Light from distant stars, as it travels through cosmic dust, loses some of its energy. Dust grains absorb and scatter light, and the way they do so at different wavelengths reveals their composition and size. Astronomers call this the dust extinction curve. For the galaxy M33, such curves in the ultraviolet have been obtained for the first time — previously they had only been measured in four other Local Group galaxies.

New data from the Hubble Space Telescope show that dust in M33 behaves differently than in the Milky Way. Along the four studied lines of sight, the extinction curves differ greatly in shape, and the famous ultraviolet bump at 2175 angstroms is sometimes prominent and sometimes nearly absent. On average, dust in M33 absorbs ultraviolet light more weakly than in other Local Group galaxies, and its behavior does not fit the familiar dependence on the parameter R(V), which describes the overall reddening of light.

Interestingly, the variations in the extinction curves in M33 correlate better with the gas-to-dust ratio than with the galaxy's overall metallicity. This reinforces the idea that dust properties are more strongly influenced by local conditions (radiation density, shock waves from supernova explosions) rather than by global galaxy characteristics like heavy-element abundance. Dust turns out to be a sensitive indicator of local cosmic weather.

Such a spread in extinction curves reminds us that cosmic dust is not uniform: its composition and grain sizes depend on where it formed and what has happened to it since. By studying dust in different galaxies, astronomers are learning to read the history of star formation and chemical evolution across the universe.