The Fourth Doctor, Leela and K9 Mark I once encountered such a nebula on "the edge of the cosmos". Although its high gravity was necessary to bring together the requisite matter to form star systems, it was extremely perilous to any ships that might have gotten too near. The Doctor's TARDIS immediately reacted to the proximity of the nebula by sounding alarms. In trying to explain the problem to Leela, the Doctor said that it was "sucking everything around it like a gigantic whirlpool — including us."