When Starman wears "his" old cap from the film, Jenny (not knowing who he really is) becomes upset. It's a touching, sad moment.
Overlapping with heartwarming, from the conclusion of the two-part episode where Starman and Jenny Hayden are reunited; Scott misses the chance to see his mother again, so as Jenny's friend gives Scott her message for him, Starman uses his sphere to make her look like Jenny, so that Scott can have a present-day memory of how she is.
The comic book
David Knight's death at the very start of the series. It's very sudden and undignified. He's shot through the chest while preparing to fly into the night, never even sees his attacker, and he falls to his death, landing in a dirty alleyway below.
The fight between the Knight Family the day before. This is heartbreaking for multiple reasons. One, the last words Jack says to his brother, while his brother's alive, are him mocking his brother and Starman costume he's chosen to wear. Two, Jack's harsh words are not only mocking his brother, but his father as well and the Starman legacy that he created. Finally, even Jack is filled with shame at his words, but is unable to build up the courage to simply apologize, keeping his true feelings to himself.
Jack: I'm sorry. I mock the costume I mock my father...his life. I...am a fool. Why...why am I such a fool?
The exchange also makes it clear just how different Jack is from both his father and his brother, his only surviving relatives at this point.
Starman Annual #1, which features the story of Prince Gavyn. The story's conclusion see's Gavyn apparently dying attempting to stop a wall of antimatter that is threatening to consume his planet. The wall stops after his death and the people of his planet assume that it was his sacrifice that saved them. However, as the story's narrator, The Shade, reveals it wasn't Gavyn that stopped the wall at all. It was the rest of the heroes back on Earth during the Crisis on Infinite Earths event who managed to do it, thereby saving all of reality. The tragedy of it all? Gavyn's sacrifice was unnecessary, as the threat would have been stopped with or without him and he contributed nothing to doing so.
Shade: And there lies the true sadness to this tale. If he had lingered by his love. If Gavyn had flown into danger a moment later, the antimatter might have vanished before he fell. He might have lived.
Starman Annual #2. Brian Savage, Scalphunter, falls in love with a woman from high society and begins courting her. While the two are happy for a time, they eventually split up after an altercation which sees Savage killing a criminal who came after him. After the killing is done Savage and his girlfriend simply share a single look before he turns from her and walks away, since the two of them realized in that moment that regardless of how they felt about one another they were simply too different to make things work.
In the same issues its revealed that the night before he died, David Knight called broke up with his fiance, due to fearing for the danger that she might have been placed in as Starman's wife. He makes the choice to continue being Starman rather than to be with her.
Ted Knight's death and even worse, funeral at the end of the '90s Starman run, where the characters walk up and say their piece.
Jack's visit to Krypton, where he encounters a very young Jor-El is this both in-universe and for the reader, because Jack and the reader both know that Krypton is doomed.
Starman #54 gives us the tale of Herman Moll, the inventor of the spaceship that Jack and Mikaal will one day use to traverse the stars. After their plot to have him killed by an assassin is thwarted by Brian Savage, with some warning from the Shade, Moll's cousins simply hire a doctor to declare him insane and have him carted off to an asylum, where he presumably dies. This is made easier for them to do because, as even Savage admits, in the year of 1899 it's quite impossible to believe that any ship could ever go to the stars. The experience breaks Moll and Savage is certain that he'll end up crazy by the end of the night that he's being taken away, while Moll's cousins get away with their crimes.
Brian Savage: Had Moll carted away-straightjacket'n'all. Saw him being taken. Shock of what was happening had touched him. Could see that even if he weren't loco that morning, he'd sure as hell be raving by mightnight.
Starman #72: Ted Knight and Jack Knight's final conversation with one another. Not counting their in the final Talking with David issue that is. This naturally doubles as a moment of Heartwarming as well.
Ted: You asked me a long time ago to study my cosmic energy. That was our deal. So you'd be Starman in my stead. I've done as you asked, son. But now i'm done. My studies will benefit the world. I've outlined examples...ways the energy will improve Earth And the accompanying patents will add to the Knight fortune, so you'll never need worry for the future.
Jack: No, dad. I don't like this. What are you saying?
Ted: I'm dying, Jack. Dr. Phosphorus gave me cancer and I only have a few painful months left. This way I die a hero's death. And more important a quick one.
Ted: But what? It's me or the city. I've been Opal's guardian for so many years it seems only right that it will be my final act...my final act as Starman one last time.
Jack: ...Here, dad. This is your grandson. Hold him.
Ted: I hold the future. Jack, know that in death, your old man died happy. Content. Jack...when you tell your boy about me, please lie a bit to make me sound "cool".
Jack: I won't have to lie.
Starman #73, Matt O'Dare's death, along with Jack, who'd been describing his heart as feeling "numb" after his father's death, finally breaking down and crying over the death of his father when he receives Sadie's letter telling him that she's left him and Opal City behind.
Starman #80- While all sweet and heartwarming, Jack's farewells to each of his friends is also extremely sad, as its the end of the wonderful series:
Jack's final meeting with the O'Dare family
Clarence O'Dare: I'm an Opal cop. I've got a great life and a great wife. And no matter where you are, i'll always think of you as one of ours.
Mason jumps in the way of an arrow to save Jack's life, dying in the process, but he is saved by Zatarra from beyond the grave, as a final bit of magic and a wedding present to Mason and Charity, his student.
Jack and Mikaal's last conversation
Jack: I have one brother. Now I like to think that I have two.
The Shade's parting gift to Jack
Shade: Do you see the shape? The flare of the petals? Smell its aroma? And the dark blue hue like an Opal night sky? These bushes are Opal's natives born and bred. Take this and Opal goes with you wherever you are. Remember Opal is a city. Beautiful, but in the end just a city. San Francisco is just a city, too. You made Opal what it is to you. You can do the same in any place if you have love and happiness.
Jack passing on the Cosmic Rod to Courtney
Courtney: I've never been sure you even liked me.
Jack: I've never been sure myself...no, that isn't true. As corny as it sounds, you remind me of me. Brash like me. Difference being, I wasn't nice. I wasn't a hero. I was bad news. You...you're 24 karats, baby. As bright as the sun. Seeing that made me feel...of, whatever...whatever I may have thought of you, i've had time to reconsider the matter. Courtney, if you accept this...you honor me.
Courtney: I'll be the best I can.
And Jack's goodbye to Opal itself
Jack: I'm numb. I thought this would be more emotional. There was a time I never would have given up my city. But now the die is cast, I find I do so without the creeping terror i'd always imagined. Sadie. I'm going to be with Sadie. I'm going to be a father all over again. And suddenly te city's spires, while beautiful...are only that. I look around the store. Old things. Beautiful old things. Each piece I pass kisses my cheek or strokes my are as we part. Handshakes and farewells of bakelite and rayon and franciscan pottery. But then...light falls upon a fiestaware bowl, a Matt Mason figure and a hopalong lunchbox that i'd clumped together. It's a still life. I paint what I see. Using a set of oils i'd long thought lost. And then and there...I resolve to be a painter once again. Then and there. Art. My new life. I take my boy to see his grandfather one last time. And all those who bore the name between my father and me. I pack the things that matter and leave the things that don't. Then I make like a banana...and split. As I feel Opal...my wonderful Opal ebbing away...I start to cry. But I manage not to look back. For i've more to see on the road ahead.