Astronomical Objects – Galaxies

Besides planets and the moon, galaxies are the most widely studied astronomical objects.
Typically, a galaxy refers to an extensive collection of gases, stars, interstellar gaseous elements,
stellar remnants, and their respective solar systems – all held up by gravity. Galaxies usually
differ in size from goliaths containing nearly a hundred trillion stars to dwarfs carrying just a few
hundred million stars. Galaxies are classified based on their morphologies into four major
categories: irregular, spiral, elliptical, and spiral barred galaxies (Faulkner, 2018). These galaxies
are further subdivided into numerous subcategories, although other galaxies are classified based
on individual unique characteristics and size. The commonest ‘morphological classification’
model was devised by Edwin Hubble and is referred to as the Hubble Sequence. In the modern
day, the classification of galaxies is primarily done through computation techniques and physical
morphology. This discussion will describe the four main galaxies proposed by Hubble, including
irregular, spiral, elliptical, and spiral barred.

1. Spiral Galaxies

Spiral galaxies contain flat, rotating disks with bulging midpoints enclosed by spiral
arms. It is estimated that the spinning speeds of spiral galaxies can sometimes reach up to several
hundreds of miles or kilometers per second, transforming matter in disks into spiral shapes
similar to cosmic pinwheels (Faulkner, 2018). The central bulge is composed of dimmer and
older stars. Experts often assume that this central bulge comprises supermassive dark holes.
Another notable feature of spiral galaxies is the disks of stars enclosing the bulge that tend to
split into arms orbiting the galaxy. The spiral arms are composed of extensive amounts of dust
and gases and, thus, several ‘young stars’ are born in these areas. The young stars are known for
shining brightly before quickly disappearing. Usually, spiral galaxies are categorized based on

3
the size of the central bulge, arm lumpiness, and spiral tightness. Andromeda is the commonest
example of a spiral galaxy (Seigar, 2017).

2. Elliptical Galaxies

Unlike a spiral galaxy, an elliptical galaxy has a stretched spherical shape and often lacks
a bulge or nucleus at the midpoint. Despite lacking a bulge, an elliptical galaxy still shines
brightest at the center, with the light dimming towards the edges or outer surfaces. The light
illuminated by these galaxies originates from older red stars. Additionally, materials (dust and
gases) and stars seem to spread uniformly across elliptical galaxies. Several theories posit that a
larger percentage of the weight of an elliptical galaxy originates from a black hole located at the
center. Like spiral galaxies, elliptical galaxies vary in size from giants like ESO 325-G004 to
dwarfs such as Leo I (Faulkner, 2018).

3. Irregular Galaxies

Unlike elliptical and spiral galaxies, irregular galaxies lack a definite shape, spiral arms,
or nuclear bulges, although they are constantly in a motion similar to other galaxies. It is
theorized that most irregular galaxies were elliptical or spiral galaxies but experienced
deformation due to unbalanced gravity. They are also believed to contain vast amounts of dust
and gases with a relatively smaller appearance (approximately 10 percent of the Milky Way
galaxy). The three types of irregular galaxies include Irr I (have some structure), Irr II (lack any
structural appearance), and DIrr III (irregular dwarf galaxy).

4. Barred Spiral Galaxies

The only difference between barred spiral galaxies and regular spiral galaxies is that they
contain a string or bright stars lying across their central bulges and extending into the external
disks (Hodge, 2021). These bright bulges lack activities are carry mainly aged, red stars.

4
However, the arms and central bar are typified with plenty of activities, including the formation
of young stars. The Milky Way is a quintessential barred spiral galaxy (Faulkner, 2018).
Scientists have also further classified barred spiral galaxies into three, including barred spiral
galaxies types A, B, and C. Type A or SBA contain tightly held arms, while type B or SBB
contain both loosely and tightly held arms. Type C or Sbc are comprised of only loosely held
arms.

5

References

Faulkner, N. (2018). The Milky Way and other galaxies. Britannica Educational publishing.
Hodge, P. W. (2021, Oct. 11). Galaxy: Astronomy. Britannica.
https://www.britannica.com/science/galaxy
Seigar, M. S. (2017). Spiral structure in galaxies. Morgan & Claypool Publishers.