Thanks to Gaia, Astronomers are Able to Map Out Nebulae in 3D
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Thanks to Gaia, Astronomers are Able to Map Out Nebulae in 3D

The submillimetre-wavelength glow of dust clouds is overlaid on a visible-light view of nebulae in the Orion Molecular Complex in this 2D image. The Orion A portion of the Complex is represented by the large orange bar extending down to the lower left. The well-known Orion Nebula, also known as Messier 42, is the large bright cloud in the upper right. (Credit: ESO/Digitized Sky Survey 2.) Astronomers now have a new tool for studying nebulae like this one: 3D mapping with Gaia data.

Have you ever imagined flying through the Orion Nebula with all of its newborn stars? Or how about flying through the California Nebula? Of course, we’ve seen simulated “fly-throughs” of nebulae in sci-fi TV and movies, as well as planetarium domes. But what if we had a warp-speed spaceship and could browse the real thing? The first thing we’d need is precise information about that region of space. A 3D model with precise distance measurements to stars and other objects would be extremely useful in this situation.

Today, we don’t travel to the stars in spaceships, but we do have a navigator to show us the way. The Gaia spacecraft performs space-based astrometry using a pair of telescopes. The goal is to calculate distances from Earth’s LaGrange 2 point to over a billion stars in our galaxy. Having precise distances to objects all over the galaxy allows for a complete 3D view of the Milky Way.

Gaia Helps Measure Nebulae

However, Gaia’s work is useful for more than just astronomy. Its data allowed scientists to create realistic 3D images of the Orion A and California nebulae. Those models will assist them in comprehending why one is a star-forming behemoth and the other is a quiet.

2D false-color images of dust distribution inside the California (top) and Orion A Clouds (bottom). The data was collected using the Herschel Space Telescope. Lombardi et al. (2014), doi: 10.1051/0004-6361/201323293 (bottom); Lada et al. (2017), doi: 10.1051/0004-6361/201731221 (top). The structures of the two nebulae differ significantly, which may explain their differences in star formation.

Revealing 3D Nebulae

Sara Rezaei Khoshbakht (Max Planck Institute in Heidelberg) and Jouni Kainulainen created the new 3D reconstructions shown in the video below (Chalmers University, Gothenberg). Their findings shed new light on the Orion A (in the Orion Molecular Cloud complex) and California nebulae. From Earth, these clouds appear similar, with a few structural differences. Astronomers, on the other hand, didn’t have a good sense of their three-dimensional sizes and shapes.

This animation illustrates the 3D reconstruction of the Orion A Molecular Cloud using 60,000 stars as extinction probes with precise distance estimates. The calculations are based on the Gaia EDR3 star catalog. The axes x, y, and z are given in parsecs, with the earth as the zero point. The spatial resolution is 5 parsecs (15 light-years). Courtesy Sara Rezaei Khoshbakht, Jouni Kainulainen, Max-Planck-Institut für Astronomie

As seen in this video, Orion A in 3D provides a very different perspective (above). It has a complex filamentary structure with a very dense filamentary structure. It has thick condensations along a prominent ridge of gas and dust and is much denser than the California Nebula. It’s also a busy star-forming region, as we all know.

The California Nebula is nearly edge-on from our point of view. Because it is approximately 500 light-years across, astronomers do not have a single primary distance measurement for it. However, in 3D (below), it appears fairly flat with a small amount of filamentary structure and dense regions. A large bubble extends beneath the main cloud as well. According to the Gaia data, it’s not nearly as dense as astronomers thought. That explains why it isn’t as active as the Orion A cloud as a starbirth region.

This animation depicts a 3D reconstruction of the California Molecular Cloud using 160,000 stars as extinction probes with precise distance estimates. The calculations are based on the Gaia EDR3 star catalog. The axes x, y, and z are given in parsecs, with the earth as the zero point. 5 parsecs is the spatial resolution (15 light-years). Sara Rezaei Khoshbakht and Jouni Kainulainen of the Max-Planck-Institut für Astronomie contributed to this article.

Reconstructing Nebulae

The two scientists used a method developed by Rezai Khoshbakht for her Ph.D. research to create their 3D reconstructions. They used data collected by Gaia while studying starlight as it passed through a nebula’s gas and dust. “We analyzed and cross-correlated the light from 160,000 and 60,000 stars for the California and Orion A Clouds, respectively,” she explained in a recent press release.

They were able to reconstruct the shapes of the clouds and their densities from this data at a resolution of only 15 light-years. It’s an excellent addition to the nebula-measuring toolbox. For one thing, it explains why some nebulae are star-forming monsters while others are not. Rezaei Khoshbakht intends to use the same method to study the entire Milky Way galaxy after this current study. She will map its dust distribution and determine how it relates to star formation throughout the galaxy.

Nebula Density 101

Nebulae are clouds of gas and dust that can be found throughout our galaxy and in many other galaxies. Not all of them become stars. Those who do, however, have just the right amount of “star-maker material” to do the job. Star formation in a typical star-forming region, such as the Orion Nebula, begins with cold, dense pockets of material. Low temperatures cause dense regions to collapse even more. The thick regions then begin to heat up under increased pressure until conditions are ideal for the formation of a star. If a nebula has a lot of material, it can be a very efficient star-maker. That efficiency is low if it is not dense.

Traditionally, astronomers had to approximate a nebula’s density when determining its star formation efficiency. That is the amount of gas and dust in the cloud. However, “approximate” is not synonymous with “accurate.”

“Density is the amount of matter compressed into a given volume; it’s one of the crucial properties that determine star-formation efficiency,” Rezaei Khoshbakht explained. She also mentioned that some clouds of gas and dust produce more stars than others. Density plays a significant role in this.

Kainulainen’s work involves calculating the densities of objects in space, which is a difficult task. “Everything we see when we observe objects in space is their two-dimensional projection on an imaginary celestial sphere,” he said. Conventional observations lack the depth required by astronomers to obtain accurate measurements in space. They had to rely on column density until now. The pair’s new 3D reconstructions provide an advanced astrometric tool for understanding star formation in nebulae across the galaxy.

Gaia’s Astrometrical Role

Using Gaia data (along with data from other observations), astronomers will be able to create a more complete 3D view of the Milky Way. This includes the distribution of stars throughout the spiral arms and core. Furthermore, the Sagittarius Dwarf Galaxy is currently “ingesting” the galaxy. Gaia’s stars should be able to map themselves as they merge with the Milky Way. Gaia’s study of starlight as it passes through nebulae is also significant. It hints at the starbirth engines that hatch newborn stars in stellar nurseries.

It’s not a stretch to believe Gaia’s data will be included in stellar databases for a long time. Perhaps one day it will aid humans in their long-awaited journey through a nebula.

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