However one may ask where is the Galactic center?
The question of the finding where the galactic center is not something that is easily calculated. However being located at the point that our solar system is, we are able to readily identify the center. Radio observations made of our galactic center reveal a highly sophisticated structure.
|This image of the galactic center reveals regions of star birth never before seen.|
A large number of globular clusters and gas nebulae are found at center. The above X-Ray image shows numerous stars and nebulae at the heart of our galaxy, the lighter waves on the left represent nebula's.
|Simulation of star rotation around a black hole. The closer the star
to the black hole the faster its rotation and the more space time is
|Plot of orbits that stars orbiting Sgr* A. For some of these stars the orbit is of a high velocity while others take 10 - 14 years.|
|The above figure shows orbits of stars around Sagittarius A*, as determined by their proper motion and radial velocity. The black hole potential Sagittarius* A is in the center. Sagittarius A* has a proper motion of about 20km/s. You can find the full research here http://arxiv.org/pdf/0810.4674v1.pdf|
A technical answer to explain a black hole would be to say, it is a solution to Einstein's theory of relativity which describes the gravitational field of mass zero. A black hole is a compact mass that has a radius smaller than Schwarzschild radius.
The radius where the escape velocity of the black hole reaches the speed of light is called the Schwarzschild radius, named after physicist Karl Schwarzschild. The speed of light is measured to be approximately 300,000 kilometers per second.
|Simulation of black hole, with gravitational lensing effect|
The first discussion of black holes can be traced back to famous French mathematician Pierre Simon Laplace. Fellow engineers may recall having studied Laplace Transform which is useful in analyzing electronic circuits.
|Simulation of black hole creating a gravitation lensing effect, making it look as though the stars behind are bending around it.|
Laplace considered that if we reduce the radius (r) of a celestial body of a mass (M), the escape velocity (Vesc) at its surface will increase. This can be written in its mathematical form below:
From this we can see that for a small radius, (Vesc) will roughly equal to the speed of light. This happens when the radius (r) decreases.
|The above image shows small spiral galaxy NGC 7742. It is theorized that such type of galaxies called Seyfert type 2 are powered by a black hole in its core.|
So what does this Schwarzschild equation tell us?
This equation gives us a black hole's size and the corresponding event horizon.
If we were to imagine an astronaut fall toward the event horizon, if he carried a watch it would run more and more slowly and his progress toward the event horizon would slow down as well. At the event horizon, the gravitational redshift would be so intense that the watch would stop altogether. Our astronaut would have reached the event horizon, but from our perspective he would never have made it. For the astronaut he would he would go pass the event horizon, continuously falling into the black hole for perhaps infinity. However this is our imagined scenario, because in reality the gravity at the event horizon would be so intense that our astronaut would be ripped apart atom by atom.
The above video shows very interesting simulation of a black hole with (rs) having a radius of 6.03.
At the moment it is quite difficult for us to observe the center of our galaxy but perhaps in the near future we may be able to observe this radius.
|This comparison above shows four elliptical galaxies that have a
massive central bulge of stars, the larger the bulge the larger the
black hole. The galaxies are part of 30 galaxies searched by astronomers
using NASA's Hubble Space Telescope.|
The column of black-and-white pictures at left, taken by ground-based telescopes, shows the galaxies. The boxes define the central regions of stars. Close-up images of these regions, as seen by Hubble's Wide Field and Planetary Camera , are in the middle column. The column at right lists the masses of the black holes and illustrates the respective diameters of the event horizons. An event horizon defines the black hole's boundary. The event horizons cannot be seen in the Hubble images because they are about 25 million times smaller than the scale of the pictures.
Astronomers determined the mass of each black hole by measuring the motion of stars swirling around it: the closer the stars approach the black hole, the faster their velocity. The faster the stars are moving, the more massive the black hole is. This information suggests that the galaxy and the black hole grew at the same time.
Black holes seem to behave like very efficient vacuum cleaners with an infinite limit. But can a black hole go away over time?
There are three stages of a black hole; stellar, primordial and super massive.
A stellar black hole is a region of space into which a star collapse's. So how does a star collapse? A start collapses once it has run out of fuel, this fuel consists of hydrogen and helium which we discussed in a previous post in Interstellar Star Formation.
It collapses to a critical size where any other type of force is overcome by a single force, gravity.
Primordial black holes are said to have been created at the time of the big bang. These mini black holes are theorized to be of the order of 1cm. In such small objects quantum theory becomes very important. Primordial black holes could perhaps be very hot and from the outside they may actually look like a whit hole as they expel intense radiation and gamma ray bursts.
Super massive black holes are what we have been discussing earlier. These are the type that harbor in the center of galaxies. In the order of 100 million solar masses, 100 million times the size of our solar system lay at galactic nuclei. For stellar black holes they may have a potential lifetime of 10^67 years. Most of these will have gone away by now or perhaps many lurk in far distances where there is an infancy of the universe.
|Black hole as represented in 2014 sci-fi epic Interstellar|
There are still many questions about black holes that are yet to be answered. With future development of the space telescope to succeed the Hubble Space Telescope, the James Webb Telescope is sure to uncover many more facets of the hidden secrets in universe.
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Goodbye for now and see you next time.