© Jeffrey J Wolynski

Mercury is a dead star. It is the left over remains of a star many tens of billions of years old. It has been wandering the galaxy for some time having being pelted by numerous asteroids over it's dead history. We can tell how long it has been dead by dating the surface craters, as which ever surface crater is the oldest will tell when it stopped having the ability to regenerate its surface via magma interiors/oceans of water. Like the Moon, it has probably been dead for about 50 billion years.

Its iron core is roughly the same size as Earths' thus meaning it was probably the same size as Earth at one point, but the surface erosion and many billions of years of wandering the galaxy have eroded it considerably.