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Chapter 18: Problem 5

What are H II regions? Near what kinds of stars are they found? Why do only these stars give rise to H II regions?

Short Answer

Expert verified
H II regions are areas of ionized hydrogen found in interstellar medium. They are usually found around very hot and highly luminous stars, specifically of types O and B, as only these types of stars have the necessary heat to ionize surrounding hydrogen gas and create these regions.

Step by step solution

01

Definition of H II Regions

H II regions are areas of interstellar medium that consist mainly of ionized hydrogen. They are named so due to the presence of large amounts of singly-ionized hydrogen, denoted as H II.
02

Stars Found Near H II Regions

H II regions are typically found around hot, bright stars. These stars are generally of spectral type O or B, the hottest and most luminous types of stars.
03

Why Only These Stars Give Rise to H II Regions

The reason why only stars of types O and B give rise to H II regions is due to their high temperatures. The immense heat of these stars provides enough energy to ionize the surrounding hydrogen gas, creating a region of ionized hydrogen. Stars of lower temperature don't have sufficient energy to fully ionize hydrogen, and therefore don't lead to the formation of H II regions.

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Most popular questions from this chapter

Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to examine the Milky Way Galaxy. Open the Favourites pane and click on Stars \(>\) Sun in Milky Way to display our Galaxy from a position \(0.150\) million light-years above the galactic plane. (You can remove the astronaut's feet from this view if desired by clicking on View \(>\) Feet.) You can zoom in or out on the Galaxy using the + and - buttons at the upper right end of the toolbar. You can move the Galaxy by holding down the mouse button while moving the mouse. You can also rotate the Galaxy by putting the mouse cursor over the image and holding down the Shift key while holding down the mouse button and moving the mouse. (a) You can identify H II regions by their characteristic magenta color. Describe where in the Galaxy you find these. Are most found in the inner part of the Galaxy or in its outer regions? (b) Where do you find dark lanes of dustin the inner part of the Galaxy or in its outer regions? Do you see any connection between the locations of dust and of H II regions? If there is a connection, what do you think causes it? If there is not a connection, why is this the case? You can examine the location of Galaxy in relation to neighboring galaxies by turning the Milky Way edge-on and by increasing the distance from the Earth using the up key below the Viewing Location on the toolbar.

Why is the daytime sky blue? Why are distant mountains purple? Why is the Sun red when seen near the horizon at sunrise or sunset? In what ways are your answers analogous to the explanations for the bluish color of reflection nebulae and the process of interstellar reddening?

Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to investigate a star-forming region. Use the Find ... command in the Edit menu to find and center on M20 (the Trifid Nebula, shown in the figure that accompanies Question 31) as seen from your location. Zoom out as far as possible using the Zoom controls at the righthand end of the toolbar. Set the Time appropriately and adjust the Month and Day in the Date to answer the following questions. (Hint: You may want to remove daylight and display the local meridian to provide precise answers.) (a) On what day is M20 highest in the sky at noon? Explain how you determined this. (b) On what day is M20 highest in the sky at midnight, so that it is best placed for observing with a telescope? Explain how you determined this.

What are T Tauri stars? How do we know that they eject matter at high speed? How does their rate of mass loss compare to that of the Sun?

In the direction of a particular star cluster, interstellar extinction allows only \(15 \%\) of a star's light to pass through each kiloparsec \((1000 \mathrm{pc})\) of the interstellar medium. If the star cluster is \(3.0\) kiloparsecs away, what percentage of its photons survive the trip to the Earth?
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