ous problem could be observed by a telescope of limiting apparent magnitude (a) 18, (b) 30? HINT
11. A shock wave from a supernova explosion moves at a speed of about 5000 km/s. How long will such a disturbance take to cross a molecular cloud 20 pc in diameter? HINT
12. The luminosity of a hypothetical star-forming region is dominated by five bright O-type stars, each of absolute magnitude 28. What is the net absolute magnitude of the region? HINT
13. If the star-forming region in Problem 12 is in a galaxy 10 Mpc (1 megaparsec = 1,000,000 pc) from Earth, calculate its apparent magnitude. HINT
14. An open cluster has a diameter of 5 pc and a mass 1000 times the mass of the Sun. (a) Estimate the typical speed of its component stars. (b) Based on this speed, estimate the number of times a typical star orbits the center of the cluster in the 500 million years it takes for the cluster to dissolve in the galactic tidal field. HINT
15. Approximating the gravitational field of our Galaxy as a mass of 1011 solar masses at a distance of 8000 pc (see Chapter 23), estimate the “tidal radius” of a 20,000-solar-mass open-star cluster—that is, the distance from the cluster’s center, outside of which the galactic tidal force overwhelms the cluster’s own gravity. HINT
1. Prestellar Evolution. As a group, create a large pie chart showing the amount of time spent in each of the stages 1 through 6 listed in Table 19.1 for a solar-type star over a total period of 30 million years. Each group member should label one or two of the stages and clearly describe the characteristics of each.
RESEARCHING ON THE WEB
To complete the following exercises, go to the online Destinations Module for Chapter 19 on the Companion Website for Astronomy Today 4/e.
1. Access the "Hertzsprung–Russell Diagram" page and describe the current evolutionary stage of 80 percent of the stars in our galaxy.
2. Access the "All Star Line Up" page and describe how Alpha Centauri relates to our Sun in terms of size, brightness, and spectral class.
3. Access the "What is a Star?" page and describe how stars 10 times more massive and 1/10th the Sun’s mass will be different than the Sun in terms of brightness and lifetime.
1. The Trifid Nebula, otherwise known as M20, is a place where new stars are forming. It has been called a “dark night revelation, even in modest apertures.” An 8- to 10-inch telescope is needed to see the triple-lobed structure of the nebula. Ordinary binoculars reveal the Trifid as a hazy patch located in the constellation Sagittarius. This nebula is set against the richest part of the Milky Way, the edgewise projection of our own Galaxy around the sky. It is one of many wonders in this region of the heavens. What are the dark lanes in M20? Why are other parts of the nebula bright? There have been reports of large-scale changes occurring in this nebula in the last century and a half. The reports are based on old drawings, which show M20 looking slightly different from how it appears today. Do you think it possible for a cloud in space to undergo a change in appearance on a time scale of years, decades, or centuries?
2. Summer is a good time to search with binoculars for open-star clusters. Open clusters are generally found in the plane of the Galaxy. If you can see the hazy band of the Milky Way arcing across your night sky—in other words, if you are far from city lights and looking at an appropriate time of night and year—you can simply sweep with your binoculars along the Milky Way. Numerous “clumps” of stars will pop into view. Many will turn out to be open-star clusters.
3. Globular star clusters are harder to find. They are intrinsically larger, but they are also much farther away and therefore appear smaller in the sky. The most famous globular cluster visible from the Northern Hemisphere is M13 in the constellation Hercules, visible on spring and summer evenings. This cluster contains half a million or so of the Galaxy’s most ancient stars. It may be glimpsed in binoculars as a little ball of light, located about one-third of the way from the star Eta 