The object has survived its swing around the Milky Way’s supermassive black hole, but the questions of what it is and where it comes from remain unanswered.

This spring, astronomers around the world watched a mysterious, gaseous object called G2 slingshot around the Milky Way’s supermassive black hole. They pointed instruments from radio dishes to X-ray satellites at our galaxy’s center, hoping for insight into the object itself and the environment around the black hole.

Of course, as with most scientific endeavors, the results have raised a bunch of questions.

Star and/or Cloud?

Cloud nears breakup near black hole
In 2011 astronomers spotted a gaseous object (red-yellow blob above center, with orbit shown in red) zooming toward the supermassive black hole in the Milky Way's core. The stars orbiting the black hole are also shown, along blue lines marking their orbits. The stars and the object, called G2, are shown in their actual positions in 2011.

Astronomers first spotted the bit of infrared-emitting fuzz that is G2 in 2011. It’s been a subject of debate since then, largely because of its crazy orbit. Although many stars and gas streamers crowd in near the black hole, objects generally have nondescript, elliptical orbits around that central beast. But G2 follows a “supersquinched ellipse,” making a beeline for the black hole, whipping around it, and shooting straight back out again, says galactic center researcher Daryl Haggard (Amherst College).

To do that, the object must have lost enough momentum that, instead of following a more circular orbit, it started to fall headlong toward the black hole. You can think of it like a coin rolling around a bowl: put enough speed on the coin, and it’ll follow an orbit around the rim; without that momentum, the coin will just roll straight to the bottom. So something must have robbed G2 of that energy. “And that’s just very difficult to understand,” Haggard says.

Much ink (both print and electronic) has been spilt trying to explain what stripped G2’s momentum. The answer is twisted up in the nature of G2: is it merely a gas cloud, or does G2 hide a star inside the cloud?

Two teams are spearheading the infrared observations that peer through the dust, enabling astronomers to see the stars and gas in the galactic downtown. One team, led out of the Max Planck Institute for Extraterrestrial Physics in Germany, favors the cloud idea; the other, out of the University of California, Los Angeles, favors the star idea. Even here at the office my colleague Monica and I have a running bet on which side is right. (The winner gets a plate of homemade cookies.)

Comparing Apples and Oranges

stars arond Sgr A*
The central parts of our galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope. By following the motions of the most central stars, astronomers can determine the mass of the supermassive black hole that they orbit.
ESO / Stefan Gillessen et al.

The hope was that G2 would reveal its nature during its close encounter this past March with the black hole, Sagittarius A*. However, the data are still inconclusive.

The most serious consequence of the continuing mystery is that I’m still waiting for my plate of cookies. My tummy has to wait because it’s hard to unite the two teams’ observations into a coherent picture. The teams depend on different types of observations. The MPI team has fabulous measurements from the SINFONI spectrograph at the European Southern Observatory’s Paranal site in Chile. The spectroscopic data for G2 from late summer 2013 through spring 2014 show an extended gas tail disrupted during the close pass.

On the other hand, the UCLA team has exquisite images from the Keck Observatory on Mauna Kea. The images for spring and summer 2014 show an unresolved compact object that didn’t brighten and stuck to its orbit during the pass, as you’d expect for a dust-enshrouded star.

The UCLA team suggests that the discrepancy comes from the fact that the two types of observations are looking at different features: the spectra catch gas that’s stretched out from the orbit, while the images catch the dusty shell heated by the star within. The MPI observations thus don’t rule out a star; they just show that gas is yanked out during the close pass.

If Sgr A* slowly brightens in the next few years, it’ll indicate that it did indeed tear gas off G2 and that this gas fell through the accretion flow toward the black hole. That could reveal G2’s nature and how much stuff it lost during the pass.

To Shock Or Not To Shock

Milky Way's central arcsecond
The orbits of stars within the central 1 square arcsecond of the Milky Way, centered on the location of the central black hole (yellow star symbol). The fuzzy blobs are diffraction-limited star images in a frame taken by the 10-meter Keck telescope in 2004. While every star in this image has been seen to move, estimates of orbital parameters are only possible for those that have had significant curvature detected. The annual average positions for these seven stars are plotted as colored dots, which have increasing color saturation with time. Also plotted are the best fitting simultaneous orbital solutions. These orbits provide the best data yet on the mass of the central black hole. Click here for the movie.
Keck Observatory / UCLA Galactic Center Group

Unfortunately for now, the conversation is largely limited to those working with infrared and ultraviolet data and theoretical simulations: those working in X-ray and radio have seen nothing. “We’ve all just been staring at the data, wishing it would appear,” says Haggard. “And it hasn’t.”

This absence is a surprise. Astronomers expected X-ray and radio emission to appear as G2 rammed its way through the hot gas around the black hole, creating a shock wave. But that didn’t happen.

This might not be the first time, either. Dozens of stars whiz around the galactic center, and one of them, S2, comes closer to Sgr A* than G2 did. That star should also have created a shock during its closest approach in 2002, but spotty data from NASA’s Chandra X-ray Observatory showed no sign of one. Astronomers hope that much better observations during S2’s next pass in 2018 will settle the question.

If S2 creates a shock, then G2 is weird. This scenario would strengthen the MPI team’s argument that G2 is a clump in a cloud streamer torn from a star, falling along a path that its (not-yet-found) parent star cleared out for it. That suggestion builds off simulations by James Guillochon (Harvard-Smithsonian Center for Astrophysics) and his colleagues that show G2 could be a cloud stripped off a star during its own close pass around the black hole.

But if S2 doesn’t create a shock, then there’s something funky about the accretion flow around the black hole — maybe the material in that flow is too hot or too diffuse (or both) to be shocked. It’s fair to say that, while G2 has captured the imagination, understanding it is only part of the much greater adventure of exploring the galactic center.



O. Pfuhl et al. “The Galactic Center Cloud G2 and Its Gas Streamer.” Posted to July 16, 2014.

G. Witzel et al. “Detection of Galactic Center Source G2 at 3.8 Microns During Periapse Passage.” Posted to October 7, 2014.

R. A. Murray-Clay and A. Loeb. “Disruption of a Proto-Planetary Disk by the Black Hole at the Milky Way Centre.” Nature Communications, September 11, 2012.

J. Guillochon et al. “Possible Origin of the G2 Cloud from the Tidal Disruption of a Known Giant Star by Sgr A*.” Posted to January 13, 2014.

What's inside a black hole? Explore these questions and more in Sky & Telescope's Astronomy's 60 Greatest Mysteries.


g2 Sgr A*


Image of Fred Shuman

Fred Shuman

October 17, 2014 at 7:18 pm

"the object must have lost enough momentum" -- don't you mean, "... lost enough angular momentum"?

BTW, I hope one of you gets that plate of cookies soon!
Let's get this mystery solved, so we can move on to the next dozen or so!

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