FINAL EXAM STUDY GUIDE
1. Astronomy is a science that studies the physical universe and all natural objects beyond Earth’s atmosphere.
2. Why is astronomy different from astrology? Astronomy is a science, whereas astrology is a pseudoscience claiming divination of human affairs by interpreting the movements of heavenly bodies. 3. Constellations are patterns of bright stars in the night sky named by human imagination. Are they visible with the naked eye, that is, without the aid of a telescope? (Yes/No)
4. Name three constellations visible from El Paso. Orion, Big Dipper, Cassiopeia, and Leo 5. The Sun, the Moon, stars, planets, galaxies and all astronomical sources rise in the east and set in the west every day. It takes the celestial sphere and all of them twenty-four hours to go once “around us”. 6. The three brightest celestial bodies seen from Earth are: The Sun, the Full Moon and planet Venus. 7. The only source that never rises or sets is a star that remains fixed in the sky. What is it called? Polaris 8. Approximately, how high in the sky is the star on #7 above as seen from El Paso? 32° 9. How high in the sky is the star on #7 above as seen from Earth’s North Pole? 85° to 90° How high in the sky is the star on #7 above as seen by someone near Earth’s equator? 0° to 5° 10. What are the coordinates (altitude and azimuth) of a planet 15° above the western horizon? altitude: 25° azimuth: 270°
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11. The apparent magnitudes (m) of four objects are given. Sort them from brightest to dimmest: m = 0.03, m = 8.7, m = -2.1, m = -0.04. m = -2.1, m = -0.04, m = 0.03, m = 8.7 Which is/are visible to the naked eye? m = 8.7
12. The apparent (m = 3.7) and absolute (M = 1.8) magnitudes of a star are given. How far away is it? (Hint: no formula is needed; a simple comparison of m and M tells you if the star is exactly 10 parsecs [m=M], less than 10 parsecs [m<M] or more than 10 parsecs [m>M] away). more than 10 parsecs 13. During what lunar phase is the Moon unobservable from Earth? new moon
14. During what lunar phase does the Moon rise when the Sun sets? full moon
15. Why does the Moon show phases? (the answer is not that Earth blocks it out or casts its shadow on it!) The Moon has phases because it orbits Earth, which causes the portion we see illuminated to change. 16. The tilt of Earth’s axis explains the seasons … but how?
Earth has seasons because the axis is tilted, the planet rotates on its axis as it orbits the Sun. 17. Earth’s orbit about the Sun is a slightly eccentric ellipse, almost a circle. This means Earth gets slightly closer and slightly farther away from the Sun throughout the year. On average, how far away from the Sun is planet Earth in kilometers? 150 million km We call this distance 1 astronomical unit (1 AU). 18. This semester, two astronomers were periodically mentioned in class practically from beginning to end: the first man in history who used a telescope with scientific goals and the discoverer of the three laws that govern the basic motions of planets, asteroids, comets, stars, etc. They are Galileo Galilei and Johannes Kepler. Don't forget about the age old question of economic models are simplified versions of reality
19. In a time when observational proof was particularly important to confirm whether the Sun and the planets orbit the Earth or the planets and Earth itself orbit the Sun, the telescope provided evidence that indeed Earth and the planets orbit the Sun. Describe the two telescopic observations/discoveries by Galileo in support of this heliocentric (sun-centered) model. Hint: They involve observations of planets Jupiter and Venus.
Galileo observed that Jupiter has four moons and that Venus has phases
20. Two basic, observable differences between planets and stars in the sky are:
a. Planets appear [dimmer, redder, rounder] brighter than stars. That’s because being in the solar system, they are much closer to us than stars.
b. Planets “wander” (move) against the background of the stars whose positions in the sky, being so remote, do not seem to change. If you want to learn more check out arts and entertainment management pace
21. Since the very first naked-eye observations of the planets, it was found they not only “wander”, but also trace “loops” in the sky every now and then. These observed loopy trajectories confounded astronomers. Such trajectories cannot be simply described as smooth, single circles, as originally assumed by the Greeks.
a. How were these trajectories understood still in terms of circles in the old geocentric model where the Sun and planets were assumed to orbit the Earth? (Hint: Two circles)
The Greeks introduced the idea of epicycles and deferents – the circle-over-circle trajectories. The center of a small circle, the epicycles, orbits around a larger circle, the deferent, while the planet resolves about the small one. We also discuss several other topics like
b. How do we explain these loopy trajectories today once we knew Earth and the planets orbit the Sun? (Hint: Different planets orbit the Sun at different speeds)
Seen from Earth, planets make “loops” in the sky, an apparent motion caused when one planet overtakes another as they orbit the sun at different speeds, which came to be known as retrograde motion.
22. Nicolaus Copernicus was as Danish astronomer of the late 1500’s, before the telescope era. He provided Kepler with the best naked-eye observations of the motions of planets that led him to the discovery of three laws that explain how planets and other objects orbit the Sun. Don't forget about the age old question of hun 1201 final exam
23. Briefly describe Kepler’s first law of motion.
Planets have elliptical orbits with the sun at one focus.
24. Briefly describe Kepler’s second law of motion.
Planets speed up in their orbits as they approach the Sun and slow down as they recede from it. 25. Briefly describe Kepler’s third law of motion.
The size of a planet determines its orbital period; the farther out a planet is, the bigger its orbit. 26. An ellipse is an oval, a squashed circle. Would you say the elliptical orbits of the planets in the solar system are little eccentric or very eccentric? Why? (Hint: Eccentricity numerical values) The elliptical orbits of the planets in the solar system have little eccentricity because the closer a planets eccentricity is to zero, the more circular its orbit is. We also discuss several other topics like
Fill in the three blanks on #27 below with this list of people and countries:
Newton Einstein Copernicus Galileo France Holland Italy
27. The telescope was invented in Holland in the early 1600’s. Galileo was the first human being who systematically pointed a telescope to the skies. Newton invented the reflecting telescope in the mid 1600s.
28. Traditionally, telescopes come in two types: reflecting, those based on lenses and refracting, based on mirrors.
29. Among those based on mirrors, two optical designs are most popular: refracting, the original design by Isaac Newton and reflectors, the one preferred for professional research at modern observatories. These are characterized by the circular “hole” at the very center of the telescope’s primary mirror.
30. What does a telescope do? By collecting and focusing light, it creates an image of a distant source that can be stored, photographed, analyzed, etc. The quality of this image is given by the telescope’s two most important optical properties:
a. Light-Gathering Power: How much light the telescope can collect. The larger the telescope’s aperture (diameter), the more light is collected and the brighter and sharper the resulting image. That’s why a larger telescope is better than a small one.
b. Resolving Power: The telescope’s power to see fine details in astronomical sources. The larger the telescope, the more details the telescope can see in astronomical objects.
In short, larger apertures (diameters) imply both brighter and more detailed images. 31. At least from the astronomical standpoint, a telescope’s magnifying power or magnification is not as important as its light-gathering or resolving power (See #30 Above), since images can be magnified or demagnified at will in the observatory by means of a computer program.
32. The largest ground-based optical telescopes to date are:
GTC (Canary Islands) and the Keck Observatory (Hawaii).
33. Name two forms of high-energy light. (Example: visible light, infrared, etc.) gamma ray, x-ray 34. Name two forms of low-energy light. (Example: visible light, x-rays, etc.) radio, microwave 35. Light takes different forms, all traveling at the same speed of 300,000 km/s. Although their speed is
constant, they show different frequencies and wavelengths, such that, a radio wave has a long (short/long) wavelength and a low (low/high) frequency. A gamma ray has a short wavelength and a
high frequency. As one increases, the other decreases, and vice versa. In the end, the product of wavelength times frequency (the speed of light), remains the same for all forms of light. 36. What are the most important features of the New Generation Telescopes (NGT’s)? New Generation Telescopes feature larger apertures with diameters of 8m or larger; segmented, thinner, and lighter mirrors made of low thermal expansion glass like zerodur; and adaptive optics which are deformable mirrors and computer-controlled pistons that are used to reshape mirrors and compensate for the turbulence in the Earth’s atmosphere.
37. The color of a star is simply determined by its surface temperature.
38. Antares is a red star in constellation Scorpius. Vega is a blue star in constellation Lyra. Which of the two stars is cooler? Antares That’s because the cooler the star, the dimmer and redder its color, or the hotter the star, the brighter and bluer its color.
Fill in the six blanks below with these six words:
chemical spectrum dark rainbow atmosphere light
39. Absorption lines are dark lines observed in the spectrum of a star once its light (collected by a telescope) is broken up into the colors of the rainbow. It turns out these lines reveal the chemical composition of the star’s outermost gaseous layer, its atmosphere.
40. The Doppler effect explains why the dark lines in a star’s spectrum appear red (blue/red)shifted in a source receding from us in space, whereas those from an approaching source show a characteristic blue (red/blue)shift.
41. Name the five planets visible from Earth to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Since they are all detectable without a telescope, they were reported by civilizations in the past. 42. The brightest planet and third brightest astronomical source after the Sun and the Full Moon is Venus. That’s because this planet is fully covered in gasses, which reflect the sunlight very efficiently; besides, it’s the brightest planet to Earth.
43. Uranus was the first planet discovered with a telescope, it’s too distant and faint to see with the naked eye. Unlike the rest of the planets of the solar system, Uranus is practically “tipped on its side.” 44. Neptune was originally a pure theoretical prediction by two independent European astronomers, who applied Newton’s Gravitational Theory to conclude that the mismatch between the observed and precise orbit of Uranus could only be due to the presence of a massive planet beyond, not yet discovered. They provided the predicted coordinates of this remote object and Neptune was finally discovered in 1846. 45. Pluto was discovered by Clyde Tombaugh on photographic plates in 1930. He compared two photos of the same region of the sky, one week apart in time. He then noticed the only little dot that moved against the background of stars. It was Pluto, reclassified in 2006 as a dwarf planet, the first of this new kind of objects.
46. Name three dwarf planets of the solar system. Ceres, Pluto, Planet X
47. Suppose a new object is discovered orbiting the Sun beyond Pluto. It shows a near-spherical shape and it’s found to be embedded in the Oort Cloud. What kind of object is this (planet, dwarf planet, comet, asteroid)? Why? The new object would be considered a dwarf planet because it meets the three criteira: it orbits the Sun, it acquired a spherical shape, and its surrounded by debris.
48. What kind of concrete evidence today tells us there was liquid water on Mars in the past? The evidence of ice in the polar regions and the evidence of water and ice trails on the planet’s surface indicates that water was once present on Mars.
49. In a system of two gravitationally-bound objects such as a binary system of stars, the center of mass (CM), also called center of gravity, is a point of balance between the two objects, such that the most massive (heaviest) one lies closer to the CM, tracing a smaller ellipse about it, whereas the least massive one is farther away, tracing a bigger ellipse. Also, the heaviest member of the system moves slower as the least massive one has to catch up and necessarily move faster.
50. In a binary system like #49 above, the time both objects take to orbit their common center of mass is known as the origin, and it’s the same for both. If, for example, the binary system consists of a star accompanied by an extrasolar planet, and it takes the star 35 days to orbit the CM, then the extrasolar planet will orbit the CM in 35 days too.
51. Explain why we can’t just point the largest telescopes to the stars and discover as many planets as we want around those stars by taking pictures of them.
Earth’s atmosphere acts like a filter for most forms of electromagnetic radiation, light, from celestial bodies. These frequencies cannot make it to the ground because they are absorbed by the atmosphere. 52. Approximately, how many extrasolar planets have been discovered to date? 3500 53. Fundamentally, two indirect methods have been used since 1995 to discover planets around other stars: the radial velocity (Doppler) technique and the transit method.
54. The radial velocity technique to discover extrasolar planets works like this:
a. A nearby star is observed and its light is broken up to get its spectrum. (Hint: Colors of the rainbow)
b. Spectral, dark (dark/bight) lines are identified, superimposed to the bright background of colors. c. Observed over several days or weeks, such lines seem to show periodic shifts to the blue and red parts of the spectrum, indicating these stars approach us and recede from us in a periodic fashion, according to the well-known Doppler effect.
d. From c. above, we conclude the star must be wobbling in space.
e. If the star is wobbling, there must be an invisible object (most likely, a planet) around it whose gravitational tug on the star is strong enough to make it wobble.
f. The invisible planet’s mass and period are directly measured from the velocity curve obtained at the observatory.
55. Describe how the second method to discover extrasolar planets on #53 above works. Changes in the intensity of light from stars while being blocked by an orbiting opaque body can be measured to reveal the planet’s existence and size.
56. What additional information of the discovered planet do we get from the second method on #53 above? The planet’s presence, mass, orbital period and size are all derived from the periodic Doppler shifts of lines in the star’s spectrum and the luminosity drop in the star’s light curve.
57. What’s 51 Peg b? A Hot Jupiter? A Super-Earth? A “normal” Jovian planet? Hot Jupiter 58. What are the two most populated categories of planets found to date? (Hot Jupiter’s, Super-Earths, etc.) Hot Jupiter’s and Super-Earths
59. The formation of rings around Jovian planets is related to the planet’s Roche Limit. Explain how a ring forms. A ring forms when the distance which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body’s tidal forces exceeding the first body gravitational self attraction.
60. The Moon always shows the same hemisphere to us, known as the near side of the Moon. That’s because the Moon spins about its own axis in the same time period it orbits the Earth (approximately 27 days). This is called synchronous rotation.