Week #7 Notes:

1. Week #7 Notes: Our Solar System, Others, and the Sun

2. The Formation of the Solar System • How did the Earth and the rest of the Solar System form? • We can accurately date the formation by studying the oldest objects we can find in the Solar System and allowing a little more time. • For example, astronauts found rocks on the Moon older than 4.4 billion years. • We have concluded that the Solar System formed about 4.6 billion years ago.

3. Collapse of a Cloud • Our best current idea is that 4.6 billion years ago, a huge cloud of gas and dust in space collapsed, pulled together by the force of gravity. • What triggered the collapse isn’t known. • It might have been gravity pulling together a random cloud, or it might have been a shock wave from a nearby supernova (see figure). • As the gas pressure increased, eventually the rate of collapse decreased to a slower contraction.

4. Collapse of a Cloud • In the disk of gas and dust, we calculate that the dust began to clump (see figures). • Smaller clumps joined together to make larger ones, and eventually planetesimals, bodies that range from about 1 kilometer up to a few hundred kilometers across, were formed. • Gravity subsequently pulled many planetesimals together to make protoplanets (pre-planets) orbiting a protosun (pre-Sun).

5. Collapse of a Cloud • The protoplanets then contracted and cooled to make the planets we have today, and the protosun contracted to form the Sun (see figure). • Some of the planetesimals may still be orbiting the Sun; that is why we are so interested in studying small bodies of the Solar System like comets, meteoroids, and asteroids. • Most of the unused gas and dust, however, was blown away by a strong solar wind.

6. Extra-solar Planets (Exoplanets) • People have been looking for planets around other stars for decades. • Many times in the last century, astronomers reported the discovery of a planet orbiting another star, but for a long time each of these reports had proven false. • Finally, in the 1990s, the discovery of extra-solar planets (planets outside, “extra-,” the Solar System) seemed valid. • They have also become known as exoplanets.

7. Discovering Exoplanets • Since exoplanets shine only by reflecting a small amount of light from their parent stars, they are very faint and extremely difficult to see in the glare of their parent stars with current technology. • So the search for planets has not concentrated on visible sightings of these planets. • Rather, it has depended on watching for motions in the star that would have to be caused by something orbiting it.

8. Astrometric Method • The earlier reports, now rejected, were based on tracking the motion of the nearest stars across the sky with respect to other stars. • The precise measurement of stellar positions and motions is called astrometry, so this method is known as the astrometric method. • If a star wobbled from side to side, it would reveal that a planet was wobbling invisibly the other way, so that the star/planet system was moving together in a straight line. • Astrometric measurements have been made over the last hundred years or so, and a few of the nearby stars whose motions in the sky were followed seemed to show such wobbling.

9. Timing of Radio Pulsars • The first extra-solar planet was discovered in 1991 around a pulsar, a weird kind of collapsed star that gives off extremely regular pulses of radio waves with a period that is a fraction of a second. • The pulses came more frequently for a time and then less frequently in a regular pattern. • So the planet around this pulsar must be first causing the pulsar to move in our direction, making the pulses come more rapidly, and then causing it to move in the opposite direction, making the pulses come less often.

10. Transiting Planets • Since 1999, astronomers have not had to resort only to periodically changing Doppler shifts to detect exoplanets. • One planet was discovered to have its orbit aligned so that the planet went in front of the star each time around—that is, it underwent a transit. • The dip in the star’s brightness of a few per cent can be measured not only by professional but also by amateur astronomers (see figures).

11. Transiting Planets • During the past few years, several additional examples of transiting planets have been found. • Many more exoplanets will be discovered in this way from the ground and from space. • This method is analogous to observing the transit of Venus across the disk of our Sun (see figure). • Progress in the search for exoplanets is so rapid that you should keep up by looking at the Extrasolar Planets Encyclopaedia at http://www.obspm.fr/planets and the Marcy site at http://exoplanets.org, both linked through this book’s web pages.

12. The Nature of Exoplanet Systems • We have discovered enough exoplanets to be able to study their statistics. • About 1 per cent of nearby solar-type stars have jovian planets in circular orbits that take between 3 and 5 Earth days. • These are sometimes called “hot Jupiters,” since they are so close to their parent stars (within 1/10 A.U.) that temperatures are very high. • Another 7 per cent of these nearby stars have jovian planets whose orbits are very eccentric. • As we observe for longer and longer periods of time, we have better chances of discovering planets with lower masses or with larger orbits.

13. Brown Dwarfs • Some of the objects found with the Doppler shift technique might actually be too massive to be true “planets” (more than 13 Jupiter masses). • If so, they are brown dwarfs, which are in some ways “failed stars.” • Each of the brown dwarfs has less than 75 Jupiter masses (or 7.5 per cent the mass of the Sun), which is not enough for them to become normal stars. • Brown dwarfs can be thought of as the previously “missing links” between normal stars and planets.

14. Brown Dwarfs • Most exciting, a planet next to a brown dwarf (presumably orbiting it) has been imaged (see figure), the first planet to be imaged around any star. • The star with its planet is 200 light-years away from us; observations from the ground and from space have shown that the two objects are moving through space together. • The planet is 55 A.U. away from its parent star.

15. Planetary Systems in Formation • We are increasingly finding signs that planetary systems are forming around other stars. • One of the first signs was the discovery of an apparent disk of material around a southern star in the constellation Pictor (see figures, right). • The best observations of it were made with the Hubble Space Telescope, and may show signs of orbiting planets. • An even nearer planetary disk has been found, enabling observations with higher resolution (see figure, left).

16. Planetary Systems in Formation • Other images of regions in space known as “stellar nurseries” show objects that appear to be protoplanetary disks (see figures, top). • Observations reported in 2005 show that these objects contain about as much mass as a planetary system, clinching the idea that they are locations where planets are forming. • Locations where there are planets in formation glow in the infrared (see figure, bottom).

17. Planetary Systems in Formation • The Hubble Space Telescope has imaged such a ring of dust around the nearby star Fomalhaut (see figure), recording signs that a planet is tugging on it gravitationally.

18. Our Star: The Sun

19. What Is the Sun’s Basic Structure? • We think of the Sun as the bright ball of gas that appears to travel across our sky every day. • We are seeing only one layer of the Sun, part of its atmosphere. • The layer that we see is called the photosphere, which simply means the sphere from which the light comes (from the Greek photos, “light”); it is typically called the Sun’s “surface.”

20. What Is the Sun’s Basic Structure? • The photosphere is about 110 Earth diameters across, so over a million Earths (110 cubed) could fit inside the Sun! • As is typical of many stars, about 92 per cent of the atoms and nuclei in the outer parts are hydrogen, just under 8 per cent are helium, and a mixture of all the other elements makes up the remaining approximately two tenths of one per cent. • We basically find out about the chemical composition of the Sun and stars by studying their spectra.

21. What Is the Sun’s Basic Structure? • The Sun is sometimes considered a typical star, in the sense that stars much hotter and much cooler, and stars intrinsically much brighter and much fainter, exist. (Actually, there are many more stars that are fainter and cooler than there are stars like our Sun or hotter.) • Radiation from the photosphere peaks (is brightest) in the middle of the visible spectrum. Eyes can see!

22. What Is the Sun’s Basic Structure? • The Sun is white, not yellow, though it is often thought of as being yellow. • It only appears yellow, or even orange or red, when it is close to the horizon: The blue and green light is selectively absorbed and scattered by Earth’s atmosphere. The scattered blue light produces the color of the daytime sky.

23. What Is the Sun’s Basic Structure? • Beneath the photosphere is the solar interior (see figure). • All the solar energy is generated there at the solar core, which is about 10 per cent of the solar diameter at this stage of the Sun’s life. • The temperature there is about 15,000,000 K and the density is sufficiently high to allow nuclear fusion to take place.

24. What Is the Sun’s Basic Structure? • The photosphere is the lowest level of the solar atmosphere. • Though the Sun is gaseous throughout, with no solid parts, we still use the term “atmosphere” for the upper part of the solar material because it is relatively transparent. • Just above the photosphere is a jagged, spiky layer about 10,000 km thick, only about 1.5 per cent of the solar radius. • This layer glows colorfully pinkish when seen during an eclipse (see figure), when the photosphere is hidden, and is thus called the chromosphere (from the Greek chromos, “color”).

25. What Is the Sun’s Basic Structure? • Above the chromosphere, a ghostly white halo called the corona (from the Latin, “crown”) extends tens of millions of kilometers into space (see figure). • The corona is continually expanding into interplanetary space and in this form is called the solar wind. • The Earth is bathed in the solar wind, a stream of particles having many different speeds.

26. The Photosphere • The Sun is a normal star with a surface (photospheric) temperature of about 5800 K, neither the hottest nor the coolest star. • The Sun is the only star sufficiently nearby to allow us to study its surface in detail. • We can resolve parts of the surface about 700 km across, roughly the distance from Boston to Washington, D.C. • When we study the solar surface in white light—all the visible radiation taken together—with typical good resolution (about 1 arc second), we see a salt-and-pepper texture called granulation (see figure).

27. The Photosphere • The effect, caused by convection, is similar to that seen in boiling liquids on Earth: Hot pockets (cells) of gas are more buoyant than surrounding regions, so they rise and deposit their energy at the surface. • This causes them to become denser, and so they subsequently sink.

28. The Photosphere • On close examination, the photosphere as a whole oscillates—vibrating up and down slightly—as can be studied using the Doppler effect. • The first period of vibration discovered was 5 minutes long, and for many years astronomers thought that 5 minutes was a basic duration for oscillation on the Sun. • However, astronomers have since realized that the Sun simultaneously vibrates with many different periods, and that studying these periods tells us about the solar interior. • The method works similarly to the way terrestrial geologists investigate the Earth’s interior by measuring seismic waves on the Earth’s surface; the studies of the Sun are thus called, by analogy, “solar seismology” or helioseismology (after Helios, the Greek Sun god).

29. The Photosphere • Studies of solar vibrations thus far have told us about the temperature and density at various levels in the solar interior, and about how fast the interior rotates (see figure). • Since other stars probably behave like the Sun, we are learning about the interiors of stars in general. • Helioseismology can even be used to image, though not with high resolution, what the back side of the Sun looks like.

30. 10.1a The Photosphere • The spectrum of the solar photosphere, like that of almost all stars, is a continuous spectrum crossed by absorption lines (see figure; Also see the discussion in Chapter 2). • Hundreds of thousands of these absorption lines, which are also called Fraunhofer lines, have been photographed and catalogued. • They represent sets of spectral lines from most of the chemical elements. • Iron has many lines in the spectrum. • The hydrogen lines are strong but few in number. • From the spectral lines, we can figure out the relative abundances (the percentages) of the elements. • Not only the Sun but also all other ordinary stars have only absorption lines in their spectra.

31. The Chromosphere • When we look at the Sun through a filter that passes only the red light from hydrogen gas, the chromosphere is opaque. • Thus through a hydrogen-light filter (see figure), our view is of the chromosphere. • The view has brighter and darker areas, with the brighter areas in the same regions as “sunspots”, but the chromosphere looks different from the photosphere below it.

32. The Chromosphere • For example, under high resolution, we see that the chromosphere is not a spherical shell around the Sun but rather is composed of small “spicules.” • These jets of gas rise and fall, and have been compared in appearance to blades of grass or burning prairies (see figures).

33. The Chromosphere • Spicules are more or less cylinders about 700 km across and 7000 km tall. • They seem to have lifetimes of about 5 to 15 minutes, and there may be approximately half a million of them on the surface of the Sun at any given moment. • They are about the same size as the granules, and are matter ejected into the chromosphere, presumably from the boiling effect that also makes the granules.

34. The Chromosphere • Chromospheric matter appears to be at a temperature of 7000 to 15,000 K, somewhat higher than the temperature of the photosphere. • Ultraviolet spectra of stars recorded by spacecraft have shown unmistakable signs of chromospheres in Sun-like stars. • Thus by studying the solar chromosphere, we are also learning what the chromospheres of other stars are like.

35. The Chromosphere • Studying the solar chromosphere once led to a major discovery: that of helium. • A yellow spectral line was seen in the chromosphere at a nineteenth-century total solar eclipse, and didn’t quite match the known yellow lines from sodium. • The gas was called “helium,” after the Greek Sun god, Helios, since it was known only on the Sun. • It took decades before helium was isolated on Earth by chemists.

36. The Corona • During total solar eclipses, when first the photosphere and then the chromosphere are completely hidden from view, a faint white halo around the Sun becomes visible. • This corona is the outermost part of the solar atmosphere, and technically extends throughout the Solar System. • At the lowest levels, the corona’s temperature is about 2,000,000 K. • The heating mechanism of the corona is magnetic.

37. The Corona • The Extreme-ultraviolet Imaging Telescope (EIT) on the SOHO spacecraft images the corona every few minutes. • By looking through filters that pass only light given off by gas at a very high temperature, the spacecraft can make images of the corona even in the center of the Sun’s disk (see figure).

38. The Corona • On the occasional dates of solar eclipses, these images can be lined up with the eclipse images, to allow astronomers to trace many of the coronal streamers seen during an eclipse back to their roots on the Sun’s surface (see figure). • Even though the temperature of the corona is so high, the actual amount of energy in the solar corona is not large. • The temperature quoted is actually a measure of how fast individual particles (electrons, in particular) are moving.

39. The Corona • The corona has less than one-billionth the density of the Earth’s atmosphere, and would be considered a very good vacuum in a laboratory on Earth. • For this reason, the corona serves as a unique and valuable celestial laboratory in which we may study gaseous “plasmas” in a near-vacuum. • Plasmas are gases consisting of positively and negatively charged particles and can be shaped by magnetic fields.

40. The Corona • Beautiful long streamers extend away from the Sun in the equatorial regions. • The shape of the corona varies continuously and is thus different at each successive eclipse. • The structure of the corona is maintained by the magnetic field of the Sun.

41. 10.1c The Corona • Among the major conclusions of this research is that the corona is much more dynamic than we had thought. • For example, many blobs of matter were seen to be ejected from the corona into interplanetary space, one per day or so. • These “coronal mass ejections” sometimes even impact the Earth, causing surges in power lines and zapping—even occasionally destroying the capabilities of—satellites that bring you television or telephone calls. • SOHO and other spacecraft far above the Earth in the direction of the Sun give us early warning when solar particles pass them.

42. The Corona • The gas in the corona is so hot that it emits mainly x-rays, photons of high energy. • The photosphere, on the other hand, is too cool to emit x-rays. • As a result, when photographs of the Sun are taken in the x-ray region of the spectrum (from satellites, since x-rays cannot pass through the Earth’s atmosphere), they show the corona and its structure rather thanthe photosphere.

43. 10.1c The Corona • The x-ray images also reveal very dark areas at the Sun’s poles and extending downward across the center of the solar disk. • These dark locations are coronal holes, regions of the corona that are particularly cool and quiet (see figure). • The density of gas in those areas is lower than the density in adjacent areas. • There is usually a coronal hole at one or both of the solar poles. • Less often, we find additional coronal holes at lower solar latitudes.

44. The Corona • The most detailed x-ray images support the more recent ultraviolet high-resolution images in showing that most, if not all, the radiation appears in the form of loops of gas joining separate points on the solar surface (see figure).

45. The Scientific Value of Eclipses • We discussed solar eclipses and eclipse expeditions in Chapter 4. • In these days of orbiting satellites, why is it worth making an expedition to observe a total solar eclipse for scientific purposes? • There is much to be said for the benefits of eclipse observing. • Eclipse observations are a relatively inexpensive way, compared to space research, of observing the outer layers of the Sun. • Artificial eclipses made by spacecraft hide not only the photosphere but also the inner corona. • And for some kinds of observations, space techniques have not yet matched ground-based eclipse capabilities.

46. What Are Those Blemishes on the Sun? • If you examine the Sun through a properly filtered telescope, you may notice some sunspots (see figure), which appear relatively dark when seen in white light. • Sunspots were discovered in 1610, independently by Galileo and by others shortly after the telescope was first put to astronomical use; an occasional sunspot had been seen with the naked eye previously, and Kepler even saw one in 1607 with a pinhole projection.

47. What Are Those Blemishes on the Sun? • Sunspots look dark because they are giving off less radiation per unit area than the photosphere that surrounds them. • Thus they are relatively cool (about 2000 K cooler than the photosphere), since cooler gas radiates less than hotter gas (recall our discussion in Chapter 2). • A sunspot includes an apparently dark central region, called the umbra (from the Latin for “shadow”; plural: umbrae). • The umbra is surrounded by a penumbra (plural: penumbrae), which is not as dark. • Some large sunspots are quite long-lived, lasting for over a month. • If you follow them day after day, you can see how the Sun rotates at their latitudes.

48. Sun Spots:

49. What Are Those Blemishes on the Sun? • To explain the origin of sunspots, we must understand magnetic fields. • When iron filings are put near a simple bar magnet, the filings show a pattern (see figure). • The magnet is said to have a north pole and a south pole, and the magnetic field linking them is characterized by what we call magnetic-field lines (after all, the iron filings are spread out in what look like lines). • The Earth (as well as some other planets) has a magnetic field that has many characteristics in common with that of a bar magnet. • The structure seen in the solar corona, including the streamers, shows matter being held by the Sun’s magnetic field.

50. What Are Those Blemishes on the Sun? • George Ellery Hale showed, in 1908, that the sunspots are regions of very high magnetic-field strength on the Sun, thousands of times more powerful than the Earth’s magnetic field (see figure). • Sunspots usually occur in pairs, and often these pairs are part of larger groups. • In each pair, one sunspot will be typical of a north magnetic pole and the other will be typical of a south magnetic pole. • The strongest magnetic fields in the Sun occur in sunspots. • The magnetic fields in sunspots are able to restrain charged matter; they keep hot gas from being carried upward to the surface. • As a result, sunspots are cool and dark.