why do atoms of the same element always have the same number of protons but sometimes have different mass number? What do you call these atoms?

Which contributions did galileo make to the model of the solar system? check all that apply. a mathematical model for the orbits of the planets scientifically accurate data on planetary orbits data that showed that planets reflected sunlight similar to earth's moon up the idea that stars are other suns that do not move u observations of the moons of jupiter rotating around the gas giant