Milky Way’s magnetic field mapped

Planck telescope records light reflecting off interstellar dust grains

NATURAL ATTRACTION  The galaxy’s magnetic field (brown lines) swirls across the sky in a map from the Planck telescope. Darker colors show stronger polarization of light. The dark band through the middle is the plane of the Milky Way. Patches with no data have very low signal and have not been fully analyzed.

ESA and the Planck Collaboration

If the Milky Way were one giant magnet, sprinkling iron filings around it would trace the galaxy’s magnetic field. Scientists have found a more practical way to map the field using the Planck telescope.

To do so, Planck measured the polarization of microwave light that permeates space. When light is polarized, its electric fields all point in the same direction. Light reflecting off interstellar dust grains becomes polarized in the direction the grains are aligned; that direction, in turn, is steered by the galaxy’s magnetic field.

Planck’s map, reported in four papers posted May 5 at, shows the entire sky with a dark band through the center representing the plane of the galaxy. Darker shading reflects more-polarized light. The lines mark the direction of the magnetic field.

The galactic magnetic field is about 100 million times as weak as a refrigerator magnet, and yet it may be crucial to the formation of stars. The field map is also important for understanding the polarization of the cosmic microwave background, the flash of light emitted 380,000 years after the Big Bang, which the BICEP2 team recently used to see gravitational waves from the primordial universe (SN: 4/5/14, p. 6).

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