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What keeps the warm gas filaments in an H II region from spreading out?

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H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

The warm gas filaments within an H II region are prevented from spreading out by the pressure of tenuous, million-degree material located between the filaments.1

Although this hot material is significantly less dense than the visible "warm" (8,000 K) gas, its pressure is comparable, which effectively keeps the filaments from expanding into the surrounding vacuum. Additionally, in regions where stellar winds from massive stars create large cavities or bubbles, this hot, tenuous gas fills the majority of the cavity and serves to separate and confine the denser filaments of the H II region.1