Where do elements, such as iron in our blood or calcium in our bones, come from? Astronomers say they come from thermonuclear reactions in hundreds of millions of stars that burn at high temperatures in our galaxy. Stars that are 10 or more times more massive than the sun eventually explode as supernovas, leaving traces of elements in the space between the stars of the Milky Way. When our solar system was created, astronomers say the trace elements were drawn from interstellar gases to form the Earth.