The Physics Factbook
Edited by Glenn Elert -- Written by his students
An educational, Fair Use website
|Kraan-Korteweg, Renée C. & Ofer Lahav. "Galaxies Behind The Milky Way." Scientific America. October 1998.||"These measurements, confirmed by the Cosmic Background Explorer satellite in 1989 and 1990, suggest that our galaxy and its neighbors, the so-called Local Group, are moving at 600 kilometers per second (1.34 million miles per hour) in the direction of the constellation Hydra."||600 km/s|
|Does the Milky Way move in space or does it just stay put? Archive of Ask the Space Scientist FAQs. NASA/GSFC.||"The total speed is about 300 kilometers per second or so."||300 km/s|
|AstroFile -- Future Fate of the Milky Way Galaxy. Association of Universities for Research in Astronomy. 21 October 1997.||"The Milky Way and the Andromeda galaxy are approaching each other with a speed of 300,000 miles per hour."||130 km/s|
As we all know, a galaxy is a massive ensemble of hundreds of millions of stars. The galaxy where we live in today is called the Milky Way. The name itself came from the ancient Greek galaxies kyklos, or ring of milk, due to its faint milky appearance. Our Milky Way is a large spiral galaxy. Its diameter is at least 100,000 light-years, and may contain as many as 200 billions stars today.
Ever since four hundred years ago the settlement that the Earth is moving about the sun, and one hundred and fifty years ago that the sun is moving about the center of the Galaxy, it shouldn't be surprising if we learned that the Galaxy is also moving. The Milky Way is part of a cluster of galaxies call the Local Group. Two chief members are the Milky Way and the Andromeda galaxy, the Andromeda galaxy is known to contain at least 300 billion stars. We can presume that in every cluster of galaxies, the individual galaxy itself move about some sort of center of gravity. However, how do the clusters themselves move?
In 1928, an American astronomer Milton La Salle Humason found a galaxy that was receding at a speed of 3,800 km/s, and by 1936, when he observed the same galaxy again, he found it receding at a speed of 40,000 km/s. It didn't make any sense that the galaxies be receding from us and yet the recessions would be faster as they get farther way from each other. "Was there something special about our galaxy? Did it repel other galaxies, and did this repulsion grow stronger with distance?
If our galaxy exerted a repulsive force, that force should be felt with the local groups, however it wasn't. Furthermore, a repulsive force that grew stronger with distance is highly unlikely. For example, as we've learned in the past, a magnetic pole can repel another magnetic pole like itself, and an electric charge can repel another electric charge like itself, but in each of the cases, the repulsion weakens with the increase of distance. Hubble, an American astronomer, concluded in 1929 that the "entire universe was steadily expanding"and that the galaxies were moving apart from one another as part of this expansion and not because of any repulsive force. In addition, in 1916, Albert Einstein as part of his general theory of relativity, had prepared a set of equations that were intended to describe the properties of the universe as a whole, that showed that the universe would have to be expanding.
In conclusion, galaxies experience neutral attractions on one other. Due to relativity, the speed of the Milky Way varies when compared with different objects in space. For example, I have learned from my research that the Milky Way and Andromeda galaxy are approaching each other with a speed of about 130 km/s, however the collision of these two galaxies will not occur for about 5 billion years (AstroFile). Another result I found was that our galaxy and neighbors are moving at 600 km/s in the direction of the constellation Hydra (Scientific American). Finally, I found that the Milky Way moves through space within the cluster of galaxies it is a member of, and this cluster in turn moves through space towards yet another larger cluster of galaxies off in the direction of the constellation Virgo. This speed is approximately 300 km/s (Ask the Space Scientist). Therefore, the speed of the Milky Way galaxy is not a single number, its value is relative to the speed of other objects.
Patricia Kong -- 1999
|Hall, Stephen S. Mapping the Next Millennium. New York: Random House: 1992, 328, 334.||"The Great Attractor is … incredibly dense and unimaginably large, lying off at a distance of 150 million light years, in the direction of the Hydra Centaurus supercluster …. Our galaxy, the Milky Way, is caught in the tug of this Great Attractor, and so are we, hurtling -- though we'd never know it -- toward this mysterious mass at a speed of 370 miles per second."||600 km/s|
|"That exact fate befell Vera Rubin and W. Kent Ford, Jr., of the Carnegie Institution of Washington and two colleagues in the mid-1970s. Using complicated and (to hear some astronomers tell it) not entirely convincing measurements, they reported that the Milky Way, our galaxy, had a peculiar velocity of about 500 kilometers per second. In other words, the Milky Way was hurtling about 370 miles per second off the plumb of the big bang. Rubin in fact had suggested the same thing way back in 1950, but her data had been even sketchier, the reaction even chillier."||600 km/s|
|Rhett Herman, How Fast is the Earth Moving? Scientific American: Ask the Experts. 26 October 1998||"The galaxies in our neighborhood are also rushing at a speed of nearly 1,000 kilometers per second towards a structure called the Great Attractor, a region of space roughly 150 million light-years (one light year is about six trillion miles) away from us. This Great Attractor, having a mass 100 quadrillion times greater than our sun and span of 500 million light-years, is made of both the visible matter that we can see along with the so-called dark matter that we cannot see."||1,000 km/s|
The motion of galaxies is determined from an apparent change in the color of light they emit. There are gaps within the spectra of the light emerging from every galaxy. These gaps, called absorption lines, are not located in the spectra at random. The patterns in the missing wavelengths tell us something about the elements present in the stars. In a sense, spectra are like fingerprints. Each element has its own specific set of absorption lines. When a star or group of stars (which is what a galaxy is) are moving relative to us on earth, these elemental fingerprints get shifted from their usual location in the spectra. When a galaxy is moving towards us, these fingerprints get shifted toward the blue end of the spectrum and when a galaxy is moving away from us, they get shifted toward the red end. The amount of shift can be used to determine speed. The greater the shift, the faster the galaxy is moving relative to us on earth.
This phenomena is an example of the Doppler effect -- an apparent change in the frequency of a wave when either the source or observer of the wave are moving -- named for the Austrian physicist Christian Döppler who first described it in the 1840s. The Doppler effect is familiar to anyone who has heard a horn or siren sounding while it went past them. As the horn approaches the listener it has one sound -- a pitch that's higher than normal (although most people are unaware of this). When the horn passes it has another sound -- one that's obviously lower in pitch (and that most people notice). The Doppler effect is used to measure the speed of many things: baseball pitches, tennis serves, speeding cars, and blowing rain (thus the name, Doppler radar).
Of the billions of galaxies in the observable universe, nearly all of them have spectra with a red shift; that is, they are moving away from us. Moreover, the degree of red shift, and thus the speed, of the galaxies in the universe is very nearly directly proportional to their distance from the Milky Way (an observation first reported by the American astronomer Edwin Hubble in the 1920s). This is generally interpreted as evidence for the overall expansion of the universe. It's not that the galaxies are rushing away from us. Rather, it's that the very space between them is getting larger. They aren't moving, nor are we. The universe is just getting larger and we happen to be in the universe.
Note that I wrote that the speed of the galaxies in the universe is very nearly directly proportional to their distance from us. Superimposed on the "recessional velocity"associated with the expansion of the universe is a "peculiar velocity"associated with the individual wandering motions of galaxies. If you imaging being on a train with a group of people, the recessional velocity is something like the motion of the train as a whole, while the peculiar velocity is something like the motion of the passengers milling about within the car. The first is a global property while the second is a local property, although "local"in this sense is hardly "small".
These peculiar velocities were always assumed to be randomly distributed since it was always assumed that the universe was homogeneous; that is, well mixed with no lumps or pockets. In the 1980s, astronomers began systematically cataloging the location and peculiar velocities of thousands and then millions of galaxies. What they found was the opposite of the assumption: the universe is not well mixed and the peculiar velocities are not randomly distributed. The universe isn't creamy, it's foamy; like a collection of bubbles, with galaxies being the "liquid"web that surrounds the "bubbles"of empty space. The galaxies aren't wandering, they're going somewhere; not all of them, but certainly several million of them in the space around the Milky Way.
In 1987, a group of seven astronomers uncovered this coordinated motion of the Milky Way and our several million nearest galactic neighbors -- Alan Dressler, Sandra Moore Faber, Donald Lynden-Bell, Roberto Terlevich, Roger Davies, Gary Wegner and David Burstein. Their results were so astounding they acquired the equally astounding nickname of "The Seven Samurai"(the name of a classic Japanese Samurai movie that spawned the classic American Western movie "The Magnificent Seven"). The place towards which we all appear headed was originally called the New Supergalactic Center or the Very Massive Object until one of the discoverers, Alan Dressler, decided they needed a catchier name and came up with "The Great Attractor".
The mass of the Great Attractor truly is great. Whereas our galaxy contains the equivalent of 1011 solar masses, the Great attractor is estimated to be on the scale of 1017 solar masses; a million times heavier than the Milky Way. If the Milky Way were a piece of gravel, the Great Attractor would be a truck. It's attraction is so strong that we are being sucked into it at the rate of 600 km/s. In comparison, the earth moves around the sun at the relatively pokey rate of 30 km/s and rockets escaping the earth's gravitational pull barely move at 11 km/s.
The Great Attractor is something on the order of 150 million light years from earth. One light year is the distance a ray of light would travel in the vacuum of space in one year -- about 1013 km. At the rate stated, we should arrive at the center of the Great Attractor in something like 15 billion years. Those of you who can't wait will be pleased to know that this is the upper limit on the estimated time of arrival. Since forces accelerate objects we will surely arrive there a few billion years earlier.
What we will find there remains uncertain. Pictures of the Great Attractor are hard to come by as this region in the sky is obscured by the dust and debris of our own galaxy. We can see where these millions of galaxies are headed, but we don't know what's there. Infrared images have revealed galaxies clustered at the center of the attractor, but they contain nowhere near the mass predicted by calculations. The problem of missing mass is one that is now common in astronomy. The large scale structure and behavior of the universe just can't be explained in terms of the mass that is visible to astronomers. Candidates to fill the vacancy include such bizarre entities as MACHOs (massive compact halo objects), WIMPs (weakly interacting massive particles), and other forms of so-called "dark matter". I have to put dark matter in quotes, because I think it's a terrible name. Dark matter is not dark or black, if it were somebody would have seen it, or rather somebody would have noticed a patch of sky that had nothing in it. Dark matter would better be called "invisible matter"as it doesn't interact with photons (the particles that make up visible light). Whatever the case, explaining away the missing matter -- explaining what it is that makes up the heart of the Great Attractor -- will surely rank as one of the greatest discoveries in the history of science.
General sources for further edification are …
Some papers of the Seven Samurai are …
Editor's Supplement -- 2000
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