Simon Helberg, Jim Parsons, Kunal Nayyar

[WARNING: The following story contains spoilers about Thursday's episode of The Big Bang Theory. Read at your own risk.]

RIP Professor Proton.

Bob Newhart returned to The Big Bang Theory as Arthur Jeffries, aka Professor Proton, on Thursday, but his appearance — at least in this world — was short-lived. He dies off-screen, leaving Sheldon (Jim Parsons), predictably, in denial and refusing to attend the funeral because it was on Star Wars Day. But when Sheldon takes a nap, Professor Proton appears as his personal Obi-Wan Kenobi in his dream to help Sheldon, who's lost his grandfather, father and "had to say goodbye to 11 Dr. Who's," how to grieve.

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"It's OK to be sad about them," he says. "Just make sure you appreciate those who are still there for you."

When Sheldon awakes, he immediately embraces Leonard (Johnny Galecki) after having rejected his hug earlier.

"It seemed like an opportunity to see Sheldon deal with that kind of grief, which is something we haven't explored on the show before," executive producer Steve Molaro tells TVGuide.com. "I really liked the way we did it [that tied in] with Star Wars, where we were able to sort of make it work both ways. It's bittersweet, but it's nice to see Sheldon grow, as he always does, a tiny little bit. It's contained in this one episode to how he appreciates the people around him. Moving forward, I don't know how much long-lasting impact it has, but we definitely want to get that across in this episode."

Molaro said producers had planned to kill of Professor Proton, a role for which Newhart finally won his first ever Emmy last year, for a while, as it "seemed like the right time" to broach another benchmark in Sheldon's social development. "As you get older, you lose more people in your life. We wanted to touch on that," Molaro says. "Professor Proton is a father figure to Sheldon, so it's a huge loss. He struggles with it. He tries to convince himself that there's nothing he can do about it. It's bothering him a lot more than he would admit at first and doesn't go [to the funeral]."

Leonard and Penny (Kaley Cuoco-Sweeting) do attend the funeral, where a discussion about life regrets morphs into a contest to even up the number of proposals between them, which was 2-1 for the former. Although Leonard turns down Penny's faux proposal,  that doesn't mean they won't get engaged for real — and soon.

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"There's a lot of discussion in the last few episodes and they both realize they have to decide how serious they are about doing this," Molaro says. "Leonard and Penny are capable of proposing to each other at any time on Big Bang Theory, for better or worse. They will talk about ... how often they propose and should they really be doing that anymore or should they just do it for real and get engaged? They can't keep playing this proposal game forever. It comes to a head in the last two episodes."

Even if Leonard doesn't finally put a ring on it this season, the show has at least three more years to tackle the story line — and to welcome back Newhart. Thanks to their Obi-Wan vision device, Professor Proton is not truly gone.

"We can definitely bring him back and it's just story dependent," Molaro says. "We love Bob. He's a part of the family forever and if we find a story that we're excited about that involves him coming back in this way, we'd be more than happy to do it. He's a great character and he means a lot to Sheldon, so just in that, I would think it has an impact on most viewers. But the way it's handled where he's not gone forever from the show, I think makes it OK."

The Big Bang Theory airs Thursdays at 8/7c on CBS. Catch up episodes here.

(Full disclosure: TVGuide.com is owned by CBS.)