# How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

The ESA video ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole 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:

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

• Perhaps a "conjunction" of individually fainter stars? Same flash is seen at left when two stars "merge". Dec 30, 2018 at 10:11
• @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 commons.wikimedia.org/wiki/File:Galactic_centre_orbits.svg
– uhoh
Dec 30, 2018 at 10:16
• @RobJeffries I've added an annotated sub-GIF
– uhoh
Jul 26, 2019 at 4:53

## 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)

Over the whole observation period multiple telescopes and imaging instruments were used. Early observations were accomplished using the NTT. Since around the year 2002 the VLT observed Sagittarius A* with the NACO and GRAVITY instruments too.

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

(Source)

# 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

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)