Irregular Galaxies

Irregular galaxies are the final major type of galaxies. These galaxies are the smallest of the three types.


The best way to describe an irregular galaxy is to compare them to open clusters. Open clusters are groups of stars that don't have an overall structure, but are gravitationally bound to each other. Irregular galaxies are large groups of stars that have no overall structure, but are also gravitationally bound to each other.


Obviously, an irregular galaxy is much larger than an open cluster, but are usually found orbiting a larger galaxy. These satellite galaxies were formed around the same time as their parent galaxy were formed, but since they don't have as much mass, they didn't grow into a spiral galaxy or elliptical galaxy. In irregular galaxies, we do find that they have a lot of gas and dust, which means they undergo active star formation. This also means that generally these are younger galaxies, with the stars forming after the galaxy formed.


The best examples of irregular galaxies (stylized as Irr) are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), which can both be seen in the southern night sky.

• LMC is at -69° declination (seen only at latitudes south of 21°N)

NASA

• SMC is at -72° declination (seen only at latitudes south of 18°N)

ESA/Hubble and Digitized Sky Survey 2

Galaxies

We know about that can be grouped together in open or globular clusters. These stars are grouped in this way because they formed from the same cloud and gravity kept them in close proximity to each other. What happens if these clouds are much larger? Galaxies form.


In fact, it is believed that galaxies were the first large-scale features to form before stars and clusters. Older galaxies (i.e. galaxies formed long ago) also tend to be active (very bright to be seen over long distances).


Galaxies come in three basic types:

• Elliptical galaxies which look like globular clusters but on a much larger scale

Elliptical Galaxy NGC 4150

Image Credit:

NASA, ESA, R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore), R. O'Connell (University of Virginia, Charlottesville), and the WFC3 Scientific Oversight Committee

• Spiral galaxies which look like whirlpools

Andromeda Galaxy (nearest major galaxy to ours)

Image Credit:

NASA/JPL-Caltech

Barred Spiral Galaxy NGC 1300
Image Credit:

APOD

 Hubble Heritage Team, ESA, NASA



• Irregular galaxies which are kinda like open clusters, but again on much larger scale.

Large Magellanic Cloud

Image Credit:

APOD

Yuri Beletsky (ESO)

Small Magellanic Cloud with two globular clusters

Image Credit:

APOD

Stéphane Guisard

LMC (upper right center) and SMC (lower left)

Image Credit:

NASA, Axel Mellinger (Central Michigan University)

We will discuss the three types in more detail over the next few days.