Sagittarius A

The constellation Sagittarius (from “Urania’s Mirror” a set of constellation cards published in England circa 1825)

My apologies for the blogging break last week.  Usually I try to write a new post every weekday, but last week was a blogging holiday.  To reinvigorate things after the lost week, let’s turn to a big subject—in fact a super-massive subject!  Long ago, Ferrebeekeeper featured a post about Eta Carinae, a blue hypergiant with a hundred times the mass of the sun (which is itself a million times more massive than Earth).  Stars like Eta Carinae are rarely formed and short lived—there are probably less than a dozen in our galaxy.  However compared to the most massive object in the galaxy, Eta Carinae is puny and common.  Twenty six thousand light years away from the solar system there exists a truly monstrous space object!

In 1974, Astronomers discovered an astronomical feature which was emitting exotic radio waves in the Sagittarius constellation. The scientists named the feature “Sagittarius A” and set out to determine what it was.  Part of the feature seems to be the remnants of a star which had gone supernova.  A second part of the feature is a cloud of ionized gas surrounded by an even larger torus of molecular gas.  In the middle of Sagittarius A is something which is emitting most of the high energy electromagnetic radiation visible to radio telescopes.  The cloud of ionized gas seems to be emptying into it and nearby stars orbit it with greater velocity than stars move anywhere else in the galaxy (in fact the object affects the proper motion of thousands of nearby stars).  And yet the space object at the center of Sagittarius A has a diameter of only 44 million kilometers–a bit less than the distance between the middle of the sun and Mercury at its perihelion (when the rocky planet is closest to the sun).  By calculating the proper motion of thousands of nearby stars, scientists determined that the mysterious object at the center of Sagittarius A (which they took to calling Sagitarrius A*) has mass of 4.31 million suns (i.e. solar masses). Whatever lies at the center of Sagittarius A–which I probably should have mentioned, is also the center of the Milky Way Galaxy–is smaller in volume than a large star, but has a mass which exceeds by many orders of magnitude even exotic hypergiants like Eta Carina.

Of course the only kinds of discrete objects which we know (or even hypothesize) to be capable of attaining such mass are black holes.  It is believed that most (indeed probably all) galaxies have super-massive black hole at their centers.  Smaller galaxies have small super massive black holes (forgive the oxymoron) but large galaxies have immense central black holes which can equal billions of solar masses.  Radio astronomers have observed plumes of exotic electromagnetic radiation coming from the center of other galaxies, and they wondered where the Milky Way’s galactic center was located.  It seems that a supernova near the galactic center blew away a great deal of the dust and gas on which the black hole would otherwise “feed” thereby making the galactic center of the Milky Way less energetic than the active center of farther (e.g. older) galaxies.

Artist’s Conception of Galactic Center

The super massive black holes which lie at the center of galaxies may be a result of the accretion of matter around stellar-sized black holes (which could grow quickly in matter-rich galactic cores) but most astrophysicists believe they are instead a primordial feature of the Big Bang around which galaxies themselves coalesced.  The ultimate nature of super massive black holes remains unknown and seems to be tied to the nature and shape of our universe.

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