<h2>Question: How are these images obtained?</h2>

<p>Later in the video the narrator says they took the images using ESO's VLT.</p>

<blockquote>
  <p>03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very Large Telescope to the limits.</p>
</blockquote>

<p>(Source: <a href="https://www.eso.org/public/archives/videos/script/eso1825a_en.pdf" rel="nofollow noreferrer">ESO transscript</a>)</p>

<p>Over the whole observation period multiple telescopes and imaging instruments were used. Early observations were accomplished using the <a href="https://www.eso.org/public/teles-instr/lasilla/ntt/" rel="nofollow noreferrer">NTT</a>. Since around the year 2002 the VLT observed Sagittarius A* with the <a href="https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/naco/" rel="nofollow noreferrer">NACO</a> and <a href="https://www.eso.org/sci/facilities/paranal/instruments/gravity.html" rel="nofollow noreferrer">GRAVITY</a> instruments too.</p>

<p>This graph puts the rotation period of Sagittarius A* in relation to the time and the observing instruments:</p>

<p><a href="https://i.sstatic.net/q2mKf.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/q2mKf.jpg" alt="&quot;S2&quot; orbit with instruments and timescale"></a></p>

<p>(<a href="https://www.eso.org/public/images/eso1825c/" rel="nofollow noreferrer">Source</a>)</p>

<h1>Question: What process is it that is believed to be causing that flashing?</h1>

<p>ESO provides older footage over a minutes-timescale showing a flare of Sagittarius A* in May 2003: <a href="https://www.eso.org/public/videos/eso0330a/" rel="nofollow noreferrer">Flashes of light from disappearing matter</a></p>

<blockquote>
  <p>ESO Press Video eso0330 shows the detection of a powerful flare from the centre of the Milky Way galaxy. These and other adaptive optics (AO) images (with resolution 0.040 arcsec in the near-infrared H-band at wavelength 1.65 µm) of the central region of the Milky Way were obtained with the NACO imager on the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory on May 9, 2003. [...] The position of the 15-year orbiting star S2 (cf. ESO Press Release eso0226) is marked by a cross and the astrometric location of the black hole is indicated by a circle.</p>
</blockquote>

<p>The cause of the flickering is probably the same for the video sequence you showed.</p>

<p>A October 2018 ESO publication states:</p>

<blockquote>
  <p>ESO’s GRAVITY instrument on the Very Large Telescope (VLT) Interferometer has been used by scientists from a consortium of European institutions, including ESO [1], to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, [...]. The observed flares provide long-awaited confirmation that the object in the centre of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole’s event horizon — making these the most detailed observations yet of material orbiting this close to a black hole.</p>
</blockquote>

<p>(<a href="https://www.eso.org/public/news/eso1835/" rel="nofollow noreferrer">Source</a>)</p>

<p>ESO also has <a href="https://www.youtube.com/watch?v=DRCD-zx5QFA" rel="nofollow noreferrer">another video</a> showing the therory with more detail: skip to about 1:00.</p>

<p>Flares associated with Sgr A* are also described in <a href="https://astronomy.stackexchange.com/a/28241/7982">this answer</a>.</p>

<p>Artist's illustration of heated gas orbiting the black hole:</p>

<p><a href="https://i.sstatic.net/xpg0G.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/xpg0G.png" alt="Screengrab from Youtube DRCD-zx5QFA @ 1:25"></a></p>

<p>(Screengrab from 1:25 of that video)</p>

<hr>

<p>See also </p>

<ul>
<li>Phys.org (2019): <a href="https://phys.org/news/2019-09-black-hole-center-galaxy-hungrier.html" rel="nofollow noreferrer">Black hole at the center of our galaxy appears to be getting hungrier</a></li>
<li>Do et al. 2019 <a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab38c3" rel="nofollow noreferrer">Unprecedented Near-infrared Brightness and Variability of Sgr A*</a></li>
<li>Chen et al. 2019 <a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab3c68" rel="nofollow noreferrer">Consistency of the Infrared Variability of SGR A* over 22 yr</a></li>
</ul>

<p><img src="https://i.sstatic.net/AyNc5.jpg" alt="Figure 1 from Do et al. 2019, Unprecedented Near-infrared Brightness and Variability of Sgr A*"></p>

<p>Figure 1 from Do et al. 2019</p>

<blockquote>
  <p>Figure 1. Top row: a series of K' images taken on 2019 May 13 centered on Sgr A* showing the large variations in brightness throughout the night. The first image from the left is the brightest measurement ever made of Sgr A* in the near-infrared. Also labeled are nearby stars S0-2 (K' = 14 mag) and S0-17 (K' = 16 mag) for comparison. Bottom panel: K' (black) and H-band light curves of Sgr A* from 2019 May 13. On this night, we alternated between H and K' observations. The H-band magnitudes are offset using H − K' = 2.45 mag. There appear to be no significant color changes during the large change in brightness. Red circles show the location of the four images in the panels above.</p>
</blockquote>

CC BY-SA 4.0
2018-12-30T13:48:28.763Z
Thank you for your nice answer! About the first part; the *gravitational red shift* is the topic of this video, but that is done *spectroscopically. My question is about the short clip which is not necessarily the same thing; it is made from images collected over years or decades. They just show the clip to help the viewers understand how S2 is moving. But the clip isn't really the data that the video is about. Maybe they also used gravity for the imaging as well, I'm not 100% sure yet though.
uhoh
CC BY-SA 4.0
2018-12-30T13:51:55.950Z
[In fact, the observation has been conducted for over 26 years with the SINFONI and NACO adaptive optics instruments in the VLT **while the new approach in 2018 also used the beam-combiner instrument GRAVITY**](https://en.wikipedia.org/wiki/Very_Large_Telescope#Scientific_results)
uhoh
CC BY-SA 4.0
2018-12-30T13:57:56.693Z
It looks like M. Parsa et al. 2017 [Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center](https://www.eso.org/public/archives/announcements/pdf/ann17051a.pdf) explains the instruments that were used before GRAVITY came on line, and GRAVITY Collaboration 2018 [Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole](https://arxiv.org/abs/1807.09409) talks about the gravitational red shift done once GRAVITY came on line.
uhoh
CC BY-SA 4.0
2018-12-30T14:03:50.607Z
also see the IAS video [Investigating the relativistic ...the galactic center](https://youtu.be/3Q9rn-zIjps) found [here](https://video.ias.edu/workshop/2017/1129-Eckart)
uhoh
CC BY-SA 4.0
2018-12-30T21:09:53.213Z
@uhoh thanks for pointing out the mistakes on the instruments. I clarified the answer. I also added another ESO publication as reference to the second question.
try-catch-finally
CC BY-SA 4.0
2018-12-30T13:24:35.023Z
2019-09-12T05:21:29.500Z
2019-09-12T05:21:29.500Z
try-catch-finally
<p>The ESA video <a href="https://www.youtube.com/watch?v=if2opecmev8" rel="noreferrer">ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole</a> includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a presumed supermassive black hole. This isn't visible light because it's obscured by dust, so it may be radio or long wave infrared, but I don't know.</p>

<p>In the middle, I can see something flashing at whatever wavelength this image has been produced from.</p>

<p><strong>Question:</strong> </p>

<ol>
<li>How are these images obtained, and</li>
<li>what process is it that is believed to be causing that flashing?</li>
</ol>

<p>GIF made from video at around <code>02:50</code>:</p>

<p><img src="https://i.sstatic.net/eXQ0F.gif" alt="flashing at the center of the Milky Way galaxy and SgrA*"></p>

<hr>

<p>Six annotated frames from GIF highlighting the flashing that I'm seeing.</p>

<p><img src="https://i.sstatic.net/RdyAG.gif" alt="flares? from Sgr A*"></p>

CC BY-SA 4.0
2018-12-30T10:11:34.297Z
Perhaps a "conjunction" of individually fainter stars? Same flash is seen at left when two stars "merge".
Alchimista
CC BY-SA 4.0
2018-12-30T10:16:56.077Z
@Alchimista the location may turn out to be the massive object about which the stars are orbiting. It might be handy to find a plot or map of their orbits and compare. For example https://commons.wikimedia.org/wiki/File:Galactic_centre_orbits.svg
uhoh
CC BY-SA 4.0
2019-07-26T04:53:12.047Z
@RobJeffries I've added an annotated sub-GIF
uhoh
<supermassive-black-hole><dust><accretion-discs><identify-this-object><sagittarius-a>
How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?
uhoh
# How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

(Asked by: username_0 on 2018-12-30)

The ESA video [ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole](https://www.youtube.com/watch?v=if2opecmev8) includes a clip of images of stars at the center of our galaxy orbiting around SgrA\*, a presumed supermassive black hole. This isn't visible light because it's obscured by dust, so it may be radio or long wave infrared, but I don't know.

In the middle, I can see something flashing at whatever wavelength this image has been produced from.

**Question:**

1. How are these images obtained, and
2. what process is it that is believed to be causing that flashing?

GIF made from video at around `02:50`:

![flashing at the center of the Milky Way galaxy and SgrA*](https://i.sstatic.net/eXQ0F.gif)

---

Six annotated frames from GIF highlighting the flashing that I'm seeing.

![flares? from Sgr A*](https://i.sstatic.net/RdyAG.gif)

**Tags:** supermassive-black-hole, dust, accretion-discs, identify-this-object, sagittarius-a
> **Comments:**
> **username_1**: Perhaps a "conjunction" of individually fainter stars? Same flash is seen at left when two stars "merge".  
> **username_0**: @Alchimista the location may turn out to be the massive object about which the stars are orbiting. It might be handy to find a plot or map of their orbits and compare. For example https://commons.wikimedia.org/wiki/File:Galactic_centre_orbits.svg  
> **username_0**: @RobJeffries I've added an annotated sub-GIF  

---

## 1 answers

### Answer by username_2 on 2018-12-30. Score: 15 | Accepted answer

## Question: How are these images obtained?

Later in the video the narrator says they took the images using ESO's VLT.

> 03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very Large Telescope to the limits.

(Source: [ESO transscript](https://www.eso.org/public/archives/videos/script/eso1825a_en.pdf))

Over the whole observation period multiple telescopes and imaging instruments were used. Early observations were accomplished using the [NTT](https://www.eso.org/public/teles-instr/lasilla/ntt/). Since around the year 2002 the VLT observed Sagittarius A\* with the [NACO](https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/naco/) and [GRAVITY](https://www.eso.org/sci/facilities/paranal/instruments/gravity.html) instruments too.

This graph puts the rotation period of Sagittarius A\* in relation to the time and the observing instruments:

[![](https://i.sstatic.net/q2mKf.jpg)](https://i.sstatic.net/q2mKf.jpg)

([Source](https://www.eso.org/public/images/eso1825c/))

# Question: What process is it that is believed to be causing that flashing?

ESO provides older footage over a minutes-timescale showing a flare of Sagittarius A\* in May 2003: [Flashes of light from disappearing matter](https://www.eso.org/public/videos/eso0330a/)

> ESO Press Video eso0330 shows the detection of a powerful flare from the centre of the Milky Way galaxy. These and other adaptive optics (AO) images (with resolution 0.040 arcsec in the near-infrared H-band at wavelength 1.65 µm) of the central region of the Milky Way were obtained with the NACO imager on the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory on May 9, 2003. [...] The position of the 15-year orbiting star S2 (cf. ESO Press Release eso0226) is marked by a cross and the astrometric location of the black hole is indicated by a circle.

The cause of the flickering is probably the same for the video sequence you showed.

A October 2018 ESO publication states:

> ESO’s GRAVITY instrument on the Very Large Telescope (VLT) Interferometer has been used by scientists from a consortium of European institutions, including ESO [1], to observe flares of infrared radiation coming from the accretion disc around Sagittarius A\*, [...]. The observed flares provide long-awaited confirmation that the object in the centre of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole’s event horizon — making these the most detailed observations yet of material orbiting this close to a black hole.

([Source](https://www.eso.org/public/news/eso1835/))

ESO also has [another video](https://www.youtube.com/watch?v=DRCD-zx5QFA) showing the therory with more detail: skip to about 1:00.

Flares associated with Sgr A\* are also described in [this answer](https://astronomy.stackexchange.com/a/28241/7982).

Artist's illustration of heated gas orbiting the black hole:

[![Screengrab from Youtube DRCD-zx5QFA @ 1:25](https://i.sstatic.net/xpg0G.png)](https://i.sstatic.net/xpg0G.png)

(Screengrab from 1:25 of that video)

---

See also

* Phys.org (2019): [Black hole at the center of our galaxy appears to be getting hungrier](https://phys.org/news/2019-09-black-hole-center-galaxy-hungrier.html)
* Do et al. 2019 [Unprecedented Near-infrared Brightness and Variability of Sgr A\*](https://iopscience.iop.org/article/10.3847/2041-8213/ab38c3)
* Chen et al. 2019 [Consistency of the Infrared Variability of SGR A\* over 22 yr](https://iopscience.iop.org/article/10.3847/2041-8213/ab3c68)

![Figure 1 from Do et al. 2019, Unprecedented Near-infrared Brightness and Variability of Sgr A*](https://i.sstatic.net/AyNc5.jpg)

Figure 1 from Do et al. 2019

> Figure 1. Top row: a series of K' images taken on 2019 May 13 centered on Sgr A\* showing the large variations in brightness throughout the night. The first image from the left is the brightest measurement ever made of Sgr A\* in the near-infrared. Also labeled are nearby stars S0-2 (K' = 14 mag) and S0-17 (K' = 16 mag) for comparison. Bottom panel: K' (black) and H-band light curves of Sgr A\* from 2019 May 13. On this night, we alternated between H and K' observations. The H-band magnitudes are offset using H − K' = 2.45 mag. There appear to be no significant color changes during the large change in brightness. Red circles show the location of the four images in the panels above.

> **Comments:**
> **username_0**: Thank you for your nice answer! About the first part; the *gravitational red shift* is the topic of this video, but that is done *spectroscopically. My question is about the short clip which is not necessarily the same thing; it is made from images collected over years or decades. They just show the clip to help the viewers understand how S2 is moving. But the clip isn't really the data that the video is about. Maybe they also used gravity for the imaging as well, I'm not 100% sure yet though.  
> **username_0**: [In fact, the observation has been conducted for over 26 years with the SINFONI and NACO adaptive optics instruments in the VLT **while the new approach in 2018 also used the beam-combiner instrument GRAVITY**](https://en.wikipedia.org/wiki/Very_Large_Telescope#Scientific_results)  
> **username_0**: It looks like M. Parsa et al. 2017 [Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center](https://www.eso.org/public/archives/announcements/pdf/ann17051a.pdf) explains the instruments that were used before GRAVITY came on line, and GRAVITY Collaboration 2018 [Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole](https://arxiv.org/abs/1807.09409) talks about the gravitational red shift done once GRAVITY came on line.  
> **username_0**: also see the IAS video [Investigating the relativistic ...the galactic center](https://youtu.be/3Q9rn-zIjps) found [here](https://video.ias.edu/workshop/2017/1129-Eckart)  
> **username_2**: @uhoh thanks for pointing out the mistakes on the instruments. I clarified the answer. I also added another ESO publication as reference to the second question.  

---

