Sections
But the Milky Way as a vast system of stars and other galaxies as some sort of common astro-bodies and then as other "Milky Ways" had to be discovered.
In IAL 26: The Discovery of Galaxies, we give the story of that discovery after brief introductions to relationship of distance and lookback time, the local universe concept, galaxies in general, and the Milky Way (which is OUR galaxy or the Galaxy).
The figure above (local link / general link: galaxy_hcg_87.html) and figure below (local link / general link: galaxy_hcg_87.html) of a Hickson Compact Group of galaxies is a preview for galaxies in general.
php require("/home/jeffery/public_html/astro/galaxies/galaxy_hcg_87_2.html");?>
Recall that the observable universe and the local observable universe are spheres surrounding us---see the figure below (local link / general link: cosmos_artist_conception.html).
An aspect of cosmology is where and when of astronomical objects in cosmological spacetime: e.g., the where and when of galaxies, quasars, and the source of the cosmic microwave background (CMB).
First, we note that electromagnetic radiation (EMR) has a finite speed: the vacuum light speed---which is 2.99792458*10**8 m/s exactly by the definition of the meter---approximately it's 3*10**8 m/s.
So when we look out into space, we always look back in time and see things as the were in the past.
So the amount of time in the past for an observed astronomical object is a function of distance from us.
This amount of time in the past is called lookback time.
For relatively nearby objects,
lookback time
is a very simple function of distance.
Divide distance in light-years by
vacuum light speed
in light-years per year
(which is 1 ly/yr) and that is the
lookback time:
If an object is about 100 ly away at this instant in
cosmic time,
the object is observed
as it was 100 years ago usually in place NOT far from where it is today.
So everything is simple and
lookback time
is hardly worth mentioning.
For an example of nearby objects, let's consider those
within the Milky Way.
The
Milky Way galactic disk
is ∼ 30 kpc in diameter (i.e., 100 kly, where a
kilo-light-years (kly)
is 1000 light-years).
So lookback times
for inside the
Milky Way
are at most of order 10**5 years.
The time scale of the example is long in terms of
human history,
but in
cosmic time
it is like an instant.
On average, little cosmic evolution occurs on this time scale
and most objects barely move relatively speaking.
To illustrate this point, note that
the fastest Milky Way relative
motions are only of order 200 km/s
(see Milky Way: Galactic Rotation)
which is much less than the
vacuum light speed.
In 10**5 years at 200 km/s, an object moves
which is tiny compared to 100 kly.
So relative motions in the
Milky Way
during lookback times
are negligible for most purposes.
To reiterate, everything is simple for nearby
astronomical objects and
lookback time
is hardly worth mentioning.
When the
mass media
or some smart aleck
makes an issue of an astronomical event as actually happening
100,000 years ago or a million years ago
in cosmic time,
it is like making an issue
of something having happened a second ago or a minute ago in
everyday life time.
Astronomers speak of such astronomical events as if they have just happened---they
have just happened.
As you look progressively farther out,
lookback times
become progressively bigger, and so more important since the
universe has evolved more
since the light signals started out.
In particular, because of the
universal expansion
(which we take up
in IAL 30: Cosmology),
the objects farther out at this instant in
cosmic time
have moved significantly since light signals started out that
are reaching us now.
The objects were closer when those light signals started out.
The universal expansion
is illustrated in multiple ways in the figure below
(local link /
general link: expanding_universe.html).
Because of universal expansion,
the relationship between
distance at once instant in
cosmic time---which
is called cosmological physical distance
or just physical distance
for short---and lookback time
becomes complex.
For relatively short physical distances r
(e.g., r ∼ < 1 Gpc), the simple rule t_lookback = r/c is still approximately valid (e.g.,
for r = 1 Gpc, accurate to ∼ 10 %).
But still progressively fails as r increases and progressively improves as r decreases becoming
exactly accurate as r goes to zero.
In fact, the exact relationship between
physical distance
and lookback time
becomes dependent on the
cosmological model of the
universe one adopts.
In the NOT-so-distant past (e.g., before 1998
and the discovery of the
acceleration of the universe),
the question of the right cosmological model to adopt (and therefore
of the relationship
between
physical distance
and lookback time)
was rather open within
the paradigm
of the expanding universe.
Fortunately, now we have the
Λ-CDM model (AKA concordance model)
which at present passes all tests (except for one
tension to be discussed
in IAL 30: Cosmology)
and which describes the
observable universe
very well as far as it goes.
We will take up the
Λ-CDM model
in general and its limitations in IAL 30: Cosmology.
Here we are just interested in the fact that the
Λ-CDM model
allows us to make PRECISE specifications for
the expression observable universe
which are probably reasonably accurate even if the
Λ-CDM model needs revision
(which it probably does).
But as a preview, see the insert
Big Bang Cosmology and Λ-CDM Model Limitations
below
(local link /
general link: big_bang_cosmology_limitations.html):
t_lookback=r/c , where
c is the vacuum light speed
and line-of-sight distance.
The calculation is elementary:
100 kly divided the vacuum light speed
in light-years per years
=10**5 ly/(1 ly/yr)
=10**5 years.
Δr ≅ 200 km/s * 10**5 yr = 200 km/s * (1 ly/yr)/(3*10**5 km/s) * 10**5 yr
≅ 70 ly = 0.07 kly
php require("/home/jeffery/public_html/astro/cosmol/expanding_universe.html");?>
So for the most part, here we will only discuss galaxies as they are at present in cosmic time or nearly at present.
But remember whenever you look out, you also look back.
So the nearly present is also the nearby.
So we should a have specification for the expression local universe (meaning nearby or nearly present or modern universe).
We will need the local universe concept when we take up cosmology in IAL 30: Cosmology.
In fact, the expression local universe is often used vaguely or in a context-dependent sense. The reader/listener is supposed to know what how local the local universe is meant to be.
We will often use local universe is in a vague or context-dependent sense.
However, yours truly also has a favorite meaning for local universe which we introduce below in subsection Specifying the Local Universe Vaguely and Precisely.
But before this introduction we need to disgress on cosmological redshift z in subsection Cosmological Redshift z and the Λ-CDM model and cosmological distance measures in subsection The Λ-CDM Model and Cosmological Distance Measures.
Cosmological redshift z is a direct observable obtained from line spectra of galaxies and other extragalactic objects.
In general, cosmological physical distance and lookback time must be obtained from the cosmological redshift z and a cosmological model which at present is nearly always the Λ-CDM model.
Astronomers conventionally use cosmological redshift z to express cosmological distance in space and time from Earth at our instant in cosmic time.
The expansion of the universe causes space to grow under a light signal as it propagates to us from a cosmologically remote object. This growth causes the wavelength of the light signal to increase. The growth literally stretches the light signal, but it's NOT a forced stretching---the wavelength just grows as space grows.
The space growth is the cosmological redshift with its mathematical specification symbolized by z.
The cosmological redshift is further explicated in the figure below (local link / general link: cosmological_redshift.html).
As mentioned above, given a cosmological model,
cosmological redshift z
can be used to calculate
the distance to where the object is at the present instant in
cosmic time
(i.e.,
the physical distance)
and the
lookback time.
The Λ-CDM model
is the model of choice at present for the reasons given above
in section
Cosmology, Distance, and Lookback Time.
There are several
cosmological distance measures.
Here we only need to consider
cosmological redshift z
(the direct observable) and
the model-dependent
physical distance
and lookback time.
The figure below
(local link /
general link: cosmos_distance_z_10000.html)
shows
physical distance,
and lookback time,
and other
cosmological distance measures)
as functions of
cosmological redshift z
for the Λ-CDM model.
The expression local universe is often
used vaguely or in a context-dependent sense.
The reader/listener is supposed to know what how local
the local universe is meant to be.
That is the vague specification.
For the precise specification, we will say the
z ≤ z_specified local universe, where
z_specified is a specified value suitable for the occasion.
Yours truly considers the
z ≤ 0.5 local universe often very
useful for the following reasons:
The time period 5 Gyr may seem long to considered nearly present given that the
age of the observable universe = 13.797(23) Gyr (Planck 2018)
since the
Big Bang
according to Λ-CDM model
(see Λ-CDM model parameters).
However, the rate of cosmic evolution in many respects
(e.g., changing properties of stars
and galaxies)
slows down as
cosmic time passes since
the Big Bang
and by Big Bang about 9 Gyr
(which is lookback time about 5 Gyr)
the evolution is sufficiently slow that
5 Gyr ago does NOT seem to be too different from the present.
So crudely speaking we can regard
z ≤ 0.5 local universe defined as we have defined
as nearly present for many purposes.
So it is certainly convenient if we can also regard these objects
as typical of the nearly present phase of cosmic evolution.
And we can with our choice of the
z ≤ 0.5 local universe
with its lookback time
of about 5 Gyr
since
Sun and
Solar System
formed 4.6 Gyr ago
(see Wikipedia:
Formation and evolution of the Solar System: Timeline of Solar System evolution).
As z
increases from 0 (which is essentially the neighborhood of the
Milky Way
and Local Group of galaxies: see below),
the deviation between t_lookback and r/c grows.
However at
z = 0.5,
the Λ-CDM model
shows the
lookback time is 5 Gyr
and the physical distance
(at the present instant, NOT at the
lookback time epoch)
is about 6.2 Gly = 2 Mpc
(see the
cosmological distance measure
figure above local link /
general link: cosmos_distance_z_10000.html
and
Cosmic Calculator
with
Hubble constant
equal to 70 (km/s)/Mpc).
So out to z = 0.5
the deviation from the simple relationship has only grown to ∼ 20 %.
For crude purposes, this deviation can be neglected.
And the deviation gets smaller as z
decreases as is
illustrated in the
cosmological distance measure
plots at local link /
general link: cosmos_distance_z_10000.html,
where physical distance
is labeled "LOS comoving".
This means that with the vacuum light speed
is measured in Gly/Gyr and has value 1 Gly/Gyr, the
simple approximate relationship it t_lookback=r/(1 Gly/Gyr).
Thus, the plot shows deviations directly since insofar as the simple approximate relationship
holds, the curves for lookback time
and physical distance should be identical.
As the simple approximate relationship with increasing
z, the curves increasingly
deviate.
At z = 0.1,
the deviation between the two quantities is only a few percent and is negligible for
many purposes.
The approximation gets better as
z goes to zero
and becomes exact for z = 0.
The
naive Hubble distance
is illustrated in the
cosmological distance measure
plots at local link /
general link: cosmos_distance_z_10000.html,
where it is labeled "naive Hubble".
As one can see---with a some mental extrapolation for the first graph---the
naive Hubble distance
is NOT so bad an approximation out to
z = 0.5.
Beyond z = 0.5
deviation of the
naive Hubble distance
from the lookback time
and physical distance continues
to grow and, depending on one's purposes,
the naive Hubble distance
sooner or later becomes useless as an approximation
to
physical distance
and lookback time.
By choosing z = 0.5
as the boundary for the local universe, we
have guaranteed that the
naive Hubble distance
will NOT be a bad approximation for the
z ≤ 0.5 local universe.
To recapitulate,
z ≤ 0.5 local universe
corresponds to physical distance ∼< 6 Gly ≅ 2 Gpc
and lookback time ∼< 5 Gyr.
But after all that elaboration, remember
z ≤ 0.5 local universe
is just one choice for a specification for
the local universe.
php require("/home/jeffery/public_html/astro/cosmol/cosmological_redshift.html");?>
php require("/home/jeffery/public_html/astro/cosmol/cosmos_distance_z_10000.shtml");?>
So those are the reasons for
thinking the z ≤ 0.5 local universe
is a useful version of the
local universe.
The graphs measure lookback time
in gigayears and physical distance
in giga-light-years
with gigayears (Gyr) and giga-light-years (Gly)
scaled to be the same size.
At z = 0.3,
lookback time is ∼ 3.3 Gyr
and the physical distance
is ∼ 3.9 Gly = 1.3 Mpc
(see
Cosmic Calculator
with
Hubble constant
equal
to 70 (km/s)/Mpc).
This is only about a 13 % deviation.
Cimatti (2022, p. 28)
choose z ≤ 0.1 corresponding to
physical distance ∼< 1.2 Glr &cong 0.4 Gpc
and lookback time ∼< 1.5 Gyr
where the physical distance
and lookback time values
come from the look-up table of
Sergey V. Pilipenko, 2021,
Paper-and-pencil cosmological calculator, arXiv:1303.5961.
It has good features, and so it's NOT an arbitrary
Procrustean bed---see
figure below
(local link /
general link: procrustean_bed.html).
php require("/home/jeffery/public_html/astro/art/art_p/procrustean_bed.html");?>
Galaxies are large, relatively dense, gravitationally-bound systems of baryonic matter (i.e., ordinary matter consisting of protons, neutrons, and electrons in the form of stars and interstellar medium (ISM): dust and gas) and dark matter (see figure below: local link / general link: dark_halo.html).
Galaxies usually are considered to have multiple generations of stars and this sets the them apart from star clusters which usually have only one main burst of star formation: the one that formed them out of a star formation region. However, the smallest galaxies and largest star clusters (all globular clusters) overlap in size scale: globular clusters are usually higher density in stars. Note also that star clusters do NOT sit in concentrations of dark matter.
In the modern/local observable universe, many galaxies continue to have star formation, but many also have ceased to have significant star formation and are called quenched galaxies.
We will NOT refer much more to dark matter in this lecture. But we preview it in the figure below (local link / general link: dark_halo.html).
Galaxies
are a fairly diverse lot, but there does NOT seem to be much
difficulty identifying in what objects are galaxies.
Of course,
galaxy mergers are probably
sometimes ambiguous as to when two galaxies
have become one
and one's person's
galaxy fragment left from a
galaxy interaction
may be another's irregular galaxy.
But people do NOT seem to worry about these ambiguities.
They acknowledge that to one degree or another all
galaxies are unique.
See the galaxy interaction
in the figure below
(local link /
general link: galaxy_mice.html).
There are 3 main types of
galaxies: each
type itself being reasonably well specified.
The types were set empirically and make up the
Hubble sequence (of galaxy types).
See the cartoon
illustrating the 3 main types
in the figure below
(local link /
general link: galaxy_types_main.html).
The figure below
(local link /
general link: galaxy_hubble_sequence.html)
illustrates the subtypes of
the Hubble sequence
but it unaccountably omits the irregulars.
A fuller discussion of
galaxy morphological classification
is given in
IAL 28: Galaxies: Types of Galaxies.
NOT counting some unknown number of
dwarf ellipticals,
ellipticals
make up ∼ 20 % of all
galaxies
in the local universe
(FK-582).
One can't do everything---sometimes there's
A Bridge too Far.
Irregular galaxies make up
∼ 3 % of all
galaxies
in the local universe
(FK-582).
Yours truly thinks these fractions are NOT counting
dwarf galaxies, but
yours truly has yet to check this out carefully.
Galaxies
range in mass
(counting both dark matter
and baryonic matter)
from ∼ 10**5 M_☉
for dwarf ellipticals
to ∼ 10**13 M_☉
for
cD galaxy (AKA supergiant elliptical galaxy)
(see Wikipedia:
Elliptical galaxy: Sizes and shapes).
Actually, this all-at-one time rule was thought to hold for all kinds of
star clusters.
But circa 2018
there is evidence for some continuing
star formation
in open clusters????.
We will consider how
galaxies are
distributed in space (i.e., the
large-scale structure of the universe)
in IAL 29: The Large-Scale Structure of the Universe.
Here one can say that they seem be distributed on
filaments or cell walls
surrounding voids.
A more trivial question is how are galaxies
oriented in space.
Answer: randomly and we can only see them from one direction.
In images, yours truly always thinks
galaxies
are hanging in space like
mobiles over
a baby's crib---as illustrated by the figure below (local link /
general link: galaxy_hcg_87.html).
But unlike terrestrial mobiles,
we can only ever see galaxies in one orientation.
There is a big distinction between thinking about
stars
and thinking about galaxies.
Stars are pinpricks compared to the distances between
nearest neighbors, unless they are in a
close binary system
or other close multiple star system.
Galaxies
are typically NOT pinpricks compared to the distances
between nearest neighbors.
Since galaxies range
in size scale in
stellar matter
(i.e., baryonic matter
which is NOT
baryonic dark matter)
from about 1 kpc to 250 kps
(FK-582),
the ratio of typical intergalactic spacing
(which is of order a megaparsecs)
to galaxy size scale
is typically of order 1000/1 to 1/1.
Galaxies
are NOT like stars where the ratio of size scale
to spacing (except in multiple star systems) is typically
of order
php require("/home/jeffery/public_html/astro/cosmol/dark_halo.html");?>
php require("/home/jeffery/public_html/astro/galaxies/galaxy_mice.html");?>
php require("/home/jeffery/public_html/astro/galaxies/galaxy_types_main.html");?>
There are actually subtypes and types NOT included in the
Hubble sequence.
php require("/home/jeffery/public_html/astro/galaxies/galaxy_hubble_sequence.html");?>
We are using the expression
local universe
vaguely here since yours truly hasn't tracked
down exactly what the authors use for
local universe.
Spiral galaxies
make up about 77 % of all
galaxies
in the local universe
(FK-582).
php require("/home/jeffery/public_html/astro/galaxies/m87_virgo.html");?>
php require("/home/jeffery/public_html/astro/galaxies/galaxy_hcg_87_2b.html");?>
We can put some numbers on these comparisons.
1 pc/R_☉
≅ 3*10**16/(7*10**8)
≅ 10**8 to within factor of 10**3 or so.
Form groups of 2 or 3---NOT more---and tackle Homework 26 problems 2--5 on galaxies.
Discuss each problem and come to a group answer.
Let's work for 5 or so minutes.
The winners get chocolates.
See Solutions 26.
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_026_discovery_galaxies.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_hot_2.html");?>
php require("/home/jeffery/public_html/astro/galaxies/milky_way_death_valley_3.html");?>
The Milky Way as a naked-eye object has always been with us: known since prehistory.
Galaxy is derived from the ancient Greek name for the Milky Way, galaxias kyklos (milky circle): gala is ancient Greek/Greek for milk and kyklos is ancient Greek/Greek for cirle.
In Latin, the name is via lactea of which Milky Way is a straight translation.
After or maybe sometime before???? other galaxies were proven to exist, the term galaxy was made generic for star systems like the Milky Way.
The Milky Way is quite visible in dark skys, but even in the city under very clear conditions its faint luminous band can sometimes be seen---maybe with a bit of imagination. For the Milky Way seen in a dark sky, see the figure above/below (local link / general link: galaxies/milky_way_death_valley.html).
php require("/home/jeffery/public_html/astro/galaxies/milky_way_death_valley_2.html");?>
In the winter sky (see the figure below),
the Milky Way passes
over the north-east shoulder of
Orion
(where one finds
Betelgeuse)
and
through the constellation
Cassiopeia
in the northern sky (which can easily be recognized: it's the big W).
Cassiopeia is
circumpolar at mid-northern latitudes, and so can be found
pretty much at any time of the year.
Orion
can be easily located in the winter sky: it's a very obvious constellation.
In the summer sky, it passes through the Summer Triangle formed by the bright stars Vega, Altair, and Deneb (FK-S-8, S-13).
The Galactic center is in the direction of Sagittarius which is low in the southern sky in summer (Shu-257; HI-405; ???).
The Milky Way is a
spiral
with a disk
diameter of ∼ 30 kpc
(CK-379)
or ∼ 50 kpc
(FK-559)
depending on how one delimits the
Galactic disk.
The Milky Way mass ∼ 1.3*10**12 M_☉
(i.e.,
total mass which is mostly the mass of
dark matter halo)
and its
baryonic matter mass
(i.e., mostly stellar matter
mass)
is ∼ 5*10**10 M_☉
(see Wikipedia: Milky Way: Mass).
In fact, the total mass is of order the
golden mass ≅ 10**12 M_☉
which suggests that the
Milky Way is in the process of becoming
a quenched galaxy:
i.e., one in which star formation has turned off.
However,
galaxy quenching
takes 1 or more gigayears, and so it will probably be awhile before the
Milky Way quenches.
We have trouble seeing the structure of the
Milky Way:
we are EMBEDDED in
the Galactic disk
and that is laced with obscuring
interstellar dust.
It's a classic "can't see the forest for the trees" situation.
Before about 1650, no one had thought of a
universe of
galaxies
(No-405)
and before
1924, when solid evidence appeared for it
(see Wikipedia:
Edwin Hubble: Universe goes beyond the Milky Way galaxy;
No-510),
relatively few believed in it---or so it seems---it's hard to tell without a
poll from the past.
The spiral arms
are in
the Galactic disk.
A map of the Milky Way is shown
in the figure below
(local link /
general link: milky_way_map.html).
php require("/home/jeffery/public_html/astro/sky_map/sky_map_winter.html");?>
Other galaxies didn't help us understand
the Milky Way in the old days.
The figure below
(local link /
general link: milky_way_cartoon.html)
gives the basic structure of the
Milky Way and
the Solar System
location and orientation in it.
php require("/home/jeffery/public_html/astro/galaxies/milky_way_cartoon.html");?>
The Milky Way is a
barred spiral galaxy
although the existence
of the bar has only been established since circa 1990.
php require("/home/jeffery/public_html/astro/galaxies/milky_way_map.html");?>
All the
bright naked-eye stars
we are familiar with are actually just in our
neighborhood in the Milky Way
is illustrated in the figure below
(local link /
general link: milky_way_local.html).
php require("/home/jeffery/public_html/astro/galaxies/milky_way_local.html");?>
Form groups of 2 or 3---NOT more---and tackle Homework 26 problems 2--5 on galaxies.
Discuss each problem and come to a group answer.
Let's work for 5 or so minutes.
The winners get chocolates.
See Solutions 26.
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_026_discovery_galaxies.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_easter_bunny_2.html");?>
The Milky Way as a milky band on the sky has been known forever, of course, as we discussed above in subsection The Milk Way on the Sky.
The speculative hypothesis 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 ideas, we only know the first person to have left a permanent historical record of the idea. Others, in many cases many others, may have had the idea earlier, but no record survives of their thinking. This usually goes without saying.
Democritus, however, apparently thought of the stars pasted on a sphere of the stars: a real celestial sphere (Fu-136--146, esp. 140).
See the imaginative portrait of Democritus (c.460--c.370 BCE) in the figure below (local link / general link: democritus.html).
Galileo (1564--1642)
(see figure above:
local link /
general link: galileo_ottavio_leoni.html)
and probably other early telescopic observers (just
after 1608
when the telescope was invented) discovered that
much of the
Milky Way is
indeed resolvable into stars
(No-401).
It probably occurred to many people in the decades after
1608 that
the Milky Way
is something like an
a very oblate spheroid
or maybe a
torus
of stars
around the Solar System.
However, interest in and discussion of the overall structure of the
Milky Way was
slow in developing it seems until the mid 18th century.
The obscure surveyor, antiquarian, amateur theologian, and architect
Thomas Wright (1711--1786)
of Durham from 1742,
the NOT-so-obscure philosopher
Immanuel Kant (1724--1804)
shortly thereafter and following
Wright's lead,
and the mathematician
Heinrich Lambert (1728--1777)
by his own account
in 1749 all speculated on structure of the
Milky Way
(No-404--407).
I believe that they were all thinking of the
Milky Way
as a very oblate spheroid
or a least a flattened structure in which
the Solar System
was embedded.????
Wright
even proposed that the
Milky Way
was supported against gravitational collapse by
orbital motion about the center (the "divine center") of the
Milky Way
(No-405).
An example of
Thomas Wright's (1711--1786)
architecture
is shown in the figure below.
Caption: "Wright's Observatory/Folly:
Westerton,
Spennymoor,
Durham,
Great Britain.
The Tower is a circular structure,
in a Gothick revival
style of the 18th century.
Built as an observatory
by Thomas Wright (1711--1786)
of nearby Byres Green.
He was a mathematician,
astronomer,
(famous for his explanation of the Milky Way),
architect,
and garden designer.
The observatory appears in a document of
1744, but does NOT appear to have
been completed until after
Wright's
death in 1796.
A plaque dated 1950 was erected to
commemorate the 200th anniversary of his publication
The Original Theory of the Universe of
1750."
Perhaps, this is
Wright's version of the
Tower of Babel.
Credit/Permission: ©
Hugh Mortimer,
2007 /
Creative Commons
CC BY-SA 2.0.
This is essentially true: rotational kinetic energy
holds up the
Milky Way
from collapse to its center.
But this idea was NOT completely accepted
until the 1920s???.
The stars moved
around a bit with their own peculiar motions, but somehow on average the
Milky Way (which
they may have been still thinking of as the whole universe)
was static on average.
Albert Einstein (1879--1955) thought this
and constructed his
Einstein universe
in 1917
to be consistent with it.
We'll take up the subject of the
Einstein universe
in
IAL 30: Cosmology:
Einstein, General Relativity, and the Einstein Universe.
The greatest observational astronomer of the
18th century
and early 19th century
was
William Herschel (1738--1822).
For some details of
William Herschel's (1738--1822) life,
see the figure below
(local link /
general link: william_herschel.html).
The project and its results are explicated in the figure below
(local link /
general link: milky_way_william_herschel.html)
and this completes our discussion of
William Herschel (1738--1822).
Observations and statistical methods continued to develop through
the 19th century
and early 20th century---or, as in the old movies, one could
say time passed.
In 1901,
Jacobus Kapteyn (1851--1922)
relying on statistical methods
(descended from those of Herschel)
proposed a model of the
Milky Way
that put the
Sun, more or
less, in the center of disk-shaped structure of stars
that was 10 kpc in diameter and ∼ 2 kpc thick at
its thick point
(No-453,490).
By 1922,
Kapteyn
had revised his model and gave it dimensions
of about 17 kpc by 3 kpc with the
Sun about 0.65 kpc
from the center in the central plane of the disk
(No-491).
For a diagram os his model,
see the figure below
(local link /
general link: milky_way_jacobus_kapteyn.html).
Kapteyn
and other early modelers were fooled by
their ignorance of extinction caused by
interstellar dust---just
as Herschel was.
Interstellar dust
was only conclusively demonstrated exist in the 1930s
(No-491).
As aforementioned, interstellar dust
limits the distance we can see in
the Galactic disk
to less than 3 kpc in most directions
in the Galactic disk
(FK-563).
Harlow Shapley (1885--1972)
took a different approach to determining the structure of the
Milky Way
than Kapteyn.
He measured the distances to
globular clusters
in the
Milky Way halo (AKA Galactic halo)
using
Cepheids.
The figure below
(local link /
general link: galaxy_sombrero.html)
illustrates a swarm of
globular clusters
in the galactic halo
of the Sombrero Galaxy (M104, NGC 4594).
Interstellar dust
is much less significant away from the
Galactic disk.
Yours truly think Shapley
probably had the idea that
globular clusters and
the Milky Way stars
were in orbit around the
Galactic center
as suggested the figure below:
(local link /
general link: milky_way_cartoon.html).
But I need to check this detail in the
history of astronomy.
Although
Shapley's
determinations had their own errors, he was on the
more correct path than
Kapteyn
and other early Milky Way modelers.
But this was NOT clear to everyone circa 1920.
After the nature of
galaxies was
effectively discovered in the
1920s
(see the following sections),
the nature of the Milky Way
was clarified: it was
very probably a spiral galaxy
like the other spiral galaxies
that were then discovered.
So the discovery of
galaxies
really completes the discovery of the
Milky Way as
a spiral galaxy.
Of course, the exact structure of the
Milky Way
was hardly known in the 1920s.
The folks then still couldn't see in the
visible
beyond ∼ 3 kpc in most directions in the
Galactic disk
because of
interstellar dust
(FK-563)
which they did NOT yet know.
Yes, they knew about obvious dark patches and they could
see
dust lanes
in other spiral galaxies,
but they did NOT yet know how severe the opaqueness problem was in
the Galactic disk.
Only in the
1930s
mostly through the work
of Robert Julius Trumpler (1886-1956)
did the severity of the opaqueness of
interstellar dust
inside the Galactic disk
become clear.
After
World War II,
radio astronomy
provided a way around the
interstellar dust problem.
The interstellar dust is largely
transparent in the radio band.
In the
1940s
and
1950s,
radio observations of the
hydrogen 21-centimeter line
emission from
neutral atomic hydrogen gas
allowed a beginning to made in determining the structure of the
Milky Way
(see Wikipedia:
21 centimeter radiation: In radio astronomy).
A lot is now known about the structure both
from the radio band
and other electromagnetic radiation
bands, but there is still lots more to discover.
The current understanding of the
Milky Way
is summarized in the figure below
(local link /
general link: milky_way_map.html).
Instead, we leave off this thread to pick up another one: the
discovery of other galaxies
in the following sections.
However, if you like you may peruse
Milky way videos
below
(local link /
general link: milky_way_videos.html):
php require("/home/jeffery/public_html/astro/ancient_astronomy/democritus.html");?>
php require("/home/jeffery/public_html/astro/galileo/galileo_ottavio_leoni.html");?>
Image link: Wikipedia:
File:WrightsObservatoryWesterton(HughMortimer)Jan2007.jpg.
It seems some people anyway, until circa 1920,
thought that the
Milky Way
was essentially held up from collapse by some means NOT known in
terrestrial physics.
The most accepted idea that emerged from the various speculations of
Wright,
Kant,
and
Lambert
was it seems that the Milky Way
was something like a very oblate spheroid
and the Solar System was
embedded in it, and hence it
looked like a broad ring
straddling a great circle
on the celestial sphere.
php require("/home/jeffery/public_html/astro/astronomer/william_herschel.html");?>
William Herschel's (1738--1822)
and
his 0.48-m reflector telescope
are shown in the figure below
(local link /
general link: telescope_william_herschel.html).
php require("/home/jeffery/public_html/astro/telescope/telescope_william_herschel.html");?>
In his own day, Herschel's
discovery of Uranus
was probably the achievement that was most recognized---it was the first
planet discovered in historical times.
See a modern image of Uranus
in the figure below
(local link /
general link: uranus_rings.html).
php require("/home/jeffery/public_html/astro/uranus/uranus_rings.html");?>
In regard the Milky Way,
Herschel
attempted to map it---from his own backyard---using
star counts (star gages he called them)
and statistics
(No-407--408).
php require("/home/jeffery/public_html/astro/galaxies/milky_way_william_herschel.html");?>
php require("/home/jeffery/public_html/astro/galaxies/milky_way_jacobus_kapteyn.html");?>
php require("/home/jeffery/public_html/astro/galaxies/galaxy_sombrero.html");?>
Cepheids
are very luminous post-main-sequence stars
that are used as distance indicators.
Cepheids are
explicated in the figure below
(local link /
general link: star_hr_cepheids.html).
php require("/home/jeffery/public_html/astro/star/star_hr_cepheids.html");?>
The period-luminosity
relation of Cepheids
was discovered by
Henrietta Swan Leavitt (1868--1921).
For Herself, see the figure below
(local link /
general link: henrietta_swan_leavitt.html).
php require("/home/jeffery/public_html/astro/astronomer/iohn_a_wheeler.html");?>
We will NOT go into
Shapley's
method in detail, but we can mention a couple of points.
php require("/home/jeffery/public_html/astro/galaxies/milky_way_cartoon.html");?>
By 1916,
Shapley
had estimated that the diameter of the system of
globular clusters
of the
Galactic disk
was of order 100 kpc (which is ∼ 3 times too large)
and that the Sun
was far from the
Galactic center.
php require("/home/jeffery/public_html/astro/galaxies/milky_way_map.html");?>
A real image of a galaxy
that resembles the
Milky Way
(i.e., the Milky Way
twin)
is shown in the figure below
(local link /
general link: milky_way_ngc_6744.html).
php require("/home/jeffery/public_html/astro/galaxies/milky_way_ngc_6744.html");?>
We will NOT detail the post-WW II
history of exploration
of the Milky Way: it's long, elaborate, and
rather tedious for non-specialists.
EOF
php require("/home/jeffery/public_html/astro/galaxies/milky_way_videos.html");?>
The story starts with nebulae (in the historical sense).
From Classical Antiquity on, there had always been a few recognized "cloudy stars" which were eventually called nebulae which is just Latin for clouds.
Ptolemy (c.100--c.170 CE) (see figure below (local link / general link: ptolemy_armillary.html) listed 6 cloudy stars (plus one cloudy object NOT associated with any star) in his 2nd century CE catalog of 1022 stars in 48 constellations (No-113,402; Wikipedia: Nebula: Observational history; SEDS: Ptolemy (about 85-165 AD)). Since Ptolemy wrote in ancient Greek, he did NOT use exactly the word nebula. He used "nephele" which became a loanword in Latin as nebula. Note nebula (L.) = nephele (G.) = νηφηλη (G. letters).
php require("/home/jeffery/public_html/astro/ptolemy/ptolemy_armillary.html");?>
Ptolemy
surprisingly missed one of the most obvious of the
nebulae:
the Andromeda nebula:
see the figure below
(local link /
general link: galaxy_andromeda_m31.html).
php require("/home/jeffery/public_html/astro/galaxies/galaxy_andromeda_m31.html");?>
Medieval Islamic astronomer
al-Sufi (903--986)
(see figure below
local link /
general link: simon_marius.html)
in his
Book of the Fixed Stars (circa 964)
is the first to put the
Andromeda nebula
in the historical record
as a nebula
(see No-188,402;
Wikipedia: Andromeda Galaxy:
Observation history;
Wikipedia:
Book of the Fixed Stars: Influence).
Simon Marius (see figure below local link / general link: simon_marius.html) was the first to telescopically observe and describe the Andromeda nebula (i.e., the Andromeda Galaxy (M31)) which he did 1612 (see Wikipedia: Simon Marius: Discoveries; Wikipedia: Andromeda Galaxy: Observation history; No-402).
php require("/home/jeffery/public_html/astro/astronomer/simon_marius.html");?>
After the invention of the telescope
in 1608 and its
nearly immediate utilization in astronomy, more
nebulae
were gradually discovered, but even well into the
18th century
they still numbered only a few tens.
The first person in the historical record
to speculate that
nebulae (in the historical sense)
were other galaxies is none other than
Christopher Wren (1632--1723).
See the figure below
(local link /
general link: christopher_wren.html).
Caption: Wright,
Kant,
and Lambert.
Our three old friends who speculated on the nature of the
Milky Way---Thomas Wright (1711--1786),
Immanuel Kant (1724--1804),
and
Johann Heinrich Lambert (1728--1777)---also
all speculated that there might be
OTHER MILKY WAYS (i.e., other galaxies)
besides our own
Milky Way
(see
Wikipedia: Thomas Wright: Astronomy;
Wikipedia: Immanuel Kant: Early Work;
To-131;
No-404--407).
The tenable alternative hypothesis was that space was infinite
emptiness beyond the boundary of the
Milky Way.
Of course, one can imagine other alternative hypotheses too, but without empirical or
theoretical guidance, one is just wandering in a sea of speculation.
But if there are other
galaxies, what
are they observationally?
Our three friends speculated that other galaxies
could be
nebulae.
Credit/Permission: ©
David Jeffery,
2005 / Own work.
Image link: Itself.
The first person to take a strong observational
interest in nebulae was
Charles Messier (1730--1817)
(see figure below
(local link /
general link: charles_messier.html),
who was the observing assistant at the Marine Observatory in Paris.
He made a determined
effort to discover all the brightest
nebulae
(No-403--404).
Caption: M2 (NGC 7089)
is a Galactic
globular cluster
in Aquarius.
M2 is the 2nd object in the
Messier catalog.
It is about 15 kpc away and is located almost directly
below the south Galactic pole in the
Galactic halo.
M2 is one of the richer denser
globular clusters
with about 10**5 star and a diameter of about 50 pc.
It has an overall visual magnitude of 6.3 which makes
it just barely unobservable with the naked eye.
But it is a good object for small telescopes and binoculars.
Like all Galactic globular clusters,
M2 is very old.
Current calculations put the ages of
Galactic globular clusters
at about 12.5 Gyr
(FK-638).
This age is a lower bound on the age of the
observable universe
which in the
Λ-CDM model
is 13.7±0.2 Gyr
(FK-653).
This image was made using the Kitt Peak National Observatory
0.9-meter telescope.
Credit/Permission: ©
Doug Williams, REU Program/NOAO/AURA/NSF,
1997 /
NOAO/AURA Image Library Conditions of Use.
Messier's
aim in making his catalog was to be able avoid mistaking
nebulae for
comets.
Using his large, powerful
reflector telescopes (the
best available at the time for his work), he had found
∼ 2000
nebulae by
1800
(No-404).
It seems the fainter you go (the deeper you go in
astro-jargon),
the more
nebulae you find:
this is still true today, of course---you keep finding less
intrinsically luminous or farther ones---there is no practical limit.
Herschel
himself verified that some nebulae were resolvable into
stars and
that others clearly seemed to be gas surrounding
a single star.
The former turned out to be Galactic
star clusters
and the latter planetary nebulae???
(No-407).
I think
Herschel
also believed that there could be other
galaxies
(No-407): he was familiar
Lambert's
ideas
(No-407).
Also, I think, he must have thought some
nebulae were
other
galaxies, but
I can't find a source that says so explicitly.
Time passes again.
In the 19th century,
Lord Rosse (1800-1867),
a wealthy Irish
landowner, was a believer in other
galaxies
and was also a pioneer in large
reflector telescope construction.
For Himself,
see the figure below
(local link /
general link: lord_rosse.html).
In the figure below
(local link /
general link: galaxy_whirlpool_lord_rosse.html)
is one of Himself's
own drawing of a
spiral nebula.
Whenever a new advanced instrument is developed, it is natural that it's
new capabilities are used to discover new things very quickly.
The new discoveries that are possible are then often exhausted.
Smaller telescopes did NOT have the:
The closest
spiral galaxies
are quite big objects on the sky. They don't need much
magnification or
angular resolution
in order to be just seen.
For example, Andromeda Galaxy (M31)
has a disk angular diameter of ∼ 3° on the sky
(Cox-578).
But it and the other
spiral galaxies
are rather faint and look like clouds in
small telescopes used
for visual astronomy.
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 doesn't look like
the pictures which result mainly from
long exposure photography.
You need a lot of
light-gathering power
to see the structure of
spiral galaxies
visually.
With better reflective coating on your mirror than
speculum,
you would NOT need a
Leviathan
(a 1.83-meter mirror), but probably still a meter-size mirror.
In a battle of technologies,
Leviathan's size
allowed visual astronomy beat out
long-exposure astrophotography
for the discovery of the spiral nebulae---but
NOT by many years.
A sort of last hurrah for
visual astronomy.
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).
Without a poll from the past, it is hard to tell what
astronomers really thought way back when.
In the time from
Lord Rosse
to the
1920s considerably
more data and theory were developed about the nature of
spiral nebulae.
This included data from astrophotography and
spectroscopy.
Some thought they were other galaxies
and some just little whirlwinds of gas and dust inside the
Milky Way.
We will NOT here rehearse the argument about the
nature of
spiral nebulae.
By the 1920s, however, the argument was coming to a head.
This debate has gone down in history as
The Great Debate
(AKA the Shapley-Curtis Debate, 1920, Apr26)
Curtis was
pro
on the subject of some
nebulae being
other galaxies;
Shapley
was con.
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).
php require("/home/jeffery/public_html/astro/astronomer/christopher_wren.html");?>
The first persons speculate that
nebulae (in the historical sense)
were other galaxies
with an impact on the development of
astronomy
(unlike Christopher Wren (1632--1723))
are none other than
our three old friends who had
speculated on the nature of the Milky Way.
See the figure below
and the figure below that
(local link /
general link: immanuel_kant.html).
php require("/home/jeffery/public_html/astro/astronomer/immanuel_kant.html");?>
php require("/home/jeffery/public_html/astro/astronomer/charles_messier.html");?>
An example of a Messier object is in
the figure below.
Download site: NOAO: M2, NGC 7089.
Image link: Itself.
As aforesaid, Messier
was actually interested in
comets and discovered more than a dozen.
See the beautiful comet shot
in the figure below
(local link /
general link: comet_lovejoy.html).
William Herschel (1738--1822)
was interested in
nebulae, and
did NOT stop just with the bright ones---he as interested in them for their own sake.
php require("/home/jeffery/public_html/astro/comet/comet_lovejoy.html");?>
Faint comets and
nebulae look much
alike---they both look like fuzzy little clouds.
Question:
Nebulae
can be
distinguished from comets by:
Answer 1 is right.
php require("/home/jeffery/public_html/astro/astronomer/lord_rosse.html");?>
In 1845,
Himself
completed the construction of the
Leviathan of Parsonstown,
the record largest telescope to date.
See the figure below
(local link /
general link: telescope_leviathan.html).
php require("/home/jeffery/public_html/astro/telescope/telescope_leviathan.html");?>
With the Leviathan,
Himself
made his greatest discovery: that some
nebulae
had spiral structure: they are, of course, the
spiral galaxies as we now know.
php require("/home/jeffery/public_html/astro/galaxies/galaxy_whirlpool_lord_rosse.html");?>
See the figure below
(local link /
general link: galaxy_whirlpool.html)
for two modern images of M51---which shows
Himself didn't do too badly.
php require("/home/jeffery/public_html/astro/galaxies/galaxy_whirlpool.html");?>
The Leviathan continued in operation until
circa 1890, but
its initial discovery of the
spiral nebulae
was its greatest achievement.
It is NOT unusual that the greatest discoveries were made early on.
It may that the
Leviathan stopped doing
any real research when
Danish-Irish
with the departure of astronomer
J. L. E. Dreyer (1852--1926)
(who worked at the
Lord Rosse's
observatory
1874--1878).
Question: Why did it take the Leviathan
to discover the
spiral nebulae?
We mean discover that there are spiral nebulae since
many spiral nebulae were known before their
spiral structure was known.
Lord Rosse's
belief that
spiral nebulae
were other galaxies
was probably a minority belief, but NOT an insignificant minority belief.
Answer 3 is right.
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).
There were arguments both ways.
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.
The Roaring Twenties, the age of
the great Moderns---they are now
in redux.
In fact, 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 some
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).
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
(Shu-286--291).
Heber Curtis (1872--1942)
of Lick Observatory writing to his family,
1920 May15,
about The Great Debate
(see M. A. Hoskin, 1976,
Journal for the History of Astronomy,
"The Great Debate: What Really Happened, Vol. 7, p. 169").
php require("/home/jeffery/public_html/astro/art/art_g/gladiator.html");?>
Form groups of 2 or 3---NOT more---and tackle Homework 26 problems 6--11 on the discovery of the Milky Way and the discovery of galaxies.
Discuss each problem and come to a group answer.
Let's work for 5 or so minutes.
The winners get chocolates.
See Solutions 26.
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_026_discovery_galaxies.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_fountain_2.html");?>
Edwin Hubble (1889--1953) came from Missouri---but nevertheless took a law degree from Oxford and practiced law in Kentucky before leaving all that to do a Ph.D. in astronomy (1917) (No-508). See images of Edwin Hubble in the figure below (local link / general link: edwin_hubble.html).
php require("/home/jeffery/public_html/astro/astronomer/edwin_hubble.html");?>
He was also an athlete and is the only known astronomer to have
boxed a world light heavyweight champion:
Georges Carpentier (1894--1975)---a non-title
bout one assumes (No-508).
After serving in the infantry in WWI, he joined the staff of Mount Wilson Observatory in southern California in 1919 (No-509).
Ah, California: see the figure below (local link / general link: pfeiffer_beach.html).
php require("/home/jeffery/public_html/astro/art/art_p/pfeiffer_beach.html");?>
Hubble, remarkably
for an astronomer, became a
well known person in Hollywood
during its golden age and he
possibly turns up in small parts in Hollywood
novels
under different names: e.g., in
James Hilton's (1900--1954)
Morning Journey (1951).
The great discoveries
Hubble
was to make at
Mount Wilson
were predicated on the facts that
Mount Wilson
in those days
(the 1920s) was one of the best observing sites in the world---this
was before the 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).
Hubble
from his Ph.D.
student days had been interested those
nebulae
that we now classify as
galaxies
(No-508).
He had developed what we now call the
Hubble sequence galaxy types
by 1923
(No-508--509) which is illustrated in
the Hubble tuning-fork diagram:
the figure below
(local link /
general link: galaxy_hubble_sequence.html).
Recall, we discussed
galaxy types
above in subsection Types of Galaxies
and a fuller discussion of
galaxy morphological classification
is given in
IAL 28: Galaxies: Types of Galaxies.
In the early 1920s
using the 100-inch Hooker telescope,
Hubble was able to resolve
stars
in the outer regions of the
Andromeda nebula (M31)
(which is now known as the Andromeda Galaxy)
and the Triangulum nebula (M33)
(which is now known as the Triagulum Galaxy)
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 (M31)
and the Triangulum nebula (M33),
just in itself, proved that
spiral nebulae
were NOT just whirlpools
of gas
in space---though they
could still have some gas, of course---as
indeed they do.
By 1924
using the approximately known luminosities of
Cepheids
and
period-luminosity
relation,
he was able to put the
Andromeda nebula
well beyond the confines of the
Milky Way
as established by
Shapley
determining the distances
to Milky Way globular clusters
also using Cepheids
(see Wikipedia:
Edwin Hubble: Universe goes beyond the Milky Way galaxy;
No-510).
This is NOT a very accurate result.
Hubble had various
systematic errors
in his measurements and calibrations that are entirely understandable given his time.
The modern distance to the
Andromeda Galaxy (M31)
is about 765 kpc (a href="http://en.wikipedia.org/wiki/Andromeda_Galaxy">Wikipedia: Andromeda Galaxy
But even if
Hubble's contemporaries
suspected large errors---and they may have---they did concede??? that
the Andromeda Galaxy (M31)
had to be a remote large system of stars comparable
in size to Milky Way.
php require("/home/jeffery/public_html/astro/telescope/telescope_hooker.html");?>
php require("/home/jeffery/public_html/astro/galaxies/galaxy_hubble_sequence.html");?>
php require("/home/jeffery/public_html/astro/galaxies/local_group.html");?>
The Andromeda Galaxy
is shown up close in the figure below
(local link /
general link: galaxy_andromeda_m31.html).
php require("/home/jeffery/public_html/astro/galaxies/galaxy_andromeda_m31.html");?>
In the Andromeda nebula,
Hubble found that
34 stars were Cepheids
by identifying their the known
period-luminosity
relation
(No-510).
Incidentally,
Hubble and
Shapley
didn't get along all that well---perhaps because
they were both from Missouri
(No-496).
Recall, Cepheids
are very luminous post-main-sequence stars
that are used as distance indicators.
Cepheids are
explicated in the figure below
(local link /
general link: star_hr_cepheids.html).
php require("/home/jeffery/public_html/astro/star/star_hr_cepheids.html");?>
From his observations of
Cepheids in
the Andromeda nebula,
Hubble had discovered that
said Andromeda nebula
was, in fact, the
Andromeda Galaxy (M31),
a giant system well outside of
the Milky Way---see
the figure below
(local link /
general link: alien_hubble.html).
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By the 1924,
Hubble had established the
distance to the
Andromeda Galaxy (M31)
to be 285 kpc
(No-510).
Now if
Andromeda Galaxy (M31)
is another galaxy
comparable in size to Milky Way,
then:
The first point above, showed suddenly in 1924 that there were thousands of spiral galaxies extending out as far as the 100-inch Hooker telescope could see.
There were also other nebulae with star-like spectra (which spiral nebulae have too) that were NOT obviously gaseous/dust nebulae inside the Milky Way.
It was probably argued before Hubble's discovery that they too could be other galaxies too---just NOT spiral galaxies---since these other nebulae were often found in obvious physical clusters with spirals---now known in 1924 to be remote and large---they had to be remote and large too.
The other nebulae with star-like spectra are the elliptical galaxies (see M87 in the Virgo Cluster in the figure below) and irregular galaxies---yours truly does NOT know how quickly this became this was recognized---but it was probably immediately in 1924.
It was an obvious conclusion for Hubble himself who was the world's expert on the nebulae with star-like spectra.
Caption: M87 (NGC 4486) in constellation Virgo.
M87 is the giant elliptical galaxy at the center of the Virgo Cluster, the nearest large cluster of galaxies at about 15 Mpc away (FK-593,615).
M87 is about 100 kpc in diameter and since it is rather spherical it has much more stellar matter (i.e., baryonic matter which is NOT baryonic dark matter) than the Milky Way. It probably grew so large by galactic cannibalism.
M87 is surrounded by a rich system of globular clusters which are fairly clear in the image. (FK-615).
Credit/Permission: ©
NOAO/AURA/NSF,
NOAO/AURA /
NOAO/AURA Image Library Conditions of Use.
Download site: M87, NGC 4486.
Image link: Itself.
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).
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.
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).
Question:
The Virgo Cluster
is in:
Answer 3 is right.
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Hubble
used his distances to make his second great discovery:
the observational discovery of the
expansion of the universe
in 1929
(No-523)
which we discuss in
IAL 30: Cosmology:
The Expansion of the Universe.
Edwin Hubble (1889--1953) videos
(i.e., Edwin Hubble (1889--1953)
videos):
Form groups of 2 or 3---NOT more---and tackle Homework 26 problems 6--13 on the discovery of the Milky Way and the discovery of galaxies.
Discuss each problem and come to a group answer.
Let's work for 5 or so minutes.
The winners get chocolates.
See Solutions 26.
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