Among irregular galaxies what makes the Large and Small Magellanic cloud

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PUBLISHED April 17, 2019

A composite image of the Messier 81 (M81) galaxy shows what astronomers call a "grand design" spiral galaxy, where each of its arms curls all the way down into its center. Located about 12 million light-years away in the Ursa Major constellation, M81 is among the brightest of the galaxies visible by telescope from Earth.

Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also contain supermassive black holes at their centers. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion starsthat spin around Sagittarius A*, a supermassive black hole that contains as much mass as four million suns.

The deeper we look into the cosmos, the more galaxies we see. One 2016 study estimated that the observable universe contains two trillion—or two million million—galaxies. Some of those distant systems are similar to our own Milky Way galaxy, while others are quite different.

Types of galaxies

Before the 20th century, we didn't know that galaxies other than the Milky Way existed; earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere.

In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.

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A galaxy is a large group of stars, gas, and dust bound together by gravity. Our solar system resides in the Milky Way galaxy, a spiral galaxy that is part of a group of galaxies called the Local Group.

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There are billions of galaxies in the Universe, but only three outside our Milky Way Galaxy can be seen without a telescope - the Large and Small Magellanic Clouds and the Andromeda galaxy. The Large and Small Magellanic Clouds are about 160,000 light years away and are satellites of the Milky Way. They can be seen from the southern hemisphere. The Andromeda Galaxy is a larger galaxy that is about 2.5 million light years away and can be seen from the northern hemisphere with good eyesight and a very dark sky. The other galaxies are even further away from us and can only be seen through telescopes.

The smallest galaxies may contain only a few hundred thousand stars and be several thousand light years across, while the largest galaxies have trillions of stars and may be hundreds of thousands of light years across. Galaxies can be found by themselves, in small groups and in large clusters. It is very rare to find stars in the space in between galaxies.

Galaxies sometimes collide with each other, with interesting results. These collisions can trigger bursts of star-formation in addition to changing the shapes of the galaxies that collide. However, when galaxy collisions occur, individual stars do not collide, due to the vast distances between them.

Interacting galaxies imaged by the Hubble Space Telescope. (Credit: NGC 4676: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA; Antennae: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration UGC 8334: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University))

Galaxy classification

Galaxies are classified by shape. There are three general types: elliptical, spiral, and irregular.

Perhaps the most familiar kind of galaxy are spiral galaxies. They have a distinctive shape with spiral arms in a relatively flat disk and a central "bulge". The bulge has a large concentration of stars. The arms and bulge are surrounded by a faint halo of stars. The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. Our Milky Way Galaxy is a spiral galaxy.

Some spiral galaxies are what we call "barred spirals" because the central bulge looks elongated – like a bar. In barred spirals, the spiral arms of the galaxy appear to spring out of the ends of the bar.

Examples of spiral galaxies imaged by the Hubble Space Telescope. (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)

As their name suggests, elliptical galaxies are round or oval, with stars distributed fairly uniformly throughout. They have a bulge and halo, like spiral galaxies, but don't have the flat disk of stars. The stars in ellipticals tend to be older.

Irregular galaxies have no identifiable shape or structure to them. They are often chaotic in appearance, without a bulge or any trace of spiral arms. The different shapes and orientation of galaxies are a result of their history, which may have included interactions with other galaxies.

Examples of an elliptical galaxy (NGC 1132) and an irregular galaxy (NGC 1472A). (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)

Updated: February 2016

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For Educators

• Hidden Lives of Galaxies Booklet

Lucky Southern Hemisphere observers get to see something that many northerners never see: the Magellanic Clouds. The Large and Small Magellanic Clouds are satellite galaxies of the Milky Way. As some of the closest galaxies to our home galaxy, they stand out as big, misty blobs of light under dark skies. Scientists estimate the Small Magellanic Cloud contains around 3 billion stars, while the Large Magellanic Cloud houses some 30 billion stars.

History usually credits 15th-century Portuguese voyager/astronomer Ferdinand Magellan with “discovering” these hazy star-clouds. And, in later years, the Clouds became known by his name. But, in addition to Magellan, many early navigators used these two small galaxies to find their way across southern oceans. And they were always noticeable in the sky to the people of the Southern Hemisphere, and so they figured in early southern legends and myths.

The Magellanic Clouds from the Southern Hemisphere

If you want to see the Magellanic Clouds, you have to head south. They’re not visible north of about 17 degrees north latitude. If you are in the Southern Hemisphere, you can see these satellite galaxies any night of the year because they’re south circumpolar, that is, they’re close enough to the South Celestial Pole that they never set.

The Large Magellanic Cloud is one of the closest galaxies to us at about 160,000 light-years away. It’s about 40,000 light-years closer than the Small Magellanic Cloud.

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How to see the Magellanic Clouds

The Large Magellanic Cloud is easier to spot than the Small Magellanic Cloud, but they both require dark skies.

The Large Magellanic Cloud shines at magnitude 0.9. Keep in mind that it’s stretched out over an area of sky about 9 by 11 degrees. A star of that magnitude would appear very bright because of its pinpoint source of light, but for the Large Magellanic Cloud, its diffuse, spread-out light means it appears as a hazy smudge on the sky.

The Large Magellanic Cloud lies in both the constellations Dorado and Mensa. Draw a line from Sirius past the right side of Canopus to find it.

The Small Magellanic Clouds shines at magnitude +2. Its brightness is spread over about about 13 square degrees of sky. You can find it in the southeast corner of the constellation Tucana the Toucan.

The Clouds in myth

In the Southern Hemisphere, Australian Aborigines, the Maori people of New Zealand and the Polynesian people of the South Pacific were familiar with both the Large and Small Clouds. They used them as navigational markers during their oceanic expeditions. They considered these hazy star-clouds predictors of the winds. The website OzSky.org explains:

Many tribes of Australian Aboriginals have ‘dreamtime stories,’ which they have passed down from generation to generation, to explain the universe as they perceive it. One such legend describes the Clouds as the campfires of an old couple, the Jukara. The Jukara relied on other star people to supply them with fish and lily bulbs caught in the Milky Way to survive. The old couple cooked the food over their campfire, which was the star Achernar. The Large Cloud represented the old man while the Small Cloud was the old woman.

Another myth comes from South Africa. The nearby constellation Mensa (“Table”) got its name from South Africa’s Table Mountain. One story says that the Large Magellanic Cloud is a puff of smoke from a pipe-smoking contest held on the mountain.

In history, Al Sufi, the Persian astronomer, described the Large Magellanic Cloud in his Book of Fixed Stars in AD 964. He called it Al Bakr, describing it as the White Ox of southern Arabia.

Science of the Magellanic Clouds

Henrietta Swan Leavitt – famous for her work on the Cepheid variable stars – studied the Large and Small Magellanic Clouds from Harvard College Observatory in Southern Peru. In the early 1900s, she published her work on variable stars in the Small Magellanic Cloud, famously showing the relationship between the periods (cycles) of the stars’ variability and their luminosities. Her study was titled 1777 variables in the Magellanic Clouds. The period-luminosity relationship later became a reliable gauge for astronomers trying to parse the riddle of star and galaxy distances.

Astronomers believe that the Large and Small Clouds formed around the same time as our Milky Way, some 12 to 13 billion years ago. Due to their repeated interaction with our larger Milky Way galaxy, it’s thought that great galactic tides might have caused their irregular shape.

The ongoing Dark Energy Survey found a dark stream of interacting matter between the Large and Small Magellanic Clouds. Simulations performed by a team of scientists at the University of Arizona suggested that the two galaxies might be interacting with each other and might eventually merge.

Bottom line: The Large and Small Magellanic Clouds are two of the closest galaxies to the Milky Way. They can be seen without optical aid from southern skies.

Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.