Table Of ContentShrouds of the Night C H A P T E R
16
hrouds of the ight
Masks of the Milky Way
and Our Awesome New View of Galaxies
by
David L. Block
and
Kenneth C. Freeman, FRS
Prof. Dr. David L. Block Professor Dr. Kenneth Freeman
Director: Anglo American Cosmic Dust Laboratory Research School of Astronomy and Astrophysics
School of Computational & Applied Mathematics
The Australian National University
University of the Witwatersrand, Johannesburg
Mount Stromlo Observatory
1 Jan Smuts Avenue
Johannesburg Cotter Road
2001 South Africa Weston Creek PO
ACT 2611
Australia
Cover Photo: [Figure 202] secured with the Hubble Space Telescope, courtesy NASA, ESA, Natan Smith at the University of California, Berkeley,
and his collaborators, together with the Hubble Heritage Team at the space Telescope Science Institute, Baltimore.
ISBN: 978-0-387-78974-3 e-ISBN: 978-0-387-78975-0
DOI: 10.1007/978-0-387-78975-0
Library of Congress Control Number: 2008931168
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reamble
Grains of carbon based cosmic dust – the stuff of which every reader is made – were rec-
ognized in the early photographs of nebulae and galaxies. How pervasive were these dusty
Shrouds of the Night in galaxies external to our Milky Way? It was only in 1994 that the
dilemma facing astronomers, as to the existence and amounts of cold and very cold dust
grains in galaxies other than our Milky Way, was defi nitely solved by author David Block and
his collaborators. The temperatures of these grains may be minus 253 degrees Centigrade –
and even colder. There is one story of the enigmatic masks themselves, and another of the
awesome New View of spiral galaxies behind their masks. The duality in the structure of
spiral galaxies was featured on the cover of the British journal Nature in 1991.
Then there is the mysterious “dark matter” enveloping galaxies. Matter which is there, but
which emits no light whatsoever. In 1970, a research paper on galaxies by Ken Freeman
pointed out that “…there must be in these galaxies additional matter which is undetected …
Its mass must be as least as large as the mass of the detected galaxy …” John Bahcall, astro-
physicist at the Institute of Advanced Study in Princeton, wrote to Freeman that this 1970
paper “is the earliest explicit recognition of the [dark matter] problem that I know about
from rotation curves.”
In the following pages, join our authors David Block and Kenneth Freeman, as they explore
enigmatic Shrouds of the Night. The thoughts contained in this book have occupied their
Preamble
minds and the minds of their collaborators for a total of more than 60 years. They will
vii
introduce many of the modern ideas about galaxies and explain how these concepts were
discovered. Although this book is not a historical treatise, you will meet some of the older
ideas and also many of the key players in galactic astronomy over the past few centuries,
and learn how they made the discoveries which gradually led to our present level of under-
standing. Some of these pioneers were very infl uential in their time but their names have
now been almost forgotten, like the British amateur astronomer John Reynolds to whom
Edwin Hubble wrote for assistance in the classifi cation of spiral galaxies. Some surprises lie
in store, such as the person who took the First Photograph (ca. 1826) well before the public
announcement of photography in 1839, and the remarkable equipment used at Birr Castle
to draw objects of our night skies in the dark.
Preamble
viii
Preface:
era Rubin
Over many millennia, civilizations have been curious about the Universe in which they fi nd
themselves, so stories about origins were devised: how the Milky Way formed, why there are
seasons, what causes the rising and the setting of the Sun and the stars. These stories were
handed down throughout generations, and became an important part of science history. As
tools and understanding progressed, questions were answered but new discoveries and new
questions arose to take their places.
In recent times, the pace of science and technology has increased enormously. Before 1950
we lived in a Universe that we detected almost entirely with our eyes, or with substitute eyes,
such as telescopes or cameras. These instruments were generally sensitive only to the wave-
length region seen by the eye. But in the last fi fty years or so, the pace of new astronomical
discoveries has been enormous. Every decade or so, a new discovery has forced scientists
to revise their understanding of the history and evolution of the Universe. Some of these
phenomena were surprises, enabled by new technologies. Some advances came from using
known technology, some from using new ideas, some from both.
Consider this increase in our knowledge of astronomy in only 50 years:
1950s: New detectors sensitive in the radio region of the spectrum returned images of stars
and galaxies as seen by their emission of radio waves. Astronomers discovered radio galaxies,
galaxies that emit more of their radiation at radio wavelengths than at optical wavelengths.
Within a few decades or so, astronomers could detect and study galaxies also by their ultra- Preface
violet, infrared, x-ray, and gamma ray radiation. ix
1960s: Quasars were discovered, now understood as energetic cores of galaxies, many enor-
mously distant from our Galaxy. They are also called quasistellar objects, due to their point-
like nature.
1970s: Studies of rotation velocities for stars and gas in galaxies, some acquired from optical,
some from radio observations, indicate that most of the matter in the Universe is dark. Now
called dark matter, radiation is not one of its attributes. Most of the dark matter cannot be
composed of conventional matter.
1980s: Distant galaxies appear to be expanding at velocities faster than predicted from sim-
ple cosmological models. Dark energy, an unknown energy, is invoked to explain the high
velocities.
1990s: Astronomers increase the dust mass in spiral galaxies by ninety percent. The fi rst
extrasolar planets are also detected, planets orbiting nearby stars. The number of known
extrasolar planets now numbers almost 250.
What discoveries will the next 50 years bring astronomers and the interested public? We
can only guess how our view of the Universe will be altered. Earlier science history suggests
an accelerating rate of discoveries. The work of David Block and Kenneth Freeman already
forms an important part of this accelerating knowledge. As they describe in this book, their
discovery that cold cosmic dust pervades space makes it necessary for astronomers to rede-
sign their classifi cation scheme for galaxies. They suggest that symmetry should be at the
heart of a new classifi cation scheme in the near-infrared. One look at the stunning images
produced by astrophotographer David Malin in Chapter 11 is suffi cient to convince any
skeptic.
Along the way, this book takes the reader on a whirlwind tour of astronomical photo-
graphic history beginning with the world’s fi rst heliographs (one of these newly unveiled
by David Block) to the present day diffi culties of classifying galaxies. But the described
route is not linear. Instead, the reader is exposed to nights at a telescope, travels to talk
Preface and to learn, biographies of early and recent scientists who have contributed to the path
x David and Ken follow. Their story is part of the history as they describe their work as
astronomers. And they write about their discoveries in a manner that makes it fun to
read. David and Ken’s book is unconventional. It mixes history, geography, physics,
geometry, biography, art, poetry, plants and religion, with ground based and space pho-
tographs of galaxies. Some readers may question the discussion of religious beliefs, but
this is their story.
Even to an astronomer who studies galaxies, the comparison of the early and even recent
images with the newly processed ones can only be described as breathtaking. The authors
correctly call these “the new view of galaxies.” But some of the knowledge is old, only
uncovered by the authors. The book contains extended quotes from the scientists them-
selves. Many of Sir John Herschel’s drawings from the Cape of Good Hope concern the
Magellanic Clouds observed by eye, and their details are unforgettable.
I congratulate the authors. As a tribute to them, I add a quote from Marcel Proust:
The real voyage of discovery consists not in seeking new landscapes but in having
new eyes.
Vera C. Rubin is an observational astronomer who has studied the motions of gas and stars
in galaxies and motions of galaxies in the Universe for seventy-fi ve percent of her life. Her
work was infl uential in discovering that most of the matter in the Universe is dark. She is a
graduate of Vassar College, Cornell University, and Georgetown University; George Gamow
(George Washington University) was her thesis professor. A staff member at the Depart-
ment of Terrestrial Magnetism, Carnegie Institution of Washington since 1965, she is now
a Senior Fellow. She is a member of the U.S. National Academy of Sciences and Pontifi -
cal Academy of Sciences. President Clinton awarded her the National Medal of Science in
1993. She received the Gold Medal of the Royal Astronomical Society (London) in 1996.
The previous woman to receive this medal was Caroline Herschel in 1828. She has numer-
ous honorary degrees, including Harvard, Princeton, Yale and Smith College. In 1994 she
delivered the Henry Norris Russell Lectureship; previous recipients of this esteemed Lec- Preface
tureship have included Nobel laureates Enrico Fermi and Charles Townes. Vera is active in xi
