I am trying to get my head abound something and I am hoping you guys can shed some light on this for me. When discussing objects moving an relativistic speeds, its often mentioned that as an object approaches the speed of light, it gains mass. My question is: is this mass real? If you somehow managed to impart enough energy on a probe to move it fast enough that it has the mass of a star, would that probe have the same gravitational pull of a star? Or is the increase in mass simply something we use to discuss how much kinetic energy the object contains?