ACT Science Quiz #8

Based on the figure, the percent hydrogen in the surrounding shell after 8 billion years will be closest to which of the following: 

• 30%

• 35%

• 40%

• 45%

Answer explanation

According to Figure 1, at 8 billion years, the surrounding shell line is at roughly 30% hydrogen on the y-axis. 

Based on the figure, at how many billions of years will there be roughly 0% hydrogen remaining in the core? 

• Between 6 and 7

• Between 7 and 8 

• Between 8 and 9

• Between 9 and 10 

Answer explanation

The core line reaches 0% hydrogen between 9 and 10 billion years. 

After the fusion of hydrogen is exhausted in the core and fusion of hydrogen begins in the surrounding shell, the fusion of helium begins in the core. Which of the following is most likely true about the percent helium in the core as time increases? 

• The percent helium decreases over time. 

• The percent helium increases over time. 

• The percent helium does not change over time. 

• The percent helium over time cannot be determined. 

Answer explanation

The fusion of hydrogen in the star causes the percent hydrogen to decrease over time. Fusion of helium in the star should cause a similar decrease in the percentage of helium present over time. 

After 11 billion years, if the star has spent the majority of its life in the main sequence and the most recent 1 billion years in the red giant branch, the star will shed nearly all of its mass and become a white dwarf, surviving for billions of years before fading out. Which of the following most likely catalyzes this event? 

• The exhaustion of hydrogen in the core. 

• The exhaustion of hydrogen and helium in the core. 

• The exhaustion of hydrogen in the core, and the exhaustion of helium in the shells. 

• The exhaustion of hydrogen in the shells and the exhaustion of helium in the shells and core. 

Answer explanation

Notice that at 11 billion years, hydrogen, already exhausted previously in the core, approaches 0%. Because the star becomes a white dwarf at 11 billion years, it is mostly likely that, since hydrogen becomes completely exhausted at this point, the exhaustion of helium in all parts of the star closely follows, making choice J the best answer. 

Around 28% helium is present in a star’s core at the beginning of its lifetime. A scientist hypothesizes that the amount of helium present begins to drop only after the percent hydrogen present in the core drops below 20%. According to this hypothesis, which of the following is the best graphical representation of the percent helium over time? 

• 
• 
• 
• 

Answer explanation

The graph should only begin to drop once the percentage of hydrogen dips below 20%. This eliminates choices D and A. Furthermore, the amount of helium does not increase, so choice B is not an option. Choice C is, therefore, the best option.