  The interaction of an object with its surrounding bath can lead to a coupling
between the object's internal degrees of freedom and its center-of-mass motion.
This coupling is especially important for nanomechanical oscillators, which are
amongst the most promising systems for preparing macroscopic objects in quantum
mechanical states. Here we exploit this coupling to derive the internal
temperature of a levitated nanoparticle from measurements of its center-of-mass
dynamics. For a laser-trapped silica particle in high vacuum we find an
internal temperature of $1000(60)\,\mathrm{K}$. The measurement and control of
the internal temperature of nanomechanical oscillators is of fundamental
importance because blackbody emission sets limits to the coherence of
macroscopic quantum states.
