Normal GalaxiesMilky Way GalaxyNormal Galaxies - SpiralNormal Galaxies - EllipticalHome PageGALAXIES
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Q1. How is the mass of a galaxy measured? What is the unexpected result of such measurements? Answer
Q2. All large galaxies are composed of two basic components: a disk and a spherical component. Describe the properties of these two parts of a galaxy. Answer
Q3. How do we know that galaxies are much more massive than indicated from the number of stars they contain? Answer
Q4. Compare and contrast the observed properties of spiral and elliptical galaxies, including shape, color, amount of gas & dust, and range of sizes or masses. Answer
Q5. What is the evidence for dark matter in galaxies? Answer
Q6. Most observed galaxies are spiral galaxies, yet astronomers believe they make up only about 15% of all galaxies. How can you explain that discrepancy? Answer
Q7. What is meant by the term "missing mass" in a galaxy? How do we know it is there if we can’t see it? Answer
Q8. Two galaxies are observed. The first has large clouds of gas and dust with active star formation; the second is composed of old stars and has no active star formation occurring. Which of these galaxies is a spiral and which and elliptical? Explain. Answer
Q9. Broadly defined, what are the two major differences in the appearance of spiral and elliptical galaxies? Answer
Q10. Why are spiral galaxies bluer than elliptical galaxies? Answer
Q11. How do we know that galaxies contain massive halos? How do we know that there are supermassive black holes in their centers? Answer
Q12. How can it be that 70% of all observed galaxies are spiral galaxies, but spirals account for only about 15% of all galaxies? Answer
Q13. Why are spiral galaxies the most commonly observed galaxies even though they are not the most common type of galaxy? Answer
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Q1. If we could see the Milky Way from the outside, what shape would it have? What simple observation (using just the naked eye) tells us about the shape? Answer
Q2. Describe the appearance of the Milky Way galaxy, both as seen from the inside and as seen from the outside. Answer
Normal Galaxies - Spiral
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Q1. By which two processes do spiral arms form in a galaxy? Answer
Q2. Why do we see spiral arms in that type of galaxy, that is, what is it about the objects in a spiral arm that make them noticeable? Answer
Q3. Two mechanisms are thought to be responsible for the formation of spiral patterns in galaxy disks. For either one of these mechanisms, explain how it produces a spiral pattern. Answer
Normal Galaxies -
Elliptical Questions
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Q1. Why are giant galaxies usually located near the center of a large cluster of galaxies? Answer
Normal Galaxies -
Colliding Questions
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Q1. How can a galaxy increase the amount of matter it contains? Answer
Normal Galaxies -
Formation Questions
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Q1. Rotation plays a critical role during the formation of galaxies. What two principles describe the properties of a rotating body? Answer
Q2. Observations indicate that galaxies in the very early universe were much smaller than galaxies today, with only a few percent the mass of an average modern galaxy. Discuss this new finding in the context of the theories of galaxy formation. Answer
Q3. Why does the rate of rotation of a collapsing object increase? Answer
Q4. What role does the time at which star formation begins in a galaxy have on the type of galaxy it becomes? Answer
Q5. Explain why rotation flattens an object. Answer
Q6. Why does a collapsing object begin to spin more rapidly? Answer
Normal Galaxies -
Black Holes Questions
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Q1. Recent observations suggest that supermassive black holes exist in the centers of galaxies. What is the nature of those observations and how do they lead to this conclusion? Answer
Normal Galaxies Answers
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A1. The mass of a galaxy is measured by determining how fast the galaxy rotates at a given distance from the center of the galaxy. The faster it rotates, the stronger gravity is at that point, which means there is more matter inside that position. When the rotation is measured far outside the visible galaxy, the rate of rotation is still quite large, indicating that the mass of the galaxy is much larger than can be accounted for by the visible stars and gas.
A2. The disk of a galaxy is a thin, pancake shaped distribution of stars with lots of gas and dust clouds. The stars are often distributed in a spiral pattern in the disk. The spherical component is more rounded, and can be anywhere from perfectly spherical to significantly flattened. It contains many stars but no clouds of gas and dust, and therefore appears much smoother. The overall appearance of a galaxy depends upon which of these two components dominates the light.
A3. The mass of a galaxy can be determined by observing its rotation curve – the speed of rotation as a function of distance from the center of the galaxy. All galaxies exhibit high rotation velocities at large distances from their centers, which suggests that there is a strong force of gravity far from their center. This force of gravity can only be explained if the mass of the galaxy is much greater than would be exerted by the visible stars of the galaxy.
A4. Spiral galaxies have a prominent disk of stars, gas, and dust, with a central bulge. Elliptical galaxies are more rounded and uniform in shape, ranging from perfectly spherical to fairly elongated. The disks of spiral galaxies are blue, because of the massive hot stars that have formed in the clouds of gas and dust found there. The central bulge of a spiral galaxy is red in color, because no star formation occurs there. Elliptical galaxies have no clouds of gas and dust, no star formation, and no hot blue stars. Hence, they are red in color. Elliptical galaxies span a wide range of masses, from tiny galaxies of only a million solar masses to huge giant galaxies with a trillion solar masses of matter. Spiral galaxies cover a more narrow range of masses, generally from a billion to a few hundred billion times the mass of our sun.
A5. The rate of rotation of objects found outside the visible edge of galaxies does not diminish with increasing distance as it should. This implies that the force of gravity experienced by this matter is just as large as that experienced by matter closer to the galaxy. The only way this could occur is for there to be a significant amount of matter outside the visible galaxy to exert this extra force of gravity.
A6. All spiral galaxies are at least modest in size and brightness, and are thus easy to observe even at fairly large distances. Many elliptical galaxies are quite small and dim. They can only be seen when they are very close to us. Because of this difference in average brightness, we see many more spiral galaxies in casual surveys of the sky. However, when care is taken to include all galaxies within a certain volume, elliptical galaxies outnumber the spirals.
A7. When the mass of a galaxy is measure, it is much greater than can be accounted for by all the stars and luminous matter visible within the galaxy. We know this extra matter is present from the gravitational force it exerts on the matter which we can see.
A8. The first galaxy is a spiral galaxy. The clouds of gas and dust and clumps of newly formed stars will give it the patchy appearance characteristic of spiral galaxies. Elliptical galaxies have no clouds of gas and dust and no star formation. Hence, they appear more uniform and have no active star forming regions today. The second galaxy is an elliptical galaxy.
A9. Elliptical galaxies do not possess a disk and are uniform in brightness; spiral galaxies do have a disk and a patchy in appearance.
A10. Spiral galaxies contain enough gas for stars to form within them. Their light is dominated by the light of young, massive, blue stars. Elliptical galaxies are not presently forming stars. Without any young, blue stars, their light is dominated by older red giant stars.
A11. The rotation velocity of clouds of gas can be measured far from the center of a galaxy. Since the velocity of even the most distant clouds remains fairly high, this tells us that the force of gravity at that distance remains high, which can only mean there is a great deal of unseen matter creating that force of gravity. Rapid orbital motion very near the center of a galaxy also tells us that the force of gravity there must be quite large. Stars near the center of the galaxy are orbiting an unseen, massive object in the center of the galaxy.
A12. Spiral galaxies tend to be fairly large and bright, and are therefore easy to see. Elliptical galaxies may be either very large or very small. The small elliptical galaxies are so dim they can only be seen when very close to us. As a result, we do not see many of the elliptical galaxies.
A13. Spiral galaxies tend to be fairly large in size and brightness, which makes them more easily visible to astronomers. Elliptical galaxies are much more numerous, but span a much wider range of sizes and brightnesses. The very common but very small elliptical galaxies can only be seen when they are very close to us, so general surveys of galaxies often miss them.
Milky Way Galaxy Answers
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A1. From the outside, the Milky Way would look like a pancake – thin as seen from the side and round as seen from above – with a small central bulge – like a dollop of butter in the center of the pancake. The appearance of the visible Milky Way across the sky (the faint, narrow band of stars visible from a dark location) tells us that the galaxy is flat.
A2. The Milky Way appears in our sky (from the inside) as a faint fuzzy band of light which stretches across the sky. In a small telescope, this fuzzy band is resolved into millions of distant stars. Seen from the outside, this band of light would be seen as a disk of stars (with a spiral pattern), shaped like a pancake, with a noticeable bulge in the center.
Normal Galaxies - Spiral Answers
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A1. Some galaxies have spiral density waves in the gas of the disk. A density wave is simply a moving region of slightly greater density. These higher densities encourage star formation. Spiral arms may also be formed when one massive star forms and then dies in a supernova explosion. The expanding cloud from this event will trigger more star formation. The rotation of the galaxy will slowly spread this group of stars into a little fragment of a spiral arm because those stars closer to the center will rotate a bit faster than those further out. This process may repeat for several generations of stars.
A2. The spiral arms of a galaxy show the location of current star formation. Since some new stars are very bright upper main sequence stars, they are very noticeable and out shine a larger number of dimmer stars around them. There are just as many stars in between the spiral arms, but the very bright stars that might have been there once have long since died and disappeared.
A3. Density waves of gas may be set up in the rapidly spinning disk of a galaxy. The gas in the disk will be slightly more concentrated in these waves, so star formation will be favored there. A spiral pattern will result from the very bright massive stars that will occasionally form there.
The differential rotation of the disk (inside rotates faster than the outside) will gradually stretch a large cloud of gas into an elongated shape. As stars form in this cloud, they will form a small piece of a spiral arm. After many of these clouds have been stretched out this way, the galaxy will contain many fragments of spiral arms.
Normal Galaxies - Elliptical Answers
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A1. A large galaxy in the center of a dense cluster of galaxies will have many encounters with other galaxies. Some of those galaxies will be captured by the larger one, merging with it and allowing it to become even larger. Over time a giant galaxy will be formed this way.
Normal Galaxies - Colliding Answers
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A1. If two galaxies collide, they may merge into a larger galaxy. A large galaxy in the center of a large cluster of galaxies can grow this way to become a giant galaxy.
Normal Galaxies - Formation Answers
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A1. The amount of rotation of an isolated body is fixed. The amount of rotation equals the product of the mass, the size, and the rate of rotation. Rotation also creates an apparent outward force. This force is the result of a particle’s tendency to continue to move in a straight line as the rest of the body turns in its rotation.
A2. Astronomers have debated the relative significance of the role of the gravitational collapse of a large cloud of gas to form a galaxy and the cannibalism of small galaxies by larger ones in the formation of large galaxies. This recent observations suggests that large galaxies do not form at once from the collapse of a cloud of gas, but rather that galaxies tend to be rather small when they first form. This suggests that large galaxies must grow after their formation through galactic cannibalism.
A3. For an isolated object the amount of rotation, defined as the product of the mass of the object, its size, and the rate of rotation, must remain constant. If the body collapses (without changing its mass) the rate of rotation must increase so the amount of rotation can remain constant.
A4. Once a star forms, it will remain in whatever orbit it had at the time of its formation. Hence, the visible shape of a galaxy is determined by when during the collapse of the galaxy significant star formation occurred. If star formation began early, the galaxy will today still have the round shape of an elliptical galaxy. If star formation was delayed, the galaxy will flatten more before stars form.
A5. A rotating object feels an outward force due to the rotation, because any object tends to move in a straight line until a force causes to change. This outward force of rotation more directly counteracts the inward force of gravity at the equator, where the two forces are directly opposite each other, than it does near the axis of rotation, where the outward force from rotation is not as strong and not exactly opposite gravity. Hence, material near the axis of rotation falls more quickly and the object becomes flattened.
A6. The amount of rotation, defined as the product of the size of the object times its mass times the rate of rotation, is constant for an isolated body. If the mass stays the same while the object collapses, its rate of spin must increase so the product of all three stays the same.
Normal Galaxies - Black Holes Answers
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A1. Spectra taken of matter very close to the center of a galaxy shows unexpectedly large Doppler shifts, to the blue and red on opposite sides of the center, indicating that the matter is orbiting the center of the galaxy at high speed. Such rapid orbital motion so close to the center of the galaxy requires that the matter be orbiting a very large concentration of mass, which is small enough to fit inside the orbit. This results suggests that black holes of 100's of millions or billions of solar masses reside in the centers of galaxies.