New Evidence May Identify Mystery Object at Milky Way Galaxy's Core

Dusty Cloud G2
This annotated composite image shows the motion of the dusty cloud G2 as it closes in, and then passes, the supermassive black hole at the center of the Milky Way, and remains compact. The blobs have been colorized to show the motion of the cloud, red indicated that the object is receding and blue approaching. The cross marks the position of the supermassive black hole. (Image credit: ESO/A. Eckart)

New observations may finally reveal the identity of a mystery object circling around the monster black hole at the center of our Milky Way galaxy — or not.

Known to many as "G2," the unidentified object could be a cloud of gas, or it could be a star, depending on who you ask. Discovered in 2011, G2 captured the attention of scientists because it was on its way to making a tight swing around the black hole — potentially providing the dark monster with a snack. You can see a video of G2's movement here.

The new observations of G2 show that it has remained compact during its swing around the black hole, according to the authors of the new research. Since a gas cloud would likely be smeared out by the gravitational pull of the black hole, the scientists conclude that the object is a star. But the group that discovered G2 says the new results are not enough to make a definitive statement about the identity of this peculiar blob. [Images: Black Holes of the Universe

A star or a gas cloud

The strange mystery object G2 is a fuzzy-looking thing in most photographs. That's due partly the resolution of those images — it is very difficult to take pictures of the center of the galaxy from Earth's location on the outer edge of the galactic disk.

G2 is also fuzzy-looking because it has an outer layer of gas and dust around it, and that's one of the few things scientists studying it can seem to agree on. What lies inside that dust cloud is the subject of an intense scientific debate.

Leading the inquiry are two groups. The first is led by Stefan Gillessen at the Max Planck Institute, who first identified G2 back in 2011. Gillessen thinks there's enough evidence to show that G2 is a gas cloud, and that it has been smeared out by its close encounter with the black hole.

The second group is led by Andrea Ghez at the University of California, Los Angeles. Ghez was one of the scientists to first demonstrate that there is a monster black hole at the center of the Milky Way (the black hole is called Sagittarius A*, pronounced "Sagittarius A-star"). Ghez says there's more than sufficient evidence to show that there is a star inside G2's dusty outer shell, and that it has remained compact as it swings around Sagittarius A*.

For the past year it has felt like these two groups have been engaged in a tug of war. Even though more observations have rolled in, they haven't been enough to change anyone's mind. 

The third party

Now, a third group has weighed in and picked a side: G2 is a star.

A group of scientists at the University of Cologne in Germany claims the researchers' most recent data shows that G2 remains compact after its closest approach to the black hole. It hasn't been smeared out or torn apart the way a gas cloud likely would be under such a strong gravitational force, they say.

"For us, everything points at it being a young star," said Andreas Eckart, a co-author on the new paper.

The new research paper includes an analysis of data taken by Eckart's group with the SINFONI instrument on the European Southern Observatory's Very Large Telescope (VLT) — the same instrument used by Gillessen and colleagues. The study appears in the Feb. 20 issue of the Astrophysical Journal Letters. 

Eckart and colleagues say their data shows that G2 has passed its point of closest approach to the black hole. Prior to 2014, they measured the gas cloud moving away from Earth. Now it can be seen moving in the direction of Earth, and at a much faster clip: from 6.2 million miles per hour (10 million km/h) on closest approach, to 7.4 million miles per hour (12 million km/h) after.

The group also made a size measurement of G2 based on data taken between 2008 and 2013, and compared it with the size of G2 today.

"If you take that size estimate and assume it's a freely expanding cloud on its orbit, it gets orbitally stretched and we calculated that it would be four times larger along the orbit," he said. "But we see that it's compact."

Gillessen initially predicted that not only would G2 smear out in its closest approach to Sagittarius A*, but some of its matter would fall in and radiate light. But the galactic fireworks never appeared.

The new paper by the Cologne group also includes an independent analysis on publicly available data from both the UCLA group (taken with the Keck telescope in Hawaii) and the Max Planck group. That data comes from before G2 made its close approach to the black hole.

"We don't see any stretching of the cloud that was claimed previously," Eckart said. "We get a much more coherent picture of a single object." [8 Baffling Astronomy Mysteries]

The debate continues

Gillessen's mind has not been changed by the new paper.

Among other things, the Eckart group did not have access to the data taken by his group in 2014, the year of G2's closest approach, Gillessen said in an interview with The Eckart group's original data was also taken with shorter integration times, meaning that the researchers looked at the object for a shorter period of time. This was done, according to the Cologne paper, to reduce noise in the images.

But Gillessen said these factors mean that the researchers in the Eckart group are only looking at "the tip of the iceberg," when it comes to G2's physical features. They may not, he said, be seeing the whole picture.

Despite their disagreements, all three of the groups observing G2 belong to a relatively small community of researchers focusing on this strange object at the galactic center. The Cologne group and the Max Planck group are collaborating on a telescope instrument; one of Eckart's former graduate students now belongs to the group at UCLA.

Gillessen said he and his colleagues have more observation time planned for the spring and the summer. The identity of G2 may finally be settled in 2015 — or not.

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Calla Cofield
Senior Writer

Calla Cofield joined's crew in October 2014. She enjoys writing about black holes, exploding stars, ripples in space-time, science in comic books, and all the mysteries of the cosmos. Prior to joining Calla worked as a freelance writer, with her work appearing in APS News, Symmetry magazine, Scientific American, Nature News, Physics World, and others. From 2010 to 2014 she was a producer for The Physics Central Podcast. Previously, Calla worked at the American Museum of Natural History in New York City (hands down the best office building ever) and SLAC National Accelerator Laboratory in California. Calla studied physics at the University of Massachusetts, Amherst and is originally from Sandy, Utah. In 2018, Calla left to join NASA's Jet Propulsion Laboratory media team where she oversees astronomy, physics, exoplanets and the Cold Atom Lab mission. She has been underground at three of the largest particle accelerators in the world and would really like to know what the heck dark matter is. Contact Calla via: E-Mail – Twitter