Philosophical Society of Washington

Minutes of the 2111th Meeting

Speaker: Henry T. Freudenreich, NASA Goddard Spaceflight Center, Greenbelt, MD
Topic: “Deconstructing the Milky Way”

President Garavelli called the 2111th meeting to order at 8:15 p.m. on December 10, 1999. The Recording Secretary read the minutes of the 2110th meeting and they were approved.

The speaker for the 2111th meeting was Henry T. Freudenreich of the NASA Goddard Spaceflight Center, Greenbelt, MD. The title of his presentation was “Deconstructing the Milky Way.”

Our sun and its handful of planets reside in an unremarkable part of a vast stellar system we call the Milky Way Galaxy. Our galaxy is shaped like a great circular disk, with a breadth nearly ten times greater than its thickness. Within the plane of the disc are long “arms” — dense concentrations of bright, young stars dust and giant gaseous clouds — spiraling outward from its center. Astronomers think there are four spiral arms in the Milky Way and we appear to be on a spur connecting two of them. This is not exactly right in the middle of things. Beyond this simple description, we know surprisingly little about the Milky Way Galaxy in which we live.

It's a peculiar comment on our place in the universe that we know more about the structure of galaxies that are millions of light-years away than we do about our own galaxy. This is primarily because dust, stars, and giant molecular clouds within the galaxy itself obscure much of the Milky Way. Our situation can be likened to describing the outlines of a forest from the inside. All we can see are the trees immediately around us. Fortunately for astronomers, certain wavelengths of light are able to pass through the obscuring dust to varying degrees. With some clever subtracting and guesswork astronomers are able to “cut down the nearest trees” so as to see distant reaches of the galaxy that are otherwise obscured. The speaker provided an update on what astronomers have learned about the grand-scale shape of the Milky Way using these techniques.

While we can clearly observe galaxies up to hundreds of millions of light-years away, a fog of interstellar dust obscures the greater portion of our Milky Way. The tenuous matter between the stars in our galaxy is 90% hydrogen, 9% helium and 1% heavier elements collected into fine particles astronomers call dust. Most of the particles are less than a micron long and are rich in carbon and silicates — a mixture of ultrafine sand and soot. Although there is a diffuse distribution of dust throughout the galaxy, most of the dust is collected into ragged clouds of various sizes and densities.

Recently astronomers have begun to penetrate this fog by means of space-based telescopes sensitive to longer wavelengths of light. One such telescope, the Diffuse Infrared Background Experiment or DIRBE created the first complete pictures of the Milky Way as seen at 10 different infrared wavelengths. The DIRBE is a near-infrared telescope was art of the Cosmic Background Explorer satellite called COBE that began its mission in 1989. Maps of the entire sky provided the clearest pictures yet of our galaxy's central bulge. Among its findings are data that show the base-plane of our galaxy is warped, and its center is occupied by a cigar-shaped bar 20,000 light-years long. However, questions remain. It will take more years of work identifying objects — stars of different types, molecular clouds and planetary nebulae — determining distances and cataloguing them before we can say with certainty, “This is what the Milky Way looks like, and this is our place in it.”

Mr. Freudenreich then closed his presentation and kindly answered questions from the floor. President Garavelli thereupon thanked Mr. Freudenreich for the society, announced the next meeting and made the usual parking announcement. He then adjourned the 2111th meeting to the Annual Meeting of the Society at 9:30 p.m.


Respectfully submitted,
Bill Spargo
Recording Secretary

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