Fly through Gaia’s 3D map of stellar nurseries

It is notoriously difficult to map and study regions in space where stars form because they are usually hidden from view by thick clouds of gas and dust, whose distances cannot be directly measured.

Gaia can’t see these clouds directly, but it can measure stellar positions and the so-called ‘extinction’ of stars. This means it can see how much light from stars is blocked by dust. From this, scientists can create 3D maps showing where the dust is, and use those maps to figure out how much ionised hydrogen gas is present – a telltale sign of star formation.

Extremely bright, young stars

The new 3D map of star-forming regions in the Milky Way is based on Gaia observations of 44 million ‘ordinary’ stars and 87 O-type stars. The map extends to a distance of 4000 light-years from us, with the Sun at the centre.

O stars are rare stars: they are young, massive, and extremely bright and hot. They shine bright in ultraviolet light. These light rays are so energetic that they can strip electrons away from hydrogen atoms when hitting them. In this way, they ‘ionise’ the hydrogen gas around the hot stars, meaning it becomes a mixture of charged particles [1]. This is one way that astronomers can identify regions in space where stars are being born.

Many telescopes have observed these regions, so we have a good idea of what they look like from our point of view. But no one really knew what they look like in three dimensions, or from an outside perspective.   

The Milky Way from above

Imagine that you are looking at the Milky Way from another galaxy. No spacecraft can travel beyond our galaxy, so we can’t take an actual photo. Fortunately, the Gaia mission is creating the most accurate multi-dimensional map of the Milky Way, giving astronomers the data to infer what it would look like. 

Gaia’s sky maps – in all three spatial coordinates (3D) plus three velocities (moving towards and away from us, and across the sky) – have revealed the precise motions and positions of millions of nearby stars. With this, the telescope has already revolutionised our view of the solar neighbourhood, allowing scientists to comprehensively map the stars and interstellar material near the Sun in a way they were unable to do before.

“Gaia provides the first accurate view of what our section of the Milky Way would look like from above,” explains Lewis McCallum, astronomer at the University of St Andrews, UK, and first author of two scientific papers explaining the new 3D model.

“There has never been a model of the distribution of the ionised gas in the local Milky Way that matches other telescope’s observations of the sky so well. That’s why we are confident that our top-down view and fly-through movies are a good approximation of what these clouds would look like in 3D.”

Lewis’s new map includes 3D views of the Gum Nebula, the North American Nebula, the California Nebula, and the Orion-Eridanus superbubble. It allows us to fly around, through, and above these areas containing stellar nurseries.

Giant cavity of interstellar matter

With the map, scientists can learn more about how the giant O stars energise gas in our galaxy, and how far out their influence reaches. Lewis and his colleagues already noticed that some of the clouds in the star-forming regions seem to have broken open, and streams of gas and dust are likely venting into a giant cavity (which can be spotted at second 4 of this video).

“This map nicely shows how radiation of massive stars ionises the surrounding interstellar medium and how dust and gas interact with this radiation. The 3D model provides a detailed look at the processes that shape our local galactic environment and helps astronomers understand interactions between the warm and cold components of the local Universe,” explained Sasha Zeegers, ESA Research Fellow and an expert on interstellar dust.

In the future, this map will span an even larger area of the Milky Way. “It required huge computational power to generate the map out to ‘just’ 4000 light-years from the Sun in high resolution [2]. We hope that the map can be expanded further out once Gaia has released its new set of data,” says Lewis.

“Gaia’s distance measurements of the nearby hot stars, and the 3D maps of dust – obtained from measuring the extinction and positions of millions of ordinary stars using Gaia data – are both crucial ingredients of this new map. Gaia’s fourth data release will contain data of even better quality and quantity, making it possible to further advance our knowledge of star-forming regions,” confirms Johannes Sahlmann, ESA’s Gaia Project Scientist.

Notes for editors
[1] Such an ionised hydrogen cloud is called a HII region by astronomers. A characteristic signal that can be picked up from these regions is the ‘hydrogen-alpha’ or ‘H-alpha’ spectral line at a wavelength of 656.3 nm.

[2] The work from Lewis McCallum and his team is based on earlier work published by Edenhofer et al in 2024, who created a dust map of our local galaxy. The new map presented today includes this previous map, and combined it with the hot (O) stars to visualise the ionised (star-forming) regions.

Two scientific papers by L. McCallum et al are published in Monthly Notices of the Royal Astronomical Society. https://academic.oup.com/mnrasl/article/540/1/L21/8085153  https://academic.oup.com/mnras/article/541/3/2324/8172009

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