We see the universe through a glass dark or more precisely through a dusty window. Interstellar dust is scattered across the Milky Way, which limits our view depending on the appearance. In some directions, the effects of dust are low, but in other regions the view is so dusty that it is called the zone of avoidance. Dust distort our view of the sky, but luckily a new study has created a detailed card with a cosmic dust so that we can better take it into account.
The main problem with interstellar dust is that it does not simply block all the light. It distorts light similar to other astrophysical effects. For example, dust tends to absorb blue colors more than red. So if there is dust between us and a distant star, the star appears red as actually. When you see a red star, does that mean that it is actually red, from a relative movement from us or because dust is in the way? It can be difficult to know the difference. Dust can also make distant galaxies appear stupid than actually. Since we use the brightness of supernovae in a galaxy to determine the distance, dust can appear more far away than they actually are.
For a long time we had a pretty good idea of where there is dust in the galaxy, so that we can blame many of these effects. But our understanding of the dust could be much better. Therefore, this new study will be so advantageous.
The team started with data from the Gaia spaceship that has collected observations of more than a billion stars in a milky way, including spectra observations of more than 220 million stars. They found that around 130 million of the spectra observations would be useful when determining the distribution of galactic dust and would concentrate on them. A restriction of the GAIA spectra is that they are not a high resolution. The main purpose of GAIA is to map the position and movement of stars for which no complete spectrum is required. Therefore, the authors looked at data from the large fiber spectroscope telescope (llamost) from the large sky, which has collected high-resolution spectra of around 1% of the Gaia stars.
The next step was to use the llamost data to extrapol spectra for the GAIA spectra with low resolution. With machine learning and Bayes' statistics, the team could model a complete spectrum from a low resolution. From this they were able to create a 3D card with galactic dust effects. In particular what is known as a dying curve.
Since light is absorbed with different rates at different frequencies of dust, astronomers not only have to know how much dust is available in a certain direction, but also how different frequencies take away by far. This exit curve can then be used to determine the impartial view backwards. From their model, the authors created the most detailed extinction card of the previous Milky Way.
They also found a surprising result. We know that the interstellar medium consists of both gas and dust. Things like neutral hydrogen and simple molecules do not absorb much light compared to dust particles, so we assumed that dust grains were the main cause of extinction curves. However, when the team looked at particularly dense regions, they found that the extinction curves were not flattened as expected. Instead, the curves were tensed even more red than less dense regions. This indicates that complex molecules, known as polycyclic aromatic hydrocarbons (PAK), play a primary role in the absorption of light. How this molecules affect our general view of the cosmos will be a focus of a future study.
Reference: Zhang, Xiangyu and Gregory M. Green. “Three-dimensional maps of the interstellar dust extracting curve in the Milky Way.” Science 387.6739 (2025): 1209-1214.
