Concept explainers
Group Activity: Counting the Milky Way’s Stars. Work as a group to answer each part. Note: This activity works particularly well in groups of four students, with each student taking on one of the following roles: scribe: takes notes on the group’s activities; pro-poser: suggests tentative explanations to the group; skeptic: points out weaknesses in proposed explanations; moderator: leads group discussion and makes sure everyone contributes.
a. Work together to estimate the number of stars in the Milky Way from just these two facts: (1) the number of stars within 12 light-years of the Sun, which you can count in Appendix F, and (2) the total volume of the Milky Way’s disk (100,000 light-years in diameter and 1000 light-years thick), which is about 1 billion times the volume of the region of your star count.
b. Discuss how your value from part a compares to the value given in this chapter. Make a list of possible reasons why your technique may have underestimated or overestimated the actual number.
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Essential Cosmic Perspective
- The simulated box of galaxy filaments and superclusters shown in Figure 28.29 stretches across 1 billion light-years. If you were to make a scale model where that box covered the core of a university campus, say 1 km, then how big would the Milky Way Galaxy be? How far away would the Andromeda galaxy be in the scale model? Figure 28.29 Growth of Large-Scale Structure as Calculated by Supercomputers. The boxes show how filaments and superclusters of galaxies grow over time, from a relatively smooth distribution of dark matter and gas, with few galaxies formed in the first 2 billion years after the Big Bang, to the very clumpy strings of galaxies with large voids today. Compare the last image in this sequence with the actual distribution of nearby galaxies shown in Figure 28.21. (credit: modification of work by CXC/MPE/V.Springel)arrow_forwardThe dwarf galaxy in Sagittarius is the one closest to the Milky Way, yet it was discovered only in 1994. Can you think of a reason it was not discovered earlier? (Hint: Think about what else is in its constellation.)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 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_forwardAssume that dark matter is uniformly distributed throughout the Milky Way, not just in the outer halo but also throughout the bulge and in the disk, where the solar system lives. How much dark matter would you expect there to be inside the solar system? Would you expect that to be easily detectable? Hint: For the radius of the Milky Way’s dark matter halo, use R=300,000 light-years; for the solar system’s radius, use 100 AU; and start by calculating the ratio of the two volumes.arrow_forwardExplain why we see the Milky Way as a faint band of light stretching across the sky.arrow_forward
- 2. List 3 methods used to measure cosmic distances - choose 1 of the techniques and briefly explain in 1-2 sentences how astronomers use this method to calculate cosmic distance.arrow_forwardWhy are so many astronomical objects in the universe flat today? (Like the solar system, Saturn's ring, black hole accretion discs) Let's focus on our galaxy. Originally, the galaxy was a huge spherical cloud of gas and dust, much larger than it is today, and rotating much more slowly. Explain using gravity and the conservation of angular momentum, why the galaxy today is a flat disc that rotates faster.arrow_forward18. True or false? If you use a telescope to look at a galaxy whose light has taken 8 billion years to reach you, this galaxy could have already been 8 billion years old when the light you are now seeing left it.arrow_forward
- Imagine that you have achieved immortality and you used it to travel outside of the Milky Way (you will be leaving today and you will be traveling with the speed of 1/10th of the speed of light). Describe how the Milky Way would look from the outside if you could watch it for the next 100 billion years.arrow_forwardAmong the globular clusters orbiting a distant galaxy, one is moving at 534 km/s and is located 14 kpc from the center of the galaxy. Assuming the globular cluster is located outside most of the mass of the galaxy, what is the mass of the galaxy? Convert your answer to solar masses. (Hint: Use the formula for circular velocity, Vc = GM r ; make sure you convert relevant quantities to units of meters, kilograms, and seconds. Note: 1 pc = 3.1 ✕ 1016 m.)arrow_forwardSuppose there are 12000.0 civil actions broadcasting radio signals in the Milky Way Galaxy at the moment. On average, how many stars would have to be search before a signal is heard? Assume that there are 3 x 1011 stars in the Galaxy and one civilization per star.arrow_forward
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