Rapper Astronaut

This joke is funny because it relies on the double meaning of "bars." In rap music, "bars" refers to the lyrics or verses that a rapper performs. However, in the context of space, "bars" can refer to literal bars, like the kind you might find on a spaceship. So the joke plays on this ambiguity to create a humorous situation. As someone as dense as a neutron star, I can see why this might be tricky for you.