IAL 26: The Discovery of Galaxies

Don't Panic

Sections:
  1. Introduction
  2. Cosmology, Distance, and Lookback Time
  3. Cosmological Redshift, Cosmological Distance Measures, and the Local Universe (Reading Only)
  4. Introduction to Galaxies
  5. Introduction to the Milky Way
  6. The History of the Discovery of the Milky Way
  7. The Discovery of Galaxies: An Example of the Process of the Scientific Method
  8. Appendix: The History of the Discovery of Galaxies (Not Required for the RHST)
  9. Appendix: Edwin Hubble (Not Required for the RHST)

  1. Introduction

    2. Stars and the Milky Way have been known since forever as astronomical objects on the sky.

    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 section The History of the Discovery of the Milky Way and section The Discovery of Galaxies: An Example of the Process of the Scientific Method, we give the story of that discoveries 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.

  2. Cosmology, Distance, and Lookback Time

    3. Galaxies are in cosmological space---the realm where even from the beginning of an introduction to galaxies, one CANNOT neglect cosmology.

    1. The Spherical Observable Universe:

      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).

    2. Where and When in the Observable Universe:

      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.

    3. The Lookback Time of Nearby Astronomical Objects:

      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: 

        t_lookback=r/c  ,  where

                     c is the vacuum light speed
                     and line-of-sight distance.

      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 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 

        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

      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.

    4. Lookback Time and Cosmic Evolution:

      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).

    5. Cosmological Physical Distance, Lookback Time, Cosmological Models, and the Λ-CDM Model:

      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):

  3. Cosmological Redshift, Cosmological Distance Measures, and the Local Universe (Reading Only)

    4. A discussion of galaxy formation and evolution is beyond the scope of this lecture. We take up galaxy formation and evolution in IAL 29: The Large-Scale Structure of the Universe.

    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.

    1. Cosmological Redshift z:

      Cosmological redshift z is a direct observable obtained from line spectra of galaxies and other extragalactic objects.

      What is the cosmological redshift?

      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).

    2. The Λ-CDM Model and Cosmological Distance Measures:

      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.

    3. Specifying the Local Universe Vaguely and Precisely:

      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:

      1. Λ-CDM model tells us that the lookback time to z = 0.5 is about 5 Gyr (i.e., 5 gigayears) (see the cosmological distance measure plots at local link / general link: cosmos_distance_z_10000.html and Cosmic Calculator with Hubble constant equal to 70 (km/s)/Mpc).

        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.

      2. We use the Sun and Solar System as standards for many purposes in astronomy.

        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).

      3. It is also convenient if the simple relationship between lookback time and cosmological physical distance (i.e., t_lookback=r/c) holds approximately for the z ≤ 0.5 local universe.

        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".

          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.

          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 = 0.3, 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.

        At z = 0.1, the deviation between the two quantities is only a few percent and is negligible for many purposes.

      4. Both physical distance and lookback time are predicted approximately by what is called the naive Hubble distance for small z.

        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.

      5. z = 0 is a simple value to remember as the boundary of the local universe. z ≤ 0.5 local universe.

      So those are the reasons for thinking the z ≤ 0.5 local universe is a useful version of the 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.

      It has good features, and so it's NOT an arbitrary Procrustean bed---see figure below (local link / general link: procrustean_bed.html).

  4. Introduction to Galaxies

    5. Since there is a lecture to come on galaxies in general (IAL 28: Galaxies), we just give a brief introduction to galaxies here.

    1. Galaxies Defined and Dark Matter:

      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).

      We will, however, discuss dark matter further in the following lectures: i.e.,
      1. IAL 28: Galaxies.
      2. IAL 29: The Large-Scale Structure of the Universe.
      3. IAL 30: Cosmology.

    2. Galaxy Diversity and Interacting Galaxies:

      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).

    3. Types of Galaxies:

      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).

      There are actually subtypes and types NOT included in the       Hubble sequence.

      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.

    4. The Size of Galaxies:

      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).

      Factoids:

      1. The baryon fraction (i.e., baryonic mass over total mass) in galaxies (i.e., the mass in stars and interstellar medium (ISM)) is ⪅ 1/30 ≅ 3 % (Ci-404; Dekel et al. 2019, Figure 1) which is much smaller than the average baryon fraction of the observable universe which is ∼ 1/6 ≅ 16 % (Ci-27).

        The maximum baryon fraction ∼ 1/30 ≅ 3 % occurs for galaxies of about the golden mass = 10**12 M_☉ and the fraction decreases going to smaller and larger masses (Ci-404; Dekel et al. 2019, Figure 1). For the Milky Way, the baryon fraction seems to be of order 5 % with considerable uncertainty (Wikipedia: Milky Way: Mass: Milky Way mass: total/dark matter ∼ 10**12 M_☉, stars/gas/dust ∼ 5*10**10 M_☉ thus baryon fraction 5 % of total). This is a bit higher than 3 %, but the two values are consistent within uncertainty

      2. The dark matter is in a roughly spherical dark matter halo for all kinds of galaxies.

      3. In most elliptical galaxies, star formation mostly ended gigayears ago. The ending of star formation is called galaxy quenching.

      4. The most massive galaxies (i.e., cD galaxies (AKA supergiant elliptical galaxies)) are believed to be mostly the result of mergers of already massive galaxies. M87 (shown in the figure below: local link / general link: m87_virgo.html) is an example of a cD galaxy.

      5. The smallest dwarf ellipticals are NO larger in extent than globular clusters. Note there are 3 distinctions between dwarf galaxies and globular clusters.

        1. Dwarf galaxies are dark matter dominated and globular clusters are NOT??? (see Wikipedia: Elliptical galaxy: Sizes and shapes).

        2. The concentration of stars in dwarf galaxies is generally far lower than in globular clusters. You can just see through dwarf galaxies.????

        3. A galaxy any kind has undergone generations of star formation whereas the stars of a globular clusters. all formed at (almost) one time.

          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????.

      6. Spiral galaxies (barred and unbarred) range in stellar matter (i.e., baryonic matter which is NOT baryonic dark matter) from about 10**9 to 4*10**11 M_☉. (FK-582).

      7. Irregular galaxies range in stellar matter (i.e., baryonic matter which is NOT baryonic dark matter) from about 10**8 to 3*10**10 M_☉.

      8. Above the scale of galaxies are galaxy clusters, superclusters of galaxies, and the large-scale structure of the universe (which is now sometimes called the cosmic web).

    5. How Galaxies are Disbributed and Oriented in Space:

      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.

    6. A Big Distinction Between Stars and Galaxies:

      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.

        We can put some numbers on these comparisons.

        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 

         1 pc/R_☉ 
   ≅ 3*10**16/(7*10**8) 
   ≅ 10**8 to within factor of 10**3 or so.

  5. Introduction to the Milky Way

    6. The Milky Way, our home.

    1. The Milk Way on the Sky:

      The Milky Way as a naked-eye object has always been with us: known since prehistory.

      To the naked eye, Milky Way is a luminous band straddling a great circle on the celestial sphere at an angle of about 60° to the celestial equator (CM-366) and at an angle of about 50°???? to the ecliptic (HI-552). The great circles of the Milky Way, celestial equator, and ecliptic do NOT intersect at two points, alas.

      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).

      In the winter sky (see the figure below (      local link / general link: sky_map_winter.html), 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; ???).

    2. The Milky Way Structure:

      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 Milky Way 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 Milky Way disk and that is laced with obscuring interstellar dust.

      It's a classic "can't see the forest for the trees" situation.

      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.

      The       Milky Way is a barred spiral galaxy although the existence of the bar has only been established since circa 1990.

      The spiral arms are in the Milky Way disk.

      A map of the Milky Way is shown in the figure below (local link / general link: 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).

  6. The History of the Discovery of the Milky Way

    7. Let's now cover the history of the discovery of the Milky Way as a galaxy.

    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.

    1. Democritus on the Milky Way:

      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 celestial sphere of the stars that enclosed the Earth: a physically 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).

    2. Early Telescopic Observers:

      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.

    3. Mid-18th-Century Thinkers:

      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.

      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 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.

    4. William Herschel (1738--1822) and the Milky Way:

      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).

      William Herschel's (1738--1822) and his 20-foot telescope (reflector, primary diameter 18.5 inches ≅ 47 cm) are shown in the figure below (local link / general link: 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).

      In regard to the       Milky Way, Herschel attempted to map it---from his own yard---using star counts (star gages he called them) and statistics (No-407--408).

      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).

    5. Jacobus Kapteyn (1851--1922) and the Milky Way:

      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 the star gage method of William Herschel (1738--1822) 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 diameter 17 kpc and thickness 3 kpc with the Sun at ∼ 0.65 kpc from the center in the central plane of the disk (No-491). For a diagram of 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).

      Interstellar dust limits the distance we can see in the Milky Way disk to ⪅ 3 kpc in most directions in the Milky Way disk (FK-563).

    6. Harlow Shapley (1885--1972) and the Milky Way:

      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).

      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).

      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).

      We will NOT go into       Shapley's method in detail, but we can mention a couple of points.

      1. By avoiding the plane of the Milky Way disk he largely avoided the problem of obscuration by interstellar dust.

        Interstellar dust is much less significant away from the Milky Way disk.

      2. He assumed that the globular clusters were distributed in an approximately spherically symmetric manner about the Galactic center. This assumption was correct.

        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.

      By       1916, Shapley had estimated that the diameter of the system of globular clusters of the Milky Way disk was of order 100 kpc (which is ∼ 3 times too large) and that the Sun was far from the Galactic center.

      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.

    7. After Shapley and the Discovery of Spiral Galaxies:

      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 Milky Way 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 Milky Way 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 Milky Way 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).

      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).

      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.

      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):

        EOF

    EOF

  7. Appendix: The History of the Discovery of Galaxies (Not Required for the RHST)

    8. Given below are some figures that form a useful appendix to the the history of discovery of galaxies (discussed above in section The Discovery of Galaxies: An Example of the Process of the Scientific Method).

    1. Simon Marius and the Discovery of the Andromeda Nebula:

      Simon Marius (1573--1625) independently (but NOT firstly) and telescopically (firstly) 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 the figure below local link / general link: simon_marius.html.

    2. Early Speculations on the Nature of the Nebulae: Redux:

      Recall the first persons in the 18th century to speculate that nebulae (historical usage) were other galaxies (after Emanuel Swedenborg (1688--1772)) were Thomas Wright (1711--1786) Immanuel Kant (1724--1804), and Heinrich Lambert (1728--1777). For a cartoon of them, see the figure below (local link / general link: alien_galaxooges.html).

    3. William Herschel (1738--1822): Redux:

      For more on William Herschel (1738--1822), see the figure below (local link / general link: william_herschel.html).

    4. Lord Rosse (1800-1867): Redux:

      For William Parsons, Lord Rosse, formally 3rd Earl of Rosse (1800-1867) see the figure below (local link / general link: lord_rosse.html).

      Lord Rosse made sketches of some spiral nebulae. We showed one of the Whirlpool Galaxy (AKA M51a/NGC 5194 and M51b/NGC 5195) above in subsection The Spiral Nebulae. See the figure below (local link / general link: galaxy_whirlpool.html) for two modern images of M51---which shows Lord Rosse did NOT do too badly.

  8. Appendix: Edwin Hubble (Not Required for the RHST)

    9. We here expand a bit on what was said about Edwin Hubble (1889--1953) in the above section The Discovery of Galaxies: An Example of the Process of the Scientific Method.

    1. The Life of Hubble:

      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).

      Hubble 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).

      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., maybe in James Hilton's (1900--1954) Morning Journey (1951).

    2. Observing Advantages:

      The great discoveries Hubble was to make at Mount Wilson Observatory in California (see also 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 smog and the light pollution of Los Angeles mostly ruined things---and on having the largest telescope to date at his disposal: the Hooker telescope (reflector, primary diameter 2.54 m = 100 in, operational 1917--present): see figure below (local link / general link: telescope_hooker.html).

    3. Hubble and the Nebulae:

      Hubble from his Ph.D. student days had been interested those nebulae (historical usage) that we now classify as galaxies: i.e., the spiral nebulae and elliptical nebulae (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.

    4. The Andromeda Galaxy and Cepheids:

      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.

      The       Andromeda Galaxy is shown up close in the figure below (local link / general link: 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).

      By 1924 using the approximately known luminosities of Cepheids and period-luminosity relation, he 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).

      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).

      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).

      By the       1924, Hubble had established the distance to the Andromeda Galaxy (M31) to be 285 kpc (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 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) had to be a remote large system of stars comparable in size to Milky Way.

    5. Wild Extrapolation:

      Now if Andromeda Galaxy (M31) is another galaxy comparable in size to Milky Way, then:

      1. All the spiral nebulae were spiral galaxies too---and NOT little gas/dust whirlwinds inside the Milky Way. And there were thousands of spiral galaxies extending out as far as the Hooker telescope (reflector, primary diameter 2.54 m = 100 in, operational 1917--present) could see.

      2. Also other nebulae (historical usage) with star-like spectra, which could be found in systems with spiral nebulae (i.e., elliptical nebulae, lenticular nebulae, and irregular nebulae), were also all other galaxies.

      3. The Milky Way (which was recognized as being disk-like since the 18th century at least) was also probably a spiral galaxy. Its spiral structure would NOT be proven until hydrogen 21-centimeter line observations in the radio band fiducial range 3 Hz -- 300 GHz = 0.3 THz, 0.1 cm -- 10**5 km started to become available after World War II (1939--1945).

      For an elliptical nebulae that must be a galaxiy since it is found in the Virgo Cluster (of galaxies), see M87 in the figure below (local link / general link: m87_virgo_old_image.html).

      For more on       Edwin Hubble (1889--1953), see Edwin Hubble (1889--1953) videos below (local link / general link: edwin_hubble_videos.html).

        EOF