Milky Way Black Hole in Sharper Focus

 

Our galaxy, the Milky Way, is just one of the 200 billion-odd galaxies strewn across the visible universe. But it’s among the biggest. For sure, there are some other spiral galaxies which outweigh ours, not to mention the even more beefy “ellipticals” formed by the merger of spirals. Nevertheless, it sits in the  club of giants. And like most others, it harbours a “supermassive” black hole. Still, that core-dwelling giant shrivels into a dwarf  next to monsters packing up to billions of solar masses at the hearts of some. Ours was credited with a “meagre” three-million until recently. But new observations, while adding new fat to the Galaxy itself, give some welcome muscle to the weakling at its center. 

In  varying estimates,  the Milky Way contains 100-to-400 billion stars (300-to-400 in later ones). The discrepancy stems from the difficulty of  spotting the “red dwarf”  stars which are much smaller and cooler than the Sun, but make up three-quarters of the star population of the Milky Way. Galaxy’s total mass, however, is calculated to be about 1.5 trillion solar masses. The difference is attributed partly to the gas and dust in the interstellar space, but mainly to a yet-unobserved mysterious matter hypothesised to surround the Galaxy as a vast spherical halo, not emitting light ─ hence called “dark matter” ─, making its presence felt through gravity. 

The majestic Galaxy is also surrounded by a retinue of about 200 “globular star clusters” which pack 10.000 to 10 million stars into extremely compact volumes, as well as  a dozen satellite dwarf  galaxies, with a  similar number of the satellites believed awaiting discovery.

As the name makes plain, a black hole cannot be visualised. As they are theorised to warp the fabric of the space-time  infinitely with their huge masses, black holes are described as mathematical points which do not allow anything, not even light, to escape back if  they fall in. They announce their presence with the X-rays emitted by the matter they attract from their vicinity, which attain great speeds and temperatures in a torus-shaped accretion disk before they are swallowed. Another telltale sign is the phenomenal orbital speeds they impart to nearby stars. 

The matter (and light) crossing a threshold called “event” horizon which varies with the black hole mass,  cannot escape back and get drawn into the “singularity” at the center where known physics laws do not apply. In the illustration, three  light rays at different distances are bent to varying degrees. A fourth rotates and defines  event horizon. The nearest fifth crosses the horizon and spirals into the singularity, with its wavelength undergoing a huge redshift.

The matter a black hole of any mass attracts from its vicinity or fom objects passing  nearby, forms an “accretion disk” around the event horizon. Approaching the horizon, the matter in the disk attains huge velocities and the friction pushes the temperature of the disk material to extreme values,  causing it to emit X-rays before crossing the event horizon. The immensely powerful magnetic fields which form in the disk, catapult part of the material to space in jets at opposing poles of the black hole at speeds approaching that of  light.  

Another category, the case for whose existence was strengthened by consistent evidence gathered in recent years, is “intermediate mass black holes”, with masses ranging from a few thousand to 30-40.000 solar.

Astronomers discovered one of the not-so-common  intermediate mass black holes in the globular star cluster Omega Centauri (above).  The black hole was found to be of  40.000 solar masses.Globular clusters are very dense and very old  assemblages of  stars located around the central bulge of  the Milky Way and other galaxies. There are some 200 of these , dispersed in the dark halo surrounding the Galaxy  (left.)  These spherical structures  60-to-300 light years wide,  are homes to hundreds of thousands (and millions in some) stars closely packed together. Omega Centauri, one of the largest , is calculated to harbour 10 million stars in a volume with a diameter of just 80 light years.

Globular clusters are among the oldest structures in the universe, with ages between 10and 12 biliion years  (the universe itself is 13.8 billion years old. ) Hence, all the massive stars they contained in the beginning have gone supernova at the end of their 30-40 million- year lives, leaving behind only the old,  red  stars with long lifespans. The black holes produced by the supernova explosions sank to the cores where they merged. In the violent processes of merger, some were hurled out of the cluster while others grew to masses  of  30-40.000 solar.

M87, at the center of  the Virgo cluster of galaxies, is one of the biggest of  ellipticals which form  when two or more spiral galaxies collide and merge.  Its mass, largely made up of gas and dark matter, could be as much as 200 times that of the Milky Way in astronomers’ estimates. The supermassive black hole at its center is also one of the biggest of its kind. It is calculated to be of 6-7 billion solar masses. The jet, extending from the black hole,  carries the particles  it takes from the disk around it to 5000 light years away with relativistic speeds (approaching to that of light.) 

With patient observations carried out nonstop for 16 years with infrared wavelengths which can penetrate dust, astronomers from the Max Planck Institute of Extraterrestrial Physics (Germany) considerably sharpened the picture in 2008.

By observing the orbital motions of 28 nearest stars rotating around the black hole, researchers obtained more accurate values as to its mass and distance from the Earth. The latest value determined for the black hole’s mass is 4 million solar. It also appears that we are a bit farther from the galactic center than we so far believed. The distance, formerly calculated to be 26.000 light years, was revised upward to 27.000. 

Of the 28 stars, observed with such modern equipment as the New Technology Telescope and Very Large Telescope of the European Southern Observatory in Chile, the trajectories of those orbiting within a radius of 1 “light month” (about 800 billion km), resemble the surroundings of a beehive. Conversely, six stars which remain outside this limit rotate on a common plane as on a disk. The most interesting of the 28 is the one named SO-2. It is so fast, that it has completed its orbit within the 16-year observation period. 

But there is a “fastest of the fast.”  The star named SO-102, discovered in 2012, was announced to have an orbital period of a mere 11.5 years around the supermassive black hole.  SO-2’s closest approach to the black hole will be in 2018, and that of  SO-102 will be in 2021. These dates will allow a test of Einstein’s theory of general relativity. If the theory is correct, the curve of the space-time will affect the motion of the stars and will cause distortions in their light reaching us.

Although latest observations have brought answers to some important questions, one still remains in the air: How is it possible that  such young stars exist in the vicinity of a supermassive black hole?

The  1-parsec-wide  (3.26 light years, or abot 30 trillion kilometres) area around the supermassive black hole at the center of  Milky Way is crowded with thousands of stars. (For comparison, nearest star to the Sun is 4.2 light years = 40 trillion kilometers away). Some of these, orbit as close as 2 light days (about 52 biliion km) to the black hole’s 11 million-km-wide event horizon and at their closest approach, are expected to come as near as 18-20 billion km to the black hole. Astronomers were struggling to explain the presence of about 100 massive stars within the gas disk around the black hole and the orbital motions of most on chaotic  inclinations  instead of  a common orbital plane.

But some newly developed theories seem to explain the puzzle. According to one, the gas raining on the black hole from stars in the extremely crowded central region of Milky Way, do not join the disk in a steady flow, but as separate squalls, spaced several million years, after accumulating. But when the temperature of the gas in the inner regions of the disk reaches millions of degrees, it causes an outward pressure which stops the inflow of the gas (Eddington limit) and in the band pressed from both sides, stars begin to form.

Another theory holds that the low-mass old stars within the disk  shed their outer shells where heavy molecules like carbon monoxide accumulate in the disk environment, and emerge with relatively pristine envelopes and “mimic the youngsters like some old  Hollywood stars.”

But the most widely accepted model provides the explanation that despite the turbulent environment inside it, the disk, a million times denser than the environment around the Sun, counterbalances the effects of the black hole with its huge gravity and destabilises  the gas, which collapses to form new stars.  

Subscribers to this theory explain the orbital dynamics of these massive young stars, which follow chaotic routes instead of an  orderly plane,  with the possibility of the black hole’s rotation on its axis. According to these astronomers, the wobbles the black hole makes as it rotates within the disk fling the stars to eccentric orbits. 

These are B-class stars with 3-15 solar masses. If one has to surmise they formed far away and later migrated in, drawn by the gravity of the black hole, they are far too young to find the time for that.  For, they are born as an association in a giant cloud of gas and dust, along with much higher numbers of smaller stars. In such a cloud, an average of 1 million low-mass stars have to be born together with  massive ones. But as it were, the number of these low-mass companions in the immediate vicinity of  Sagittarius A* was found to be about only 10.000. Furthermore, it takes aabout  1 billion years for a star born in a distant cloud  to draw near the black hole, which is 10 times the maximum lifespan of a B-class star.

Teams of astronomers  monitoring the galactic center for yearsbelieve the problem with our  giant may not be so much its current  lack of appetite as its former gluttony.  Gamma ray blobs in the form of  25.000- light year-wide symmetrical spheres, discovered on both sides of the galactic plane in 2010 by the Fermi Gamma Ray Large Area Space Telescope (Fermi-LAT), show that in the past, our black hole had emitted  intense radiation and particles for short durations like a quasar:  Some astronomers  think this violent activity  coincides with the  frantic star formation six million years ago, triggered by the latest massive gas flow from surroundings into the 1.6- light year- wide region around the black hole. According to the proposed model, the incoming gas was shared equally by the stars and the black hole. And the latter, while swallowing a fraction of  its share,  spewed most of it to space from both of  its poles by means of the intense magnetic fields on the surfaces of  the surrounding disk. Gamma rays are radiated in the lobes by energetic electrons according to some researchers, and by the energetic cosmic ray protons according to others. Furthermore, the black hole is said to have sucked in a 100 solar mass gas cloud 20.000 years ago and ejected two gamma ray emitting jets from its poles. Some astronomers maintain that  our black hole was a million times more energetic 350 years ago than it  is today.

Higher observational resolution sought by astronomers to test the theoretical explanations offered for the puzzling properties of the skinny giant of our galaxy is finally available. In February 2012, the four telescopes at the European Space Agency’s Very Large Telescope (VLT) facility at Cero Telolo, Chile, were linked by computers with a technique called interferometry. Thus the array, composed of four separate telescopes each with a mirror size of 8.2 meters, will be used as a single telescope with a 130-metre mirror, providing a resolution 10-to-100 times better than currently attained.  Thus, it will be possible to illuminate many dark secrets lurking at Milky Way’s center and elsewhere in the universe.