Astronomers at the Max Planck Institute for Astronomy have created the most comprehensive three-dimensional map of the characteristics of cosmic dust in the Milky Way galaxy, according to a study published in Science.
Visualization of the wavelength-dependence of extinction (the “extinction curve”) caused by dust, for the plane of our galaxy’s disk, out to a distance of 8,000 light-years from the Sun. Red indicates regions where extinction falls off more rapidly at long wavelengths (the red end of the spectrum), while blue indicates that extinction is less dependent on wavelength. Regions with insufficient data are shown in white. The gray contours enclose regions of high dust density. Image Credit: X. Zhang/G. Green, MPIA
The astronomers created their map using 130 million spectra from ESA's Gaia mission, data from the LAMOST spectral survey, and machine learning. Dust causes distant astronomical objects to appear redder and dimmer than they are, thus the new map will help astronomers make sense of their observations. The study also discovered unexpected features of cosmic dust, which will need additional research.
When observing distant astronomical objects, there is a potential catch: is the star genuinely as reddish as it appears, or does it only seem red due to its light passing through a cloud of cosmic dust before reaching the telescope? To ensure accurate observations, astronomers need to determine the amount of dust between themselves and their distant targets.
Dust not only gives objects a reddish appearance (called “reddening”), but it also makes them appear fainter than they actually are (a phenomenon called “extinction”). It feels as though one is peering out of a filthy window into space. To help make sense of what is being observed, two astronomers have now released a 3D map that details the characteristics of the dust everywhere in previously unheard-of detail.
This is important because, fortunately, the effect of dust on starlight can be reconstructed, allowing for more accurate observations. Not all wavelengths of light are equally absorbed and scattered by cosmic dust particles. Instead, they absorb less light at longer wavelengths (moving toward the red end of the spectrum) and more light at shorter wavelengths (moving toward the blue end).
The wavelength-dependence can be shown as an “extinction curve,” and its shape tells not only about the dust’s composition but also about its local surroundings, such as the amount and qualities of radiation in different regions of interstellar space.
Retrieving Dust Information from 130 Million Spectra
This data was used by Xiangyu Zhang, a PhD student at the Max Planck Institute for Astronomy (MPIA), and Gregory Green, an independent research group leader (Sofia Kovalevskaja Group) at MPIA and Zhang's PhD advisor, to create the most detailed 3D map of dust properties in the Milky Way galaxy. Zhang and Green used data from ESA’s Gaia project, a 10.5-year effort to acquire extremely precise measurements of locations, movements, and other attributes for over a billion stars in the Milky Way and its nearest galactic neighbors, the Magellanic Clouds.
The Gaia mission’s third data release (DR3), published in June 2022, contains 220 million spectra, and a quality assessment determined that approximately 130 million of those would be appropriate for Zhang and Green’s search for dust.
The Gaia spectra have low resolution, which means that the way light is separated into distinct wavelength bands is relatively coarse. The two astronomers discovered a method around this limitation: The National Astronomical Observatories of China’s LAMOST survey provides high-resolution spectroscopy for 1% of their chosen stars.
This offers accurate information about the basic attributes of the stars in issue, such as their surface temperatures, which determine what astronomers refer to as a star’s “spectral type.”
Reconstructing a 3D Map
Zhang and Green trained a neural network to generate model spectra depending on the features of the star and the dust that surrounds it. They compared the data to 130 million appropriate spectra from Gaia and used statistical ("Bayesian") techniques to determine the properties of the dust between us and those 130 million stars.
The discoveries enabled scientists to create the first precise, three-dimensional depiction of the Milky Way's dust extinction curve. This map was made possible by Zhang and Green's measurement of the extinction curve for an unprecedented number of stars - 130 million, in contrast to earlier research, which was based on approximately one million measurements.
However, dust is more than a mere annoyance to astronomers. It is critical for star formation, which takes place in massive gas clouds veiled from radiation by dust. When stars originate, they are surrounded by disks of gas and dust that contain planet-forming material.
Dust grains serve as the fundamental building blocks for the formation of solid structures on planets like Earth. Indeed, the majority of the elements heavier than hydrogen and helium in the galaxy’s interstellar medium are trapped in interstellar dust grains.
Unexpected Properties of Cosmic Dust
The new findings not only offer a precise 3D map but also reveal an unexpected characteristic of interstellar dust clouds. Previously, it was assumed that the extinction curve would become flatter (less dependent on wavelength) in places with increased dust density. “Higher density,” of course, is still very low in this case: around ten billionth billionth grams of dust per cubic meter, corresponding to only 10 kg of dust in a sphere with the radius of Earth. In such areas, dust granules tend to expand in size, altering the overall absorption qualities.
Instead, the astronomers discovered that in areas of intermediate density, the extinction curve gets steeper, with shorter wavelengths absorbed far more effectively than longer ones. Zhang and Green hypothesize that the steepening was driven by the rise of polycyclic aromatic hydrocarbons (PAHs), the most common hydrocarbons in the interstellar medium, which may have even had a role in the genesis of life. They have already planned to test their idea with future observations.
Journal Reference:
Zhang, X., et. al. (2025) Three-dimensional maps of the interstellar dust extinction curve within the Milky Way galaxy. Science. doi.org/10.1126/science.ado9787