Gizmodo.com's Astronomers Spot Unprecedented Flashes From Our Galaxy's Black Hole mentions Sgr A*'s companion gas cloud G2.

That Wikipedia article says:

First noticed as something unusual in images of the center of the Milky Way in 2002, the gas cloud G2, which has a mass about three times that of Earth, was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012.

but later in the same section says:

Nothing was observed during and after the closest approach of the cloud to the black hole, which was described as a lack of "fireworks" and a "flop".[57] Astronomers from the UCLA Galactic Center Group published observations obtained on March 19 and 20, 2014, concluding that G2 was still intact (in contrast to predictions for a simple gas cloud hypothesis) and that the cloud was likely to have a central star.


Professor Andrea Ghez et al. suggested in 2014 that G2 is not a gas cloud but rather a pair of binary stars that had been orbiting the black hole in tandem and merged into an extremely large star.

Question: Without knowing what this object is, how could its mass be determined to be only 3 times Earth's mass, way too small to measurably perturb the other stars in close orbits around Sgr A*?

Relevant and interesting reading: The Story of a Boring Encounter with a Black Hole


To put it simply, you can't.

Both cases are explained in these two papers:

Binary: https://arxiv.org/pdf/1410.1884.pdf
Just a Cloud: https://arxiv.org/pdf/1112.3264.pdf

In both cases you have to assume some kind of model to account for the detected fluxes in different near-infrared bands.

  • In the "Just-a-cloud" case you would need a cloud with 3 solar masses and a temperature of about 550 K.
  • In the "binary-merger" case you would need a 2 solar mass main sequence star covered in gas and dust (possibly generated by a recent merger with a companion, which, however, would somewhat distort the 2 solar mass estimate).
  • $\begingroup$ Okay this makes a lot of sense, thanks! $\endgroup$
    – uhoh
    Aug 13 '19 at 9:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.