The Hubble Sequence and 4 Important Types of Galaxies

The Hubble Sequence and 4 Important Types of Galaxies

The Hubble Sequence

The morphological classification of galaxies refers to a structure used by astronomers and researchers to distribute galaxies into sets based on their visual appearance. Scientists use several ideologies/schemes to classify types of galaxies depending on their morphologies. One of the most popular and crucial schemes is the Hubble sequence, formulated by Edwin Hubble and extended by Allan Sandage and Gerard Vaucouleurs.

Hubble sequence
The Hubble Sequence Classification
Image source: Wikipedia

Types of Galaxies

Professional and amateur astronomy uses the Hubble sequence for classifying types of galaxies.

Hubble’s scheme allocated regular galaxies into three broad groups– 

  1. Spiral Galaxies

Spiral Galaxies are a collection of stars and gas that appears like a spinning disk with a central bulge encircled by flat, rotating arms. The galaxies discovered so far are mostly spiral. This spinning motion might reach speeds of up to hundreds of kilometres per second. The central bulge that we can observe is made up of dimmer, older stars, and is presumed to contain a supermassive black hole usually.

spiral galaxies
Spiral galaxy
Image source: ESA/Hubble, M101 hires STScI-PRC2006-10aCC BY 4.0

Approximately, 2/3s of spiral galaxies happen to have a bar/straight arm structure passing through their center. The stars arrange themselves in a bar before they form the spiral orbiting disk around the central bulge. These spiral orbiting arms compose an enormous amount of gas and dust that results in birthing young stars. These young stars shine bright and decay quickly. Such galaxies are known as Barred Spiral Galaxies. One of the greatest examples of Barred Spiral galaxies in the Milky Way. 

Barred Spiral galaxy
Barred Spiral Galaxy
Image Source: NASA, ESA, and The Hubble Heritage Team STScI/AURA), Hubble2005-01-barred-spiral-galaxy-NGC1300, marked as public domain, more details on Wikimedia Commons

Structure

The formation of spiral arms is still a matter of confusion amongst scientists.

 According to one theory, the spiral arms might be a product of density waves that travel through the galaxies’ outer disk. The interactions between galaxies could result in producing such density waves as the mass of the smaller galaxy disturbs the massive galaxy’s density and structure when they combine. 

A vast bulk of spiral galaxies are observed to rotate in the motion that the arms trace the direction of the galaxy’s spin. Measurements of the spiral curves depicted that the orbital speed of the matter present in the disk does agree with the estimation if the larger part of the mass is focused near the center. This is why the visible part of the spiral galaxies is considered to contribute only a small portion to the total galactic mass. A significant portion of the spiral galaxies is packed with an extensive halo comprising of dark matter.

Spiral Galaxies Classification

Spiral galaxies are classified in accordance with the stiffness of their spiral, the total size of their central bulge and the unevenness of their arms. The volume of gas and dust present in these galaxies also affects its structure. 

At present, there are three classifications each of classical spiral galaxies and barred spiral galaxies:

1. Spiral galaxy type A – [Sa] – This type of galaxy has a characteristic massive central bulge with wide spiral arms. Approximately 2% of the mass of {Sa] galaxies consists of gas and dust. Therefore, we can say that a comparatively minor fraction of [Sa] galaxies are associated with star formation.

2. Spiral galaxy type B – [Sb] – This type of galaxy has a moderately-sized central bulge and moderately-distinct spiral arms.

3. Spiral Galaxy type C – [Sc] – This type of galaxy has a minor central bulge and thin but defined central arms. 

4. Barred spiral galaxy type A – [SBa] – This type of galaxy has firmly bound arms.

5. Barred spiral galaxy type B – [SBb] – This type of galaxy have both firmly and lightly bond arms.

6. Barred spiral galaxy type C – [Sbc] – This type of galaxy has lightly bound arms.

Future

As time passes, spiral galaxies show a tendency to evolve into elliptical galaxies. Although, all elliptical galaxies do not have a spiral origin. As time passes by, these galaxies undergo several changes in their shape, sizes and composition. Spiral galaxies, mostly found in low-density galactic clusters, may merge or transform into an elliptical or lenticular galaxy.

2. Elliptical galaxies

Elliptical galaxies are massive elongated-ball-shaped galaxies housing billions of stars. These galaxies are seen as fuzzy blobs of light in the backdrop of the night sky. 

The elliptical galactic class displays a wide range of sizes and masses. The smallest dwarf elliptical galaxies are only a few hundred light-years away and aren’t much bigger than globular clusters. The most massive elliptical galaxies range hundreds of thousands of light-years across. These galaxies can hold a trillion more stars than the Milky Way.

elliptical galaxy
Elliptical Galaxy
Image source: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); J. Blakeslee (Washington State University), Abell S740, cropped to ESO 325-G004, marked as public domain, more details on Wikimedia Commons

Structure:

Elliptical galaxies are one of the oldest creations of the universe. These galaxies reside within their uniform halos and are comparatively inactive compared to their spiral cousins. The star formation processes of most elliptical galaxies are dormant. The amount of gas and dust present in elliptical galaxies are also minimal.

The central portion of these galaxies comprises a supermassive black hole formed from the death of massive old stars. 

Formation

The old elliptical galaxies lack interstellar gases. According to estimations made with computer simulations, these galaxies are a result of galactic collisions and merging. 

When spiral galaxies collide, a significant part of the gas and dust is exposed and thrown into intergalactic space. The residue swiftly compresses, prompting a starburst (blast of star formation). This wave of young stars exhausts the galaxy’s gas reserves via supernovae and intense stellar winds that blow gas and dust clouds into deep space.

For growing elliptical galaxies, the dense regions of the universe (where galactic collisions are frequent) acts as fertile feeding grounds. Elliptical galaxies are found in the center of dense galactic clusters with spiral galaxies located near the edges. 

3. Lenticular Galaxies

Lenticular galaxies are called so because of their edge-on lens-like appearance. These galaxies are sometimes termed as armless-disks or armless-spirals. Lenticular galaxies describe the distinguishing point in the Hubble galaxy grouping theory. These galaxies depict the transition of galaxies from elliptical to spiral. Lenticular galaxies are often mistaken as spiral or elliptical when seen from different angles.

lenticular galaxy
Lenticular galaxy
Image source: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), Ngc5866 hst big, marked as public domain, more details on Wikimedia Commons

Structure:

Like spiral galaxies, galactic lenticular systems have a disk and a central bulge/nucleus and a galactic halo. Some lenticulars are also observed to possess galactic bars and even rings of star formation. However, lenticular galaxies always lack the spiral arms. This may be a result of their minimal star formation outside the central region. Lenticular galactic systems are devoid of much gas and dust; due to this, their stellar population is old, similar to the elliptical systems. 

Formation

Two different explanations govern the Formation of lenticular galaxies.

One theory suggests that they were initially spirals and have lost their tendency and capacity to form stars over time due to exhaustion of gas and dust reserves. The practical evidence of this theory is provided by the absence of recent star formation, gas and rotational support. 

The other theory suggests that lenticulars might as well form by merging. This theory is supported by the presence of bigger central bulges than most spirals. It is known that after merging, most of the stars rearrange themselves into the central region of the galaxy.

Irregular Galaxies

An irregular galaxy refers to the generic name set for any galaxy that does not entirely suit into any of the classes of the Hubble classification system. They do not have any well-defined shape or structure. These galaxies are mostly formed as a result of collisions, proximate interactions with other galaxies or violent internal activities. Irregular galaxies comprise both old and young stars, substantial volumes of dust and gas, and generally displaying bright bulges of star formation.

Irregular galaxy
Irregular Galaxy
Image source: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope), marked as public domain, more details on Wikimedia Commons

 These Galaxies differ in shape, sizes, luminosities, age, composition, etc. The closest neighbors of the Milky Way belong to the irregular-galactic-class.

Physical Significance of Classifying Types of Galaxies

Classifying types of galaxies helps us discover more about the interstellar universe and the history behind the creation of everything. (Literally!)

In Astronomy, Elliptical and lenticular galaxies are often termed “early-type” galaxies because they are known to house some of the oldest stellar populations and are relatively inactive towards new star formation. 

On the contrary, spiral and irregular galaxies are termed “late types” because they continue to form new stars and have a relatively low population of old stars. 

Criticisms

The Hubble sequence is not entirely perfect. Over the years, the scheme has been subjected to several criticisms and shortcomings. Let us have a look at some of the issues it faced:

  • The Hubble sequence assigns the types of galaxies subjectively. As a result of this, different astronomers often end up classifying the same galaxy into different groups. 
  • The Hubble sequence uses 2-D images of galaxies captured by telescopes for classification. This causes a lot of issues at times because the orientation factor of the galaxies often goes unnoticed. Galaxies may look different when seen from different angles. 
  • The Hubble sequence entirely depends on visual factors. So, classifying distant and dim galaxies prove out to be challenging. 

However, keeping these shortcomings aside, the Hubble sequence has contributed the most towards the classification of types of galaxies. 

To know more about galaxy formation and interesting facts visit https://lambdageeks.com/galaxy-definition-formation-5-interesting-facts/

To know more about telescopes visit https://lambdageeks.com/newtonian-telescope/

About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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