Universe: Stars And Galaxies
6th Edition
ISBN: 9781319115098
Author: Roger Freedman, Robert Geller, William J. Kaufmann
Publisher: W. H. Freeman
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Chapter 22, Problem 19Q
To determine
The way that orbits of stars in the galactic disk are different from the orbits of planets in our solar system and the factor that the difference implies about the way that matter is distributed in the galaxy.
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A galaxy's rotation curve is a measure of the orbital speed of stars as a function of distance
from the galaxy's centre. The fact that rotation curves are primarily flat at large galactocen-
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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-
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equal to v?/r. Here, ve is the circular orbit velocity of the star. (a) Show that the expected
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tation curve and must therefore be the general profile that dark matter follows in our galaxy.
Suppose that stars were born at random times over the last 1010 years. The rate of star formation is simply the number of stars divided by 1010 years. The fraction of stars with detected extrasolar planets is at least 18%. The rate of star formation can be multiplied by this fraction to find the rate planet formation. How often (in years) does a planetary system form in our galaxy? Assume the Milky Way contains 8 × 1011 stars.
The Sun is moving at 220 ??/? around the Galactic Center at a more-or-less constant distance of 8.5 ???. To appreciate how remarkable this is, consider the following questions:
a) How massive would the Sun have to be for the Earth to have an orbital velocity of 220 km/s at 1 AU?
b) How fast would the Earth move if it was in orbit around the Sun at a distance of 8.5 kpc? Of course, you may ignore the effects of all other stars in this calculation.
Chapter 22 Solutions
Universe: Stars And Galaxies
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- What evidence can you give that we live in a galaxy?arrow_forwardWhat will be the long-term future of our Galaxy?arrow_forwardStars form in the Milky Way at a rate of about 1 solar mass per year. At this rate, how long would it take for all the interstellar gas in the Milky Way to be turned into stars if there were no fresh gas coming in from outside? How does this compare to the estimated age of the universe, 14 billion years? What do you conclude from this?arrow_forward
- Suppose three stars lie in the disk of the Galaxy at distances of 20,000 light-years, 25,000 light-years, and 30,000 light-years from the galactic center, and suppose that right now all three are lined up in such a way that it is possible to draw a straight line through them and on to the center of the Galaxy. How will the relative positions of these three stars change with time? Assume that their orbits are all circular and lie in the plane of the disk.arrow_forwardOur Solar System is about 8.3 kpc from the centre of our galaxy. Using Newton's Universal Gravitation Law and Kepler's Third Law, calculate the approximate mass of our Milky Way if we know that the orbital velocity of the Sun around the centre of the galaxy is 225 km/s. (Hint: Use the formula for orbital velocity: v = GM -and problem , r -11 m3 Note: G is the Universal Gravitation Constant, G 6.67 × 10 kg s2' 1 kpс 1000 рс аnd 1 рс 3.1 x 1016 m. Also, pay attention to units!!! – i.e. orbital m3 velocity is in km/s and the universal gravitation constant is in kgs2 а) 8.7 х 1035 b) 2.0 x 1041kg c) 2.0 × 1030 d) 6.0 × 1024 kg kg kgarrow_forward(a) Rank the following components of the interstellar medium in order of the wavelengths at which they are observed, longest wavelength first: clouds of neutral hydrogen, coronal gas, interstellar dust, nebulae. Longest to Shortest? (b) Rank the same material in order of decreasing temperature from hottest to coolest. Hottest to Coolest?arrow_forward
- An important part of the lifecycle of galaxies like the Milky Way is the self regulation of formation of future generations of stars. Which statement best describes this process? A) Massive stars explode as Supernovae, heating nearby gas which then can't form stars, and even forcing the gas out of the galaxy in asuperbubble. B) Low mass stars like our Sun explode as Supernovae, heating nearby gas which then can't form stars, and even forcing the gas out the galaxy in asuperbubble. C) Stars fuse new elements in their cores which mix with nearby gas clouds, preventing the collapse of the clouds and hence stopping new starformation. D) The stars lock up material in their cores (like White Dwarf and Neutron Stars) meaning they can act as gravitational seeds for future starformation.arrow_forwardSuppose you want to observe the molecular gas in a galaxy with redshift z using the rotational transition of CO J=4-3. What frequency would you observe this transition at? (Hint: the CO J=1-0 emits a photon at 115.27 GHz, and higher order transitions emit photons with frequencies in multiples of J. Express your answer as an integer. Values: z = 3.7arrow_forwardThe Milky Way grew through merging with many smaller galaxies. What are the observational signatures of this process? O The motion of old stars in the bulge and halo of our galaxy are randomly orientated, meaning they were formed from collisions of small, accreted, galaxies all on different paths. O The ordered motion of the bulge / halo stars means that they came from many objects. The random motions of stars in the disk means it was formed from collisions of small, accreted, galaxies. O The motion of young stars in the disk are all in the same direction, meaning they came in as seperate objects.arrow_forward
- The Milky Way galaxy has about 5 x 10⁹ solar masses of gas in total. If 13 solar masses of that gas is turned into stars each year, how many more years could the Milky Way keep up with such a star formation rate? years (Note for comparison that the age of the universe is about 13.5 billion years, which can be written 1.35e10 years. Also, the value given is in the ballpark of how much gas in the Milky Way is used to make new stars each year.)arrow_forwardSuppose that stars were born at random times over the last 10e10 years. The rate ofstar formation is simply the number of stars divided by 10e10 years. The fraction ofstars with detected extrasolar planets is at least 9 %. The rate of star formation can bemultiplied by this fraction to find the rate planet formation. How often (in years) doesa planetary system form in our galaxy? Assume the Milky Way contains 7 × 10e11 stars. I've done this problem 3 different times from scratch and looked at similar problems here. Each time my answer is 1.587 (1.59 rounded to 2 significant figures), but when I submit, it says the answer is wrong. What do you think?arrow_forwardFigure 2 shows the "rotation curve" of NGC 2742. It plots the “radial velocity (V)" (how fast material is moving either toward or away from us) that is measured for objects at different distances (R = radius") from the center of the galaxy. The center of the galaxy is at 0 kpc (kiloparsecs) with a speed of 9 km/sec away from us. (These velocities have been corrected for the observed tilt of the galaxy and represent true orbital velocities of the stars and gas.) 200 100 U4779 -100 As you can see, one side of the galaxy is moving with a negative velocity (spinning toward us), while the other side has a positive velocity (spinning away from us). Using Newton's gravity equation, we will be able to determine the gravitational mass of the entire galaxy and how the mass varies versus distance from the galaxy's center. -200 -8 8 -4 Radius (kpc) Read the following text carefully and follow the instructions: Select five radii spaced evenly from 0-10 kpc across the galaxy. Your selections should…arrow_forward
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