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In the upcoming pages, we’ll cover the basics of our solar system and brush up on the planets and some other celestial features.
The basic tool that astronomers use to study celestial objects is the telescope. People tend to be most familiar with optical telescopes, but astronomers use telescopes that span the entire electromagnetic spectrum.
There are three major functions of an astronomical telescope. They are (in order of importance):
Think about trying to fill a canteen with water during a rain shower by allowing rain to fall into the opening of the canteen. Inserting a funnel into the opening of the canteen will allow you to fill it more quickly. The funnel increases the water-gathering power of the canteen. A funnel with a larger area will have more water gathering power. Analogously, telescopes act as light funnels to gather light from a larger area down to the smaller area of the eyepiece or instrument on the telescope.
The light-gathering power of a telescope then allows astronomers to observe fainter stars or galaxies than would be possible with less light-gathering power. The light-gathering power of a telescope depends on the area, or the square of the diameter, of the primary mirror or lens. The primary purpose of most large astronomical telescopes is increased light-gathering power.
The resolving power of a telescope is the ability to see sharp detail. A telescope with good resolving power allows us to see smaller details than a telescope with poor resolving power. The resolving power depends on the diameter of the primary mirror or lens.
However, turbulence in Earth’s atmosphere limits the resolving power of large telescopes. The reason that images from the Hubble Space Telescope are so sharp is that the resolving power is not limited by the atmosphere.
The magnifying power of a telescope is simply how many times bigger the image appears than it actually is. Magnification does nothing to improve the quality of the image; hence it is less important than the other two functions of a telescope. The amount of magnification is determined by the eyepiece. Changing eyepieces changes the amount of magnification.
Many people incorrectly think that magnifying power is the most important function of a telescope. However, magnifying a blurry image will simply produce a bigger blurry image; the other two functions that improve the image quality are more important.
There are two basic types of telescopes. Reflecting telescopes use mirrors as the main light-gathering element; refracting telescopes use lenses. Because there are engineering limitations on how large a lens can be made, most large astronomical telescopes are reflecting telescopes.
Astronomers use telescopes that cover the entire electromagnetic spectrum because celestial objects emit radiation throughout the entire spectrum. At infrared wavelengths, ground-based optical telescopes with minor modifications and special instrumentations are used as infrared telescopes. Some infrared wavelengths are absorbed by the atmosphere and require telescopes launched by satellites.
Most ultraviolet, X-ray, and gamma ray wavelengths are absorbed by the atmosphere. At these wavelengths, astronomers must use satellite-borne telescopes. X-ray and gamma ray telescopes borrow detection techniques from high-energy elementary particle physics and bear little resemblance to optical telescopes.
Radio telescopes are basically large concave radio antennas that look similar to the antennas for satellite TV reception. Radio telescopes can be as large as hundreds of feet in diameter.
Contrary to popular belief, astronomers seldom look through telescopes. Rather, they mount various types of instruments on the end of telescopes. Digital cameras have largely replaced traditional photography. To study chemical compositions of celestial objects, astronomers mount a spectroscope on the end of the telescope. Other types of instruments have more specialized functions, but generally, data is recorded digitally and analyzed by computers.
According to the International Astronomical Union’s (IAU) latest definition (2006), a planet is “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighborhood around its orbit.” The planets in order out from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are two basic classifications of planets in the solar system. The inner planets, which are Mercury, Venus, Earth, and Mars, are all roughly similar in size and composition. Being somewhat like Earth, the inner planets are classified as the terrestrial planets. Earth’s Moon is also classified as a terrestrial planet, and many astronomers consider the Earth and Moon system as a double planet system. Terrestrial planets are basically small balls of rock. Jupiter, Saturn, Uranus, and Neptune form the other major group of planets. Being similar to Jupiter, they are called Jovian planets. They are basically big balls of gas, so they are often referred to as gas giants. Pluto does not fit into this classification scheme. Pluto has properties more like the major moons of the Jovian planets, and in 2006 was reclassified as a dwarf planet.
Astronomers think that the solar system formed when a large cloud of gas and dust began to collapse. Such a cloud is called a nebula, so this theory is called the nebular theory. Once the collapse starts, gravity pulls the cloud particles closer together and toward the center. As it collapses, it begins to spin faster and flatten. Most of the matter was used to form the Sun, and a small amount of the material in the flattened disk spun into eddies and formed the planets. In the inner part of the solar system, the Sun’s heat evaporated most of the hydrogen and helium and other gases. Hence these inner planets are mostly metal and rock, while the outer planets are mostly gases such as hydrogen, helium, ammonia, and methane. Because planets are natural byproducts of the formation of stars, most stars should have planets orbiting them.
As large as it may seem to us, our solar system is on a cosmic scale quite small. Our Sun is one of hundreds of billion stars in our Milky Way galaxy. Our galaxy is only one of an unknown number of billions of galaxies in the universe.
Which best describes the relative size and composition of terrestrial planets compared to Jovian planets?
Answer B is correct. Remember that the four Jovian planets are larger and primarily made up of gas. The Jovian planets are: Jupiter, Saturn, Uranus, and Neptune. The terrestrial planets are: Mercury, Venus, Earth, and Mars.
Although it’s counter-intuitive, the changing distance between the Sun and Earth does not cause the seasons. The earth does follow an elliptical orbit around the Sun, but it is close enough to being circular that the difference in distance cannot cause the seasons.
The tilt of the Earth’s axis and the revolution of the earth around the sun causes the seasons. As the Earth orbits the Sun, the tilt of Earth’s rotational axis causes an apparent north-south movement of the Sun in the sky. On the summer solstice, June 21, Earth’s northern hemisphere is tilted 23.5° towards the Sun and the Sun is in its northernmost position in the sky. Hence in the northern hemisphere, we see the sun higher in the sky, the Sun’s rays strike the northern hemisphere more directly, the daylight periods are longer, and it gets hot.
On the winter solstice, December 21, the reverse is true. The northern hemisphere is tilted 23.5° away from the Sun. Hence we see the Sun lower in the sky, the Sun’s rays strike the northern hemisphere less directly, the daylight periods are shorter, and it gets cold out. On the spring and autumn equinoxes, March 21 and Sept 21, the Sun is directly over the equator. Clearly, it is the tilt of the Earth’s axis and the revolution of the earth around the sun that causes the seasons and also causes the seasons to be reversed between the northern hemisphere and the southern hemisphere. The changing distance between the Earth and the Sun does not cause the seasons.
As the Moon orbits the Earth every month, its position in the sky changes. It rises about forty-eight minutes later every day. Hence depending on where it is in its orbit, the Moon might be seen at any time of the day or night. At any given time, half the Moon is lighted by the Sun and half can be seen from the Earth. But these are not always the same parts of the Moon. If the Moon is directly opposite the Sun in the sky, we see the daylight side of the Moon and call it a full Moon.
If the Moon is between the Sun and Earth, we see only the nighttime side of the Moon and call it a new Moon. If the Moon is at a ninety-degree angle from the Sun in the sky, we see one of the quarter phases. Less than ninety degrees gives us the crescent phases, and more than ninety degrees gives us the gibbous phases. The lunar phases are not caused by the Earth’s shadow falling on the Moon. That is an eclipse.
When the Moon passes between the Sun and Earth and blocks the sun from view, the Moon’s shadow falls on the Earth. That is a solar eclipse. A solar eclipse can only occur during the new Moon, but not during every new moon. The Moon’s shadow may fall above the north pole or below the south pole.
During the full Moon, when the Moon is opposite the Sun in the sky, Earth’s shadow might fall on the Moon. This is a lunar eclipse. We don’t have a lunar eclipse during every full Moon because the shadow may fall above the Moon’s north pole or below its south pole.
Which moon phase is most likely to cause a solar eclipse?
The correct answer is D. A solar eclipse can only occur during the new moon, and a lunar eclipse can only occur during the full moon. Neither solar nor lunar eclipse can occur during the phases listed.
When Copernicus suggested that the planets orbit the Sun, he did not have any idea why. The Copernican model at this point was not a very good scientific model because it lacked a physical explanation. Galileo recognized this fact and began to work on the physics of motion. Newton put it all together with his three laws of motion and his law of gravity.
You can find the words stating these laws in most textbooks, but many people recite the words without understanding them. Don’t be one of those people. Concentrate on the physical meaning of these laws.
In physics, speed and velocity are not the same thing. Velocity includes direction; speed does not. Acceleration is any change in velocity. Note the distinction here; a change in direction at a constant speed is also an acceleration.
The meaning of Newton’s first law, the inertial law, is that any change in velocity requires an external force. An object will move at a constant velocity—rest is a constant velocity of zero—until an external force acts on it. As long as there are no external forces acting on an object, its velocity will not change. If something is slowing down, when no force seems to be acting on it, friction is the external force acting on it.
The second law provides a formula for telling how much force is needed to get a given amount of acceleration. Force equals the mass times the acceleration:
force = mass · acceleration (or) F = ma
You should know from experience that it takes a constant force longer to accelerate a more massive object.
Most people can recite Newton’s third law:
For every action, there is an equal and opposite reaction.
Few people really understand it. If a 200-pound person is standing on a floor, Earth’s gravity is pulling the person down with a force of 200 pounds. The floor holds the person up with a force of 200 pounds. These are equal and opposite forces. They are the set of action-reaction pair required by Newton’s third law. The floor pushes up against the weight of the person, and the person pushes down against the floor.
Remember that an action-reaction pair does not act on the same object. A slightly different situation is if the person is standing directly on the Earth. Now the action force is Earth pulling down on the person, and the reaction force is the person pulling up on the Earth. The Newton’s third law reaction to the Earth pulling the person down is the person pulling the Earth up. A 200-pound person pulls the Earth up with a force of 200 pounds, and the Earth pulls the person down with a force of 200 pounds. If object A acts on object B, the reaction is object B acting on object A. Always! Newton’s third law never requires a third object.
Newton’s law of gravity states that there is a gravitational force between any two objects in the universe. The amount of force is proportional to the product of the masses and inversely proportional to the square of the distance between them.
It is this gravitational force that causes the planets to orbit the Sun, the Moon to orbit the Earth, and so on. According to Newton’s first law, any object, including a planet, will travel in a straight line at a constant velocity unless an outside force acts on it. The Sun’s gravity acts on the planets causing them to deviate from their straight-line motion. Hence they orbit in a circular or elliptical path around the Sun.
Abbé Georges Lemaître, a Belgian who was both a cleric and a scientist, was the father of the big bang model. Lemaître proposed a theory that is consistent both with Hubble’s observation that the universe is expanding and with the theological idea that the universe had a definite beginning point.
According to the big bang theory, the universe had a moment of formation when it appeared as an initial fireball. What caused this primeval fireball and what existed before the fireball came into existence is beyond the realm of questions science can answer. The fireball caused everything we can see in the universe today to expand away from the site of the fireball; we still observe that the universe is expanding.
This expanding fireball is often presented as a giant explosion, but this needs some rethinking. In an explosion, such as a bomb or firecracker, matter expands to fill up space that is already there. In the expanding universe, space is expanding so that the galaxies in space are moving farther apart. A better analogy than an explosion is a loaf of raisin bread. The raisins are spread throughout the bread and move farther apart as the dough rises. Blowing up a balloon with little galaxies drawn on it also makes a handy classroom analogy.
During the first few minutes after the primeval fireball began expanding, the matter in the universe was made. Protons, electrons, and neutrons were made. The only elements made during the big bang were hydrogen, helium, and trace amounts of lithium and beryllium. All other elements were manufactured later in stars.
As the universe continued to expand and cool, matter began to clump. Large clumps formed galaxies, such as our own Milky Way galaxy, which contains hundreds of billions of stars. We think all galaxies were formed about the same time during the early history of the universe. When galaxies are young, they often go through very energetic stages when they emit far more energy from their nuclei than can easily be explained. Galaxies in this stage are quasars. The energy source for quasars, which is probably a supermassive black hole, tends to reduce the amount of energy it emits as it ages. Hence quasars existed only during the early history of the universe. Because of light travel time from the far reaches of the universe, we only see quasars at great distances. Closer galaxies have finished with the quasar stage.
The first generation of stars that formed in our galaxy were mostly hydrogen and helium, because that was what was made during the big bang. The nuclear fusion reactions that create stars fuse these lighter elements into heavier elements. When stars run out of hydrogen fuel in their cores, they expand into red giants, which might be about the size of Earth’s orbit around the Sun (or larger). Stars of about the Sun’s mass can then fuse helium to carbon, and some oxygen, in their cores. These stars will then collapse into slowly cooling, burned-out stars about the size of Earth that are called white dwarfs. If the white dwarf is more than 1.4 times the mass of the Sun, it can not be a stable white dwarf and collapses into a neutron star. The protons and electrons are squeezed together into neutrons, and the neutron star collapses to about the size of a city. During the transition from red giant to white dwarf, many stars gently blow off their outer layers to form a planetary nebula, which is a shell of gas around a dying star.
Stars that are ten or more times the mass of the Sun can fuse elements heavier than carbon in their cores. The extra mass provides enough gravitational force needed to squeeze these heavier elements to undergo fusion, until elements about as heavy as iron on the periodic table are made. Iron is the boundary between fission and fusion; so fusing elements heavier than iron requires, rather than releases, energy. When the iron core builds up in these massive stars, they explode in a humongous explosion called a supernova.
The Crab Pulsar lies at the heart of the nebula and spins thirty times each second.
A supernova releases as much energy in a year as the Sun does in its 10 billion year lifetime. There is plenty of energy to make all the elements that will normally not fuse. The supernova also blasts these heavy elements out into space to be recycled into the next generation of stars. The Crab Nebula is the remnant of a supernova that occurred on July 4, 1054. The material, enriched in heavy elements, will be recycled into nebulas such as the Orion Nebula that is in the process of forming new stars. Our solar system formed from a nebula that was also enriched in heavy elements by a prior generation of stars that exploded as supernovas.
The core that remains after the supernova explosion will collapse into a neutron star, which is a ball of neutrons with the mass comparable to the Sun but compressed to the size of a city. If the core is more than about two to three times the mass of the Sun, it will collapse into a black hole. To become a black hole, the Sun would have to be compressed to a radius of fewer than three kilometers. A black hole is so highly compressed that its escape velocity exceeds the speed of light. Hence nothing can escape!
Because they manufacture and recycle heavy elements, supernovas play a crucial role in how you got here. The atoms in your body heavier than hydrogen were manufactured in the cores of stars and recycled by supernova explosions.
