We human beings love to travel. We love to see new places and explore spectacular locations. Inside every Human there lives a Columbus, a Vasco da Gama, an explorer. But travelling only around the World is not the end of our journey. Let us come out of the Earth and make a journey to the center of our Galaxy, the Milky Way. Lets imagine that you are the Sun and then you will feel a huge inward centrifugal force which makes you go round a particular center. This center is the center of our galaxy. It is located 25,940 light years away from the Sun, in the constellation Sagitarius. But this region cannot be observed optically because this region is crowded with stars and interstellar clouds whose extreme opacity and brightness blocks our view to the center.
But in the mid February of 1974, two astronomers, Bruce Balick and Robert Brown detected a Radio source emanating from the center, using the baseline interferometer of National Radio Astronomy Observatory. They named it Sgr A*. From then onwards Astronomers throughout the world started to reaserch on this object at the center. They found that it was not a star. Curiosity grew among the astronomers. Then they tried to look at the stars around or in the neighborhood of the Sgr A*. They presicely tracked the motion of the stars S1, S2, S8, S12, S13 and S14 which are in close proximity of the central object Sgr A*. Except S1 all the stars stated above have their pericenter with SgrA* very small, that means much closer to Sgr A*. After tracking the stars for few years they found that the star S2 has the smallest orbit around the center Sgr A* with a time period of 15.2 years and eccintricity of 0.88. The mass of Sgr A* was recorded to be 4.1 million times the solar mass within a volume of radius 6.25 light hours. Now let us calculate the apicenter and pericenter of S2, from the observed data.
We know that the square of time period (T) of the orbit is directly proportional to the qube of semi-major axis (a): 
Thus, 
----------------(1)
where G =
= Gravitational constant
M = 4.1 million solar mass.
= 
Now substituting the values of G and M in equation (1) we get,
= 
=
Therefore, 
Thus pericenter distance is rper = (1- e )a = (1- 0.88) x 995 = 119.4 AU = 17 light hours. This is the closest distance of S2 from Sgr A*. The Sgr A* is definitely a very power full object which has very large mass and which is holding all the stars and whose high speed rotation is rotating the entire galaxy. Thus the central object must be a very powerful black hole.
Now let us calculate the Schwarzchild radius of the central object.
= 
Now if this were the radius of the central object then it won't be a black hole or so mach powerful as it is. If its radius is smaller than its Schwarzchild radius than its definitely a Black Hole. The radius of Sgr A* as observed is 6.25 light hours or 45 AU which almost half of its Schwarzchild Radius. Thus the center of Milky Way is a Super Massive Black Hole holding a mass of about 4.1 million solar masses within a volume of radius 6.25 light hours or 45 AU.