The Role of Images in Astronomical Discovery by Rene Roy

Author:Rene Roy [Roy, Rene]
Language: eng
Format: epub
Published: 0101-01-01T00:00:00+00:00

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Fig. 6.3 Map of starlight polarization for 7,000 stars across the sky. The short lines indicate the strength and direction of the polarization (E-vector), indicative of the projected magnetic field permeating the interstellar medium. The galactic latitude is shown as the y-axis, and galactic longitude as the 14:05:49

x-axis, with 0° corresponding to the direction of the galactic center. From Mathewson and Ford (1970), Memoirs of the Royal Astronomical Society.23

, subject to the Cambridge Core

23 D. S. Mathewson and V. L. Ford, Polarization Observations of 1800 stars, Memoirs of the Royal Astronomical Society, 1970, Vol. 74, pp. 139–182.

6. Galaxies in Focus

139

charged particles, mainly protons and electrons, microscopic bullets traveling at velocities

close to the speed of light. They are a natural source of radioactivity, and when they collide

with living cells, they can trigger mutations. Hence cosmic rays may have played a signifi-

cant role in the evolution of living species. Details of how energetic particles also produce

electromagnetic waves in the radio domain of the electromagnetic spectrum are given in

Chapter 7.

A special imaging technique, polarimetry, gives us the means to measure and map the

magnetic fields. Here is how it works. The small dust grains are weakly magnetized. As tiny

magnets, they line up with the general magnetic field, like the needle of the compass lines

up with the Earth’s magnetic field. Light going through or scattered by the dust varies in

intensity with orientation as viewed against the plane of the sky: light is polarized, i.e. it is

more intense at a certain angle as it passes more easily through dust grains aligned in a given

direction. The degree of polarization is a measure of strength of the magnetic field that lines

up the grains. By analyzing the polarization of starlight over many directions in the sky, a

map of the Milky Way’s magnetic field can be made, an informative non-homomorphic

representation (Fig. 6.3; see also Plate 7.2).

Furthermore, an active surface chemistry makes the interstellar dust grains micro-

factories of complex molecules. The interstellar molecular products go from rather simple

radicals, such as OH or CH+, or molecular hydrogen, to a whole range of molecules like

carbon monoxide, water or more complex molecules made up of as many as 17 atoms,

and even amino acids. The interstellar medium is particularly efficient at making water ice.

For example, water on Earth was produced in interstellar clouds prior to the formation of

our solar system. Molecules emit mostly in the infrared and radio domain of the electro-

magnetic spectrum, and astronomers are able to make images of molecular clouds at those

wavelengths.

Shaping the Milky Way

If the nearest star is the Sun, the nearest galaxy is the Milky Way, and we are embedded

in it. In one of the great breakthroughs of early twentieth-century astronomy, the Amer-

ican astronomer Harlow Shapley proved, in 1918, that we were not at the center of this

giant system of stars as many believed until then.24 Shapley assumed zero dust and perfect

transparency. He also mistook the short-period variable stars, which he used for determin-

ing the distance, for brighter ones, and hence grossly overestimated the size of our Milky

Way by a factor of three.

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