In the previous article we explored the anatomy of black holes, how you can build one, and what strange phenomena takes place in their immediate surroundings.
This time, we are going to deal with how they originated in the Universe, and how they behave, in particular the Milky Way supermassive beast located in the direction of Sagittarius. Its presence betrayed long ago by peculiar radio emissions from a point source by the name Sagittarius A* (colloquially pronounced A-star).
To be fair, Sgr. A* is not the only black hole around here. There is sound theoretical foundation, and convincing observational evidence, for the existence of probably millions of smaller ones, those that are what’s left after the demise of giant, short lived stars or neutron star mergers, matters which we promise to talk about in the near future.
In the beginning
Primordial matter in the early universe was not distributed evenly. It consisted mostly of hydrogen, a little bit of helium, and copious amounts of an as yet unidentified form of matter generically known as “dark matter”. All other elements were synthesized from hydrogen and helium inside the nuclear furnaces of earlier stars. Dust of which we are all made, comes from probably 2 or 3 supernova blasts in successive generations of giant stars, the ones which live short, seed surroundings with fresh new material, and leave behind neutron stars or small black holes. Primordial unprocessed hydrogen though, is still by far the most abundant element in the Universe.
We learned matter (normal and dark) was not distributed uniformly, by scrutiny of echoes from the Big Bang. The analysis of this echoes (yet another future story) has been refined for the last 10 – 15 years, culminating with early results from the Wilkinson Anisotropy Probe spacecraft. It happens all too frequently with a major new experiment to yield unexpected results that send theoreticians scrambling back to their equations trying to figure out what they messed up with. Remarkably though, this time the biggest news was there was no news. The distribution and size range of primordial matter clumpiness – as inferred from background microwave radiation analysis – did fit nicely with standard cosmological models.
Being uneven in the first place, and being subject as usual to mutual gravitational forces, the clumpiness tended to aggravate instead of to even out. Since matter attracts matter, denser regions exerted more attraction on their surroundings pulling in more matter and so on, giving rise to the first generations of stars and galaxies .
Matter (stars, gas, dust) tends, because of gravitation, to spiral inward, so most older stars are to be found in a more or less rounded galactic central bulge. Though the issue of origin for galactic black holes is not settled, as long as we have been able to measure giant black ones at the core of other galaxies, a very linear relation among their weight and the size of the old star’s central bulge has been detected. This relation hints that the crowded inner regions of the galactic core probably invite successive mergers among stars, crossing the black hole threshold which then grows as much as available material there is. Other hypothesis proposes black holes form first from gas and dust, and provides a nucleating environment for the central bulge.
Things heat up
Notwithstanding how giant galactic core black holes come to be, their extreme influence on surrounding objects give rise to the most powerful phenomena in the Universe. When matter in sizable amounts is trapped within their area of influence, it spirals in at ever increasing speeds crashing into itself before being swallowed. In the last stretch before crossing the Event Horizon, gravitational energy released accelerates its movement at close to the speed of light, temperatures rise to multimillion degrees Kelvin igniting thermonuclear flares, ionized gas moving at relativistic speeds generate gigantic magnetic fields that shape mammoth jets of energetic particles squirting along the rotation axis.
Active galactic cores (powered by multimillion sun’s weight black holes)were common in the Universe’s early stages. We know this for we are seeing it as it happened, since this chaos emits powerful electromagnetic radiation across the whole spectrum, from radio waves to X rays. How can we probe so far into the past? The farther we look from home, the earlier we see in time. Now just arriving photons, were emitted from the most distant objects we can observe when the Universe was a tenth or less than its current age. It took that much time for them to travel, and the information they carry is appropriately outdated. Quasars, the extremely far, extremely brilliant light sources known for long, are now generally acknowledged as being primeval galaxies harboring supermassive black holes, feeding greedily on gas, dust and stars unlucky enough to have drifted hopelessly near their reach.
Behind the curtain
If the central beast has no food to ingest, it stays low profile, like our own black hole. That it is big we know, for we have been able just recently to see in infrared light and with adaptive optics, stars orbiting very close despite the fact the beast hides itself inside a cocoon of gas and dust along the galactic plane blocking direct visible light observation. From their movements we can infer how large is the invisible mass to which they are tethered, and it weights in at about 2,600,000 times the Sun’s mass. The Chandra X ray telescope on the other hand, afforded for the first time observation at high resolution of this energetic radiation able to penetrate the dust curtain, unveiling brilliant point sources and extremely hot gas in the Galactic Core.
Once in a while, some matter happens to dare too close, and gets trapped and swallowed. May be an unlucky star, a clump of dust or gas. Whatever falls prey, we hear briefly its agony wails in the radio, infrared and X ray bands.
How dangerous is to be sharing a galaxy with an almost 3 million suns heavy black hole? It sits at about 25,000 light years away, which said that way means next to nothing. Now, the snack ingested as Chandra recently reported, took place more or less when the first humans crossed the Bering Isthmus and begun to disperse in America, for it took that much for light to travel the span.
Put it another way, if you could board a conventional jetliner and cruise at normal speed nonstop to the galaxy core, you should be living on airline fare for about 30,000 million years. If somebody can endure that. (The Universe is about 13,000 million years old, Earth about 4,500 million years old). This was to put into perspective how far we are located from it. Yet, in the end it is not distance alone what keeps us out of harm’s way, but the fact that the Milky Way is a relatively mature galaxy with not much fuel left to feed an active quasar kind core.
Next Northern Hemisphere Summer, Southern Hemisphere Winter, when Sagittarius rises at dusk in the East, look at the sprout of the teapot asterism. Imagine a brief puff of steam rising, and inside the dusty darkness, at its end, there is A*.
Fasting for eons, feasting once in a while. Our black hole.