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BASIC EXTRAGALACTIC ASTRONOMY - Part 6: Galaxies, Discovery and Classification
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BASIC EXTRAGALACTIC ASTRONOMY
Rudy E. Kokich, Alexandra J. Kokich, Andrea I. Hudson
22 Feb, 2020
Part 6: Galaxies, Discovery and Classification
The term galaxy is derived from Greek words for a "milky circle". When capitalized, Galaxy refers to our own Milky Way galaxy. Since human beginnings, the sublime splendor of the Milky Way spanning a dark night sky served as evidence that the scope of creation is unimaginably greater than mortal affairs. The spectacle inspired mythology, religion, art, philosophy, architecture, and ultimately experimental science and technology. Throughout history, speculations about its nature ranged from metaphysical follies to surprisingly accurate scientific insight. For example, as early as the 5th century BC, Greek philosopher Democritus, the father of the Atomic Hypothesis, the Void Hypothesis (between atoms), exobiology, and remarkably factual cosmology, proposed that the Milky Way is composed of innumerable distant stars, too faint to be seen individually. It would be two thousand years before Democritus's insight could be confirmed by Galileo with the newly invented telescope.
A number of nebulous objects visible with unaided eye were known to the ancient astronomers. In his books on the motions of 1022 fixed stars, around 150 AD, Claudius Ptolemy recorded five "nebulous stars", all of which are actually open clusters. The first true nebula was documented by Persian astronomer al-Sufi around 964 AD, who described a "little cloud" at the location of the Andromeda galaxy. He also documented the two Magellanic Clouds which are not visible from Iraq, but can be seen from the Arabian Peninsula. The number of reported nebulae increased rapidly with the use of telescopes. In 1610, French astronomer Peiresc discovered the Great Nebula in Orion. By 1781, Charles Messier completed his catalogue of 103 nebulae. And by 1802, William and Caroline Herschel published three catalogs of nebulae totalling 2,510 entries. In spite of a sizable collection of identified objects, the nature of nebulae remained uncertain for over a century. The Herschels thought them to be composed of clouds of innumerable unresolved stars.
The first step toward identifying galaxies among the large set of unclassified nebulae was taken by Anglo-Irish astronomer William Parsons, 3rd Earl of Rosse. Using his 72 inch (1.83 m) telescope known as Leviathan of Parsonstown, the largest instrument in the world until 1917, Parsons observed that some nebulae had a distinct spiral structure. His drawings of spiral nebulae, especially of M51, closely resemble modern astrophotographs.
Fig. 29: Rosse's drawings of spiral nebulae observed through his Leviathan telescope.
Like the Herschels, Rosse theorized that all nebulae consist of large numbers of unresolved stars. In fact, he reported that his telescope could resolve the Orion Nebula into stars. This allegation was not entirely untrue since the region is a stellar nursery strewn with nearly 3,000 young stars belonging to the Orion Nebula Cluster, which can cause an illusion of partial resolution in a large telescope. Astronomers who hypothesized that nebulae are composed of clouds of interstellar gas from which new stars formed were met with some resistance because the idea implied a Universe which is not constant, but changing and evolving. Various interpretations of the constancy of the Universe persisted well into the middle of the 20th century.
In 1864, English astronomer William Huggins, an early pioneer of astronomical spectroscopy, photographed the spectrum of planetary nebula NGC 6543 (Cat's Eye Nebula) and found it to have a bright emission line spectrum typical of fluorescing gas. Over his career, assisted by his wife, Margaret, he discovered that approximately one third of the 70 nebulae examined showed the emission spectrum of a gas, while the rest had a continuous spectrum characteristic of stars. Incidentally, in 1868 Huggins measured a redshift in the spectrum of Sirius, and suggested it could be used to calculate the star's recession velocity relative to Earth.
A third type of nebula was identified in 1912 by American astronomer Vesto Slipher. He showed that the spectrum of the nebula surrounding the star Merope was of the absorption type, precisely matching that of the star itself. This proved that the nebula was illuminated by light reflected from the star (reflection nebula) rather than by gas fluorescence (emission nebula).
Between the 1860's and the 1920's the nature and distances of nebulae - spiral nebulae especially - remained under intense analysis and speculation. It is important to keep in mind that the scientific world of that period developed a wealth of hypotheses, but had no confirmed knowledge regarding the dimensions of the Milky Way, the size or the age of the Universe, expansion of the Universe, particle physics, the origin of elements, nuclear reactions, energy generation mechanisms within stars, the etiology and differences between novae and supernovae... Progress was facilitated by the development of photographic techniques and larger, more precise instruments, while it was hindered by dubious findings of several highly reputable astronomers. One such confused the absolute magnitude-luminosity period relation of Cepheid variables (used as standard candles) by alleging them to be occulting binary stars. Another one successfully claimed to have photographed evidence of rotation in the Pinwheel spiral nebula, which imposed a limit on the nebula's distance and size, lest the stars fly apart. In spite of some truly prescient hypotheses, the prevalent scientific opinion during these decades was that the Milky Way comprised the entire Universe, including all the nebulae, and that it was in a number of properties constant.
Progress was made gradually. In 1912, Slipher was the first to measure the shift of spectral lines of nearby spiral nebulae, allowing him to estimate their motions relative to Earth. Two years later, at the onset of World War I, he reported radial velocity of -300 km/s for M31, and demonstrated that spiral nebulae rotate by measuring that one side of the spiral arms exhibited a redshift, while the other exhibited a blueshift relative to the center.
Fig. 30: Spectroscopic detection of the rotation of spiral nebulae. For nearby galaxies, tangential velocity can be calculated by using equations (4) and (5) in Section 2).
In 1915, he published a survey of the motions of 15 spiral nebulae, and found that the average radial velocity was 400 km/s, about 25 times greater than the average velocity of stars within the Milky Way.
In 1917, American astronomer Heber Curtis discovered evidence of 12 novae in the photographic record of the Great Andromeda Nebula, and found them to be on average 10 magnitudes fainter than those detected in the Milky Way. This allowed him to estimate the distance of nearly 500,000 light years - a number five times lower than the current value, but sufficiently large to prove that the Andromeda Nebula lies far outside the confines of the Milky Way. As a result, Curtis became a major advocate of the Island Universe Hypothesis which proposed that nebulae are in fact very distant independent galaxies comparable in scale to our own. The speculation about stellar systems of equal rank outside the Milky Way actually dates back to the early 1700s. It was independently proposed by Swedish philosopher Emanuel Swedenborg and English astronomer Thomas Wright of Durham. It is frequently mentioned in the form of "sidereal systems" in William Herschel's writing. And, it was adopted by German philosopher Immanuel Kant (1755) and Swiss polymath Johann Lambert (1761), even as it was not possible at the time to observationally distinguish between aggregations of stars and gaseous nebulae.
On April 26, 1920, the Great Debate took place in the hall of the US National Academy of Sciences in Washington DC proposing to resolve the question of the size of the Milky Way, and whether it constituted the entire Universe, or was merely one of the thousands of similar spiral nebulae.
On one side was Harlow Shapley, who had been measuring the size of the Milky Way using Cepheid variables as standard candles, and had calculated its size to be at least 10 times larger than previously thought. He claimed that, if the Andromeda Nebula were of similar dimensions, it would have to lie at the distance on the order of 100 million light years - a number unacceptable to most contemporary astronomers. He pointed out that, in 1885, a nova with apparent magnitude of 5.8 had been observed in M31. It outshone the entire nebula, requiring an impossible output of energy if the nebula were extragalactic. (The "nova" was actually of a previously unknown type - a much more powerful and brighter Ia supernova, SN 1885A).
On the other side was Heber Curtis who emphasized that, according to his estimates, the Andromeda Nebula lies far outside the confines of the Milky Way, that it has prominent dust lanes which resemble those displayed by the Milky Way, and that its radial velocity and rotation speed are far faster than any star within our galaxy.
Although, already for several years, most astronomers had been favoring the idea that spiral nebulae were in fact distant galaxies, no definite winner was declared in the end because none of the arguments were regarded as conclusive.
One line of decisive evidence came from redshift measurements of spiral nebulae made with a new generation of large telescopes. In 1925, Belgian priest and astronomer Georges Lemaitre began writing a report titled A Homogeneous Universe of Constant Mass and Growing Radius Accounting for the Radial Velocity of Extragalactic Nebulae, which was published in April, 1927 in a little-known Belgian journal. Having discovered a set of solutions to Einstein's field equations which allowed for an expanding Universe, he presented mathematical proof that galactic redshifts can be explained by their recession velocity, and should be proportional to their distances. Based on available data and on his own observations, he provided the first estimate of the expansion rate of the Universe (later named the Hubble constant).
From the start, Lemaitre's idea was met with skepticism which lasted for decades. Most influential cosmologists at the time, Einstein and Hubble included, believed in a static Universe in which galactic redshifts had to be explained by a still unknown phenomenon of nature. When Lemaitre submitted his report to Einstein during their meeting in Brussels in 1927, Einstein commented, "Your calculations are correct, but your physics is atrocious." The influential man refused to accept the possibility that the Universe might be expanding, which he would later in life describe as his greatest blunder. As for Hubble, unwilling or unable to challenge the prevailing concept of a static Universe, he always prudently referred to the term recession velocity as apparent recession velocity.
In 1931, Lemaitre received wider attention when Arthur Eddington published a commentary on his 1927 article in the Monthly Notices of the Royal Astronomical Society. Lemaitre was invited to London to participate in a British Science Association meeting where he confounded the world with another radical idea. If the Universe is expanding, he reasoned, and if we project the expansion backward in time, then the Universe must have originated from a single point in space of infinite density, which he named The Primeval Atom. The hypothesis emerged to confront public scrutiny in a 1931 report published in the British journal Nature, and in an article for general readers in the December 1932 issue of Popular Science. Eddington found Lemaitre's hypothesis unpleasant. Einstein regarded it unjustifiable from the physical point of view. Even decades later, after Lemaitre's observations had been confirmed by Edwin Hubble, notable British astrophysicists Fred Hoyle, Hermann Bondi, and Thomas Gold found Lemaitre's conclusions incompatible with the scientific method. While he had accepted the idea of an expanding Universe, Hoyle declared the notion that the Universe had a beginning at which it had arisen from nothing an irrational argument in favor of a Creator. In a 1949 BBC interview, Hoyle referred to the hypothesis of the primeval atom as the Big Bang, possibly intending to convey derision. And, the name stuck.
Lemaitre's response to criticism was that the Universe must have had a beginning because the prevailing assumption of a static Universe could not be sustained into the infinite past.
Another line of convincing evidence that spiral nebulae are in fact distant galaxies arose from the work of American astronomer Henrietta Swan Leavitt who published in 1912 a close relationship between the period and the apparent magnitude of Cepheid variables in the Small Magellanic Cloud.
Since all the stars lay at approximately the same distance, their absolute magnitudes could be deduced from apparent magnitudes as soon as the scale was calibrated with parallax measured distances to some nearby Cepheids. The resulting Leavitt's Law made Cepheid variables the first "standard candles" by which distances could be measured to other galaxies too remote for parallax observations.
Fig. 31: The current relationship between the period and absolute magnitude for two major types of Cepheid variables. At the time of discovery, the existence of two major types of Cepheids was unknown, leading to errors in distance estimates of nearby galaxies.
American astronomer Edwin Hubble established himself as one of the most influential astronomers of all time by using Leavitt's Law to determine distances to nearby galaxies and conclusively prove them to lie far outside the Milky Way. Working on the recently completed 100 inch Hooker telescope at the Mt. Wilson observatory, Hubble sought evidence of Cepheid variable stars in nearby spiral nebulae, including M31 and M33. Between October 1923 and February 1924 he identified in the Andromeda nebula the first extragalactic variable star, M31-V1, whose light curve was typical of a Cepheid, with a brief peak, prolonged decline, long trough, and a rapid rise. His observation notes indicate a period of 31.415 days which, by the Leavitt's Law of those days, corresponded to an absolute magnitude of -5.0. From the median apparent magnitude of 18.5, and estimated color index of +0.9 he calculated the distance to the star and the nebula to be 220,000 parsecs, or 717,200 light years. This is much less than the distance we know today, but it was quite sufficient to prove to Hubble that the Andromeda nebula was a galaxy in its own right, far outside the bounds of the Milky Way.
Fig. 32: The second most influential star in history, M31-V1, the Cepheid variable Hubble identified in the Andromeda nebula, which allowed him to estimate its distance to be far outside the Milky Way
Actually, Hubble may have made an error in his observation notes when he initially calculated the distance of 220,000 parsecs. If we enter his values, corrected for the color index, into the distance modulus equation, the result is 331,000 parsecs, or 1,079,000 light years. He amended his result before announcing the discovery in a New York Times article on 23 November 1924, along with 35 other Cepheid variables in M31 and M33. The newspaper carelessly misspelled his name as Dr. Edwin Hubbell.
Urged by Shapley and astronomer Henry Russell, Hubble wrote a paper titled Extragalactic Nature of Spiral Nebulae which was presented in December 1924 at a joint meeting of the American Astronomical Society and American Association for the Advancement of Science. The paper shared the first prize, but did not cause a sensation because leading astronomers had been notified of his findings months earlier. Hubble's results regarding the Andromeda galaxy would not be formally published in The Astrophysical Journal until April 1929.
The current value for the distance of the Andromeda galaxy is 2.537 million light years, about 2.5 times greater than the distance estimate in Hubble's report. A major source of error was that in Hubble's time the difference between Type I (Classical) and Type II Cepheids had not been discovered, which distorted the Leavitt's Law curve. Also, Hubble worked with insensitive film emulsions, and lacked accurate photometric instruments. According to a 2011 study, M31-V1 has a period of 31.397 days, color index of 0.6, median apparent magnitude 19.0, and absolute magnitude -5.3 in the visual band.
Applying these magnitudes in the distance modulus equation results in the distance of 724,400 parsecs or 2.362 million light years, "subject to reduction if star is dimmed by intervening nebulosity," in Hubble's own words. Averaging measurements of a large number of M31 Cepheids would produce a more accurate result.
Hubble proceeded to measure distances to nearby galaxies using the Cepheid method. He then analyzed redshift measurements for 46 galaxies collected by Vesto Slipher and by his own assistant astronomer Milton Humason. When galaxy distances were plotted against their CZ recession velocities, a roughly linear relationship emerged, where the slope of the line reflected the expansion rate of the Universe (see section 5, Fig. 6). His first estimates yielded the expansion rate, later known as the Hubble Constant, of 500 km/s/Mpc, about 7 times higher than the current value, because his distance measurements had been too short by a factor of 7. But, inaccuracies aside, Hubble convincingly proved a law (now known as the Hubble-Lemaitre Law) that the recession velocity of a galaxy is proportional to its distance.
It is difficult to believe that, after years of painstakingly correlating galaxy distances with redshifts, and after estimating the expansion rate of the Universe, Hubble remained sincerely doubtful of Lemaitre's interpretation that redshift measured actual recession velocity. But, it seems that he genuinely shared with the majority of renowned cosmologists the conviction in the constancy of the Universe. In his own words, he used "the term 'apparent' recession velocity to emphasize the empirical features of the correlation," leaving the issue of the causal mechanism of redshifts to the theoreticians, "who are competent to discuss the matter with authority."
After Einstein heard of Hubble's findings, in April 1931 he published a paper for the Prussian Academy of Sciences in which he renounced the cosmological constant, which he disliked for a number of reasons, and came around to the idea of a dynamic, expanding Universe model described by Friedman and Lemaitre. He then had a meeting with Hubble, during which he failed to convince Hubble that the Universe can not be static because that would require an impossibly precise balance.
As reported in a Los Angeles Times article on 31 December 1941, Hubble announced to the American Association for the Advancement of Science that a six-year Mt. Wilson telescope survey revealed a uniform distribution of galaxies in space, which can not be consistent with the theory of an expanding Universe. "Explanations which try to get around what the great telescope sees fail to stand up. The Universe probably is not exploding but is a quiet, peaceful place and possibly just about infinite in size.”
According to American astronomer Allan Sandage, to the very end of his writing Hubble favoured "the model where no true expansion exists, and therefore that the redshift represents a hitherto unrecognized principle of nature."
If Hubble could not accept that redshift is due to actual recession velocity, he was in very illustrious company. The idea of a static Universe, infinite both temporally and spatially, first proposed by English astronomer Thomas Diggs in the 16th century, appeals to important facets of human mentality. Fritz Zwicky maintained the tired light hypothesis whereby photons lose energy by interacting with matter. In 1976, quantum field theorist Irvin Segal proposed that redshift is due to the curvature of space. A number of scientists showed that redshift can be caused by light escaping from deep gravitational wells. Throughout his career, until 2001, the father of the theory of stellar nucleosynthesis, Fred Hoyle, promoted the steady-state cosmology in which the density of an expanding Universe remains constant by continuous creation of matter. As recently as 2013, a group of Chinese astronomers headed by Ming-Hui Shao suggested that redshift is due to the interaction of photons with intergalactic electromagnetic fields. And, there are many other examples of truly brilliant minds devising interpretations outside the sphere of the simplest explanation. Skepticism, speculation, and criticism lie at the foundation of the scientific method.
31) Morphological Classification
Working with the world's largest telescope at Mt. Wilson, Hubble was in a unique position to begin morphological classification of galaxies. In 1926, he published in the Astrophysical Journal an article titled Extra-Galactic Nebulae in which he reported the properties of 400 objects, and classified them "based on the forms of the photographic images."
He divided galaxies into:
-Ellipticals, with featureless light distribution, labeled E1 - E7, where the integer indicates ellipticity x 10.
-Lenticulars, showing a featureless disk without spiral structures surrounding a bright nucleus, labeled E0.
-Normal Spirals, showing spiral arms surrounding unbarred nuclei, labeled Sa - Sc.
-Barred Spirals. showing spiral arms surrounding a barred nucleus, labeled SBa - SBc.
-Irregular Galaxies, which lack a dominating nucleus and rotational symmetry, labeled Irr.
He assigned subscripts a, b, and c to spiral galaxy labels based on stages he defined as early, intermediate, and late respectively. He described lenticular galaxies as a "hypothetical intermediate" between normal and barred spirals. It is probable that he thought this classification scheme, from left to right, reflects actual evolutionary pathway of galaxy formation. This assumption was generally accepted for decades, resulting in presently obsolete terms "early-type galaxies" for the ellipticals and the lenticulars, and "late-type galaxies" for the spirals.
Fig. 33: Hubble's visual galaxy classification
In the relatively small sample of spiral galaxies accessible to him, Hubble made a key observation that galaxies with larger nuclear bulges presented with more tightly bound spiral arms. Over subsequent decades this led to the density wave model of spiral arm formation whereby stars randomly move around the galaxy, but are delayed within density waves of matter in the thin galactic disk, much as waves of traffic form on a congested highway.
New models, suggested by intuition and supported by computer simulations, propose that collections of stars and interstellar matter within spiral arms are locally bound by gravity, also constituting actual comoving structures rather than only density waves.
In 1959, French astronomer Gerard de Vaucouleurs recommended that additional criteria be used in the morphological classification of galaxies. These included the size and the appearance of the nuclear bar, rings in the disks, and lens shapes of edge-on spirals.
The Hubble-Vaucouleurs classification divides lenticular galaxies into unbarred (S0A) and barred (S0B), with S0 retained for those galaxies in which it is impossible to tell. Spiral galaxies are divided into normal spirals (SA), barred spirals (SB), and intermediate spirals (SAB) with weakly barred nuclei. Galaxies without a central bulge are denoted (m). Those with rings are denoted ®, without rings (s), and transitional galaxies with partial rings (rs). Irregular galaxies are denoted (I), and dwarf galaxies as (d).
For example, an unbarred galaxy with loosely wound arms and no rings would be classified as SA(s)c. And dIm would be an irregular dwarf galaxy with no central bulge.
Fig. 34: Hubble-Vaucouleurs morphological galaxy classification diagram
In many cases galaxy classification is subjective, and in some cases impossible due to the orientation of the galaxy relative to the observer, or to the quality of its image. For this reason SIMBAD Astronomical Database specifies a data quality flag ranging from A (best) to E (worst) for every object's morphological type listing. A very prominent galaxy M 101 is classified as SABc, but only with intermediate confidence of C. Even on excellent photographs, the difference between ellipticals and lenticulars may be difficult to distinguish. The same is true for galaxies without a central bulge labeled (m).
It is important to note that the galaxy classification diagram, from left to right, does not represent morphologial evolution of galaxies. Quite the opposite is true. With numerous exceptions, galaxy evolution models follow the reverse path on the diagram - from right to left, or from the "late-type" galaxies to the "early-type". Generally speaking, primordial irregular dwarf galaxies merge to form large gas-rich Sd spirals with loosely wound arms. These deplete gas through new star formation to become tightly wound Sa spirals. Finally, mergers of large galaxies give rise to the ellipticals. Lenticular galaxies are intermediate between the spirals and the ellipticals. Two mechanisms - not mutually exclusive - have been proposed for their formation. One is the process of galaxy mergers. The other is gas depletion and loss of spiral arms in tightly wound Sa spiral galaxies.
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