Three classical Cepheid variable stars in the nuclear bulge of the Milky
  Way
  The nuclear bulge is a region with a radius of about 200 parsecs around the
centre of the Milky Way. It contains stars with ages ranging from a few million
years to over a billion years, yet its star-formation history and the
triggering process for star formation remain to be resolved. Recently, episodic
star formation, powered by changes in the gas content, has been suggested.
Classical Cepheid variable stars have pulsation periods that decrease with
increasing age, so it is possible to probe the star-formation history on the
basis of the distribution of their periods. Here we report the presence of
three classical Cepheids in the nuclear bulge with pulsation periods of
approximately 20 days, within 40 parsecs (projected distance) of the central
black hole. No Cepheids with longer or shorter periods were found. We infer
that there was a period about 25 million years ago, and possibly lasting until
recently, in which star formation increased relative to the period of 30-70
million years ago.
