LitCharts assigns a color and icon to each theme in The Sun is Also a Star, which you can use to track the themes throughout the work.
Immigration and the American Dream
Passion vs. Reason
Interconnectedness and Destiny
Isolation vs. Connection
Summary
Analysis
Peter and Natasha's parents go ahead to the airport, and Daniel and Natasha take a cab together. In the backseat, they decide that if she weren't leaving, they would've worked out. Daniel says it would've taken Dae Hyun a long time to come around, and Natasha thinks of comforting Daniel on their wedding day about his dad's absence. She asks Daniel how many kids he wants and shares that she's not sure she wants kids at all. Daniel says he'd think about that.
For the moment, Daniel and Natasha's conversation about the future is purely theoretical, as they're choosing to not consider the fact that, given their beliefs in interconnectedness, things could still work out. This shows how negative emotions can cloud how someone thinks about their destiny and connections with those around them.
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Themes
Natasha tells Daniel he shouldn't be a doctor and confesses that maybe she won't be a data scientist after all—being passionate about something would be nice. After a moment, she asks how many questions they have left. They have two and then the staring exercise. When Daniel suggests they kiss instead, their driver reminds them that he can hear and see them. However, he leaves them alone while they kiss.
Everything Natasha says in this exchange shows how much she's changed over the course of the day. Now, she recognizes that passion is important and isn't in opposition to reason in the least. This will allow her to have a more nuanced and well-rounded view going forward.
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Themes
For the final two questions, Natasha isn't sure what she'd save from a fire, but Daniel would save his notebook. Natasha would find Samuel's death the most disturbing, and Daniel would find Natasha's death disturbing. He insists that she's part of his family, thinking about the multiverse theory. Somewhere, they're married and maybe have kids. Natasha finally tells Daniel she loves him.
Here, Daniel's jump to thinking about the multiverse theory shows that he also has also learned a lot from Natasha and considers science more than he did previously. He finds the idea that he's living many lives at once comforting, as he can have many different connections with many different people.