Sagi the Finicky Eater

A while back now I put up information about the black hole at the center of our galaxy. To reintroduce that creature, its name is Sagittarius A* (read A-star). My post (here) reported that the hole was drawing, to itself, a vast cloud of gas. And the NYT story that I’d read and quoted there assured us cheerfully that Sgr A* will eventually eat the whole galaxy. I confessed problems believing that.

Well, the other day, we saw a story that appeared in Science, originating from the University of Massachusetts, Amherst. The chief investigator was Daniel Q. Wang, professor of astronomy and leader of a team researching Sgr A* using NASA’s X-ray telescope Chandra. Wang and company discovered that this, our most potent black hole, actually rejects more than 99 percent of the matter flowing toward it. Why it does so—when other black holes are thought to be swallowing masses and masses of matter (or so they say, so they say)—is not yet understood.

Reading that I got to thinking about some future, galactically rich man who decides to commit suicide by plunging his rocket into Sagi’s center—and gets rejected as inedible.

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The image is NASA’s picture of the day today (link - but it may change tomorrow). It is titled The Quiet Sagittarius A*. The credit line reads: X-ray - NASA / CXC / Q. Daniel Wang (UMASS) et al., IR - NASA/STScI.

Dark Matter? Maybe Not.

Great orthodoxies in astrophysics are like the Tower of Siloam (link). They grow cracks and begin to crumble in almost unnoticed ways—until, one day, they crash down with a roar. I became aware of another little crumble this morning (ht to Montag). It takes the form of a press release from Case Western Reserve University reporting on a study which appears to legitimize Modified Newtonian Dynamics (MOND) and thus throws doubt on the existence of dark matter (link).

MOND, a theory introduced by Mordehai Milgrom, the Israeli astrophysicist, in 1983, suggests that Newton’s theory of gravity may not operate uniformly at all stellar distances, hence Newton’s theory needs “modification.” Wikipedia’s article on MOND (link) brings a graphic that neatly illustrates the problem.

Expected (A) and observed (B) star velocities as
a function of distance from the galactic center.

The dotted blue line illustrates the expected rotational velocities of stars at increasing distances from a galactic core. The red line illustrates the actually observed velocities. In order to explain that gap, between velocities predicted by Newton’s law of gravitation and what is actually seen, astrophysics has invented dark matter to supply sufficient amounts of mass to justify the observations—while keeping Newton’s laws untouched. The problem, of course, is that dark matter has not been physically observed. It is simply a projection of gravity-producing mass into a void which may be empty.

The current study is headed by Stacy McGaugh and Mordehai Milgrom and deals with the rotational behavior of seventeen dwarf galaxies that circle around Andromeda. This sort of thing interests me—because I feel quite uneasy about a cosmological picture in which virtually the whole universe (96% of it) is supposedly made up of dark energy and dark matter—of which we have not even the slightest knowledge in a touch-and-feel sense.

The word mond in German means “moon.” Well, it isn't full moon yet in astrophysics, but the light is certainly increasing.