The Role of Images in Astronomical Discovery Page 22
universal force permeating interstellar space. The small grain can become magnetized.
A large-scale and low-intensity magnetic field pervades the interstellar space of galax-
ies. Its strength is weak, a million times weaker than Earth’s magnetic field, but it spreads
on a scale of light-years. As clouds of interstellar gas and dust move around, they collide
with each other. Like invisible springs, the embedded magnetic fields become compressed.
Expanding bubbles, driven by the stellar winds and supernovae explosions of massive stars,
also compress the gas and dust and its magnetic field. As this happens, electrically charged
particles, trapped in the magnetic field lines, are accelerated to higher energies, like peb-
bles spun with a slingshot. It was the Italian physicist Enrico Fermi (1901–1954) who
showed that moving magnetic fields have the net effect of accelerating charged particles
to extremely high energies.22 When the accelerated particles reach high enough energies,
they escape their cloud and propagate through space as “cosmic rays.” These are electrically
21 The near-infrared domain corresponds to wavelengths of 1 to 5 microns, and the mid-infrared to wavelengths of 5 to 25 microns.
22 E. Fermi, Galactic Magnetic Fields and the Origin of Cosmic Radiation, The Astrophysical Journal, 1954, Vol. 119, pp. 1–6.
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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
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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. However, his basic approach and his conclusion were correct. He
derived that our Sun and solar system are in orbital motion around the galactic center that
lies in the direction of the constellation of Sagittarius. It is now well established that the
Sun and its planetary system is located at about 27,000 light-years from the center of the
24 H. Shapley, Studies Based on the Colors and Magnitudes in Stellar Clusters – Seventh Paper: The Distances, Distribution in Space and Dimensions of 69 Globular Clusters, The Astrophysical Journal, 1918, Vol. 48, pp. 154–181.
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Fig. 6.4 Projection of 334 open star clusters (within 1,000 parsecs of the Sun) projected on a plane perpendicular to the galactic plane. The dotted line marks the plane of symmetry of the open clusters. From Trumpler (1930), Lick Observatory Bulletins.
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6. Galaxies in Focus
141
Milky Way. At the average speed of 828,000 km/h (230 km/s), it takes us about 225 to 250
million terrestrial years to complete a full galactic revolution.
Knowing our Milky Way and its constituents has been an essential step in understanding
other galaxies, including those of different histories, shapes and masses. But the epistemic
process also works in reverse. It became easier to chart our own galaxy and describe its
shape once the extragalactic nature of “nebulae” was established. The Lick Observatory
astronomer Robert Trumpler found inspiration in images of other spirals. He figured out
the overall shape of the Milky Way system by mapping the distribution of star clusters. In
his second seminal paper, also published in 1930, Trumpler presented the results of a study
of 334 open star clusters, groups of stars that formed coevally from the same molecular
cloud. By mapping the distribution of these clusters, Trumpler showed that the Milky Way
had a flattened disk shape (Fig. 6.4). “The hypothesis supports the view that our Milky Way
system is a highly resolved spiral nebula, a right-handed spiral as seen from the galactic
north pole, of dimensions similar to those of the Andromeda nebula.”25
Crystallizing Galaxy Shapes
As has been shown in the previous chapters, eye observations, drawings and photographs
helped to find order in the span of galaxy shapes. In his 1811 article, William Herschel drew
forms of “nebulae” where basic galaxy silhouettes are recognizable (Chapters 1 and 2). Half
a century later, the Birr Castle observers sketched most of the key shapes that twentieth-
century observers used in designing classific
ation schemes (Chapter 9). With the ability to
measure distances and the understanding of how dust affects our viewing, exploring the
shapes of the various galaxies became an important area of research. Ellipticals and disks
were the two main categories of galaxies quickly identified, both presenting varying degrees
of flattening for ellipticals and central concentration for spirals or disk systems.
Contrary to Curtis’ early insight, there were more than just spirals in the extragalactic
world. As discussed briefly above, the contents and shapes of galaxies are determined by
nature, inherited conditions at birth, and by nurture, later events from interaction with their
environment. Galaxies can be rich or poor in interstellar gas. Their present gaseous and
dust content reflects the conditions that have prevailed throughout galactic history. These
are driven, first, by initial states such as the mass and angular momentum of the proto-
galaxy, and second, by the environment, factors such as intergalactic density, or interaction
with neighbors at birth or later.
The New Zealander–American astronomer Beatrice Tinsley (1941–1981) produced fun-
damental work that linked stellar evolution, gas consumption and the integrated properties
of galaxies as they evolved. She found that “while these calculations cannot prove that the
sequence irregular–spiral–elliptical is not an evolutionary order, they show that all the prin-
cipal galactic types may have originated at the same time, but with some differences in
25 R. J. Trumpler, Preliminary Results on the Distances, Dimensions and Space Distribution of Open Star Clusters, Lick Observatory Bulletin, 1930, No. 42, pp. 154–188.
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Part II – Images as Galaxy Discovery Engines
physical conditions that led to different stellar birth rates.”26 Modern astrophysicists have
been able to disentangle these various factors using a range of techniques. Images of galax-
ies have helped provide insight into the process of proto-galaxies transforming into the
galaxies seen today.
Galaxies as Interstellar Gas Processors
Although their morphology and mass may have changed considerably over aeons, most
galaxies appear to have been in place for billions of years. In addition to billions, and
sometimes trillions of stars, galaxies hold variable amounts of gas and dust, current con-
tents being mostly dictated by how galaxies initially came together. Primordial collapse of
giant clouds took place at rates that depended on the large-scale properties of the primor-
dial units and on the environment. These materials have been recycled in and out of stars
to form successive generations of stars. This lifecycle rhythm has determined the morpho-
logical properties of the galaxies we observe today, as inferred by Tinsley.
The process continues today at various rates. With each generation of stars, the heavy-
element content of stars and gas clouds is enhanced. Adding to this “closed-box” process-
ing, the merging of galaxies, cannibalizing each other, has affected their evolution to a
degree that we are still trying to understand. In today’s galaxies, a large fraction of the pri-
mordial gas has been used up, having condensed into stars and planets, or lost to intergalac-
tic space. As this process has happened at different rates, interstellar gas and dust represent
a varying fraction of the visible mass of galaxies of the present day. In some galaxies, ellip-
ticals for example, the gas fraction is almost zero. In the flatter spirals, the collapse of the
proto-galaxy was slower; the gas content is still relatively high, several percent of the total
mass.
“Starburst galaxies” or galaxies currently undergoing intense star-formation episodes
make for spectacular images (see Plate 6.2 and Plate 11.1). We find them at both ends of
the scale of mass, among small irregulars and giant ultraluminous galaxies. They harbor a
high proportion of young, massive stars that sculpt the interstellar medium of these galax-
ies. The irregular and filamentary appearance of these galaxies is indicative of a highly
turbulent interstellar medium ploughed by shocks and expanding super-bubbles, phenom-
ena associated with the evolution of giant clusters of massive stars.
Order from Chaos
If a proto-galaxy originated from an initially slowly rotating cloud, its collapse was fast
and led to an intense firework with a huge number of stars formed in rapid sequences of a
few hundred million years long: most of the initial gas condensed into stars of all masses.
This scenario can be viewed as a grand scale version of what Immanuel Kant and Pierre
26 B. J. Tinsley, Evolution of the Stars and Gas in Galaxies, The Astrophysical Journal, 1968, Vol. 151, p. 558.
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Simon Laplace imagined for the formation of the solar system. Then, the large number of
very massive stars evolved rapidly into supernovae and blew away the unused gas into the
circum-galactic and intergalactic environment. The dried-up descendants of these rapidly
forming stellar fireworks are observed today as elliptical galaxies, or elliptical as per their
silhouettes. Physically, they are spheroidal or oblate systems (see Plate 6.3).
In ellipticals, stellar orbits occupy no preferential plane; they spread out in all directions
and the multitude of stars move either in prograde or retrograde directions. One can imagine
such systems as being somewhat analogous to gigantic swarms of bees collecting around a
beehive, or for the stars, around a common center of mass.
Selected directions or planes may host a denser traffic due to dynamic resonances, or
gravitational potential asymmetry induced by the capture of a smaller galaxy. Internal reso-
nant modes determine a variety of three-dimensional shapes, from the perfect spheroidal to
ellipsoidal or even prolate. The whole galaxy slowly spins on itself, especially in the cen-
tral parts where rapidly rotating cores have been mapped. There are even cases of counter-
rotating cores, the inner core rotating in reverse from the main body of the galaxy.
Ellipticals encompass the whole range of galaxy masses, including the most massive in
the universe. The mammoth elliptical Messier 87, near the center of the Virgo cluster of
galaxies, 53 million light-years away, has more than one trillion stars and is surrounded by
30,000 globular clusters (Fig. 6.5). The least massive galaxies are also ellipticals: the dwarf
spheroidal Leo I galaxy, 820,000 light-years away, is a tiny member of the Local Group of
galaxies. It has a few tens of millions of stars and three known globular clusters. Likewise,
the Sculptor dwarf galaxy harbors only a few million stars and it can hardly be distinguished
from a blown-up globular cluster.
Disks from Orderly Traffic
Proto-galaxies with significant initial angular momentum collapsed more slowly than those
that produce ellipticals. This led to a slower assembly of stars, with most of the gas assem-
bling into a flat disk. The accumulation of gas continued over a much longer period than
for rapidly collapsing spheroidal systems. Despite mass loss, disk galaxies have been able
to retain a generous
gaseous reservoir. Hence, star formation is still continuing in gas-rich
galaxies such as ours, particularly in the spiral arms, regions of higher gas density (see
Plate 6.4).
Consequently disk galaxies are much flatter than ellipticals and most of their stars are
revolving in the same direction around the center of mass. All spirals and many large irregu-
lars are disk galaxies. Like pizzas, some are flatter, some bulkier.27 One of the most striking
features of disk galaxies is their spiral arms as discovered in Messier 51 by William Parsons
in 1845. The spiral pattern is not due to stars or gas flowing along the arms, inward or out-
ward. The “spirality” is a resonance pattern (Fig. 6.6). The spiral shape arises as a vibration
27 In spirals, the diameter-to-thickness ratio of the disks varies from about 100 to 1 for the most flattened disks to 10 to 1 for the bulkier disks.
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Part II – Images as Galaxy Discovery Engines
Fig. 6.5 Messier 87 in the Virgo cluster about 54 million light-years away. It is the most massive
galaxy in the local universe. This image by the Hubble Space Telescope shows the jet of energetic
charged particles coming out of the center. Credit: NASA, ESA.
mode of the whole disk system, which induces a concentration of mass at certain loci, which
lines up; this leads to the beautiful spiral pattern. It is like a traffic jam, with a pile-up of gas
at specific locations along the galactic orbit. The resonances last a few million years and
are accompanied by an increase of density of the interstellar gas by a factor of at least two
or three, enough to locally enhance dust and star formation. With the loci of overdense gas
and dust, spiral arms are cradles of young stars and nebulae, which enhance their visibility.
While spiral arms correspond to transient regions of stellar formation, the underlying older
stellar population remains more smoothly distributed. Infrared images, which highlight the
older stellar population, do not show the spiral pattern as strikingly.
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Fig. 6.6 The formation of spiral arms. Graphic illustrating how slightly elliptical orbits pile up to