milky way

A New Survey of the Milky Way Reveals Billions of Objects, Helping to Map Our Surroundings in Three Dimensions

The Dark Energy Camera Plane Survey 2 (DECaPS2) is out. This is the second data release from DECaPS, and the survey contains over 3 billion objects in the Milky Way. As the leading image shows, there are so many stars it appears as if there’s no space between them.

Throughout most of human history, the nature of the Milky Way has confounded us. Aristotle thought that it resulted from the ignition of Earth’s upper atmosphere, similar to how comets produce tails. The famous Persian scholar Al-Biruni thought the Milky Way was made up of countless fragments of nebulous stars. There are countless other ideas of what the Milky Way might have been.

Our modern understanding of the Milky Way is exquisitely detailed. Large-scale surveys of the galaxy have played a huge role in our growing understanding of the Milky Way. The ESA’s Gaia mission, an ongoing survey of the Milky Way, has collected detailed data on over one billion stars, including their ages, masses, chemical compositions, colours, temperatures, and metal content.

Will DECaPS2 make a similar contribution? It might, based on raw data alone.

“When combined with images from Pan-STARRS 1, DECaPS2 completes a 360-degree panoramic view of the Milky Way’s disk…”

Edward Schlafly, paper co-author, Space Telescope Science Institute.

A new paper in The Astrophysical Journal Supplement describes the new data release. It’s titled “The Dark Energy Camera Plane Survey 2 (DECaPS2): More Sky, Less Bias, and Better Uncertainties.” The lead author is Andrew K. Saydjari, a graduate student at Harvard University and a researcher at the Center for Astrophysics | Harvard & Smithsonian.

DECaPS2 contains 3.32 billion objects built from 34 billion detections. The detections are in 21,400 exposures which added up to 260 hours of open shutter time with the Dark Energy Camera (DECam) at the Cerro Tololo observatory. Of the 3.32 billion objects, about 2 billion are stars. It took two years and produced over 10 terabytes of data. The first data set was released in 2017, and it contained 2 billion objects, most of which were also stars.

The survey focuses on the galactic disk, where most of the galaxy’s stars and dust are located. But the high density makes the region difficult to observe. The DECam Plane Survey overcomes this by performing “deep photometric surveys spanning a broad wavelength range (optical to NIR).”

Astronomers have released a gargantuan survey of the galactic plane of the Milky Way. The new dataset contains a staggering 3.32 billion celestial objects — arguably the largest such catalogue so far. The survey is here reproduced in 4000-pixel resolution to be accessible on smaller devices.
Credit: DECaPS2/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA/E. Slawik Image processing: M. Zamani & D. de Martin (NSF’s NOIRLab)

“One of the main reasons for the success of DECaPS2 is that we simply pointed at a region with an extraordinarily high density of stars and were careful about identifying sources that appear nearly on top of each other,” said Saydjari. “Doing so allowed us to produce the largest catalogue ever from a single camera, in terms of the number of objects observed.”

The two data releases combined cover 6.5 percent of the night sky and span 130 degrees in length. That’s 13,000 times larger than the area of the full Moon. While 6.5 percent might not sound like a lot, it is. It’s even more impressive when combined with other sky surveys.

“When combined with images from Pan-STARRS 1, DECaPS2 completes a 360-degree panoramic view of the Milky Way’s disk and additionally reaches much fainter stars,” says Edward Schlafly, a researcher at the AURA-managed Space Telescope Science Institute and a co-author of the paper. “With this new survey, we can map the three-dimensional structure of the Milky Way’s stars and dust in unprecedented detail.”

“Astronomers will be poring over this detailed portrait of more than three billion stars in the Milky Way for decades to come.”

Debra Fischer, division director of Astronomical Sciences at NSF.

The galactic plane is difficult to observe. We’re embedded in it, and when we look toward the center, we’re looking through our arm of the Milky Way, through the central disk and beyond, into the spiral arms on the other side. It’s not just that there are hundreds of millions of stars—maybe way more—in this view. This is where most of the dust is located, too.

The image shows how beautiful and intriguing the dark lanes of dust are, but they’re also problematic. They absorb light from stars and can even block out faint stars entirely. There are many diffuse nebulae as well, and their light interferes with measurements of light from individual stars. The vast number of stars is also a challenge since they can overlap one another.

But understanding the central disk is critical to understanding the Milky Way. Observing in infrared helps overcome some of the challenges of observing the disk. Innovative data processing also helps. The team behind the survey came up with a way of predicting the background of each star. That made it easier to minimize the effects of overlapping stars and diffuse nebulae in the images.

“This is quite a technical feat. Imagine a group photo of over three billion people, and every single individual is recognizable!” says Debra Fischer, division director of Astronomical Sciences at NSF, one of the agencies that operates DECam. “Astronomers will be poring over this detailed portrait of more than three billion stars in the Milky Way for decades to come. This is a fantastic example of what partnerships across federal agencies can achieve.”

“Since my work on the Sloan Digital Sky Survey two decades ago, I have been looking for a way to make better measurements on top of complex backgrounds,” said Douglas Finkbeiner, a professor at the Center for Astrophysics, co-author of the paper, and principal investigator behind the project. “This work has achieved that and more!”

You can explore the DECaPS data and images at the Legacy Survey Viewer <Click on image>. Image Credit:

A century ago, we didn’t even know there were other galaxies. When astronomers saw the Andromeda galaxy and other spiral galaxies, they thought they were part of the Milky Way. They called them spiral nebula. Now we know better. We also know that the Milky Way is 90% dark matter and that the galaxy isn’t flat; it’s warped due to the Large and Small Magellanic Clouds tugging on it. We know that there’s a behemoth black hole lurking in the galactic center called Sagittarius A-star. We also know that galaxies grow so large by consuming and merging with other galaxies.

Large-scale astronomical surveys of the sky helped astronomers make some of these discoveries, and DECaPS promises to propel us to similar advancements. The first data release helped lead to some very interesting findings. One example is a 2018 paper that identified an old, metal-poor globular cluster in the galactic bulge. That was unusual since most globular clusters are in the galactic halo. Now, thanks to DECaPS and others, we know of many more globulars in the bulge.

The second data release will no doubt lead to many more discoveries and a filling-out of our knowledge about the galactic plane. “Combined with PS1, this completes comparable imaging of the entire Galactic plane essential for probing our Galaxy’s stars, gas, and dust,” the authors write. All of the data products from the survey “…should provide a rich, adaptable resource for the community, facilitating a variety of studies of the Milky Way,” they explain.

Another screenshot from the Legacy Survey Viewer. <Click Image to Visit> Image Credit:

We’re fortunate because our galaxy, the Milky Way, is a good model for understanding galaxies in general. About half of the stars in the Universe are in galaxies that are quite similar to ours. While nothing can be assumed, it’s a reasonable assertion that much of what DECaPS tells us about the Milky Way will have some bearing on other galaxies.

Astronomers have strived to understand galaxies and how they formed and evolved. The most comprehensive way to study them is to perform large surveys of stars like this one. Who knows what researchers will find in all of this data?


Evan Gough

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