Text copyright Robert Gendler

The Spiral Arms of the Milky Way

The interstellar medium of large spiral galaxies like the Milky Way, consists of 90% molecular hydrogen, about 10% helium and small fractions of heavier elements. Because of extremely low temperatures in interstellar space, hydrogen exists in molecular form and is contained mostly in giant molecular clouds (GMC) of tremendous mass. These giant clouds can range from 150 to 650 light years in size carrying anywhere from ten thousand to one million solar masses of material. Molecular hydrogen because it is devoid of electric charge is almost impossible to observe directly in the cold obscured interstellar regions. Fortunately carbon monoxide (CO) has played an essential role in detecting and mapping Giant Molecular Clouds. All molecular clouds contain CO and it is easily detected at radio frequencies. Radiotelescope observations have shown CO to be both a good qualitative and quantitative tracer of cold hydrogen gas within molecular clouds. CO surveys have added tremendously to our knowledge of the interstellar medium and stellar dynamics. We now know through CO surveys that virtually all star formation occurs in molecular clouds within the spiral arms of galaxies.

Our sun is located in the Orion arm of the Milky Way between the inner Sagittarius arm and the outer Perseus arm of our galaxy. Just outside the Perseus arm is the outermost Cygnus arm. Between the Sagittarius arm and the galactic center are the Scutum-Crux arm and the innermost Norma arm. A quick listing of local GMCs in our local region of the Milky Way would include the Taurus Dark clouds in Perseus, Taurus and Auriga which contain the familiar objects NGC 1499, NGC 1579, IC 348 and NGC 1333. The Ophiuchus and Lupus clouds contain the brilliant nebulosity of the Rho Ophiuchus region. The Orion Molecular clouds contain the Orion Nebula Complex and surrounding famous emission clouds.

A description of our local region within the Milky Way would not be complete without a mention of Gould's Belt. In 1879 the astronomer Benjamin Gould reported his survey of the distribution of bright stars in the local Milky Way. Gould's work showed that a true local subsystem of young stars and gas existed in a rotating flat disk inclined some 20 degrees to the proper disk of the Milky Way. The disk extends some 2000 light years across and contains some of the most famous astronomical objects including the Pleiades, the Orion Nebula and Horsehead regions, the California Nebula, the Coal Sack and the Rho Ophiuchus clouds near Antares. Gould's Belt must be a young structure between 30 and 40 million years old by virtue of the young stars it contains but its origins are still unclear. One theory is that an errant supercloud collided with a major spiral arm of the Milky Way about 100 million years ago. The shock wave resulted in the process of braking and compression of the gas of the supercloud into a flat rotating disk. The older stars drifted out of the disk leaving the younger stars to form Gould's Belt.

The Center of the Milky Way


Knowledge of what lies at the center of our galaxy may very well hold the secret to the forces which drive billions of other spiral galaxies in our universe. Our sun is only 26,000 light years from the center of our galaxy (extremely short compared to intergalactic distances) which potentially offer a bird's eye view of this tantalizing region. The view at optical wavelengths is blocked however because of massive amounts of intervening dust. Fortunately astronomers can peer through this dust utilizing instruments capable of observations at radio, infrared and x-ray wavelengths. Dust grains have a typical diameter of about 0.1 micron. Being similar in size to wavelengths of visible light, dust grains tend to block the passage of visible light. On the other hand, radio and infrared waves being much larger, and x-rays being much smaller than dust grains, allows energy of these wavelengths to pass through the dust clouds without being deflected or absorbed.

Our galaxy's central galactic bulge can be seen in the summer sky just a few degrees west of the large Sagittarius Star cloud. The true galactic center harbors an enigmatic object known as Sagittarius A. Sagittarius A has long been known as one of the most powerful radio sources in the sky, second only to the sun. Radio and X-ray observations have revealed that within the central most 30 light years surrounding Sagittarius A, things really start to get strange. The central 30 light years is an extremely compact region containing many millions of stars. As we get closer to the true center we find a number of young blue supergiant stars among older red giants and scattered supernova remnants. Astronomers postulate that multiple epochs of star formation have occurred in the galactic center spawned by collisions of closely compacted molecular clouds. High resolution imaging of the true dynamic center reveals a 7 light year wide rotating minispiral and a surrounding circumnuclear disk which delivers matter to a central object of extraordinary mass called Sagittarius A*. Several stars within a few light years of Sagittarius A* show orbital velocities so rapid that the center of the minspiral must be an object of at least 2.6 million solar masses. The incredibly massive object occupies a space of only 0.1 astronomical units (about 1 light minute). An object this compact and massive can only be explained by a supermassive black hole at the extreme center of our galaxy.

We now understand that supermassive black holes exist at the centers of most if not all spiral galaxies. The black hole known as Sagittarius A* is quiet and dim by galactic standards. The black hole is apparently starved of infalling matter as its yearly intake is less than one ten millionth of a solar mass. This is in contrast to galaxies with active nuclei which have prodigious rates of matter consumption. Recent X-ray observations have detected two lobes of 20 million degree gas extending out from the galactic center. Perhaps they represent eruptions from a remote time in the distant past when our galactic nucleus was far more active.