Date: July 5, 2009
Title: Looking Towards the Center of the Galaxy
Podcaster: Christopher Crockett
Organization: Lowell Observatory
http://www.lowell.edu/users/crockett/
Description: If you go outside tonight or tomorrow night and locate the Moon, you will be looking toward a rather remarkable patch of the sky. You will be looking towards the center of our galaxy! On today’s podcast we will use the Moon as a landmark to explore the structure of our home in the Universe, the Milky Way. Join us for a journey from the Earth, through the Milky Way’s spiral arms and right to the supermassive black hole that sits at the heart of our galaxy.
Bio: Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Wilfred Nijman, a 54-year old astronomy student who would like to remind you that it is never too late to start your studies by distance learning at http://www.studyastronomy.com.
Transcript:
Hello, this is Christopher Crockett from Lowell Observatory.
A nearly full moon hovers in the constellation Sagittarius tonight. If you go out and gaze in that direction you probably won’t notice anything of great consequence. But appearances can be deceiving. Tonight the moon serves as a landmark, a beacon lighting the way to a spectacular location in the sky. As you look towards the Earth’s only satellite, you are looking towards an area of great astrophysical significance. You are looking towards the center of our own galaxy!
The Milky Way Galaxy, our home in the cosmos, is a massive collection of roughly 200 billion stars in a thin disk spanning 100,000 light years across and 1000 light years thick. We sit about 28,000 light years from the center. One of the most striking features of our galaxy, and others like it, are the great spiral arms of gas and stars that wind around its center not unlike a cosmic pinwheel. Our Sun resides on the inside edge of the Orion Spur, a bridge of stars connecting two of the great spiral arms. When you look up under a very dark sky and see the band of the Milky Way encircling the heavens, you are seeing the diffuse light from the distant stars that fill the Orion Spur.
Out here, the Sun orbits around the center of the galaxy once every 220 million years. Thought of another way, the entire solar system has drifted one light year since the fall of the Roman Empire. Over its four-and-a-half billion year life, the Sun has looped around the entire galaxy 25 times.
From our perspective, looking toward the center of our Galaxy, we don’t actually see much. While the Galactic center is actually quite bright, nearly all of that light is blocked from our view by the numerous massive clouds of interstellar gas and dust that lie between us and the core. The intervening matter is thick enough to cut down the brightness by one trillion times!
The situation is different if we look for infrared light or radio waves; these longer wavelengths of light have an easier time navigating the hazardous maze of gas and dust clouds in the spiral arms. In sharp contrast to the visible light that our eyes can detect, infrared light emanating from the galaxy’s center is dimmed by only ten times! In addition to the high-energy X-rays that can punch through the intervening material with ease, nearly all we have learned about the center of our galaxy comes from studies of the infrared and radio light that it emits.
Lets move in towards the galactic center and take a closer look. For the first half of our journey, we pass through regions of the Galaxy that look familiar to residents of the Sun. About halfway to the center, the spiral arms end and we find them attached to what astronomers refer to as the Galactic Bar. While it sounds like a great place to stop for interstellar happy hour, the bar is actually a long, thin structure composed of gas and stars. Sitting in the plane of the disk, it pierces the center of the galaxy extending 14,000 light years outward on either side. The role of the bar appears to be a type of stellar nursery that provides a channel for funneling gas from the spiral arms into the galactic center.
Moving a bit further in, we encounter the central bulge: a distinct population of stars in the central 10,000 light years of the galaxy. Here the stars are very densely packed. The stars that sit in this region are different from the stars that make up the spiral arms. While most of the star formation in spiral galaxies takes place in the arms, the stars of the central bulge are older with little to no active star formation. It is a dead zone in the galaxy.
Moving through the dense stellar fields of the bulge and deep into the core, we find an alien environment. In the 1930s, astronomer Karl Jansky discovered a strong source of radio waves from this part of the sky. It is only in the past several decades that astronomers have been able to tease apart the secrets harbored in the heart of the Galaxy.
The radio source discovered by Karl Jansky is actually a complex of multiple structures known collectively as Sagittarius A. Looping around the center we find the largest feature: the Molecular Circumnuclear Ring, a doughnut shaped mass of gas 25 light years in diameter with a 6 light year hole carved out of its center. The ring contains vast reservoirs of hydrogen, carbon monoxide, and hydrogen cyanide flowing around a great cavity thought to have been blown out by some massive explosion in the distant past.
In addition to the ring, astronomers have identified several other components to the Sagittarius A complex. Sag A East is a shell of expanding gas thought to the remnants of a massive supernova explosion sometime in the past few thousand years. Sag A West is an unusual collection of ionized hydrogen that has the appearance of a “mini-spiral”. Gas in this spiral appears to be flowing in from the molecular ring swinging around the center at speeds of over 200,000 m.p.h.
At the center of this vortex of gas lies a small, strong source of radio waves and x-rays that happens to be coincident with the exact center of the galaxy. What sits here remained a mystery until the past 15 years when advancements in telescope technology lifted the curtain on the galactic core.
In the central tenth of a light year of the galaxy – a mere 560 billion miles from the galactic midpoint – astronomers found a few dozens stars being whipped into a frenzy. All of the stars appear to orbit an unseen mass, a mass that resides at the precise center of the Milky Way. Over the past decade or so, astronomers have been able to carefully track the orbits of these stars and, in doing so, have been able to determine the periods of the stars’ orbits. One of these stars orbits once every 15 years at speeds approaching 10 million miles per hour! By combining the orbital periods with the size of the orbits, one can calculate the mass of the central object that exerts its relentless gravitational pull on these stars. The mass astronomers have calculated is astounding.
Four million times the mass of our Sun squeezed into a volume that could be encompassed by just our Solar System!
And yet, where there should be four million Suns, we see nothing. All we see are stars orbiting an unseen object that is known to emit copious amounts of radio waves and x-rays; an object that sits at the exact center of our vast galaxy. There’s really only one type of object known to fit that bill.
A supermassive black hole!
A typical black hole forms from the supernova eruptions of the most massive stars in the Universe. The remnant cores are subject to a violent implosion. Some stellar cores are able to resist the collapse and eventually form exotic neutron stars. But for the stellar heavyweights, there is no known force capable of stopping the collapse. As the core contracts, the gravity at its surface increases until it becomes so strong that no object, not even a beam of light, can escape into space. Once that happens, the core swallows its own light and becomes invisible. A black hole is formed.
The typical black hole that forms in this way will be only a couple of times more massive than our Sun. But the beast sitting at the center of the galaxy has the mass of four million Suns! How did that come to be? We’re not entirely sure. One leading hypothesis is that in the dense and violent environment of the core, black holes may start merging with other black holes, bulking up their mass with each merger. We may be witnessing a snapshot of our galaxy’s much more violent past.
One of the truly amazing discoveries of the past decade is that, in this regard, the Milky Way is not unique. Many other galaxies appear to have supermassive black holes in their centers. Some may even have binary black holes, locked in each other’s gravitational pull. And some are much, much more massive than our own. The gargantuan galaxy M87, reigning over the core of the Virgo Galaxy Cluster 59 million light years away, harbors at its center a black hole with over six billion times more mass than our sun. That’s billion. With a B.
Despite the incredible pull of our galaxy’s central black hole, we are in no danger of being sucked in. Neither is the vast majority of the galaxy. The sphere of influence of our black hole, that is the volume in which the black hole’s gravity dominates over the surrounding stars, is only a few light years in radius. Once you are ten or a hundred light years away from the black hole, you would no longer be able to feel its influence. With the Earth at a comfortable distance of 28,000 light years, we have nothing to worry about! At least, not from the black hole.
There is so much more in the sky than your eyes alone will ever see. When you look towards the Moon tonight, you will not see the foreign environs of the hub of the Galaxy. But you will better appreciate a seemingly unremarkable patch of the sky. 100 trillion times further than the Moon lie the secrets of the galactic core. Here, beyond our Moon, there are more mysteries waiting to be unveiled. Here, there be dragons!
End of podcast:
365 Days of Astronomy
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