We Are Orbiting A Supermassive Black Hole
The Earth is orbiting around the Sun which in turn is traveling through the Milky Way in orbit around the supermassive black hole known as Sagittarius A* or "Sgr A*" for short.
A supermassive black hole is no ordinary black hole. The one at the centre of our galaxy is a monster that is 4.1 million solar masses. The mass of this object at the centre of our galaxy is precisely known because there are stars that are in fairly close orbit around it and their orbital periods have been fairly well determined.
If you know the orbital period of a satellite and the size of its orbit then you can easily figure out it using this equation:
where,
a is the semi-major axis of the orbit,
G is the gravitational constant, and
M is the mass of the object being orbited.
The object is strongly suspected to be a black hole because a star designated as "S14" is believed to come with about 6 light hours (the radius of the orbit of Uranus) of it and nothing can have a mass of 4.1 million Suns and be this small and also not be a black hole.
As well, the best evidence for Sgr A* being a black hole comes from the redshift and blueshift of matter that is in very close orbit to this object. The redshift occurs for matter that is in orbit and moving away from us, the blueshift occurs for matter that is in orbit and moving towards us. Only something that has a super-strong gravitational field would have gases orbiting at speeds high enough to generate these wavelength shifts.
Oddly, even though it has a mass of 4.1 million Suns, the "density" of this black hole is calculated to be less than that of water. This is because density for a black hole is calculated using the volume of the Schwarzchild radius (i.e. the event horizon) which increases linearly with the mass of the black hole whereas volume increases as the cube of the Schwarzchild radius.
In reality, the black hole itself is not actually less dense than water, only the average mass density calculation using the event horizon as the radius of its boundary.
Sgr A* can be found in the general direction of the Sagittarius constellation right of the tip of the teapot (i.e. the star called γ2 in the image). Unfortunately you cannot see it with the naked eye as it lies behind obscuring astronomical dust clouds and it has to be observed using radio telescopes.
The Gentle Giant
One interesting effect of black holes is that as you falls towards them the closer parts of your body will feel a stronger gravitational pull than the farther parts of your body. This means that if you were falling feet first towards the black hole your feet would feel a stronger pull than your head because your feet are closer.
At first the difference would be small but as you get closer and closer the effect gets stronger and stronger. Eventually it will feel like you are being pulled on a torture rack until eventually you are in fact literally pulled apart. This effect is charmingly called being "sphagettified".
For a regular stellar-massed black hole this will tend to happen at or near the event horizon. For a supermassive black hole however, the event horizon is far enough away from the black hole centre that tidal forces will still be relatively weak. This means that when you finally do cross the event horizon of one of these behemoths you will not notice it (but you are still doomed nonetheless).
A Time-Shifted Doom
Time is known to slow down the deeper you get into the gravitational well. This means that if you were falling feet first and being sphagettified your brain, which is farther away, would be running at a faster rate of time than your feet which are closer and being torn apart.
Maybe the pain signals would be mercifully delayed enough before they managed to reach your brain?
I imagine this would be a very strange effect to observe, that is, if it weren't for the general terror of being ripped apart by a black hole.
Other Galaxies
Supermassive black holes are believed to lie at the heart of nearly every galaxy and the mass of the black hole is fairly strongly correlated with the mass of the host galaxy (i.e. the larger the galaxy the bigger the black hole).
The black hole at the heart of Andromeda, a spiral galaxy neighbour is calculated to be 23 million Suns which is almost 6,000 times more massive than our galaxy's black hole.
The black hole at the centre of one of my favourite galaxies and shown in the image, the Sombrero galaxy (M104), is believed to have a mass of 1 billion Suns. The event horizon for this object would be 20 times the radius of the Earth's orbit. It is somewhat daunting to think that there exists a black hole so massive that its event horizon is far larger than our planet's orbit.
Finally the largest supermassive black hole lies at the centre of "TON 618" a very distant quasar and is believed to have a mass of 66 billion Suns.
The event horizon for this sucker is about 200 billion kilometres which is an astounding 1300 times larger than the Earth's orbit. This is almost 70 times larger than the orbit of Uranus. Effectively the event horizon of this monster is larger than our entire solar system.
Closing Words
So, yes, we are orbiting a supermassive black hole but no, we are not going to fall into it so you can relax.
In fact, we are in a very stable orbit going steadily and quietly around the Milky Way galaxy and the strength of the black hole's pull is really no different than the pull of anything ordinary object the happened to gave has the same mass.
These objects however interesting are no threat to life on Earth.
Post Sources
https://en.wikipedia.org/wiki/Sagittarius_A*
http://www.constellation-guide.com/sagittarius-a/
https://en.wikipedia.org/wiki/Black_hole
https://en.wikipedia.org/wiki/Orbital_period
https://en.wikipedia.org/wiki/List_of_most_massive_black_holes
https://en.wikipedia.org/wiki/TON_618
It’s intersting to think that maybe the supermassive black holes are the glue holding the galaxies together. On one hand they are the most destructive forces in the known universe but in the other, without them we couldn’t exist. Excellent post. I always enjoy reading your content.
Was the sombrero galaxy pulled into that shape because of the size of the black hole in the center?
Thx for the comment but I think that the answer to that is no. This galaxy actually has a fairly normal shape for a spiral galaxy, it is just that we are looking at it edge-on.
There is the central bulge and the spiral disk. On the edge of the disk appears to be a thick dust cloud. That in itself might be rare but I could be wrong.
I see:) my physics and astronomy knowledge is growing quickly on Steemstem
Even if someone would feel the pain of being literally destroyed by a black hole, it would only be for a few seconds, everything will happen very fast so the overall suffering will not be that much.
That is huge! Kind of difficult to imagine so much mass actually.
Yes no doubt. I would expect that it might even be a few microseconds but I have never done the math myself.
This statement got me the relief
Nice write up
Thx, and yes, we can all sleep safely at night.