Active galaxy. Coloured Hubble Space Telescope (HST) spectrograph images of the active (Seyfert) galaxy NGC 4151, thought to contain a black hole. The images show emissions from oxygen gas at a wavelength of 4959 angstroms (495.9 nanometres, left) and 5007 angstroms (500.7nm, right). The emitting gas is found in knots which are being ejected from the galaxy's nucleus at high speeds. This ejection occurs in two beams (not seen), one carrying gas towards us and one away from us. The active nucleus of this galaxy is powered by a super-massive black hole.