
The Milky Way's black hole may spring to life in 2013 - evo_9
http://arstechnica.com/science/news/2011/12/the-milky-ways-black-hole-may-spring-to-life-in-2013.ars
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william42
Considering the lightyears involved, I assume this means we'll _see_ it in
2013?

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sephlietz
Presumably, yes. It would be odd for the astronomers to use two different
temporal points of reference; time of observation is the only time that
matters. Plus, any excitement on the part of astronomers would be super
pointless if they had to wait another 24824 years to observe it.

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jarin
From the point of view of a photon originating in that gas cloud, it was
emitted by an electron in the gas cloud and absorbed by an electron in the
telescope sensor at the exact same instant.

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tikhonj
Can somebody explain why the black hole at the center of our galaxy is called
"Sgr A*"? Is there some system behind the name or are astronomers just like
programmers in naming stuff (it took me a while to figure out how to pronounce
nginx >:().

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walexander
Sgr is short for Sagitarrius, a constellation. If you mark off the boundary of
Sagitarrius, Sgr A* would be in that region. The black hole itself does not
have the name, but rather the radio source associated with it. (conjecture
alert)I'm not sure where the * comes from, but I believe the A is associated
with the fact that Sgr A is a super nova remnant. Supernova are typically
named "SN [year] [alpha]" where [year] is the year discovered and [alpha] is
an alphabetical character in order of discovery. A supernova remant wouldnt
have a year associated, so i'm guessing it's given a constellation and then
alphabetical marking.

Typically, astronomers have a mix of common names and many systematic naming
schemes, but they get obsoleted when better instruments find darker objects.
There was once a catalog of nebula called Messier with about 130 objects
(M1,M2,..). We still use these designations, but suddely they found thousands
more of these objects and had to rename using the New General Catalog,
(NGC1,NGC2,NGC496). Even further the IC goes into many thousands.

For constellations, stars in each constellation are usually named
alphabetically with greek chars in terms of their apparent brightness. Alpha
Centauri is the brightest star, for example, in Centarus. Then there are
common names like Polaris, the north star, whch is alpha ursa minoris.

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nknight
> _There was once a catalog of nebula called Messier_

Hah, I'd sort of known what the Messier catalog was, but you inspired me to
look at its Wikipedia page[1]. Apparently Charles Messier was looking for
comets, and was tired of getting distracted by things that weren't comets. M1
is _the Crab Nebula_ [2].

This amuses me.

[1] <http://en.wikipedia.org/wiki/Messier_object> [2]
<http://en.wikipedia.org/wiki/List_of_Messier_objects>

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AznHisoka
Could someone explain the implications of all this in laymen terms? Are we in
deep shit?

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ethanpoole
No. From what I understand, any material caught in the accretion disk is
merely funnelling down into the black hole's event horizon, i.e. the centre.
It will not affect any orbiting bodies, such as our solar system, but it will
make the black hole pseudo-visible to astronomers because the material in the
accretion disk will admit some light.

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michaelcampbell
It will add mass to the black hole though, right? So that will affect orbits,
no? Or is that just so infinitesimally small that it can reasonably be
ignored?

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46Bit
The mass is already roughly there. One thing to bear in mind - if you replaced
the Sun with a black hole of equal mass, planets wouldn't change orbits. It's
the same gravity and centre of mass, just a different density/volume.

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TheEzEzz
This is true for Newtonian gravity, but only approximately true for general
relativity (or so I understand it).

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dakr
This includes the effects of general relativity. As long as the mass is the
same, the orbits of planets will remain the same.

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TheEzEzz
According to wikipedia[1] the converse of the Shell Theorem is (nearly) true:

Suppose there is a force F between masses M and m, separated by a distance r
of the form F = Mmf(r) such that any spherically symmetric body affects
external bodies as if its mass were concentrated at its centre. Then what form
can the function f take?

The form of f allows Newtonian gravity but not Einsteinian.

[1]
[http://en.wikipedia.org/wiki/Shell_theorem#Converses_and_gen...](http://en.wikipedia.org/wiki/Shell_theorem#Converses_and_generalisations)

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T-hawk
The key point there is "spherically symmetric". The Sun isn't. It bulges
around its equator thanks to rotation, as does everything. That does have
effects on planetary motion; an object in an inclined orbit spends a bit more
time a bit farther away from a bit of the Sun's mass. Replace the Sun with a
black hole of equal mass and you don't have that oblateness. The effect on
planetary orbits would be very small, so macroscopically the Solar System
would still be the same, just with very slight differences in orbital speeds
and periods.

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ars
I wonder if they took time dilation into account when calculating the arrival
date.

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davidhansen
_That may be about to change, however. Astronomers have spotted a cloud of gas
with a mass about three times that of Earth that's on a trajectory that will
have it pass close to Sgr A_ in 2013*

If this is for real, I am ridiculously humbled by the power of modern
astronomy. It's pretty damned amazing to me that we have the ability to detect
the location, dimension, and trajectory of a mere 3 earth-masses of thin,
amorphous gas 27k light years away, in the noisiest and densest part of the
galaxy.

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johno215
Based on the Nature article abstract images it appears they were able to image
the cloud using the VLT telescope. I got confused about L'-band for a second.
Thought they were talking about L-band.

Feel free to correct me if someone has read the article.

[1]
[http://www.nature.com/nature/journal/vaop/ncurrent/full/natu...](http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10652.html)

[2] <http://www.eso.org/public/teles-instr/vlt.html>

[edited for mistake]

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ypcx
This is absolutely shockingly awesome. (No I don't use these words often at
all.) Based on the non-mainstream scientific evidence, the coming radiation
has the power to completely reconfigure our DNA, and in fact all DNA/RNA on
Earth's surface. _This is_ what had been predicted by numerous ancient
cultures including Mayans, and badly misinterpreted as the end of the world,
or end of the times. If you don't happen to be a hardline mainstream
scientist, I _really_ recommend picking up this book, if for nothing else,
then for all the absolutely shocking scientific research it references:
<http://www.amazon.com/dp/0525952047/>

