Why are chemical changes considered "unseen"

The “death throes” of stars are amongst the most complex phenomena in all of astronomy, and the “corpses” of stars amongst the most energetic and exotic objects in existence. beginning at the end of the core helium fusion stage in a star’s life, describe in detail the post-main sequence development of both (a) a 1.0 solar mass star, and (b) a 25.0 solar mass star. in what significant ways do they differ? what role does each of these types of stars play in the "star-gas-star" cycle and the evolution of chemical elements in our galaxy?

Anne applies a force on a toy car and makes it move forward. what can be said about the forces acting on the toy car at the moment anne applies the force?

Two polarizers are oriented at 24.0∘ to one another. light polarized at a 12.0-degree angle to each polarizer passes through both. what is the transmitted intensity (%)?