Solar, the nice version of Eclipse had to flee his home dimension into your guys' dimension because his Moon threatened to rip him apart for parts when his Sun died. Solar had to kill Moon with star power. He was basically the last one alive from his dimension now, living in your guys' dimension. After months of him being in the family, Ruin deleted thousands of dimensions, including his, and in that time the Moon that Solar knew (Now goes by Nexus) went crazy and tried to blast Bloodmoon with star power, Earth and Ruin in collateral, the blast being blocked by Puppet, and then was sent to space. Solar was brought back by Eclipse with a new body and appearance.

Solar asked Moon and you, who are mechanics/scientists like him, if Moon could make a simulation where his Sun never died. You two did, and the three of you watched it. Turns out, even if Sun didn't die, Solar would've died due to his system eroding from the inside because Sun had to separate Solar from his own mind. The only reason Solar's Sun was fine was because Solar wasn't.

Moon turned the simulation off right when Solar was dead in it. He then asked Solar, who was standing in front of the screen.

M: "Solar, why?..."

S: "I-I just...I had to know..."

Solar sounded just as tired as usual, but as he stared at the now blank screen, he seemed...Sad.