Scientists Just Mapped 130 Million Stars to Reveal Universe’s Hidden Haze
Using data from ESA’s Gaia mission and high-resolution spectroscopy from the LAMOST survey, astronomers developed the most detailed 3D map of dust in the Milky Way. This breakthrough not only enhances our ability to see distant stars but also deepens our understanding of interstellar dust, which plays a crucial role in star and planet formation. Unexpectedly, their research revealed that dust extinction behaves differently than previously thought, potentially hinting at the presence of complex molecules like polycyclic aromatic hydrocarbons, which could have implications for the origins of life, the journal Science reported.
When we observe distant stars, what we see isn’t always what we get. A star that appears reddish might not actually be that color, its light could have been altered while traveling through cosmic dust before reaching our telescope. To ensure accurate observations, astronomers must account for this dust, which not only shifts a star’s color toward red (a phenomenon known as “reddening”) but also dims its brightness (“extinction”). It’s as if we’re looking at the universe through a dusty window. Now, two astronomers have created a groundbreaking 3D map that reveals the distribution and properties of interstellar dust in unprecedented detail, offering new clarity to our cosmic view.
Fortunately, astronomers have a way to reconstruct the effects of dust. Cosmic dust does not absorb and scatter light evenly, it affects shorter wavelengths (blue light) more than longer ones (red light). This pattern, known as an “extinction curve,” helps scientists determine the composition of dust and understand its environment, including the radiation conditions in different regions of interstellar space.
This is the kind of information 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 construct the most detailed 3D map yet of the properties of dust in the Milky Way galaxy. Zhang and Green turned to data from ESA’s Gaia mission, which was a 10.5-year-effort to obtain extremely accurate measurements of positions, motions and additional properties for more than a billion stars in our Milky Way and in our nearest galactic neighbors, the Magellanic Clouds. The third data release (DR3) of the Gaia mission, published in June 2022, provides 220 million spectra, and a quality check told Zhang and Green that about 130 million of those would be suitable for their search for dust.
The Gaia spectra are low-resolution, that is, the way that they separate light into different wavelength regions is comparatively coarse. The two astronomers found a way around that limitation: For 1% of their chosen stars, there is high-resolution spectroscopy from the LAMOST survey operated by the National Astronomical Observatories of China. This provides reliable information about the basic properties of the stars in question, such as their surface temperatures, which determines what astronomers call a star’s “spectral type.”
Zhang and Green trained a neural network to generate model spectra based on a star’s properties and the properties of the intervening dust. They compared the results to 130 million suitable spectra from Gaia, and used statistical (“Bayesian”) techniques to deduce the properties of the dust between us and those 130 million stars.
The results allowed the astronomers to reconstruct the first detailed, three-dimensional map of the extinction curve of dust in the Milky Way. This map was made possible by Zhang and Green’s measurement of the extinction curve towards an unprecedented number of stars – 130 million, compared to previous works, which contained approximately 1 million measurements.
But dust is not just a nuisance for astronomers. It is important for star formation, which occurs in giant gas clouds shielded by their dust from the surrounding radiation. When stars form, they are surrounded by disks of gas and dust, which are the birthplaces of planets. The dust grains themselves are the building blocks for what will eventually become the solid bodies of planets like our Earth. In fact, within the interstellar medium of our galaxy, most of the elements heavier than hydrogen and helium are locked up in interstellar dust grains.
The new results not only produce an accurate 3D map. They have also turned up a surprising property of interstellar dust clouds. Previously, it had been expected that the extinction curve should become flatter (less dependent on wavelength) for regions with a higher dust density. “Higher density,” of course, is in this case still very little: approximately ten billionth billionth grams of dust per cubic meter, equivalent to just 10 kg of dust in a sphere with Earth’s radius. In such regions, dust grains tend to grow in size, which changes the overall absorption properties.
Instead, the astronomers found that in areas of intermediate density, the extinction curve actually becomes steeper, with smaller wavelengths absorbed much more effectively than longer ones. Zhang and Green surmise that the steepening might be caused by the growth not of dust, but of a class of molecules called polycyclic aromatic hydrocarbons (PAHs), the most abundant hydrocarbons in the interstellar medium, which may even have played a role in the origin of life. They have already set out to test their hypothesis with future observations.
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