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42 votes
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Why didn't the Event Horizon Telescope team mention Sagittarius A*?

There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make ...
HDE 226868's user avatar
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28 votes
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What is it exactly about these flares of infrared light from Sgr A* that "confirms" it is a supermassive black hole?

We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the ...
HDE 226868's user avatar
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24 votes

Why does the closest approach of star S2 to Sgr A* not appear to be near the focus of its elliptical orbit?

The orbital elements are on wikipedia: $$e=0.884\ a=0.125'',\ i=134^\circ,\, \Omega=228^\circ$$ (At an assumed distance of 8kpc, $0.125'' = 1000au$) It is the inclination that means that the black ...
James K's user avatar
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21 votes

Why didn't the Event Horizon Telescope team mention Sagittarius A*?

I've found an explanation in Dutch here by Heino Falcke, one of the EHT founders. Translation: Hard to photograph It was easiest to take a picture of M87. "It is very difficult to photograph the ...
Glorfindel's user avatar
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14 votes
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Forms of stellar orbits around the galactic center

I'm not sure what the focus is on Sgr A*? Only the stars that are very close to the Galactic center can be said to be "orbiting Sgr A*", the rest of the stars in the Galaxy orbit in the the ...
ProfRob's user avatar
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14 votes
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How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

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 ...
13 votes
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How can I observe Sgr A* with itelescope.net

I expect that all the itelescope.net instruments work at visible wavelengths. Therefore you have no chance at all to image the stars around Sgr A*, since it is behind about 25-30 magnitudes of optical ...
ProfRob's user avatar
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9 votes
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Is this image of Sgr A* correctly labelled?

The event horizon of the Milky Way black hole (if it were non-spinning) has a diameter of 24 million km and is not directly related to the dark area. The black hole shadow should have an apparent ...
ProfRob's user avatar
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8 votes
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Is the appearance of accretion disks of M87 and Sgr A* in EHT image, related to the nucleus being active or non-active?

The rings are not direct images of accretion disks. They are blurred images of the gravitationally lensed light from all around the black holes, with a central "shadow" due to photon orbits ...
ProfRob's user avatar
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8 votes

Did the Grus star get expelled from the Milky way because it was accelerated by the Sagittarius A black hole?

The linked reference 59 in the wikipedia article you quote (Koposov et al (2019)) closes with the question The fact that S5-HVS1 was ejected with a velocity almost twice that of all other known HVS ...
planetmaker's user avatar
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7 votes

Do we know at which angle the Event Horizon Telescope will look at the accretion disk of Sagittarius A*?

We don't know the answer to this (or if someone does, I'd really like to hear that answer). Our galaxy's Super-Massive Black Hole (SMBH) is an unusually quiet one, with little to no accretion disk. If ...
zephyr's user avatar
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7 votes

Why is Sgr A* moving so much?

As far as I know, the issue is not that Sgr A* itself moves around so much, but rather it is the gas around Sgr A* (that the event horizon telescope tries to image) that moves around. The typically ...
TimRias's user avatar
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6 votes
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How will they know when to start taking the picture of the black hole at the center of the Milky Way?

The Milky Way's central supermassive black hole (SMBH) is feeding, albeit at a very low level. Radio emission from the accretion disk (and/or weak jets) is responsible for the long-lived "Sgr A*&...
Peter Erwin's user avatar
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6 votes
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How did they estimate the mass of Sgr A*'s companion G2 without knowing what it was?

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 ...
SpaceCore's user avatar
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5 votes
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How did they measure the distance to the center of our galaxy to 0.3% accuracy?

The orbit of star S2 is completely determined by the astrometric observations. i.e. One has the orbital period, the angular scale of the orbit and the inclination of the orbital plane. With this ...
ProfRob's user avatar
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5 votes

At what wavelengths can black hole Sagittarius A* be observed from Earth?

From Genzel et al. (2010), here's part of Fig. 7.7.1: This is part of the spectral energy distribution of Sagittarius A*, a flot of $\nu$ (frequency) vs. $\nu L_{\nu}$ (frequency times luminosity). ...
HDE 226868's user avatar
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5 votes
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How can I obtain raw observation data of stars surrounding Sgr A*?

Observations of stars near Sgr A* are done in the near-infrared, usually with adaptive optics. Most of these are done by two groups: Andrea Ghez's group at UCLA, using the Keck Telescopes in Hawaii, ...
Peter Erwin's user avatar
  • 17.1k
4 votes

Why is Sgr A* moving so much?

If a star of mass $m$ hits a black hole of mass $M$ at velocity $v$, conservation of momentum means that the black hole now moves at velocity $$\Delta v=\frac{mv}{m+M}=\frac{1}{1+\frac{M}{m}}v.$$ The ...
Anders Sandberg's user avatar
4 votes
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Could stars like S2 near the galaxy's center have planets in a stable orbit?

Technically yes, but no. Since the Wikipedia article regarding S2 notes an orbital period of 16.0518 years, with an eccentricity of 0.88466. A typical B0V star has a mass of ~18 solar masses while Sgr ...
WarpPrime's user avatar
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4 votes

What if a Black Hole passed through an Irregular type galaxy?

Black holes (even SMBs) are actually very small objects on an astronomical scale and in practical terms probably very little would happen. There would be some perturbations of stellar orbits within ...
StephenG - Help Ukraine's user avatar
4 votes
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Is there any update on the Availability of Event Horizon Telescope Images?

Today, April 10th 2019, there was a press conference where finally an image of M87 was released: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations ...
Glorfindel's user avatar
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4 votes
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What is the rotational speed of Sagittarius A*?

According to An Upper Limit on the Spin of SgrA* Based on Stellar Orbits in Its Vicinity, by Giacomo Fragione and Abraham Loeb (2020) ApJL 901 L32: The spin of the massive black hole (BH) at the ...
PM 2Ring's user avatar
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3 votes

How feasible is it that we could see the Central Black Hole Sgr A* occlude one of it's closely orbiting stars?

I'm sure someone will write a longer/better answer, so I'll keep this short and just mention that when a black hole passes between the observer an a star, we usually say that it becomes ...
uhoh's user avatar
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3 votes

Could stars like S2 near the galaxy's center have planets in a stable orbit?

In principle such a star could have a planet. Although the star is moving fast, a planet would also be pulled by the black hole's gravity, so you only need to consider tidal effects, and the "...
James K's user avatar
  • 123k
3 votes
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How do they know the newly-spotted gas near the center of our galaxy is molecular without knowing what gas it is?

They know there's molecular gas because they observed emission from the CO molecule in two of the previously identified (atomic-hydrogen-emitting) gas clouds. From the paper: ... we targeted two ...
Peter Erwin's user avatar
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3 votes

Does the newly found orientation of Sgr A* match the theory that the Fermi bubbles might have been caused by jets?

The video discusses exactly this question around the 15 minute mark. The newly published results seem to favour an inclination between the spin axis of Sgr A* and the ...
samcarter_is_at_topanswers.xyz's user avatar
3 votes
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S0-2 (S2) Diameter/Radius

S2 is a massive main sequence star, with a spectral type between B0V and B2V (according to Paumard et.al.) Such stars have mass that is between than 8 and 18 times that of the sun and a radius that ...
James K's user avatar
  • 123k
2 votes

How can a star be "thrown out of the Milky Way" by Sagittarius A*?

It's very similar to the question noted in the comments which has a nice mathematical answer. But some comments in your question are incorrect and worth addressing. As far as I can see, the ...
userLTK's user avatar
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