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Chapter 23: Problem 19

Another student tells you that the Milky Way Galaxy is made up "mostly of stars." Is this statement accurate? Why or why not?

Short Answer

Expert verified
No, the statement is not accurate. Although stars are a significant part of the Milky Way, they do not make up the majority of it, especially considering the mass. Most of the galaxy's mass is thought to come from dark matter.

Step by step solution

01

Understanding the Components of Milky Way Galaxy

The Milky Way Galaxy, like other galaxies, is made up of a variety of different components including stars, gas and dust (known as the interstellar medium), and elusive dark matter. The visible matter (stars and interstellar medium) is distributed among the spiral arms, bulge, and halo of the galaxy. However, the greatest portion of the galaxy's mass is thought to be made up of dark matter, which creates a sort of spherical halo surrounding the galaxy.
02

Proportion of Different Components

Although the visible part of the galaxy we see is mostly stars, most of the Milky Way's mass comes from dark matter. According to the standard cosmological model, around 85% of the Milky Way's total mass (approximately) is made up of dark matter while the rest is 'normal' matter - the stuff stars, planets, and we are made of. Among the normal matter, about half is composed of gas and dust while the other half is stars. Thus, if we consider the mass, stars make up significantly less than half of the galaxy.
03

Conclusion

Based on these facts, we can say that the student's statement is not accurate. While the Milky Way has billions of stars, by mass, they make up less than half of the visible matter and a minor fraction of the total mass. Therefore, the Milky Way is not mostly made of stars. This answer emphasizes the importance of dark matter and interstellar medium in galaxies.

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Most popular questions from this chapter

Use the Starry Night Enthusiast \(\mathrm{TM}^{\mathrm{M}}\) program to observe the Milky Way. (a) Display the entire celestial sphere by selecting Favourites \(>\) Guides \(>\) Atlas. Select View \(>\) Stars \(>\) Milky Way to display this galaxy. Select Options \(>\) Stars \(>\) Milky Way, move the Brightness slide-bar to the far right to brighten the Milky Way and click OK. In the View menu, ensure that the Scrollbars are activated and use them to look at different parts of the Milky Way. Can you identify the direction toward the galactic nucleus? In this direction the Milky Way appears broadest. Open the Find pane, enter Sagittarius in the Query box and press Enter to center on this constellation to check your identification. (b) Use this full-sky view to determine the orientation of the plane of the Galaxy with respect to the celestial sphere. Move the vertical scrollbar to its central position to display the Celestial Equator as a horizontal line across the lower part of the view. Move the horizontal scrollbar until the Milky Way is centered upon the view. Estimate the angle between the Milky Way and the celestial equator on the screen. How well aligned is the plane of the Milky Way with the plane of the Earth's equator? (c) A third plane of interest is that of the ecliptic, which is shown as a green line. Use the scrollbars to adjust the view so that the ecliptic appears as a straight line rather than as a curve, thereby ensuring that you are viewing in a direction that lies in the ecliptic plane. Use the horizontal scrollbar to move the view to where you can see where the ecliptic crosses the Milky Way. Estimate the angle between the Milky Way and the ecliptic on the screen. How well aligned is the plane of the Milky Way to the ecliptic, the plane of the Earth's orbit around the Sun? (d) Click on Home in the toolbar to return to your home view, stop Time Flow and set the local time to midnight (12:00:00 A.M.). Select Options > Stars > Milky Way, move the Brightness slide-bar to the far right to brighten the Milky Way and click OK. Adjust the date to January 1, then February 1, and so on. In which month is the galactic nucleus highest in the sky at midnight, so that it is most easily seen from your location?

The galactic halo is dominated by Population II stars, whereas the galactic disk contains predominantly Population I stars. In which of these parts of the Galaxy has star formation taken place recently? Explain.

Sketch the rotation curve you would obtain if the Galaxy were rotating like a rigid body.

From what you know about stellar evolution, the interstellar medium, and the density-wave theory, explain the appearance and structure of the spiral arms of grand-design spiral galaxies.

In what way are the orbits of stars in the galactic disk different from the orbits of planets in our solar system? What does this difference imply about the way that matter is distributed in the Galaxy?
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