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All posts for the month January, 2005

Back when I used to lecture in the planetarium I would solicit questions from the audience at the end of each presentation. Every once in a while somebody would ask me what it’s like inside a black hole.

Trick question, right?

Well, probably for some of the more smart-aleck audience members it was. But, I always had an answer. I’d go into a little discussion about how we don’t know exactly, and based on a number of factors (including the laws of physics, some Einsteinian laws, etc.), we’ll probably never have a chance to explore the inside of one (and, if the gravity is so strong that light can’t even escape, is the inside of a black hole REALLY a place we want to be?). Following that there’d be a silence as people digested the idea of “being there”. Then we’d get into a discussion about what it be like to be right next to a black hole, which is a lot easier to describe, even if it IS a shrieking maestrom of radiation and searing temperatures.

At the time I was first in school, back in the dark ages of the early 60s, black holes were sort of a mathematical curiosity, a physics problem for which we didn’t have any good examples. That all changed with the advent of telescopes and detectors able to “see” the effects of black holes, including the jets that spray out from the vicinity of one as matter (stars, gas, dust) spiral into the hole. Moreover, black holes have gravitational effects on nearby stars and gas and dust that we CAN track with spectroscopic observations of the light emanating from the nearby region.

The Milky Ways Black Hole Courtesy European Space Agencys Integral Mission

The Milky Way's Black Hole Courtesy European Space Agency's Integral Mission

Which brings us to the center of our own galaxy, where a supermassive black hole about a million times the mass of our Sun lies hidden by gas and dust clouds and star clusters. This SMBH (for short), also known as Sagittarius A* (or SgrA*), radiates tremendous amounts of energy which we can detect in gamma rays. As luck would have it, we have a spacecraft called INTEGRAL that “sees” that radiation. In the image above, INTEGRAL shows us a gamma-ray view of the region near the center of the Milky Way.

Now, SgrA* is a pretty quiet and harmless black hole, and isn’t quite the powerhouse of radiation that others are—like, say, the black hole at the center of galaxy M87, which sports a very active jet. Yet, in the past, the Milky Way’s resident black hole has been restless, and whenever it acts up, the surrounding clouds light up with the evidence.

Right near SgrA* is a cloud of gas called SgrB2, and the two are about approximately 350 light-years apart. Sgr B2 is being exposed to a blast of gamma rays emitted by Sgr A* that went off about 350 years ago. The cloud absorbed the radiation and has been emitting it. Interestingly enough, the astronomers studying the data think that the whole outburst took at least ten years, possibly longer. And they’re using their studies to figure out how often and how strongly “our” black hole turns on, radiates, and then turns off again.

I have to admit, it’s pretty heady growing up knowing that these weird things that scientists once thought were probably rare are now found all over the place (in many galaxies and at the death scenes of supermassive stars). And, I find it very cool indeed that we can study the near-black-hole environment and learn so much about them.

Image of Comet Machholz by Gerald Rhemann.

Image of Comet Machholz by Gerald Rhemann.

The passage of Comet Machholz near the Pleiades a couple of weeks ago reminded me of a project I spent a number of years working on in the late 1980s and early 1990s—the International Halley Watch Atlas of Large-Scale Phenomena— a compendium of Comet Halley images. The whole thing began when applied for a job as a research assistant on an IHW team at the university. I spent the next several years measuring and studying many, many images of Comet Halley, teasing out details about its plasma tail structures. Our interest was to chart the changes in the plasma tail as the comet sampled various “regimes” of the solar wind, and use those changes to understand how the solar wind affected the charged particles in the plasma tail. It got to the point that I could look at an image of Comet Halley and tell you exactly when it was taken (date and time) and tell you something about whether or not its plasma tail was about to disconnect or was rebuilding itself after a disconnection event (when the plasma tail would break off and then re-form in response to changes in polarity in the solar wind stream).

So, it was with a great sense of memory and history that I looked at this image (and many others) of Machholz as it passed near the Pleiades star cluster in the January sky. For one thing, it was clear that the comet had just undergone a disconnection event. The new tail was sprouting out from the coma and the remnants of the old tail were streaming out in a clump highlighted against the Pleiades. (The dust tail, by contrast, is the yellowish streak that seems to point roughly downward in this image.)

Even cooler, I ran across this image from Gerald Rhemann, one of Austria’s best-known comet chasers and astrophotographers. I became quite familiar with Gerald’s work (along with his collaborator Michael Jaeger) during my Halley years, and in my subsequent role as coordinator for observations for the Ulysses Comet Watch network in the early 90s. It’s a double blast from the past—comet plasma tails and the fantastic work of a photographer whose work I’ve long admired. The scene is stunning, combining my old research interest with one of my favorite star clusters in the winter sky. Sometimes astronomy doesn’t get any better than this!