Re: Black hole at the centre of the galaxy imaged for the first time.

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Nicolaas Vroom

Jun 28, 2022, 3:28:41 AMJun 28
Op vrijdag 24 juni 2022 om 12:07:34 UTC+2 schreef Nicolaas Vroom:

> What that means that the BH is a spherical symmetrical object and that
> there exists a 'gaseous' layer outside the radius of the BH which emits
> light in all? directions.


> [Moderator's note: Almost all, or all, astrophysical black holes are not
> spherically symmetric in the sense that they rotate.

My understanding is that also all stars and planets rotate, at the same
time many of these could be called spherical symmetric like our earth
and the Sun.

> Rotating black holes are more complicated. What an observer actually
> sees when looking at a black hole is not trivial to calculate.

The first step is to observe. To calculate is a second step.

> In any case, the general
> consensus is that black holes detectable via radiation emitted from near
> them have an accretion disk and thus aren't spherically symmetric.

The question is to what extend can we conclude, based on observations,
that the BH, part of Sagittarius A*, has an accretion disk. Observing
the picture in the
ring surrounding the BH must be an 'indication' of this disk. If that is
the case the ring must be situated in a plane almost perpendicular
towards the direction of the line of sight between the earth and the
centre of the BH. (1) This direction must also be the same as the axis
of rotation of the BH.

Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.

As I mentioned before, I have my doubts. The ring does not change and
there is no prove of an accretion disk, based on this image.

What also is in favour of a sperical object is that the movement of the
stars around the BH is random. There is no preference.

I found also a different article:
This article also shows the direction of rotation of the BH.
The direction is different compared with (1) above.

My impression is that when you read this article and other articles the
accretion disks are of temporary nature and depend about source, that
causes the inflow of material. Together the BH and the source can be
considered as a binary system.

As mentioned above the movement of the stars around Sagitarrius A* are
random, as such, my guess is, that the direction of possible accretion
disks is also random, which is in contradiction with observation (1)

Nicolaas Vroom

[[Mod. note -- A few comments:
1. The Earth is *approximately* spherically symmetric, but if you look
more closely it's shape is in fact rotationally flattened. That is,
the Earth's equatorial radius is about 0.34% larger than its polar
radius, so the Earth is in fact NOT spherically symmetric.

2. While it's true that if we travel around the Sgr A* BH, its apparent
shape will change, that doesn't help us right now: our solar system
takes around 250 million years to orbit the center of our galaxy,
so we're not going to get to look at the Sgr A* BH from a
significantly different orientation any time in our lives.

3. Accretion disks (including the one around the Sgr A* BH) are indeed
temporary and depend on the availablity of source matter. But I
wouldn't say that the BH and the source are a "binary" system, because
there's no reason to think that the source is a single compact object.
Rather, the BH is embedded in a cloud of (moving) stars and
interstellar gas.

4. This 2020 article by Fragione & Loeb,
(which argues for a relatively low (slow) spin for the Sgr A* BH)
notes that past studies have given conflicting values for that spin.
I don't know enough about this subject to have an informed opionion
myself. Given the instruments now operational, we should know a
*lot* more about this in a few years, especially once ESO's
Extremely Large Telescope is operational (planned for 2027ish).
-- jt]]

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