Hubble Sequence of Galaxy Classification 

By Vedant Girnare, BS-MS'22

Hubble Sequence of Galaxy Classification

At large scales, the basic constituents of the universe are galaxies. There are ~10¹¹ in the observable universe. Galaxies have a broad range of physical properties which presumably reflect their evolutionary and formative histories and give rise to various different types of morphologies. Understanding galaxy formation and evolution is one of the main goals of cosmology.

The first step in any empirical science is to look for patterns and trends. Galaxies are classified into different groups based on their observable properties, such as their shape, size, colour, and the presence or absence of certain features. The classification of galaxies helps astronomers to better understand the nature and evolution of these objects.

One of the most common ways of classifying galaxies is based on their morphology, or shape. The most well-known classification scheme was developed by the astronomer Edwin Hubble in the 1920s, and is known as the Hubble sequence or Hubble tuning-fork diagram. This scheme divides galaxies into three main types: elliptical, spiral, and irregular. In the end, we will discuss the Colour-Morphology relation to see as an example of the multiple physical properties of a galaxy and how they relate to morphology and verify trends in the classification.

Firstly let us talk about spiral galaxies. Our own Milky Way galaxy is a spiral galaxy, the spiral structure of our galaxy was difficult to establish observationally, since we view it from within. It is easier to see this structure in other galaxies unless we are viewing it edge-on. Our nearest galaxy, labeled M31, in the Andromeda constellation has a similar spiral structure. Spiral galaxies are the most numerous among the various types of bright galaxies. Like the Milky Way, they exhibit rotation, flattening with a central bulge and dark lanes of absorbing matter.

The various classes of spiral galaxies are called Sa, Sb, Sc and so on. The sequence is in decreasing order of the importance of the central nucleus or bulge in relation to the surrounding disc. Along the sequence, the central spheroid has decreasing luminosity and the spiral arms become more loosely wound. Our galaxy and M31 are of type Sb.

In spiral galaxies, such as our Milky Way, density waves are associated with the spiral arm patterns that are often observed. These patterns appear as spiral arms sweeping outward from the galactic center, and they are made up of areas where matter is more densely concentrated (density peaks) and regions where matter is less concentrated (density troughs). These waves create regions of increased density and trigger the formation of stars within the spiral arms.

Some spirals have bars in the central region, these are called barred spirals and are categorized as SBa, SBb, and so on. Half of all disk galaxies show a central bar which contains up to 1/3 of the total light. Bars are a form of dynamical instability in differentially rotating stellar disks. Bar patterns are not static, they rotate with a pattern speed, but unlike spiral arms, they are not density waves. Stars in the bar stay in the bar.

While spirals are the most numerous amongst bright galaxies, the most numerous among all galaxies are those classified as ellipticals. These are ellipsoidal in shape, exhibit very little rotation and have very little gas and dust. The various types of ellipticals are placed in the sequence E0, E1,….E7. This sequence describes progressively flattened profiles of galaxies, E0 being nearly spherical and E7 of markedly flattened lenticular form. In En, the n is given by:

Isophotes are contours of constant surface brightness. The ratio of semi-major to semi-minor axis measures how far the isophote deviates from a circle (1-b/a). This allows to classify elliptical galaxies by type En, where n=10(1-b/a). Unlike star images which tend to be point like, galaxies have nebulous shapes like those described above. Astronomers can measure the distribution of light across a galaxy with great accuracy using solid-state instruments such as charge-coupled device (CCD). The distribution of light is conveniently described in terms of isophotes. For many galaxies especially the ellipticals, increasing sensitivity of measurement shows that the boundary of a galaxy does not come to an abrupt end; rather, there is a gradual diminution of intensity of light outwards from the center. Therefore, astronomers often use the so-called Holmberg radius, which corresponds to the isophote at which the surface brightness drops to a specific value, to indicate the optical boundary of a galaxy.

Another type of galaxy, called S0 is intermediate between the ellipticals and the spirals. Like the ellipticals, the S0 galaxies have little gas and dust, but their isophotes are more like those of the spirals. These galaxies may have formed from the collisions of the ellipticals and spirals. Collisions of galaxies are not uncommon, especially in rich clusters of galaxies. Stars may go through a collision relatively unscathed since they are widely spaced but interstellar gas and dust may be spewed out in intergalactic space. In such a case, the isophotes may remain intact. S0 galaxies have a rotating disk in addition to a central elliptical bulge, but the disk lacks spiral arms or prominent dust lanes, i.e., no active star formation. Lenticulars can also have a central bar, in which case they are labeled SB0.

And then lastly there are others broadly classified as ‘irregular’. As the name implies these galaxies have no set structural pattern. There are three major types of irregular galaxies. They are Irr I, Irr II, and dlrr. An Irr-I galaxy is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. Subtypes with some spiral structure are called Sm galaxies, while those without spiral structure are called Im galaxies. An Irr-II galaxy is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence. A dlrr is a dwarf irregular galaxy. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep-field galaxy surveys. Some of the irregular galaxies, especially of the Magellanic type, are small spiral galaxies that are being distorted by the gravity of a larger neighbor.

It is hoped that when the subject of the structure and evolution of galaxies is better understood, such classifications can be better appreciated. Meanhwhile, these may be regarded as empirical indicators of galactic physics, indicators that may help us understand it better, just as empirical classification of stars helped bring about the eventual understanding of stellar structure and evolution.

Hubble sequence turned out to be surprisingly robust: many, but not all, physical properties of galaxies correlate with the classification morphology:

E S0 Sa Sb Sc Irr
— — — — — — — — — — — — — →
Pressure support — — → Rotational support
Passive — — -> Actively star forming
Red colours — — → Blue colours
Hot gas — — → Cold gas and dust
Old — — → Still forming
High luminosity density — — → Low luminosity density 

But, for example, masses, luminosities, sizes, etc., do not correlate well with the Hubble type: at every type there is a large spread in these fundamental properties.

Now, let’s take a look at one of these trends and delve deeper into it’s nitty-gritty. A classic study of galaxies was done by Holmberg (1958) who compiled and analyzed photometric data (integrated magnitudes, colors and diameters) for 300 galaxies. One of his main conclusions was the dependence of color on morphological type. Objects of different morphological classes show clear differences in their optical colors as measured by the color indices (U-B) and (B-V). The figure below shows the well-established trend between morphology and mean color. The E and S0 galaxies are clearly redder than their spiral counterparts, and the trend from redder to bluer is nearly monotonic. At the same time, the range of colors among the Sa galaxies overlaps that of Sc galaxies: some Sc’s are as red as some Sa’s while some Sa’s are as blue as some Sc’s. It is unlikely that this overlap results from misclassification or observational errors, but rather that the scatter reflects true variations in the colors, and presumably the current star formation rates, in individual objects.

Here, we can observe that elliptical (E) and lenticular (S0) galaxies are redder in color compared to spiral galaxies. This color difference suggests differences in the stellar populations and star formation histories between these galaxy types. The redder color in ellipticals and lenticulars can be attributed to older stellar populations and a lack of ongoing star formation. This is because Spiral galaxies typically form through more orderly and less violent processes compared to elliptical galaxies. They often originate from the collapse of interstellar gas and dust clouds, which can lead to the development of a rotating disk-like structure. This disk structure is conducive to ongoing star formation because it allows gas and dust to settle into the plane of the galaxy and form new stars over time. As you can clearly infer, the spiral arms become more loosely wound and have a much more significant presence in a galaxy along the Sa, Sb, Sc sequence. Since it’s the disk/spirals that are more conducive to star formation, the younger and still-forming stars have more presence in Sc than Sa, hence we see the trend. In contrast, elliptical and lenticular galaxies are believed to have formed through more violent processes, such as mergers and interactions, which can result in the redistribution of stars and the loss of angular momentum. These processes often lead to a random, spheroidal distribution of stars. Additionally, they can disrupt the gas and dust needed for ongoing star formation. As a result, star formation in these galaxies is often quenched or significantly reduced after their initial formation, leading to older stellar populations. Also, if you can recall, since lenticulars lack spiral arms, no active star formations take place.

Then, how do you explain the overlap in colours of Sa and Sc as some Sa galaxies are as red as some Sc galaxies, while some Sc galaxies are as blue as some Sa galaxies. This suggests that the color-morphology relationship is not entirely deterministic and can vary within these subcategories. The observed color differences among galaxy types and the scatter within specific subcategories suggest that galaxies do not evolve along a strictly defined path. Instead, their evolution is influenced by a variety of factors, including interactions with other galaxies, internal processes, and the availability of gas and dust for new star formation.

Overall, Holmberg’s study provides valuable insights into the relationships between galaxy morphology and their optical colors, shedding light on the complex and diverse nature of galaxies in our universe. It also emphasizes the need for a more nuanced understanding of galaxy evolution that considers a range of factors influencing their properties and appearances.

Therefore, while the Hubble classification is a valuable tool for understanding and organizing galaxies, it may not always be a complete predictor of all galaxy properties or behaviors. To gain a more comprehensive understanding of galaxies, researchers often combine morphological classifications with additional information such as spectral data, which provides insights into the composition and physical processes within galaxies. This multidisciplinary approach helps us better understand the diversity and complexities of galaxies in the universe.

The Hubble Sequence, proposed by Edwin Hubble in the 1920s, has proven to be an indispensable framework for categorizing and understanding the staggering diversity of galaxies in our universe. By classifying galaxies into distinct types based on their visual appearances and structures, such as ellipticals, lenticulars, and spirals, with further subdivisions, the Hubble Sequence offers a systematic approach that has facilitated astronomical research for generations. This classification scheme has been instrumental in identifying broad trends in galaxy properties, such as the correlation between morphology and star formation, and has provided a foundation for studying the evolution, dynamics, and interactions of galaxies. It has allowed astronomers to make sense of the wide range of galaxy shapes and behaviors, providing a crucial guidepost for exploring the rich tapestry of the cosmos and unraveling the mysteries of galactic formation and evolution.

So to summarize, I will leave you with an image of the Tuning-fork-style diagram of the Hubble sequence: 

Or for a more objective diagram:

Sources:

• Jayant Narlikar's "Introduction to Cosmology"

• Edwin Hubble's "The Realm of the Nebulae"

• https://ned.ipac.caltech.edu

• https://www.astro.caltech.edu