriptive; galaxies do appear as if islands of stars within the vast, dark seas of region .
THE UNIVERSE OF THE VERY SMALL
The foregoing discussion has likely impressed on you that the universe is awfully large and extraordinarily empty. on the average , it is 10,000 times more empty than our Galaxy. Yet, as we’ve seen, even the Galaxy is usually empty space. The air we breathe has about 1019 atoms in each cubic centimeter—and we usually consider air as empty space. within the interstellar gas of the Galaxy, there’s about one atom in every milliliter . region is filled so sparsely that to seek out one atom, on the average , we must search through a kiloliter of space. Most of the universe is fantastically empty; places that are dense, like the physical body , are tremendously rare. Even our most familiar solids are mostly space. If we could take apart such a solid, piece by piece, we might eventually reach the small molecules from which it’s formed. Molecules are the littlest particles into which any matter are often divided while still retaining its chemical properties. A molecule of water (H2O), for instance , consists of two hydrogen atoms and one oxygen atom bonded together.
A CONCLUSION AND A BEGINNING
If you’re new astronomy, you’ve got probably reached the top of our brief tour during this chapter with mixed emotions. On the one hand, you’ll be fascinated by a number of the new ideas you’ve examine and you’ll be wanting to learn more. On the opposite hand, you’ll be feeling a touch overwhelmed by the amount of topics we’ve covered, and therefore the number of latest words and concepts we’ve introduced. Learning astronomy may be a little like learning a replacement language: initially it seems there are numerous new expressions that you’ll never master all of them , but with practice, you soon develop facility with them. At now you’ll also feel a touch small and insignificant, dwarfed by the cosmic scales of distance and time. But, there’s differently to seem at what you’ve got learned from our first glimpses of the cosmos. allow us to consider the history of the universe from the large Bang to today and compress it, for straightforward reference, into one year. (We have acquired this idea from Carl Sagan’s 1997 Pulitzer Prize-winning book, The Dragons of Eden.) On this scale, the huge Bang occurred at the essential snapshot of January1, and this moment, once you are reading this chapter would be the top of the previous second of New Year’s Eve . When did other events within the development of the universe happen during this “cosmic year?” Our system formed around September 10, and therefore the oldest rocks we will date on Earth return to the third week in September