The Harmony of the Sphere
Kant and Herschel on the Universe and
the Astronomical Phenomena
International Workshop
20 May 2011
University College London
Nebulae, Star Clusters and the
Milky Way:
From Galileo to William Herschel
Michael Hoskin
michael.hoskin@ntlworld.com
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M. Hoskin
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Nebulae, Star Clusters and
the Milky Way:
From Galileo to William Herschel
Nebulae
 The Milky Way
 Herschel’s Cosmology

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Nebulae
Among the stars listed by Ptolemy in his
catalogue in the Almagest are some seven
that he describes as nepheloeides, ‘cloudlike’, from nephele, cloud (in Latin,
nebula).
 Their diffuse shape distinguished them
from normal stars, which appeared as
points of light in the sky.

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Galileo Galilei

Galileo’s Sidereus nuncius (1610)
“A cluster that contains the nebula called Praesepe ... is
not one star only but a mass of more than forty small
stars. […] The stars that have been called by every one
of the astronomers up to this day ‘nebulous’ are groups
of small stars set thick together in a wonderful way; and
although each one of them, on account of its smallness,
or its immense distance from us, escapes our sight, from
the commingling of their rays there arises that brightness
which has hitherto been believed to be the denser part of
the heavens and able to reflect the rays of the stars or
the Sun”.
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Christiaan Huygens

In 1656 Christiaan Huygens (1629–95),
took the important step of making a careful
sketch of a nebula, namely the Orion
Nebula.
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M42 as depicted by Christiaan Huygens, Systema Saturnium (The Hague, 1659), 8
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Edmond Halley



Edmond Halley in Philosophical transactions (1715):
“[nebulae are] nothing else but the Light coming from an
extraordinary great Space in the Ether; through which a
lucid Medium is diffused, that shines with its own proper
lustre”.
Orion Nebula, the Andromeda Nebula, the galactic
cluster M11, and three globular clusters: M13, M22, and
the southern Omega Centauri which Halley himself had
observed at St Helena in 1676–78.
It was during Halley’s visit to St Helena that the
Magellanic Clouds first came under scrutiny from a
competent observer.
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Edmond Halley
Halley to Sir Jonas Moore (November 1677):
“The two Nubeculae called by the Saylors the
Magellanick Clouds, are both of them exactly
like the whiteness of the Milky Way lying within
the Antartick Circle; they are small, and in the
Moon shine, scarce perceptible; yet in the dark
the bigger is very notable.”
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Two Questions
Could star-like points of light be
confidently identified with authentic stars?
 When could success in resolving some
nebulae into stars justify the claim that all
were formed of stars?

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The Nature of Nebulae


How could one, then, ever be sure that a given
nebula would never be resolved into stars by a
more powerful telescope?
The most obvious answer was: if it was known to
have changed shape more rapidly than would
have been possible, had it been a vast star
system. A star system will appear nebulous only
if it is both so remote that the individual stars
cannot be detected, and so large that despite its
distance it appears to us to extend some
distance across the sky.
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Changing Perspective



By the 1770s Nebulae were known in considerable
numbers, and competing theoretical explanations had
been advanced for them.
But so far no astronomer had dedicated himself to the
observing of nebulae for their own sake, and to the
construction of the great reflectors which alone could
collect enough light from these faint objects to bring
them under serious examination.
With the arrival on the scene of William Herschel, this
situation would change dramatically.
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The Milky Way
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The Milky Way from Galileo to
Lambert


Galileo’s telescope confirmed that the Milky Way is
composed of innumerable faint stars, but the threedimensional structure of the star system that generates
the milky effect seems to have been of curiously little
interest to him, or indeed to any competent astronomer
of the seventeenth and early eighteenth centuries.
In about 1720, William Stukeley (1687–1765), a
physician and antiquarian but no astronomer, suggested
to Newton that the Sun and the bright stars formed a
spherical cluster and that this cluster was surrounded at
a distance by the stars of the Milky Way which together
formed a flattened ring. But his words fell on deaf ears.
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Thomas Wright
Thomas Wright (1711–
(1711–86) published An original theory or new
hypothesis of the universe in 1750: the long public silence on
the structure of the Milky Way was at last broken.
- struggle to reconcile astronomy with his highly personal
theology (to integrate the limited region of the universe that is
is
observable, into a cosmology that embraces the
supernatural).
- The centre of the universe in the moral order (which is
referred to by such titles as “the Sacred Throne”
Throne”) is also the
centre of the universe in the physical order — the
gravitational centre and the source of the laws of nature.
nature.
The rest of creation is arranged, both morally and physically,
about the Sacred Throne.
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Two attempts by Thomas
Wright to convey his
earliest conception of the
universe. All creation was
centred on the Abode of
God, here represented by
a triangle as symbol of
the Trinity. The Sun with
its planets, and the other
stars with theirs, were
distributed around this
centre on every side,
occupying a volume of
space in the shape of a
spherical shell.
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Thomas Wright

“... Upon all sides the principal stars of the visible
creation are exhibited in their natural order as seen from
ye Earth by ye naked eye. Those of ye first magnitude
nearest to our own system, and the rest proportionable
removed according to their respective phenomena.
Beyond these are others more remote crowned with a
penumbral shadow such as we call telescopic stars and
again without them more, supposed to be at immense
distance & by no means perceptible to ye human eye. At
a certain distance from ye Sun equal to a visual ray of ye
smallest visible star is a faint circle of light terminating
the utmost extent of ye visible creation, in a finite view
from ye Earth....”
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The outer sphere in this diagram
portrays Wright’
Wright’s preferred model
of the star system to which the Sun
and the visible stars belong. These
visible stars form a small segment
of a spherical system whose radius
is so vast that the boundaries of
the segment approximate to
parallel planes. An observer at A
sees only a handful of nearby stars
when looking towards B or C, but
innumerable stars whose combined
light creates an impression of
milkiness when looking towards D,
E, … From An original theory,
theory, Plate
XXVII.
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Thomas Wright


The radius of the shell he believed to be very great, so
that the shell curved imperceptibly and the restricted
segment accessible to observational astronomy
therefore approximated to a flat disk. Not through
science, but only with the aid of theology, do we discover
that our star system is distributed about a supernatural
centre.
It is a supernatural centre, but no longer the
supernatural centre. Our star system (he now holds) is
one of innumerable such systems, each of which has its
own supernatural centre. The other systems we see as
nebulae.
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Thomas Wright

In 1750 Wright acknowledges the
possibility of an alternative model for our
star system, though it is one that lacks the
spherical symmetry on which he has
hitherto set such store.
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Wright’
Wright’s alternative model of our star system. The local Divine Centre is here
shown as a sphere reminiscent of Saturn, and around it is the flattened
flattened ring of
stars to which the Sun belongs. An observer looking around within
within the plane of
the ring sees the impression of milkiness.
milkiness. As we shall see, Immanuel Kant did
not realize that Wright imagined there to be a local Divine Centre,
Centre, and therefore
saw no reason why the ring should not extend without interruption
interruption from one side
to the other, thereby becoming a diskdisk- shaped assembly of stars. From An
original theory,
theory, Plate XXVIII.
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Wright’s Influence on Kant
Wright’s theory was published in 1751 as
a summary in a Hamburg periodical. This
summary (no illustrations) came to the
eyes of the young Immanuel Kant (1724–
1804).
 In the summary Wright seemed to be
offering two alternative models for our star
system: either it was spherical, or it had
the shape of a flattened, hollow ring.

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Some Open Questions


How one is to reconcile Kant’s cosmogony with
the cosmology he derived from his creative
misunderstanding of An original theory is
unclear.
Nor is it clear how influential were Kant’s ideas,
for the work in which he set all this out — his
Universal natural history and theory of the
heavens — was on the point of publication in
1755 when the bookseller went bankrupt. The
influence of the book is hard to assess.
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Kant and Lambert



Lambert explains that the visible stars form a system that
is flattened and circular, and which we see as the Milky
Way.
This system is made up of innumerable sub-systems,
one of which comprises the Sun and those stars that we
see as individuals. The remaining sub-systems are
scattered in the plane of the Milky Way. Being finite in
number, they must avoid gravitational collapse by
orbiting about a central body.
His hierarchy extends distantly upwards; but unlike
Kant’s, Lambert’s hierarchy is finite — it has, he
suggests, perhaps a thousand steps.
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Lambert

Lambert’s attempt to explain the Milky Way and
its role in the structure of the universe belonged
to the era when cosmology was still largely the
preserve of religiously-motivated speculators,
and the impact of these explanations on the
science of astronomy was minimal. But a
dedicated observer was soon to build massive
reflectors explicitly in order to investigate “the
construction of the heavens”.
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 William
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Herschel
M. Hoskin
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William Herschel’s Early
Investigations of Nebulae

William Herschel (Friedrich Wilhelm Herschel, 1738–
1738–1822)
opened his first observing journal on 1 March 1774.

The opening page of his first observing book shows that on 1
March 1774, in addition to Saturn, he “Observed the Lucid
Spot in Orions Sword belt; but the air not being very clear it
appeared not distinct”
distinct”.
On 4 March: “Saw the Lucid Spot in Orions Sword, thro’
thro’ a 5
foot reflector; its Shape was not as Dr Smith has delineated it
in his Optics [that is, the sketch by Huygens]; tho’
tho’ something
resembling it; being nearly as follows. From this we may infer
that there are undoubtedly changes among the fixt stars, and
perhaps from a careful observation of this Spot something
might be concluded concerning the Nature of it.”
it.”

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William Herschel




Smith’s book encouraged Herschel to experiment with
telescopes.
In May 1773 he tried observing with a refractor of long
focus, but the following month he hired a small
Gregorian reflector.
Mirrors were expensive to buy: he determined to make
reflectors for himself, with Smith’s book as his guide and
help from his resourceful brother Alexander.
Herschel’s epoch-making achievements as a builder of
what we might term ‘cosmological’ telescopes were the
foundation for his investigations of “the construction of
the heavens”.
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William Herschel



Committed to astronomy beyond the solar
system, and in particular to the scrutiny of
nebulae in the hope of solving the riddle of their
physical nature.
This inevitably called for the building of reflectors
with mirrors of a size hitherto undreamed of.
Herschel recognized that to study the distant
objects that might reveal the large-scale
structure of the universe he must first be able to
see them, and for this his telescopic mirrors
must be large so as to intercept as much as
possible of the light from these objects.
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William Herschel

He alone had access to the observational
evidence on which he based his theories,
and this established a gulf between him and
his contemporaries, who had to take his
word for what was to be seen in his monster
telescopes.
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William Herschel



Herschel increased the number of known nebulae from
the hundred or so listed by Messier to over two-and-ahalf thousand.
His contemporaries were baffled by an astronomer
who broke all professional norms by importing into
astronomy the methods of natural history, and
labouring on an heroic scale to increase the number
of recorded specimens of a range of astronomical
species.
These data cried out for classification and explanation,
and classification had had virtually no place in
astronomy.
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William Herschel

Herschel, then, was in the first rank as
telescope builder, as observer, and as
theorist; and so it is not surprising that he
altered the face of astronomy dramatically.
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William Herschel


In December 1781, his neighbour and ally William
Watson Jr sent him a copy of Messier’s 1780 list of 68
nebulae.
In his first major paper on cosmology Account of some
observations tending to investigate the construction of
the heavens published in Philosophical transactions
(1784), Herschel claims that as soon as Messier’s
catalogue “came to my hands, I applied my former 20feet reflector of 12 inches aperture to them; and saw,
with the greatest pleasure, that most of the nebulae,
which I had an opportunity of examining in proper
situations, yielded to the force of my light and power, and
were resolved into stars”.
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William Herschel

Herschel did not look at another nebula
until August 1782. Even then his
observations of nebulae were desultory at
first, perhaps because of the limitations of
the mounting of his 20-ft reflector, serious
nebular studies would have to await the
completion in October 1783 of the “large”
20-ft with its 18-inch mirrors and — more
importantly — its stable mounting.
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William Herschel

A discovery of exceptional importance:
“Septr 7. 1782.... A curious Nebula, or what else to call it
I do not know. it is of a shape somewhat oval, nearly
circular, […]. It is of the same shape with 278
[magnification] but much less in appearance. with 932 it
is still the same shape but much larger. So that its
appearance seems to follow the law of magnifying, from
whence it is clear that it is of some real magnitude in the
heavens and not a glare of light. The brightness in all the
powers does not differ so much as if it were of a
planetary nature but seems to be of the stary kind, tho’
no star is visible with any power. It is all over of nearly
the same brightness. The compound eye piece will not
distinguish it from a fixt star, at least not sensibly”.
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William Herschel



This discovery of what became known as the
“Saturn Nebula” was momentous.
It was the first example of what Herschel chose
to term ‘planetary nebulae’, anomalous and
mysterious objects that seemed to have the
circular disks of planets but the pale light of
nebulae.
For a long time planetary nebulae would refuse
to fall into any simple theory of nebulae that
Herschel could devise: they kept their
unchanging place in the heavens as did the
‘fixed’ stars, and he wondered if they might be a
completely new species of celestial body.
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William Herschel

Herschel’s fine new 20-ft reflector was
nearing completion, and he resolved to
use it to examine the whole of the sky
visible from England — a task that was
to occupy him and Caroline for twenty
years and result in two catalogues of one
thousand nebulae and a third of over five
hundred.
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Herschel’
Herschel’s ‘large’
large’ 2020-ft reflector, completed in October 1783. This engraving, which he
he
published in 1794, shows the instrument equipped with an observing
observing platform which allowed
the observer to drag the tube from side to side and so track an object in the sky.
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Herschel’
Herschel’s conception of how star clusters increasingly condense over time
time under the action
of gravity. These diagrams come from his 1814 paper on “The sidereal part of the heavens”
heavens”,
but he arrived at this conception in the 1780s. We see here the start of the transition from the
stable, clockwork universe of Newton, to the universe of modern astronomy in which
everything, even the cosmos itself, has a lifelife-history.
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William Herschel



Herschel was now in some confusion as to the nature of
the nebulae. The only nebula he had examined
repeatedly over the years, the Orion Nebula, had (he
believed) altered shape and was continuing to do so.
In January 1783 he noted that “the nebulous part is quite
different from what it was last year. The 9th star very
strong, the nebula about it and the 8th being much
dispersed”. There could therefore be no question of its
being a vast star system.
He was still dealing in appearances rather than
explaining the underlying physical realities, and from this
point of view the visible nebulae and clusters could be
graded into a continuous series, with a cluster of
individual stars at one extreme and a uniform
appearance of nebulosity at the other.
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William Herschel

This procedure made Herschel very experienced in the distribution
distribution
of nebulae across the sky, and this was to have important
implications for his theories of the nature of nebulae. For example,
example,
on 30 December 1783 he commented:

“It appeared to me remarkable that in and about
the place where the many Nebulas began there
was an uncommon scarcity of stars so that many
fields were totally without a single star. If these
Nebulae should be clusters of stars it should
seem as if they were collected together from the
neighbouring spaces”.
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William Herschel

Herschel was beginning to view the Milky Way as the
optical effect of the solar system’s membership of a
(compound) layer or ‘stratum’ of stars, the stars
having been drawn together from a more primitive
and scattered distribution of stars by the action of
gravity or other attractive force.

Herschel’s 1784 paper on the construction of the
heavens was read to the Royal Society on 17 June.
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William Herschel
The feature that Herschel was
provisionally using to distinguish the Orion
Nebula and other true nebulae from star
systems disguised by distance:
 A true nebula appeared milky, while a
distant star system betrayed its nature by
displaying a ‘resolvable’ appearance.

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Herschel’s First Cosmological
Synthesis



Just five days after his paper was read to the Royal Society,
Herschel came across M17, or Omega Nebula. Seen through
his telescope, it appeared to contain both the milky nebulosity
that was supposedly characteristic of a true nebula and the
resolvable nebulosity of the distant star cluster.
He decided that if the appearance of the nebula was
confirmed, he would have to reverse his earlier judgment that
true nebulosity existed, and instead see resolvable nebulosity
as characteristic of star systems in the middle distance, and
milky nebulosity as characteristic of systems in the far
distance.
The Omega Nebula would then be “a stupendous Stratum of
immensely distant fixed stars some of whose branches are
near enough to us to be visible as resolvable nebulosity, while
the rest runs on to so great a distance as only to appear
under the milky form”
form”.
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Herschel’s First Cosmological
Synthesis



Independent confirmation of the implications of the
Omega Nebula came a month later, when Herschel
examined M27, the Dumbbell Nebula.
But what of the changes that Herschel was so convinced
had taken place in the Orion Nebula, and which
conflicted with his new theory? His solution for the
dilemma they now posed was simple: total silence.
In 1785 Herschel’s universe had a simplicity that he was
quick to exploit. Since all nebulae were star clusters,
nebulae such as those of Andromeda and Orion, which
were so very distant as to appear milky and yet despite
this distance extended across the sky, must be vast
indeed. This being so, these systems might well be
independent galaxies in their own right.
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Herschel’s First Cosmological
Synthesis

Furthermore, clustering implied that gravity or other attractive forces
were at work. Herschel now made attraction the foundation of a
complete cosmogony. In words that must have astonished his
readers, he declared his intention of investigating the universe “from
a point of view at a considerable distance both of space and of
time”
time”.

“Let us then suppose numberless stars of various sizes, scattered
over an indefinite portion of space in such a manner as to be almost
almost
equally distributed throughout the whole. The laws of attraction,
attraction,
which no doubt extend to the remotest regions of the fixed stars,
stars, will
operate in such a manner as to produce the following remarkable
effects”
effects”.
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Herschel’s First Cosmological
Synthesis


Where there happened to be a single star larger than the
rest, this star (he argued) might pull those about it so as
to generate a globular cluster. Where a number of stars
were unusually close together, they too might pull in
surrounding stars and so generate an irregular cluster.
In practice, both these effects would occur in all manner
of combinations.
The reader was not to be alarmed about the possibility of
a cosmic gravitational collapse, for the sidereal heavens
extended indefinitely; but in individual clusters a star
might now and then be destroyed, “as perhaps the very
means by which the whole is preserved and renewed.
These clusters may be the Laboratories of the universe,
if I may so express myself, where the most salutary
remedies for the decay of the whole are prepared”.
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Herschel’s First Cosmological
Synthesis

But how did the planetary nebulae fit into
this scheme? Were they really true
nebulae? In his 1785 paper Herschel is
frank about the mystery surrounding these
objects, “that from their singular
appearance leave me almost in doubt
where to class them”:
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Herschel’s First Cosmological
Synthesis

“The planetary appearance of the first two [of his list] is so
remarkable, that we can hardly suppose them to be nebulae; their
light is so uniform, as well as vivid, the diameters so small and
and well
defined, as to make it almost improbable that they should belong to
that species of bodies”
bodies”.
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Herschel’s First Cosmological
Synthesis

“If we would suppose them to be single stars with large diameters
we shall find it difficult to account for their not being brighter
brighter ...; so
that after all, we can hardly find any hypothesis so probable as that
of their being Nebulae; but then they must consist of stars that are
compressed and accumulated in the highest degree. If it were not
too hazardous to pursue a former surmise of a renewal in what I
figuratively called the Laboratories of the universe, the stars forming
these extraordinary nebulae, by some decay or waste of nature,
being no longer fit for their former purposes, and having their
projectile forces, if any such they had, retarded in each others’
others’
atmosphere, may rush at last together, and either in succession, or
by one general tremendous shock, unite into a new body.”
body.”
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Herschel’s First Cosmological
Synthesis



The explosion of Tycho’
Tycho’s nova of 1572, he suggests, might have
come about in this way.
In his third great cosmology paper of the 1780s, Herschel reflects
reflects
further on this cosmogony.
He hints at how gravity might combine with repulsive
forces to bring each star cluster eventually into
globular form, the cluster finally becoming a planetary
nebula which “may be looked upon as very aged, and
drawing on towards a period of change, or dissolution”;
and he closes this period of his career with a ringing
declaration of how his method of placing each cluster at
the appropriate stage in its life-cycle will give to mere
humans an insight into the workings of the cosmogonical
processes.
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Herschel’s Investigation of the
Milky Way

It is doubtful whether Herschel, when he first turned his
mind to the problem of explaining the Milky Way, knew
anything of the mid-eighteenth-century speculations on
the Milky Way proposed by Wright, Kant or Lambert (all
of whom in different ways saw the milky effect as
resulting from our immersion in a layer of stars).
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Herschel’s Investigation of the
Milky Way

On the other hand, there is no reason why Herschel
should not have independently perceived that the
optical effect we term the Milky Way may be due to
our immersion in a layer or ‘stratum’ of stars.
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Herschel’s Investigation of the
Milky Way



In his papers on the construction of the heavens,
Herschel realised that to achieve his goal two
assumptions were inevitable:
The first assumption, naturally, had to be that his
most powerful telescope, the ‘large’ 20-ft reflector,
could reach to the borders of the stratum in every
direction.
The second assumption would allow the observer to
convert the visual information reaching him through
his telescope when pointed in a particular direction,
into knowledge of how far the stratum of stars
extended in the direction.
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Herschel’s Investigation of the
Milky Way

Herschel set out to determine the shape of the Galaxy,
which he declared was “A very extensive, branching,
compound Congeries of many millions of stars”. But
counting stars took time, which he could ill afford to
spare from his long-term campaign of sweeping the sky
for nebulae. He therefore had to content himself with
giving a full explanation of his procedure and
implementing it around a great circle of the sky. The
result was his famous sketch of a cross-section of the
Galaxy.
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The crosscross-section of the Milky Way, from Herschel’
Herschel’s 1785 paper on the
construction of the heavens
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Herschel’s Investigation of the
Milky Way



In later years Herschel found himself forced to abandon both of the
assumptions on which his crosscross-section was based. The completion
of his monster 4040-ft reflector in 1789 brought many more stars into
view and proved that the 2020-ft had not in fact succeeded in reaching
out to the borders of the Galaxy in every direction. Nor was there
there
any good reason to argue that the 4040-ft was succeeding where the
2020-ft had failed: the Galaxy, he conceded, was in some directions
“fathomless”
fathomless”.
As the number of star clusters in his catalogues grew year by year,
year,
so he became increasingly conscious of how far from uniform is the
the
distribution of the stars, until at last he had to admit that a large star
count was more likely to be a sign of clustering than of greater
distance to the border of the Galaxy.
His diagram of the crosscross-section of the Galaxy had therefore to be
abandoned. But popular science, like nature, abhors a vacuum, and
and
Herschel’
Herschel’s sketch was being reproduced in print well into the second
half of the nineteenth century.
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Herschel’s Second Cosmological
Synthesis



Herschel’
Herschel’s cosmogony of the later 1780s had provided him with a
theory of nebulae that was satisfactory except in two respects: first,
the soso-called planetary nebulae remained a puzzle; and second, he
had had to disregard the changes he himself had (he believed)
observed in the Orion Nebula.
But on 13 November 1790 he came across “a most singular
phenomenon! A star of about the 8th magnitude, with a faint
luminous atmosphere...”
atmosphere...”. It was in fact the object known to modern
astronomy as the planetary nebula NGC 1514, which is near enough
to us for its diameter to be measured in minutes of arc rather than
than
seconds, and has a bright central star that Herschel could clearly
clearly
see.
Its appearance was therefore quite different from the tiny, seemingly
seemingly
uniform disks of the objects for which he had invented the name of
planetary nebulae.
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The planetary nebula NGC 1514, which Herschel encountered on 13
November 1790 and which convinced him of the existence of ‘true’
true’
nebulosity. He interpreted it as a star “with a faint luminous atmosphere”
atmosphere”
out of which the star was condensing. Photograph courtesy of Martin
Martin C.
Germano.
Germano.
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Herschel’s Second Cosmological
Synthesis

This, he decided, was a star with a luminous halo, a ‘nebulous star’
star’ in the
categories of his later classification.

Herschel’
Herschel’s misgivings about his current cosmogony had by now grown so
strong that he accepted the implications of this new evidence: the
the ‘nebulous
star’
star’ must be a single, partiallypartially-formed star condensing out of surrounding
nebulosity through the action of gravity; and this nebulosity was
was therefore
truly nebulous, and not merely a star cluster disguised by its great
great distance.

However, this development did nothing to undermine the fundamental
role of gravity as the agent of change in his universe:
universe: he simply
extended its operation back in time to embrace the newlynewly-discovered
embryonic stage in the life history of stellar systems, a stage before the
formation of discrete stars out of true nebulosity. In this his final cosmogony,
which he was still expounding in Philosophical transactions as late as 1818,
the universe was the setting in which light as a physical body cycled
cycled
eternally through a sequence of forms under the influence of gravitational
gravitational
attraction.
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Herschel’s Second Cosmological
Synthesis

The sequence began with thinly scattered light, which collected here
and there under the action of gravity to form diffuse clouds of
nebulosity; these then condensed further and would usually split into
a number of smaller clouds. Further condensation led to stars being
being
born out of the nebulosity, and these stars then gathered into
scattered systems, which gravity condensed further and split into
into
smaller and more compact systems, culminating in globular clusters,
clusters,
where “the exertion of a clustering power has brought the
accumulation and artificial construction of these wonderful celestial
celestial
objects to the highest degree of mysterious perfection”
perfection”.

In various ways — through the steady shining of starlight, and
perhaps through cataclysms at the end of the evolutionary sequence
sequence
— light was diffused back into distant regions, to start the whole
whole
sequence over again.
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Herschel’s Second Cosmological
Synthesis



His methodology was set out with simple clarity in lengthy papers
papers
published in 1811 and 1814, where he drew on his catalogues of
nebulae and clusters for specimens at various stages of
development, and devoted an ‘article’
article’ or section to each stage,
saying:
“... it will be found that those [celestial objects] contained in one
article, are so closely allied to those in the next, that there is perhaps
not so much difference between them, if I may use the comparison,
comparison,
as there would be in an annual description of the human figure, were
it given from the birth of a child till he comes to be a man in his
prime”
prime”.
He also applied this concept to our Galaxy, which he saw as in
process of fragmenting under the action of gravity. The clusters of
stars in the Galaxy, he explained in 1814, would become ever more
more
concentrated.
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Herschel’s Second Cosmological
Synthesis

“Till they come up to what may be called the ripening
period of the globular form, and total insulation; from
which it is evident that the milky way must be finally
broken up, and cease to be a stratum of scattered stars.
We may also draw a very important additional conclusion
from the gradual dissolution of the milky way; for the
state into which the incessant action of the clustering
power has brought it at present, is a kind of chronometer
that may be used to measure the time of its past and
future existence; and although we do not know the rate
of going of this mysterious chronometer, it is
nevertheless certain, that since the breaking up of the
parts of the milky way affords a proof that it cannot last
for ever, it equally bears witness that its past duration
cannot be admitted to be infinite”.
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Herschel’s Second Cosmological
Synthesis

There was a price to be paid for the success of
his second cosmogony:

Herschel had no way of telling whether a milkylooking nebula was a nearby cloud of true
nebulosity or a distant and vast star system. The
Orion Nebula, because it altered shape, must be
the former; but there was no means of
identifying which of the unchanging nebulae
were the latter.
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Herschel’s Second Cosmological
Synthesis

In 1813 the poet Thomas Campbell had a conversation
with Herschel in which the astronomer told him that “if
those distant bodies had ceased to exist millions of years
ago, we should still see them, as the light did travel after
the body was gone”. “I really felt at the moment”, wrote
Campbell to a friend, “as if I had been conversing with a
supernatural intelligence.”
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Herschel’s Second Cosmological
Synthesis



Herschel’
Herschel’s speculations, based on evidence to which he alone had
access and guided by methodologies as novel as the questions he
posed, found little support among serious astronomers.
He had been at the centre of controversy from his first entry onto
onto the
London scientific scene, when he knew neither how to define the
position of a star nor how to organize a scientific paper, and when
when
his claims seemed so preposterous that some declared him fit for
the insane asylum of Bedlam.
But his achievements as a telescope builder and as an observer
were indisputable — he was, after all, the first man since the dawn
of history to discover a planet. Yet his standing was such that he
was given virtually free access to the pages of Philosophical
transactions,
transactions, and this ensured that his ideas were available
worldwide — to contemporaries and, more importantly, to future
generations, who would not find his hypotheses so farfar-fetched nor
his methodology so outrageous.
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Nebulae, Star Clusters and the
Milky Way:
From Galileo to William Herschel
Thank you
Michael Hoskin
michael.hoskin@ntlworld.com
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