21ST CENT.ASTRONOMY(LL)W/CODE WKBK PKG.
6th Edition
ISBN: 9780393874921
Author: PALEN
Publisher: Norton, W. W. & Company, Inc.
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Chapter 22, Problem 38QP
To determine
The average density of the Milky Way and compare with it with the critical density.
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How astronomers determine the distance of a galaxy? Explain.
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tric distances (vrot(r) ~ constant) is the most common example of why astronomer's believe
dark matter exists. Let's work out why!
Assuming that each star in a given galaxy has a circular orbit, we know that the accelera-
tion due to gravity felt by each star is due to the mass enclosed within its orbital radius r and
equal to v?/r. Here, ve is the circular orbit velocity of the star. (a) Show that the expected
relationship between ve and r due to the stellar halo (p(r) xr-3.5) does not produce a flat
rotation curve. (b) Show that a p(r) ∞ r¯² density profile successfully produces a flat ro-
tation curve and must therefore be the general profile that dark matter follows in our galaxy.
The time for a galaxy to cross from one side of a cluster to the other is called the crossing time. Find the crossing time for a galaxy moving at speed v to cross a cluster with a diameter d. Express you answer in gigayears, using one decimal place.
Values:
v = 849 km/s
d = 1.3 Mpc
Chapter 22 Solutions
21ST CENT.ASTRONOMY(LL)W/CODE WKBK PKG.
Ch. 22.1 - Prob. 22.1CYUCh. 22.2 - Prob. 22.2CYUCh. 22.3 - Prob. 22.3CYUCh. 22.4 - Prob. 22.4CYUCh. 22.5 - Prob. 22.5CYUCh. 22 - Prob. 1QPCh. 22 - Prob. 2QPCh. 22 - Prob. 3QPCh. 22 - Prob. 4QPCh. 22 - Prob. 5QP
Ch. 22 - Prob. 6QPCh. 22 - Prob. 7QPCh. 22 - Prob. 8QPCh. 22 - Prob. 9QPCh. 22 - Prob. 10QPCh. 22 - Prob. 11QPCh. 22 - Prob. 12QPCh. 22 - Prob. 13QPCh. 22 - Prob. 14QPCh. 22 - Prob. 15QPCh. 22 - Prob. 16QPCh. 22 - Prob. 17QPCh. 22 - Prob. 18QPCh. 22 - Prob. 19QPCh. 22 - Prob. 20QPCh. 22 - Prob. 21QPCh. 22 - Prob. 22QPCh. 22 - Prob. 24QPCh. 22 - Prob. 28QPCh. 22 - Prob. 29QPCh. 22 - Prob. 31QPCh. 22 - Prob. 32QPCh. 22 - Prob. 33QPCh. 22 - Prob. 34QPCh. 22 - Prob. 35QPCh. 22 - Prob. 36QPCh. 22 - Prob. 37QPCh. 22 - Prob. 38QPCh. 22 - Prob. 39QPCh. 22 - Prob. 40QPCh. 22 - Prob. 41QPCh. 22 - Prob. 42QPCh. 22 - Prob. 43QPCh. 22 - Prob. 44QPCh. 22 - Prob. 45QP
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- Could the Milky Way ever become an active galaxy? Is it likely to ever be as luminous as a quasar?arrow_forwardIf Hubble’s constant is taken to be 70 ??? ???, and a quasar is found to have a radial velocity equal to 95% of the speed of light, how far is the quasar in Mpc? (Hint: Use Hubble’s Law and solve for the distance; and the speed of light in vacuum is: ?=3.0×105 ??/?).arrow_forwardEstimating the mass of the Milky Way a) Assuming the Sun moves in a circular orbit of radius 8 kiloparsecs around the center of the Milky Way, and that its orbital speed is 220 km/s, calculate how many years it takes the Sun to complete one orbit of the Galaxy. Remember to convert kiloparsecs to kilometers. b) Using the modified form of Kepler's third law (introduced in Lecture 13, for measuring the combined masses of binary stars), R³ m+ M = estimate the mass of the Milky Way enclosed within 8 kpc (Sun's orbit radius). The mass of the Milky Way inside p² I the Sun's orbit can be represented as a single mass (M) located at its center, and the mass of the Sun (m) can be considered infinitesimally small compared to the Milky Way's (i.e., m < M). c) Is this estimate of the Milky Way's mass an upper or lower limit? Explain your reasoning.arrow_forward
- If the active core of a galaxy contains a black hole of 106 M, what will the orbital period be for matter orbiting the black hole at a distance of 0.23 AU? Hint: Use the formula for circular velocity, V. GM V hrarrow_forwardWhat are the characteristics of an E7 Galaxy? What about E0 galaxy? Explain.arrow_forwardPlease answer within 90 minutes.arrow_forward
- = 2. Using a Hubble constant of Ho 70 km/s/Mpc, find the distance to the galaxy cluster that moves with a velocity of 6500 km/s. Give your answer in megaparsecs and light-years.arrow_forwardAssume that we have measured the distance to a close by galaxy, with apparent magnitude m1 = 6, to be d1 = 1Mpc. We now assume that all galaxies are similar and have therefore the same intrinsic or absolute, luminosity. Then measuring the apparent magnitude of a second galaxy to be m2 = 11, estimate the distance to that galaxy. Please answer within 90 minutes.arrow_forwardWhat are the two best ways to measure the distance to a distant, isolated spiral galaxy, and how would it be measured?arrow_forward
- Suppose the Sagittarius dwarf galaxy merges completely with the Milky Way and adds 150,000 stars to it. Estimate the percentage change in the mass of the Milky Way. Will this be enough mass to affect the orbit of the Sun around the galactic center? Assume that all of the Sagittarius galaxy’s stars end up in the nuclear bulge of the Milky Way Galaxy and explain your answer.arrow_forwardWhat are the two best ways to measure the distance to a nearby spiral galaxy, and how would it be measured?arrow_forwardSuppose somebody proposed that rather than invoking dark matter to explain the increased orbital velocities of stars beyond the Sun’s orbit, the problem could be solved by assuming that the Milky Way’s central black hole was much more massive. Does simply increasing the assumed mass of the Milky Way’s central supermassive black hole correctly resolve the issue of unexpectedly high orbital velocities in the Galaxy? Why or why not?arrow_forward
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