Why does the speaker say that the stars have "Hearts of fire" in "Stars" by Sara Teasdale?

Sara Teasdale's poem "Stars" describes the speaker's experience of walking up onto a hill one night to examine the blanket of stars dotting the sky above her. This poem--like the vast majority of Teasdale's work--is pretty straightforward and does not contain any complex narrative approach. Rather, she chooses to have the speaker simply describe the scene before her with figurative language. 

The poem opens with the speaker standing on the "dark hill" surrounded by a forest of pines. She then turns her head to look up at the "heaven full of stars" and begins to describe their colors, suggesting that they are "Myriads with beating / Hearts of fire." Teasdale is personifying the stars in this line, giving them the anatomical quality of a human being (a heart) and creating an emotional implication of brightness, passion, and desire (in stating that the hearts are "of fire"). This image alludes to the eternal and expansive quality of the natural world in the face of human "smallness" and mortality.