Comet 17-P-Holmes, from HST

Caption NASA:"These images taken by NASA's Hubble Space Telescope reveal Comet Holmes's bright core. The images show that the coma, the cloud of dust and gas encircling the comet, is getting fainter over time. The coma was brightest in the Oct. 29 image. It is two times fainter on Oct. 31 and nine times dimmer on Nov. 4 than during the Oct. 29 observation.

The coma is getting fainter because it is expanding. A huge number of small dust particles was created during the Oct. 23 outburst. Since then those particles have been moving away from the nucleus and filling interplanetary space.
The coma therefore is becoming more diffuse over time.

The nucleus, however, is still active and is producing a significant amount of new dust. So the region around the nucleus is still much brighter (at least 10 times brighter) than it usually is at this point in the comet's orbit.