<p>To put it simply, you can't. </p>

<p>Both cases are explained in these two papers:</p>

<p>Binary: <a href="https://arxiv.org/pdf/1410.1884.pdf" rel="noreferrer">https://arxiv.org/pdf/1410.1884.pdf</a><br>
Just a Cloud: <a href="https://arxiv.org/pdf/1112.3264.pdf" rel="noreferrer">https://arxiv.org/pdf/1112.3264.pdf</a></p>

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

<ul>
<li>In the "Just-a-cloud" case you would need a cloud with 3 solar masses and a temperature of about 550 K.   </li>
<li>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). </li>
</ul>

CC BY-SA 4.0
2019-08-13T09:34:04.603Z
Okay this makes a lot of sense, thanks!
uhoh
CC BY-SA 4.0
2019-08-13T09:14:41.263Z
2019-08-13T09:33:50.920Z
2019-08-13T09:33:50.920Z
SpaceCore
<p>Gizmodo.com's <a href="https://gizmodo.com/astronomers-spot-unprecedented-flashes-from-our-galaxys-1837181115" rel="nofollow noreferrer">Astronomers Spot Unprecedented Flashes From Our Galaxy's Black Hole</a> mentions Sgr A*'s companion <a href="https://en.wikipedia.org/wiki/Sagittarius_A*#Discovery_of_G2_gas_cloud_on_an_accretion_course" rel="nofollow noreferrer">gas cloud G2</a>.</p>

<p>That Wikipedia article says:</p>

<blockquote>
  <p>First noticed as something unusual in images of the center of the Milky Way in 2002, <strong>the gas cloud G2, which has a mass about three times that of Earth</strong>, 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.</p>
</blockquote>

<p>but later in the same section says:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>and </p>

<blockquote>
  <p>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.</p>
</blockquote>

<p><strong>Question:</strong> 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*?</p>

<p>Relevant and interesting reading: <a href="https://aasnova.org/2017/07/24/the-story-of-a-boring-encounter-with-a-black-hole/" rel="nofollow noreferrer">The Story of a Boring Encounter with a Black Hole</a></p>

CC BY-SA 4.0
2019-08-13T06:00:32.960Z
companion question: [Origins and most frequently used; perinigricon vs peribothron?](https://astronomy.stackexchange.com/q/33011/7982)
uhoh
<observational-astronomy><milky-way><supermassive-black-hole><mass><sagittarius-a>
How did they estimate the mass of Sgr A*'s companion G2 without knowing what it was?
uhoh
# How did they estimate the mass of Sgr A*'s companion G2 without knowing what it was?

(Asked by: username_0 on 2019-08-13)

Gizmodo.com's [Astronomers Spot Unprecedented Flashes From Our Galaxy's Black Hole](https://gizmodo.com/astronomers-spot-unprecedented-flashes-from-our-galaxys-1837181115) mentions Sgr A\*'s companion [gas cloud G2](https://en.wikipedia.org/wiki/Sagittarius_A*#Discovery_of_G2_gas_cloud_on_an_accretion_course).

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.

and

> 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](https://aasnova.org/2017/07/24/the-story-of-a-boring-encounter-with-a-black-hole/)

**Tags:** observational-astronomy, milky-way, supermassive-black-hole, mass, sagittarius-a

> **Comments:**
> **username_0**: companion question: [Origins and most frequently used; perinigricon vs peribothron?](https://astronomy.stackexchange.com/q/33011/7982)  

---

## 1 answers

### Answer by username_1 on 2019-08-13. Score: 6 | Accepted answer

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).

> **Comments:**
> **username_0**: Okay this makes a lot of sense, thanks!  

---

