The discovery of galaxies is a great example of the process of the scientific method.
It's NOT a typical example because it stretches over centuries.
It begins with the report in the historical record of nebulae (historical sense) in the 2nd century CE, continues with the theory that at least some the nebulae (historical sense) were other "Milky Ways" (i.e., other galaxies) in the 17th century, and ends in one sense in 1924 with the discovery that we live in universe of galaxies.
In another sense, the story goes on.
Because science studies objective things there is an
absolute gold standard---the objective things
themselves---against which theories in
science can tested.
This permits the
scientific method---which is
illustrated schematically
in the figure below
(local link /
general link: sci_method.html).
The Ptolemy (c.100--c.170 CE)
or some earlier ancient Greek astronomer
or some Babylonian astronomer
discovered there were
cloudy "stars" which came to be called
nebulae (historical usage)
(see figure below
(local link /
general link: ptolemy_armillary.html).
There were only 6 (or 7 by a broader definition)
nebulae (historical usage) in the
Ptolemaic catalogue of 1022 stars.
But what was the theory for
nebulae (historical usage)?
Time would tell. A very long time.
More
nebulae (historical usage)
were gradually
discovered as the centuries
rolled by.
One of these Ptolemy
had surprisingly missed since it is one of the most obvious of the
nebulae (historical usage):
the Andromeda nebula (now known as the
Andromeda Galaxy (M31, NGC 224)).
For the Andromeda Galaxy (M31, NGC 224),
see the figure below
(local link /
general link: galaxy_andromeda_m31.html).
Medieval Islamic astronomer
al-Sufi (903--986)
in his
Book of the Fixed Stars (circa 964)
is the first to put the
Andromeda nebula (now known as the
Andromeda Galaxy (M31, NGC 224))
in the historical record as a
nebula (historical usage)
(see No-188,402;
Wikipedia: Andromeda Galaxy:
Observation history;
Wikipedia:
Book of the Fixed Stars: Influence).
Simon Marius (1573--1625)
independently and telescopically discovered
Andromeda nebula
(i.e., the
Andromeda Galaxy (M31,NGC 224))
on 1612
Dec15
(Wikipedia: Simon Marius: Discoveries;
Wikipedia: Andromeda Galaxy:
Observation history;
SEDS:
Simon Marius (January 20, 1573 - December 26, 1624);
No-402).
His discovery was, of course,
for a long time the effective
discovery
relative to Europe.
For more on Simon Marius (1573--1625),
see
Astronomer file:
simon_marius.html.
As Julius Caesar (100--44 BCE) would say,
the answer to the question is divided into three parts:
To the naked eye
the Milky Way appears
just a band of milkiness on the sky as it's name suggests---the milky road.
And throughout most of history
that is all people knew.
However, the speculative theory that the
Milky Way was
a mass of stars
unresolved to the naked eye
goes back at least to the Presocratic philosopher
Democritus (c.460--c.370 BCE)
(No-401).
As with many ancient theories,
we only know the first person to have put this
theory in the
historical record.
Others, in many cases many others, may have had the
theory earlier,
but they did NOT succeed in putting in the
historical record.
This usually goes without saying.
Democritus,
however, thought of the stars as
pasted on a celestial sphere of the stars
that enclosed the Earth:
a physically real
celestial sphere
(Fu-136--146, esp. 140).
The theory that there was a
real celestial sphere of the stars
confining the Solar System
was held by most
astronomers in
western Eurasia from
the time of
Presocratic philosophers (c.600--c.350 BCE)
(notably
Democritus (c.460--c.370 BCE))
to circa 1600.
However, with the advent of heliocentrism
theory introduced by
Nicolaus Copernicus (1473--1543),
the situation changed dramatically.
A particularly big change in the situation was perhaps first introduced by
Thomas Digges (1546--1595) in
England who
theorized that
space was
infinite (or quasi-infinite) with stars
spread throughout in
1576.
See the figure below
(local link /
general link: copernican_cosmos_digges.html)
of
Digges' illustration of
his theory
for the universe.
Note Copernicus himself
continued to hold the
theory of a
real celestial sphere of the stars.
Very simply that they were
in 3-dimensional physical space like
the stars.
But were they closer to, mixed with, or farther than the
stars?
The situation changed again with
invention
of the telescope
in 1608
(Wikipedia:
History of the telescope: Invention).
The first observation of
Galileo Galilei (1564--1642)
solved what some of the
nebulae (historical usage)
were.
But did it solve what they all were?
The second observation led eventually to
theorizing
(though NOT apparently to
Galileo and his
contemporaries
Thus the theory
of the Milky Way
as a galaxy in our modern sense eventually evolved
(see below the subsection
Early Speculations on the Nature of the Nebulae).
Could some of the
nebulae (historical usage)
be there outside of the Milky Way?
After the invention
of the telescope
in 1608 and its
nearly immediate utilization in astronomy,
more nebulae (historical usage)
were gradually discovered, but even by
1780
they still numbered only ∼ 90
(No-403).
The first person in the historical record
to speculate that some
nebulae (historical usage)
were other galaxies outside
of the Milky Way is none other than
Christopher Wren (1632--1723).
See the figure below
(local link /
general link: christopher_wren.html).
But other theorists did eventually discuss the
theory that some of the
nebulae (historical usage)
were other galaxies with historical impact.
For these theorists,
see the figure below
(local link /
general link: immanuel_kant.html).
The first person to take a strong observational interest in
nebulae (historical usage)
was
Charles Messier (1730--1817)
who was the observing assistant at the
Hotel de Cluny Observatory
(AKA Marine Observatory) in
Paris
(No-403--404).
Messier
made a determined effort to discover all the brightest
nebulae (historical usage)
(No-403--404)
for the reasons explicated in the figure below
(local link /
general link: charles_messier.html).
Messier's
aim in making his catalog was to be able avoid mistaking
nebulae (historical usage) for
comets.
William Herschel (1738--1822),
the greatest observational astronomer of the
18th century,
was very interested in
nebulae (historical usage)
(and NOT just the bright ones)
and made a systematic search for them with
large reflector telescopes
he built himself.
They were the largest
reflector telescopes
of his time and gave him the greatest
light-gathering power
of this time,
enabling him to find faint objects no one else could.
By the end of his career,
Herschel
(aided by his sister
Caroline Herschel (1750--1848))
had discovered over 2400
nebulae (historical usage)
(Wikipedia: William Herschel:
Deep sky surveys;
Wikipedia:
William Herschel: Works with his sister Caroline Herschel).
Many of these
nebulae (historical usage)
were resolvable into star clusters
that he located as probably inside the
Milky Way which is correct
(No-407--409).
Others he
was able to determine were
single stars surrounded by
a cloud of luminous gas
which we now call planetary nebulae
which have nothing to do with with
planets, except that some look disk-like
in small telescopes like
planets, hence their name
(No-407--408,436;
Wikipedia: Planetary nebula).
Herschel was aware
of the theory
that some
nebulae (historical usage)
were other "Milk Ways"
(i.e., other galaxies)???, but
yours truly CANNOT find any reference saying what
he thought of this theory.
It may be that Herschel---who
was a careful follower of the
scientific method---simply suspended judgment.
But one thing Herschel did know was
that when he started using his
reflector telescope
of greatest
light-gathering power,
he found more stars than he had before
(No-408).
This probably caused him to wonder if there were any edge to the
Milky Way: maybe it extended forever.
However, he continued to favor the theory
that the Milky Way was caused by a
condensation under self-gravity
(which was the theory of
Immanuel Kant (1724--1804) and
Johann Heinrich Lambert (1728--1777):
see the figure above:
local link /
general link: immanuel_kant.html),
and therefore that there was an outside to the
Milky Way
(No-408).
Of course, many of the
nebulae (historical usage)
he (and his sister) discovered were
other galaxies
and eventually were catalogued as such in the
New General Catalogue (NGC, 1888)
where they are known by their
NGC numbers: e.g.,
NGC 4258 (M106), one of the key
anchors of the
cosmic distance ladder
(e.g., Riess et al 2024).
The next great discovery
in the study of the
nebulae (historical usage)
was that some
had a spiral structure: they were the
spiral nebulae
which we now call
spiral galaxies.
This discovery was made by
Lord Rosse (1800-1867)
using the largest
reflector telescope ever built
to 1845,
the Leviathan of Parsonstown
(reflector, primary diameter 1.83 m = 6 ft, operational 1845--c.1890)
(see figure below
(local link /
general link: telescope_leviathan.html).
Smaller telescopes did NOT have the:
The closest
spiral galaxies
are quite big objects on the sky.
They do NOT need much
magnification or
angular resolution
in order to be just seen.
For example, Andromeda Galaxy (M31,NGC 224)
has a disk angular diameter
of ∼ 3° on the
sky
(Cox-578).
But it and the other
close spiral galaxies
look like cloudy stars.
The Spiral arms are just
rather faint and are invisible to
visual astronomy
with the naked eye or
small telescopes.
Often all you can see is the bright central
galactic bulge.
This is why small-telescope viewing of
galaxies
is often a little disappointing.
What you see does NOT look like
the images which result mainly from
long exposure photography
or, modern times, high-sensitivity imaging.
You need a lot of
light-gathering power
to see the structure of
spiral galaxies
in visual astronomy.
With better reflective coating on your mirror than
speculum
(used by Leviathan),
you would NOT need a
Leviathan,
but probably still a telescope mirror
of order 1
meter.
Note astrophotography
using long exposures
can image
spiral arms using
smaller telescopes than
the Leviathan.
One of Lord Rosse's
sketches of a
spiral nebula
is shown in the figure below
(local link /
general link: galaxy_whirlpool_lord_rosse.html).
In the time from
Lord Rosse (1800-1867)
to the
1920, considerably
more data
and theory were developed about the nature of
nebulae (historical usage).
Th data
included astrophotography and
spectroscopy.
In fact, by 1920, it was well known
that many of the
nebulae (historical usage)
were star clusters,
planetary nebulae,
and other kinds of clouds inside the
Milky Way.
But what were the
spiral nebulae
and also the elliptical nebulae
(which were found grouped with
the spiral nebulae,
e.g., in the Virgo Cluster)?
Of the spiral nebulae,
some thought they
were whirlwinds of
gas and/or
cosmic dust inside
the Milky Way
and the elliptical nebulae
were something else in he Milky Way.
The arguments accumulated for both theories
(No-493--497).
But without a poll from the
past, it's hard
to know which was the majority view among
astronomers---most other folks probably had
NO opinion at all.
We will NOT here rehearse the arguments about the
nature of spiral nebulae
and elliptical nebulae.
However, on 1920
Apr26, there was a formal
debate between
Harlow Shapley (1885--1972)
of Mount Wilson Observatory
and Heber Curtis (1872--1942) of
Lick Observatory entitled
The Scale of the Universe which turned essentially
on whether the
spiral nebulae
and elliptical nebulae
were other galaxies
(see M. A. Hoskin, 1976,
Journal for the History of Astronomy,
"The Great Debate: What Really Happened", Vol. 7, p. 169;
Wikipedia:
The Great Debate (AKA the Shapley-Curtis Debate, 1920, Apr26)).
This debate has gone down in history as
The Great Debate
(AKA the Shapley-Curtis Debate, 1920, Apr26).
Shapley
was con
on the subject of some
nebulae (historical usage)
being other galaxies;
Curtis was
pro.
On the night of, Curtis
was probably the winner in a formal
debate sense---he thought so himself:
But the fact was that more evidence was needed to determine the nature
of the spiral nebulae.
That evidence would be provided by
Edwin Hubble (1889--1953)
(see below subsection Edwin Hubble (1889--1953)).
The great discoveries
about the nature of the
spiral nebulae
and the elliptical nebulae that
Edwin Hubble (1889--1953)
was to make in the 1920s at
Mount Wilson Observatory
in Los Angeles County, California
(see also the Mount Wilson Observatory page)
were predicated on the facts that
Mount Wilson Observatory
in those days
(the 1920s) was one of the best observing sites in the world---this
was before the
air pollution
(e.g., smog)
and the light pollution of
Los Angeles
mostly ruined things---AND on having the largest
telescope to
date at his disposal: the
100-inch Hooker telescope
(i.e., 2.54-meter Mount Wilson
reflector telescope):
see figure below
(local link /
general link: telescope_hooker.html).
In the early 1920s
using the 100-inch Hooker telescope,
Hubble was able to resolve
stars
in the outer regions of the
Andromeda nebula
(which is now known as the Andromeda Galaxy (M31, NGC 224))
and the Triangulum nebula
(which is now known as the
Triangulum Galaxy (M33, NGC 598))
by 1923
(No-509).
These galaxies
are in Local Group of Galaxies
which is shown in the figure below
(local link /
general link: local_group.html).
The fact that Hubble could resolve
stars in the
Andromeda nebula
and the Triangulum nebula,
just in itself, proved that
spiral nebulae
were NOT just whirlpools
of gas and/or
cosmic dust
in space---though they
could still have some gas
and cosmic dust, of course---as
indeed they do.
By 1924,
using the approximately known luminosities of
Cepheids
and
period-luminosity
relation,
Hubble was able to put the
Andromeda nebula
at 285 kpc (No-510)
well beyond the confines of the
Milky Way
(size scale ∼100 kpc)
as established by
Harlow Shapley (1885--1972)
in 1916
by determining the distances
to Milky Way
globular clusters
also using Cepheids
(see Wikipedia:
Edwin Hubble: Universe goes beyond the Milky Way galaxy;
No-493,510).
And if one spiral nebula
was another galaxy, all
spiral nebulae
were other galaxies.
Also all the elliptical nebulae
were elliptical galaxies since
they were found grouped with
the spiral nebulae
(e.g., in the Virgo Cluster).
Actually,
Hubble's
distance to the Andromeda Galaxy (M31, NGC 224)
was NOT very accurate
due to systematic errors
in his measurements and calibrations that are entirely understandable given his time.
The modern distance to the
Andromeda Galaxy (M31, NGC 224)
is 765 kpc (Wikipedia: Andromeda Galaxy):
this ∼ 2.7 times Hubble's value.
But even if
Hubble's contemporaries
suspected large errors---and they may have---they did concede fairly soon??? that
the Andromeda Galaxy (M31, NGC 224)
had to be another galaxy: a
remote large system of stars comparable
in size to Milky Way.
So the scientific method
in a very long time cycle of observation and
theory
had finally found out what the
nebulae (historical usage) were:
some were gas clouds in
space, some were
star clusters, and some
were galaxies.
Hubble along with collaborators
continued to work on extragalactic distance measurements for
the rest of his life.
By 1929, he had distances to
46 galaxies
beyond the Milky Way
including 4 in the
Virgo Cluster
(a nearby large galaxy cluster)
(Wikipedia:
Edwin Hubble: Redshift increases with distance;
Hubble 1929;
No-510, but this reference seems to have some errors).
But only 24 of these distances were for independent and could be used in his analysis???.
Note that Hubble could only get
Cepheid distances
to the Andromeda Galaxy (M31)
and the
Triangulum Galaxy (AKA M33) ???
(No-510).
That is about as far as he could observe Cepheids.
For more distant galaxies, he had to
use less reliable distance indicators from his early version of the
cosmic distance ladder.
How could it be anything else?
Well, actually, it extends a bit into
constellation
Coma Berenices.
See the sky map
of Constellation
Virgo
see the figure below
(local link /
general link: iau_virgo.html).
He entitled
his famous book
The Realm of the Nebulae (Edwin Hubble, 1936,
Google Books, partially online)
(see also The Realm of the Nebulae (Edwin Hubble, 1936,
NASA/ADS);
No-509).
"The realm of the nebulae", that vast realm which we inhabit---and though some suspected
as much, we never knew until 1924
(Wikipedia:
Edwin Hubble: Universe goes beyond the Milky Way galaxy;
No-510).
For more on Edwin Hubble (1889--1953),
see Edwin Hubble (1889--1953) videos below
(local link /
general link: edwin_hubble_videos.html).
The summary of the scientific method
applied to galaxies:
Having discovered galaxies,
we need:
But what does the theory of
the quasi-infinite universe
imply for the
nebulae (historical usage)?
Galileo (1564--1642)
(see the figure below
local link /
general link: galileo_ottavio_leoni.html)
was foremost of the early telescopic observers
and he made two important
discoveries relative to the
nebulae (historical usage)
reported in his
bestseller
Sidereus Nuncius (1610):
Of course, the Milky Way was always
and still is the Galaxy definitionally.
In ancient Greek,
Galaxias Kyklos
(milky circle) and in Latin,
Via Lactea (milky way).
But if there was an inside to the
Milky Way,
maybe there was an outside and the outside was NOT
just empty space.
Question:
Some
nebulae (historical usage)
are:
All answers are right in historical use of the term
nebulae (historical usage)
Otherwise, we nowadays only use nebulae
for answer 1.
Christopher Wren's
theory
of other galaxies
had NO impact on the historical evolution
of astronomy
and was first noticed it seems in
1967 by anyone other than
Christopher Wren himself and a few
contemporaries
(Robert I McLachlan 2019, arXiv:1909.02167, p. 6).
But the theory is a mark of his
genius.
An example of a Messier object is in
the figure below
(local link /
general link: globular_cluster.html).
As aforesaid in the figure above
(local link /
general link: charles_messier.html),
Messier
was actually interested in
comets and he discovered
14 of them
(Wikipedia: Charles Messier: Biography).
See the beautiful comet shot
in the figure below
(local link /
general link: comet_lovejoy.html).
Faint comets and
nebulae (historical usage) look much
alike---they both look like fuzzy little clouds.
Question:
Nebulae (historical usage)
can be
distinguished from comets by:
Answer 1 is right.
Question: Why did it take the
Leviathan of Parsonstown
(reflector, primary diameter 1.83 m = 6 ft, operational 1845--c.1890)
to discover the
spiral nebulae?
We mean discover that there are
nebulae (historical usage)
with spiral arms
since
many spiral nebulae were known before their
spiral structure was known.
In a battle of techologies, the
Leviathan's
size allowed visual astronomy
to beat out
astrophotography (advent 1840,
but with only gradually improving technique in following decades)
(Wikipedia: Astrophotography: History)
to the discovery
of the spiral nebulae.
Answer 3 is right.
Note Photography
developed in a series of steps starting from about
1816
(No-442)
and was still only being applied to bright astronomical objects
in the 1840s: i.e.,
Sun
and Moon
(No-443).
However, astrophotography did win out
eventually and hundreds???? of
spiral nebulae were
photographed by circa 1920
(No-437).
In fact, as we know in the age of
ALMA (2011--),
planet formation
in protoplanetary disks
can give rise to structures that look like
spiral galaxies, but
that has only been known since circa 2011
(see Planetary systems file:
protoplanetary_disks_alma.html).
But others thought that
spiral nebulae
and elliptical nebulae
were other galaxies just as had been
theorized for
nebulae (historical usage)
since the 17th century and
18th century.
Debate went off fine in Washington,
and I have been assured that
I came out considerably in front.
In subsequent papers, however, the two gladiators
(see figure below
local link /
general link: gladiator.html)
came off with more equal honors in a fairly sophisticated review of the evidence
(No-497).
Note Edwin Hubble (1889--1953)
had been interested in the
spiral nebulae
and the elliptical nebulae
since his Ph.D. work
and had developed
Hubble sequence
of classification for them by 1923
by which time he was working at
Mount Wilson Observatory
in California
(No-508--509).
In the Andromeda nebula,
Hubble found that
34
stars
were Cepheids
by identifying their known
period-luminosity
relation
(No-510).
Cepheids
are very bright post-main-sequence
variable stars
whose luminosities
are empirically calibrated
and whose luminosity
variation with time is well fitted by a
period-luminosity
relation.
A measurement of a Cepheid's
variation period can then be used to determine its
luminosity using
the period-luminosity
relation.
Then using the measured radiant flux (AKA flux)
for the Cepheid and its
luminosity, you can
determine its distance.
For more on Cepheids, see
Star file:
star_hr_cepheids.html.
The conclusion from Hubble's
distance determination was the Andromeda nebula
was the Andromeda Galaxy (M31, NGC 224),
a system
of stars comparable in size to the
Milky Way and well outside of it.
Question:
The Virgo Cluster
is in:
Answer 3 is right.
Hubble remained a bit
old-fashioned in that he continued to refer to
galaxies as
nebulae which
we now NO longer do, except when speaking historically.
EOF