Astronomy: The Solar System and Beyond 5th edition

1. Astronomy:The Solar System and Beyond 5th edition Michael Seeds

2. What place is this? Where are we now? - CARL SANDBURG Grass Chapter 7

3. Microscopic creatures live in the roots of your eyelashes. • For example, demodex folliculorum has been found in 97 percent of individuals and is a characteristic of healthy skin. • Indeed, everyone has them and they are harmless. • They hatch, fight for survival, mate, lay eggs, and die in the tiny spaces around the roots of your eyelashes, without doing you any harm.

4. However, the tiny beasts are not self-aware. • They never stop to ask, “Where are we?” • Humans are more intelligent. • We have the ability and responsibility to wonder where we are in the universe and how we came to be here.

5. You should study the solar system for many reasons. • You need to understand Earth as a planet because six billion people are living here and causing changes, the effects of which are unknown. • You should also study it because, as you are about to discover, there are more planets in the universe than stars. • Above all, you should study it because it is your home in the universe.

6. As humans are an intelligent species, you have the right and responsibility to wonder what you are. • Our kind have inhabited this solar system for at least a million years. • However, only within a lifetime have we begun to understand what a solar system is. • Like sleeping passengers on a train, we waken, look out at the passing scenery, and mutter: “What place is this? Where are we now?”

7. The Great Chain of Origins • You are linked through a great chain of beginnings that leads backward through time to the universe’s first instant, roughly 14 billion years ago. • The gradual discovery of the links in that chain is one of the most exciting adventures of the human intellect.

8. The Origin of Matter • The matter in your thumb came into existence within moments of the beginning of the universe. • Astronomers have strong evidence that the universe began in an event called the big bang. • By the time the universe was 3 minutes old, the protons, neutrons, and electrons in your thumb had come into existence. • You are made of very old matter.

9. The Origin of Matter • Although those particles formed quickly, they were not linked together as they are today. • Most of the matter was hydrogen and about 25 percent was helium. • Very few of the heavier atoms were made in the big bang. • Although helium is very rare in our bodies, your thumb contains many of those ancient hydrogen atoms, unchanged since the universe began.

10. The Origin of Matter • During the first few hundred million years after the big bang, matter collected to form galaxies containing billions of stars. • Astronomers understand that nuclear reactions inside stars combine low-mass atoms such as hydrogen to make heavier atoms. • Generation after generation of stars cooked the original particles, fusing them into atoms such as carbon, nitrogen, and oxygen.

11. The Origin of Matter • Even the calcium in the bone and the iron in your blood were assembled inside stars. • Atoms heavier than iron were created by rapid nuclear reactions that can only occur when a massive star explodes at the end of its life cycle. • Gold and silver are rare in your body but iodine is critical in your thyroid gland. • There are no doubt a few of those iodine atoms circulating through your thumb at this very moment, thanks to the violent deaths of massive stars.

12. The Origin of Matter • Our galaxy contains at least 100 billion stars, of which our sun is one. • The sun formed from a cloud of gas and dust about 5 billion years ago. • The atoms in your thumb were part of that cloud.

13. The Origin of Matter • How the sun took shape, how the cloud gave birth to the planets, and how those atoms in your thumb found their way onto Earth and into you is the story of this chapter.

14. The Origin of Matter • As you explore the origin of our solar system, keep in mind the great chain of origins that created the atoms. • As the geologist Preston Cloud remarked, “Stars have died that we might live.”

15. The Origin of Planets • Over roughly the last two centuries, astronomers have proposed two kinds of theories for the origin of the planets. • Catastrophic theories proposed that the planets formed from some improbable cataclysm such as the collision of the sun and another star. • Evolutionary theories proposed that the planets formed gradually and naturally along with the sun.

16. The Origin of Planets • Since about 1940, evidence has accumulated to support the evolutionary theories. • In fact, nearly all astronomers now accept that planets can form naturally as a byproduct of star formation.

17. The Origin of Planets • Stars can form from the gravitational contraction of gas and dust clouds. • In some cases, that contraction is triggered by compression of the gas cloud, perhaps by the explosion of a nearby aging star.

18. The Origin of Planets • As a star forms in a contracting cloud, it remains surrounded by a cocoon of dust and gas. • The rotation of the cloud causes that dust and gas to form a spinning disk around the protostar.

19. The Origin of Planets • When the center of the star grows hot enough to ignite nuclear reactions, its surface quickly heats up, becomes more luminous, and blows away the gas and dust cocoon.

20. The Origin of Planets • There is clear evidence that disks of gas and dust around young stars are common. • The planets in our solar system probably formed from such a disk-shaped cloud around the sun. • When the sun became luminous enough, the remaining gas and dust were blown away into space, leaving the planets orbiting the sun.

21. The Origin of Planets • This is known as the solar nebula theory, because the planets form from the nebula around the proto-sun.

22. Disks Around Other Suns • If the theory is right, then planets do form as a by-product of star formation and, thus, most stars should have planetary systems. • The first question that might occur to you is: Do astronomers see any of these disks around other stars?

23. Disks Around Other Suns • The evidence is clear. • Disks around young stars are common. • Both visible- and radio-wavelength observations detect dense disks of gas orbiting young stars. • For example, at least 50 percent of the stars in the Orion Nebula are encircled by dense disks of gas and dust.

24. Disks Around Other Suns • A young star is visible at the center of each disk. • Astronomers estimate that the disks contain at least a few times Earth’s mass in a region a few times larger in diameter than our solar system.

25. Disks Around Other Suns • The Orion star-forming region is only a few million years old, and it does not seem likely that planets could have formed in these disks yet. • Furthermore, the intense radiation from the hot stars in the area is evaporating the disks so fast planets may never have a chance to grow large.

26. Disks Around Other Suns • However, the important point for astronomers is that so many of these young stars have disks. • Evidently, disks of gas and dust are a common feature of star formation.

27. Disks Around Other Suns • The Hubble Space Telescope can detect dense disks of gas and dust around young stars in a slightly different way. • The disks are revealed by the shadows they cast in the nebulae that surround the newborn stars.

28. Disks Around Other Suns • Infrared astronomers have also found very cold, low-density dust disks around stars such as Beta Pictoris. • These stars are believed to have completed their formation, so they are clearly older than the stars in Orion.

29. Disks Around Other Suns • The dust disk around Beta Pictoris is about 20 times the diameter of our solar system and, like the other known low-density disks, has an inner zone with even lower density. • These inner regions may be places where planets have formed.

30. Disks Around Other Suns • Such tenuous dust disks are sometimes called ‘debris disks.’ • This is because they are understood to be debris released in collisions among small bodies such as comets and asteroids. • Our own solar system is known to contain such dust. • Astronomers believe it has an extensive debris disk of cold dust extending far beyond the orbits of the planets.

31. Disks Around Other Suns • Notice the difference between the two kinds of disks that astronomers have found. • The low-density dust disks such as the one around β Pictoris are produced by dust from collisions among comets and asteroids. • Such disks are evidence that planetary systems have already formed. • The dense disks of gas and dust such as those seen round the stars in Orion are sites where planets could be forming right now.

32. Disks Around Other Suns • The observational evidence gives astronomers confidence that disks of gas and dust around other stars are common. • The evidence of debris disks suggests that planets have formed in some of these disks.

33. Disks Around Other Suns • You may wonder if you can see these planets directly. • That’s not easy, but astronomers are making progress.

34. Planets Orbiting Other Suns • A planet orbiting another star is called an extrasolar planet. • Such a planet would be quite faint and difficult to detect so close to the glare of its star. • However, there are ways to find these planets.

35. Planets Orbiting Other Suns • You will remember that Earth and its moon orbit around their common center of mass. • Similarly, when a planet orbits a star, the star moves very slightly as it orbits the center of mass of the planet–star system.

36. Planets Orbiting Other Suns • Think of someone walking a dog on a leash. • The dog runs around pulling on the leash and, even if you couldn’t see the dog, you could plot its path by watching how its owner was jerked back and forth.

37. Planets Orbiting Other Suns • Astronomers can detect a planet orbiting another star, by watching how the star moves as the planet tugs on it. • The first planet detected this way orbits the star 51 Pegasi. • As the planet circles the star, the star wobbles slightly.

38. Planets Orbiting Other Suns • The very small motions of the star are detectable as Doppler shifts in the star’s spectrum.

39. Planets Orbiting Other Suns • Using the motion of the star and estimates of the star’s mass, astronomers can deduce that the planet has half the mass of Jupiter andorbits only 0.05 AU from the star.

40. Planets Orbiting Other Suns • Half the mass of Jupiter amounts to 160 Earth masses. • So, this is a large planet orbiting very close to its star.

41. Planets Orbiting Other Suns • Astronomers were not surprised by the announcement that a planet orbits 51 Pegasi. • For years, they had assumed that many stars had planets. • Nevertheless, they greeted the discovery with typical skepticism.

42. Planets Orbiting Other Suns • That skepticism led to careful tests of the data and further observations that confirmed the discovery. • Over 100 planets have been discovered in this way, including at least three planets orbiting the star Upsilon Andromidae, a true planetary system.

43. Planets Orbiting Other Suns • Another way to search for planets is to look for a change in the brightness of a star when an orbiting planet crosses in front of it. • The decrease in light is very small but it is detectable. • Astronomers have used this technique to detect a few planets. • Very large search programs are underway.

44. Planets Orbiting Other Suns • The planets discovered so far tend to be massive and have short periods. • This is because lower-mass planets or longer-period planets are harder to detect. • Low-mass planets don’t tug on their stars very much, and present-day spectrographs can’t detect the very small velocity changes that these gentle tugs produce.

45. Planets Orbiting Other Suns • Planets with longer periods are harder to detect because astronomers have not been making high-precision observations for many years. • Jupiter takes 11 years to circle the sun once. • So, it will take years for astronomers to see the longer-period wobbles produced by planets lying farther from their stars. • Consequently, you should not be surprised that most of the first planets discovered are massive and have short periods.

46. Planets Orbiting Other Suns • The new planets may seem odd for another reason. • In our own solar system, the large planets lie farther from the sun. • How could big planets form so near their stars?

47. Planets Orbiting Other Suns • Mathematical models have shown that planets that form in an especially dense disk of matter could spiral inward. • Thus, it is possible for a few planets to become the massive, short-period planets that can be detected most easily.

48. Planets Orbiting Other Suns • A few of the newly discovered extrasolar planets have elliptical orbits. • That seems odd compared to our solar system in which the planetary orbits are nearly circular.

49. Planets Orbiting Other Suns • However, theorists point out that planets may interact with each other in some young planetary systems and can be thrown into elliptical orbits. • This is probably rare among planetary systems. • However, astronomers find these extreme systems more easily because they tend to produce big wobbles.

50. Planets Orbiting Other Suns • Massive planets in small or elliptical orbits are not outrageous. • At present astronomers know only one planetary system well: our own. • As they find more extrasolar planets, they will build a better understanding of what is normal and what is rare.