Question

For Balmer series lines to show up strongly in absorption in stellar spectra, significant numbers of hydrogen atoms have to have electrons in the n = 2 energy level. What then does the appearance of these lines in a stellar spectrum tell us about the temperature of the star's surface?

A) The appearance of the lines tells us very little about the temperature because hydrogen gas will show significant Balmer absorption, whatever the surface temperature.
B) The temperature must be high enough to ionize the hydrogen atoms by collision in order that they can absorb from this level.
C) The temperature must be reasonably low so that no atoms will have electrons excited beyond this energy level (e.g., to n = 3).
D) The temperature must be reasonably high to excite the electrons to this level by collisions but not high enough to ionize the atoms.

Answer 1

The Balmer series lines indicate that the star has a surface temperature optimal for exciting hydrogen atoms to the second energy level, around 10,000 K.

Step-by-step explanation:

The presence of Balmer series lines in a stellar spectrum suggests that a star has a surface temperature that is high enough to excite electrons to the second energy level (n = 2), but not high enough to completely ionize the hydrogen atoms. This occurs at an optimum temperature of around 10,000 K. At temperatures significantly higher, hydrogen atoms are ionized and at much lower temperatures, the electrons do not get excited beyond the ground state. Therefore, the correct answer to the student's question is (D): The temperature must be reasonably high to excite the electrons to this level by collisions but not high enough to ionize the atoms.