irregular galaxy

The Span of 30 Doradus - A Nearby Galaxy

Also known as the Tarantula nebula, 30 Doradus is a region of the Large Magellanic Cloud (a nearby irregular galaxy and a satellite of the Milky Way) and is one of the most active areas of star formation in the night sky.

Image: UT/CTIO Magellanic Cloud Emission Line Survey [high-resolution]

Understanding Galaxies: An Interactive Guide

Galaxies come in many different shapes, from majestic spirals to bars to great big spheres and all the kinds in between, it can be a bit a bit hard to tell what’s what.

The European Space Agency has made a very neat interactive diagram (click through the image) in which you can examine the galaxies imaged by ESA’s Herschel and NASA’s Spitzer space telescopes and learn to tell the difference.

This finely tuned diagram allows you to open up each image to see more information about the galaxies, including the classification, distance, size, and location in the sky.

NGC 4449

NGC 4449 is an irregular dwarf galaxy located about 12.5 million light years away towards the constellation Canes Venatici, the Hunting Dogs. It is about the size of the Large Magellanic Cloud, a satellite galaxy of the Milky Way, and is currently undergoing a burst of star formation.

Ngc 4449 is also the first dwarf galaxy with a noticable tidal star stream, seen in this image on the lower right. The inset in this image shows the stream in red giant stars. This stream is made from the remains of a smaller galaxy, disrupted by the gravitational forces and now in the process of merging with NGC 4449, probably causing the current starburst. Small galaxies often have large halos of dark matter, so by studying the system astronomers can learn about how the presence of dark matter affects galactic merging.

Image and information from NASA.

NGC 3239 and SN 2012A

About 40,000 light-years across, pretty, irregular galaxy NGC 3239 lies near the center of this lovely field of galaxies in the galaxy rich constellation Leo. At a distance of only 25 million light-years it dominates the frame, sporting a peculiar arrangement of structures, young blue star clusters and star forming regions, suggesting that NGC 3239 (aka Arp 263) is the result of a galaxy merger. Appearing nearly on top of the pretty galaxy is a bright, spiky, foreground star, a nearby member of our own Milky Way galaxy almost directly along our line-of-sight to NGC 3239. Still, NGC 3239 is notable for hosting the first confirmed supernova in 2012, designated SN 2012A. SN 2012A is just below and right of the bright foreground star. Of course, based on the light-travel time to NGC 3239, the supernova explosion itself occurred 25 million years ago, triggered by the core collapse of a massive star.

Image credit: Adam Block, Mt. Lemmon SkyCenter, University of Arizona

NGC 1427A

NGC 1427A is an irregular galaxy, similar to the Large Magellanic Cloud, located about 62 million light years away in the Fornax galaxy cluster. It is over 20,000 light years long and is moving at about 400 miles per second, diving through interstellar material.

As NGC 1427A plows through the region, it encounters the dense dust and gas in the Fornax cluster. The pressure from the collision and momentum compresses material in the galaxy, leading to rapid star formation and an arrowhead shape along the leading edge. Eventually, the forces will disrupt NGC 1427A beyond its ability to recover and the galaxy will be torn apart.

Image from NASA, information from ESO and HubbleSite.

Local group galaxy NGC 6822

Nearby galaxy NGC 6822 is irregular in several ways. First, the galaxy’s star distribution merits a formal classification of dwarf irregular, and from our vantage-point the small galaxy appears nearly rectangular. What strikes astronomers as more peculiar, however, is NGC 6822's unusually high abundance of HII regions, locales of ionized hydrogen that surround young stars. Large HII regions, also known as emission nebulas, are visible surrounding the small galaxy, particularly toward the upper right. Toward the lower left are bright stars that are loosely grouped into an arm. Pictured above, NGC 6822, also known as Barnard’s Galaxy, is located only about 1.5 million light years away and so is a member of our Local Group of Galaxies. The galaxy, home to famous nebulas including Hubble V, is visible with a small telescope toward the constellation of Sagittarius.

Image credit: Local Group Galaxies Survey Team, NOAO, AURA, NSF

Neighboring Galaxy: The Large Magellanic Cloud 

Credit & Copyright: AURA/NOAO/NSF

Explanation: The brightest galaxy visible from our own Milky Way Galaxy is the Large Magellanic Cloud (LMC). Visible predominantly from Earth’s Southern Hemisphere, the LMC is the second closest galaxy, neighbor to the Small Magellanic Cloud, and one of eleven known dwarf galaxies that orbit our Milky Way Galaxy. The LMC is an irregular galaxy composed of a bar of older red stars, clouds of younger blue stars, and a bright red star forming region visible near the top of the above image called the Tarantula Nebula. The brightest supernova of modern times, SN1987A, occurred in the LMC.

Irregular Galaxy NGC 55

Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and is a well known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant and is a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. However, spanning about 50,000 light-years, NGC 55 is seen nearly edge-on, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This galaxy portrait highlights a bright core crossed with dust clouds, telltale pinkish star forming regions, and young blue star clusters in NGC 55.

Image credit: Warrumbungle Observatory, Coonabarabran, NSW, Australia

Irregular Galaxy NGC 55

Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and is a well known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant and is a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. However, spanning about 50,000 light-years, NGC 55 is seen nearly edge-on, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This highly detailed galaxy portrait highlights a bright core crossed with dust clouds, telltale pinkish star forming regions, and young blue star clusters in NGC 55.

Credit & Copyright: Don Goldman