THE BLACKHOLE AT THE CENTRE OF OUR MILKY WAY GALAXY
The term blackhole was coined by John A. Wheeler in 1965 to represent the end stage of `a gravitational collapse that cannot be halted'. When the radius of a collapsing spherical body such as a dying star drops below a critical value known as the Schwarzschild radius, further collapse cannot be halted under any circumstances. Atoms are crushed and matter gets transformed to a mysterious unknown state, as the entire mass of the blackhole contracts to a mathematical singular point. Blackholes represent the awesome triumph of gravity over the strong nuclear forces that keep the subatomic particles in their proper configuration in every atom. No matter or radiation (including light) can escape from a blackhole. The existence of a blackhole can be ascertained only from its gravitational effects on other objects.
Three types of blackholes have been described: Proton-sized primordial blackholes, thought to have been formed at the time of creation of our universe; stellar blackholes formed by the imploding core of a star that explodes as a supernova; and gigantic supermassive blackholes that reside and grow at the nucleus of galaxies.
Like most galaxies, our Milky Way galaxy also harbours a supermassive blackhole at its galactic centre called Sgr-A* and situated 26000 light-years away from the Sun in the constellation Sagittarius [One light-year, the distance travelled by light in one year, equals 9.46 million million km]. Attached herewith is a mosaic of two pictures of the galactic centre of our Milky Way galaxy.
Press Release 17/2002 of the European Southern Observatory (ESO) gives interesting information on the chaotic conditions that prevail around the supermasive blackhole at Sgr-A*. Within a radius of 1 light-year centered on Sgr-A*, hundreds of stellar blackholes (supernova remnants) and thousands of stars are trapped in a spiral `dance of death' around the supermassive blackhole. Whenever a star and a blackhole come within the range of mutual gravitational interaction, the star gets pushed outwards, whereas the blackhole gets pushed inwards. The stellar blackholes gravitate inwards and eventually merge with the supermassive blackhole at the galactic centre. The scenario at Sgr-A* is simply awesome, when viewed in the light of the fact that in our vicinity the stars are spaced several light-years apart.
The 7 bright stars near Sgr-A* seen in the lower picture can also be identified in the upper picture. One of these, designated S2 by ESO, is the star found closest to the blackhole at Sgr-A* (refer Apod-021018). This star was tracked painstakingly by ESO for more than a decade, and was found to be moving in a highly elliptical orbit around the blackhole. The exact location of Sgr-A* is identified by the frequent bursts of infrared and other radiation coming from there, as matter is gobbled up by the blackhole. The star S2 was just 17 light-hours away from Sgr-A* at its closest point, and 240 light-hours away at its farthest point. The orbital period was 15.2 years.
It is interesting to note that, in order to measure the orbital radius as 17 light-hours (0.00194 light-year) at a distance of 26000 light-years, a very big telescope with a pointing accuracy of better than 0.01 arcsecond is required. The difficulties in the mechanical design of such a big steerable telescope are no less formidable than those in its optical design!
The star's measured orbital velocity of 5000 km/sec at its closest distance of 17 light-hours enables the estimation of the mass of the supermassive blackhole residing at Sgr-A*. The distance of 17 light-hours corresponds to about 18,400 million km or 122 AU [1 Astronomical Unit, the mean distance of the earth from the sun = 150 million km]. The distance of 122 AU is just 3 times the average radius of Pluto's orbit around the Sun.
Since the entire mass of the blackhole is concentrated at a point, the attractive force on the star, according to Newton's law of gravitation, is:
G. M. m / d^2
where G is the gravitational constant (6.66. 10^-8 in cgs units), M is the mass of the blackhole, m is the mass of the star and d is the distance of the star. The centrifugal force on the orbiting star is given by:
m. v^2 / d
where v is the orbital velocity of the star. Neglecting the influences of other stars, and equating the two expressions given above, we get
M = v^2. d / G = (5000. 10^5)^2. (122. 150. 10^11) / (6.66. 10^-8) = 6.87. 10^39 g = 6.87. 10^39 / (2. 10^33) = 3.4 million Suns.
It is remarkable that the mass of a mysterious object that cannot be seen can be calculated so easily on the basis of its gravitational effects!
The above result is approximate because the orbit of the star is not circular, and the pulls of other nearby stars have not been considered. A more precise calculation gives the mass of the supermassive blackhole as 3.7 million Suns. In spite of this gigantic mass, the supermassive blackhole at Sgr-A* is referred to as a `starved blackhole' since galaxies, in general, house much heavier blackholes at their nuclei; the giant galaxy M87 has one as massive as 2.6 billion Suns.
One gets inquisitive as to how close the star S2 is to the event horizon (the surface of no return) of the supermassive blackhole at Sgr-A*. The Schwarzschild equation gives the radius of the event horizon of a blackhole as
R = 2 G. M/ c^2 Substituting... mass M = 3.7. 10^6. (2. 10^33) g gravitational constant G = 6.66. 10^-8 cgs units and velocity of light c = 3. 10^10 cm/sec, we get R = 2. (6.66. 10^-8). (7.4. 10^39)/ (3. 10^10)^2 cm = 11.0. 10^11 cm = 11.0 million km = 0.073 AU.
This is just one-fourteenth of the radius of the earth's orbit around the Sun. It is incorrect to visualize the blackhole as an object filling a sphere of radius equal to the radius of the event horizon. Its entire mass is concentrated at its centre, which is a mathematical singular point. The event horizon is pertinent only as a `surface of no return'. What happens to an object after it crosses the event horizon is `unknown and unknowable'!
Some science fiction writers have speculated that the mass devoured by a blackhole resides in another universe linked to us through a `worm-hole'. This is a speculation resting on the concept of 'parallel universes', a concept that has been introduced to explain some simple but intriguing experiments in modern Physics [including the well-known Double-slit experiment in Optics]. Whether 'parallel universes' exist or not, let us be clear that we can observe all of the gravitational effects of the blackhole in our universe.
Though the star S2 appears to be very close to the blackhole in the picture, it is actually at a respectable distance which exceeds the radius of the event horizon of the blackhole by a factor of more than thousand times. However, spiralling inwards under the tremendous gravitational pull of the supermassive blackhole, it is doomed for premature death.
It is stated that Sgr-A* gobbles up one star every million years or so, which means that the star S2 will be captured by the blackhole within 1 million years from now -- an astronomically short time! However, the infrared burst shown in the lower picture is not due to stellar capture. These frequent flares are caused by streams of infalling interstellar gas crossing the event horizon of the blackhole. As matter approaches the event horizon of the blackhole in a high-gradient gravitational field, it gets heated up so much by gravitational tidal friction that it starts emitting infrared, light and even x-ray radiation -- dubbed the `death-scream' of matter! If and when the star S2 crosses the event horizon, it will no doubt vanish with a stunning display of fireworks!
-- R.Jayaraman, <firstname.lastname@example.org>, Apr. 2008.