(This article originally appeared in the Q3 1987 star news letter)
The Great Attractor
by David Moore
In the early part of this century, V. M. Slipher and later Edwin P. Hubble measured displacements of spectral lines in the light from other galaxies. Such shifts reflect the Doppler effect: the change in the wavelength of radiation or sound that results from motion of the source with respect to the observer. Most of the galaxies showed a red shift, meaning that the spectral absorption lines were displaced toward the red, or longer-wavelength, side of the spectrum. A red shift indicates motion away from the observer. By the late
1930’s, enough data had been collected to convince nearly everyone that the universe was expanding. By the 1970’s, it was generally accepted that the observed expansion of the universe started some 15 billion years ago in an explosive creation event, known as the big bang.
Over the years, the red-shift data for distant galaxies improved, but the correlation between distance and red shift remained poor for nearby galaxies. A likely explanation for this observation is that each galaxy has a motion of its own with respect to the expanding universe. Such motions would be most evident in the red shifts of nearby galaxies, whose Hubble velocities are relatively low. If the magnitude of these peculiar velocities were a few hundred kilometers per second and were in random directions, then many nearby galaxies would show blue shifts; lines in their spectra would be Doppler-shifted toward the blue end of the spectrum, indicating motion in the direction of our galaxy. The fact that almost all galaxies show red shifts indicates that the random motion of the galaxies must be less than 100 kilometers per second.
Until 1977, astronomers were content with the idea that the individual motion of a galaxy with respect to the expanding universe had to be less than 100 kilometers per second. Astronomers began studying the Doppler shift of the microwave radiation that permeates the universe. Using the microwave radiation as the
frame of reference, astronomers determined that our solar system is moving almost 400 kilometers per second.
After correcting for our earth orbiting the sun at 30 kilometers per second, the solar system orbiting the center of the Milky Way at 230 kilometers per second, which in turn is drifting toward the Andromeda Galaxy at 40 kilometers per second, the microwave results imply that the Local Group of galaxies (the Milky Way, Andromeda, and several neighboring galaxies) is traveling through space at 600 kilometers per second. We are moving in a direction perpendicular to the line of sight between our solar system and the center of the galaxy, opposite to the direction of galactic rotation and about 27 degrees above the galactic plane. Only one reasonable explanation for our Local Group’s peculiar motion is that it must be due to a gravitational attraction by a concentration of mass. Newton’s law of gravitation indicates that the amount of mass needed to produce a given acceleration increases as the square of the distance. At a distance of 30 million light-years (the distance to the nearest cluster of galaxies) the gravitational pull of many hundreds of galaxies would suffice. If the distance was 300 million light-years, then tens of thousands of galaxies would be needed.
Our Local Group of galaxies is part of the Local Supercluster whose center is the Virgo cluster of galaxies. The center of Virgo, which is 40 to 80 million light-years away, lies about 50 degrees away from the direction our Local Group is headed. Data indicates part of our Local Group’s peculiar motion is due to the gravitational pull of our Local Supercluster, but that the entire Local Supercluster itself has a velocity of at least 150 kilometers per second towards the next-closest supercluster, Hydra-Centaurus. More recent investigations indicates that the Hydra-Centaurus Supercluster is moving even faster than our Local Group and in the same direction. Instead of being the attractor, it seems that the Hydra-Centaurus Supercluster is itself in the grip of a still more distant and more massive attractor. A number of groups of astronomers are currently trying to gather more information on the Great Attractor. What they seem to agree on is that our Local Group, our Local Supercluster, and the Hydra-Centaurus Supercluster are all heading in a particular direction toward something astronomers are calling the Great Attractor. The Great Attractor appears to be further away than anyone had dared to imagine and therefore must include tens of thousands of galaxies.
