MICHELE BRAND, Host:
A small galaxy of astronomers is gathering today in Washington. Three thousand astronomers are sharing exciting discoveries, such as a new map of the strange dark matter that surrounds our Milky Way galaxy. NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE: Astrophysicist David Law says scientists think this mysterious dark matter must be there because they can see the effects of its gravity as it pulls on the orbits of stars and galaxies.
DAVID LAW: While we might not be able to see the dark matter, it's a vitally important component of our galaxy. It makes up the bulk of what the actual stuff is in our galaxy, oftentimes greater than 70 percent or, in many cases, greater than 90 percent of the total mass of the galaxy is in this dark matter.
GREENFIELDBOYCE: Law currently works at the University of California, Los Angeles. He wondered: If you could see the cloud of dark matter enveloping the Milky Way, what would it look like? What shape would it be? He knew he could get clues from how the dark matter alters the paths of dwarf galaxies and clusters of stars as they orbit the Milky Way.
LAW: And based on the shape of those paths, we can say what the shape of the dark matter itself has to be.
GREENFIELDBOYCE: Now, you can't actually watch a dwarf galaxy go all the way around our Milky Way. That would take about a billion years. But Law and his colleagues used a kind of trick to reconstruct the path of one orbiting galaxy.
LAW: As it flies around the Milky Way, tidal forces from the Milky Way are shredding stars out of it into streams that trail behind it and even go ahead of it, along its orbit.
GREENFIELDBOYCE: He says you can picture the bright, visible part of our Milky Way as a flat, round, dinner plate.
LAW: And if you picture that sitting inside a large beach ball, then that represents the dark matter halo. And if you then came along to that beach ball and put one hand on one side and one hand on the other and squeezed it, that's something like we think the dark matter is. It's squeezed along that direction, and the dinner plate of the disk of the Milky Way is sitting inside of it.
GREENFIELDBOYCE: Law presented this new dark matter map at a major conference of the American Astronomical Society. He says it should help scientists as they try to understand the nature of dark matter, because whatever it is, it's got to be able to form this kind of squashed beach ball.
LAW: You have to have some kind of a dark matter particle which can build a halo like this, because this is generally what we see surrounding the Milky Way.
GREENFIELDBOYCE: Nell Greenfieldboyce, NPR News.
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