The Galaxies: Charms of the Universe | Space | ScienceMonk

Galaxy (adopted from Greek word ‘galactos’ meaning milk) is a complex structure of a gargantuan collection of dust, gas, billions of stars and their stellar systems, compact objects and dark matter which is held by gravity. Galaxies are the charms of the universe forming into groups, clusters and huge structures called super-clusters, appear as filaments neighboring large void regions!

Galaxy is embedded in groups or clusters and can be considered as edifice structures of Universe!

The Porpoise Galaxy from Hubble
The Porpoise Galaxy from Hubble Image Credit: NASA, ESA, Hubble, HLA; Reprocessing & Copyright: Raul Villaverde

Let us go through the History of charms of the universe, the galaxies!

Most of the early observational Astronomers were confused with the nebulae and galaxies due to a lack of observational facilities. They put forward various cosmological ideas about the galaxies and their dynamics.

Abdal al-Rahman al-Sufi, a Persian Astronomer, was the first to record the observation of the Andromeda Galaxy as a small gas cloud, following this in 964, he mentioned two gas clouds as al-Baqar in his work “Kitab suwar al-kawakib”( translated: “Book of Fixed Stars”) possibly be LMC (Large Magellanic Cloud) and SMC (Small Magellanic Cloud).

In 1610, in “Sidereus Nuncius” (translated: “Sidereal Messenger”) Galileo Galilei from his observations through the telescope illustrated that our galaxy can be determined into stars which led to modern-day cosmological ideas.

In 1636, Rene Descartes demonstrated dovetailing jigsaw puzzles of the solar system which was published as “The Island model” in The World.

In 1734, Swedish philosopher and scientist, Emanuel Swedenborg in his work “Principia” surmised the possibility of other galaxies, outside Milky Way.

In 1745, French Mathematician Pierre Louis Moreau de Maupertuis speculated that some of the gas clouds, i.e., Nebula are groups of stars with exclusive properties.

In 1750, English Astronomer Thomas Wright published his work “An Original Theory or New Hypothesis of the Universe” in which he suspected that our galaxy is a flat disk of stars that orbit the ‘divine center’ and few of the nebulae possibly be other milky ways.

In 1755, Immanuel Kant proposed that the observable universe was filled with ‘island universes’ and in 1761, Johann Lambert was first to develop fractal models of the universe. Kant considering then the idea of a flattened disk of our galaxy suggesting that flattening was due to star rotation around the disk.

In 1767, English Philosopher John Michell presented various statistical models to present binary and star clusters as real astronomical bodies.

Charles Messier from 1771-1784, catalogued 109 astronomical objects referring them as M, number, known as Messier objects distinguishing between comets and diffuse nebulae.

In 1785, William Herschel derived the structure of our galaxy, the Milky Way by counting stars in various directions with Sun nearly located at the center. The picture is a flattened disk of stars with a diameter of about five times its thickness. Herschel and his sister Caroline began systematic cataloging of the nebula.

 

The Galaxies: Charms of the Universe
Herschel’s map of the Milky Way from Herschel’s 1785 publication showing his map of the Milky Way as derived from his counting of stars in various parts of the sky.                                                    Source: On the Construction of the Heavens. By William Herschel, Esq. F. R. S. Philosophical Transactions of the Royal Society of London, Vol. 75. (1785), pp. 213-266.

 

Emanuel Swedenborg, Thomas Wright, Immanuel Kant, Johann Lambert with no observational facilities (Telescopes) speculated in their works that the gas clouds ‘nebulae’ were ‘island universes’ like our galaxy which can’t be resolved into individual stars. 

In 1845, William Parsons viewed faint nebulae directly through eye-piece of 72-inch diameter telescope and sketched their shapes. Among these, he observed few nebulae with spiral shapes, called spiral nebulae.

In 1864, John Herschel, son of William Herschel, published “General Catalogue of Nebulae and Clusters of Stars” consisting of 5079 objects based on visual observations.

In 1888, John Dreyer also published an expanded catalogue “New General Catalogue of Nebulae and Clusters of Stars,” following, Index Catalogues of 1895 and 1908 contain about 15000 objects referred with NGC or IC numbers.

In 1912, Henrietta Leavitt discovered period-luminosity relation for Cepheid variables in the Magellanic Clouds, which became a powerful tool for measuring astronomical distances.

In 1918, Harlow Shapley studied globular clusters determined the structure of the Milky Way galaxy where he used Cepheid variables.

In 1925, Edwin Hubble observed Cepheid variables in Andromeda Nebula at Mount Wilson Observatory. By using period-luminosity relation, he determined that spiral nebulae are extragalactic systems.

Classification of Galaxies

In 1936, Edwin Hubble in his book “The Realm of the Nebulae” based on inspection of various images in visual wavelengths, determined galaxies in a morphological classification which is called a Hubble sequence or Hubble tuning-fork diagram.

Figuratively (from Right to Left): E0, E1, E2, …, E7, S0, top arm of sequence: Sa, Sb, Sc, the bottom arm of sequence: SBa, SBb, SBc. This sequence has an arrangement from elliptical galaxies (which are generally predominant of old stars) through spiral galaxies to irregulars (which are predominant of young stars). This morphology gives the classification of the galaxy and not the evolution of galaxy!

In this sequence, the galaxy is broadly classified into four classes, namely ellipticals, spirals, lenticular, and irregular galaxy.

The Galaxies: Charms of the Universe
Figure 2: Hubble sequence of galaxy morphologies. Source: The Realm of the Nebulae, Edwin Hubble, 1936.
The Galaxies: Charms of the Universe
Figure 3: Hubble sequence of galaxy morphologies. Source: Abraham, 1998.

Elliptical Galaxy:

They are generally spheroidal shaped, smooth, round, featureless with no disk, no spiral arms, no particular axis of rotation. They have almost no gas or dust and show no specific features of star formations. They consist of low-mass, old population-stars. These galaxies can be huge giants which consist of about trillion stars or even more with diameters over hundred-thousand-kilo pc and small dwarfs with stars about 10 million or less with diameters 1-10 kilo pc.

They consist of absolute magnitudes ranging -8 to -23 in B-band. They appear in yellow-reddish colour. About 30% of galaxies are elliptical galaxies. Hubble designated these galaxies with letter ‘E’ followed by a number describing ellipticity of the galaxy, i.e., how flattened the galaxy seems, given mathematically as:

E= 10X [(a-b)/a]

Where “a” is semi-major axis and b is the semi-minor axis of the ellipse.

Elliptical galaxy has maximum ellipticity of b/a = 0.3.

The sub-types vary from E0 (round or early-type elliptical), E1, E2, E3, E4, E5, E6 and E7 (cigar-shaped or the most-flattened or late-type elliptical). Dwarf Elliptical galaxies (dE) galaxies are abundant in the observable universe with population II stars and much similar to globular clusters and regular ellipticals. These can be test systems and distance indicators.

E+ or late Ellipticals is another classification given by de Vaucouleurs, where these ellipticals are considered to be the first phase of the transition to the lenticular or S0 galaxies(de Vaucouleurs 1959).

Spiral Galaxies:

They are flat disk-shaped with a central bulge, spiral arms, and a halo forming 2/3rd of galaxies. They consist of a huge amount of dust and gas with a sign of star formation and are dominant of the young, blue Population I stars. They consist of absolute magnitudes raging -16 to -23 and diameters ranging from 5 to 100-kilo pc.

Hubble designated them with letter “S” and distinguished various sub-classes as a, b and c based on tightness and lumpiness of spirals arms, the degree of resolution of stars, HII regions, ordered dust lanes in the spiral arms and fraction of light in the central bulge.

  • Sa Type (early type*) with a prominent central bulge, no specific structure and unresolved into clusters, tight arms, and smooth distribution of stars with resolution into stars.
  • Sb Type with intermediate features between Sa and Sc types. These consist of central bulge much smaller than Sa type and have open spiral arms with resolution into stars.
  • Sc Type (late-type*) very less prominent central bulge, loose arms, almost open and partially distributed stars that can be resolved into star clusters and HII regions. Sc type spirals show prominent star-forming signs than Sa and Sb.

About 2/3rd of the spirals also constitute central bars and are called as Barred spiral galaxies. They are designated as “SB” and are further classified as

SBa (early type), SBb and SBc (late type) based on analogous classification of spirals with a prominence of the bar through the nucleus of the central bulge.

The modern classification includes Sd type where it consists of the less ordered spiral system and Sm, SBm types are Magellanic spirals with an almost single short arm.

(“early-type” and “late-type” words are used in just connect with the position in the spiral sequence)

Lenticular (lens-like) or S0 Galaxies:

These galaxies are armless spiral with central elliptical bulge and rotating disk. They lie between elliptical and spiral galaxies. With similar properties to the elliptical galaxy, they lack dust and gas with no sign of HII regions, i.e., no star formation. These galaxies show ellipticity b/a < 0.3.

After Hubble, based on observations, S0 galaxies are further classified into S01, S02, and S03 depending on the presence/absence of dust. Even some of these structures have a bar and are called “barred lenticular galaxies” or “SB0” galaxies. S0 galaxies are now classified as old Elliptical galaxy located in rich clusters.

Irregular Galaxies

These galaxies are late-type, shapeless with no central bulge, i.e., no dominating nuclei with the absence of rotational symmetry and no definite spiral arms. These are very small and faint with rich deposits of dust and cold gas. The sometimes referred are to as Dwarf galaxy, which is very common in the observable universe and found as satellites.

They look patchy consisting few HII regions with profuse star formation. These galaxies may be formed after collision or interaction between galaxies. They consist of with absolute magnitudes ranging -13 to -20 and diameters ranging1-10 kilo pc. They appear blue. Nearly 3% of galaxies are irregular galaxies.

These are represented as “Irr”, and modern-day extension classifies irregular galaxies as Irr I, Irr II and dIrr. Irr I galaxies exhibit organized structures such as LMC, and Irr II galaxies exhibit unorganized structures. dIrr galaxies are dwarf irregulars, which are very small and much fainter, i.e., hard to observe. These galaxies act as laboratories in understanding the formation, evolution, and dynamics of a galaxy.

Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are kinds of irregular galaxies.

Hubble did not include this class in his sequence of classification and modern days; they are extended under spiral galaxies.

The Galaxies: Charms of the Universe
Figure 4: Hubble classification of galaxies. Plate I: Ellipticals and Irregulars.                                  Source: The Realm of the Nebulae, Edwin Hubble 1936.
Charms of the Universe: Galaxy
Figure 5: Hubble classification of galaxies. Plate II: Normal and Barred spirals.                            Source: The Realm of the Nebulae, Edwin Hubble 1936.

Elliptical galaxies and S0 galaxies are referred to as early-type galaxies, while spiral galaxies and irregular galaxies are referred to as late-type galaxies. Hence, Hubble tuning fork sequence shows morphology change from early to late-type galaxies when moved from right to left.

The distribution of galaxies is not so random but collected into gravitationally bound groups and clusters. If the number of galaxies is small, then they are called galaxy groups, and if the number is relatively large, then they are called galaxy clusters.

Name of galaxy grouping Number of galaxies Ellipticals Spirals S0
Galaxy group <50 10% 80% 10%
Cluster (Poor) 50-100 10% 50% 40%
Cluster (Rich) 100-1000s 20% 20% 60%

Table data adopted from Victor Andersen 2006, University of Houston.

From observations, it is found that the majority of the giant galaxy in galaxy groups and poor clusters are spirals and in rich clusters, S0 galaxies are dominant. These groups and clusters are clustered into super-clusters. Hence, galaxies are the charms of the universe forming into groups, clusters, and huge structures called super-clusters, appear as filaments neighboring large void regions!


References:

  1. The Realm of the Nebulae, Edwin Hubble, 1936.
  2. Classification and Morphology of External Galaxies, de Vaucouleurs, Handbuch der Physik, 1959, Vol. 53, pp. 275.
  3. Galaxy Formation, Malcolm S. Longair.
  4. Galaxy Formation and Evolution, Houjon Mo, Frank van den Bosch, and Simon white.
  5. Galaxies in the Universe, L.S. Sparke and J.S. Gallagher, III.
  6. Stars and Galaxies, Seeds Backman.
  7. Introductory Astronomy, Pananides.
  8. Stellar and Galactic Astronomy Notes, Victor Andersen.
  9. Ferguson and Binggeli, 1994, A&Ap 6, 67.
  10. Van Den Bergh, 2009, ApJ 702:1502.
  11. Hubble Classification, The SAO Encyclopedia of Astronomy, COSMOS, Swinburne University of Technology.
  12. Types and Classification of Galaxies, Astronomy Courses-Academics, Cornell University.     

 

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