Author: Fraser Cain

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Our own Milky Way is an example of a grand spiral; a vast collection of 200 to 400 billion stars. Much smaller galaxies than our own are known as dwarf galaxies. They only contain a few billion stars, and have a fraction of the mass of the Milky Way.

A good example of a dwarf galaxy is the Large Magellanic Cloud, located about 160,000 light-years from Earth. It contains about 1/10th the mass of the Milky Way, and has about 10% of its stars. Two other dwarf galaxies are even closer to the Milky Way, and have been captured by our galaxy’s gravity. Other dwarf galaxies are just remnants that have been torn apart by the Milky Way’s gravity, and are currently being incorporated into the structure of our galaxy.

Some astronomers think that the largest globular cluster in the Milky Way, Omega Centauri, might have once been a dwarf galaxy that had its outer stars stripped away.

Just like their larger cousins, dwarf galaxies can be classified into three varieties: dwarf elliptical galaxies, dwarf irregular galaxies, and dwarf spiral galaxies.

The smallest dwarf galaxies in the Universe are known as ultra compact dwarf galaxies. These are a recently discovered class of galaxies not much more massive than a globular star cluster. They can be as small as 200 light-years across and contain about a hundred million stars. It’s thought that ultra compact dwarf galaxies are just the cores of dwarf elliptical galaxies that were stripped of gas and outlying stars.

Our Local Group of galaxies contains just three large spiral galaxies: Andromeda, the Milky Way, and the Triangulum Galaxy. All of the others are dwarf galaxies of varying sizes.

We have written many articles about galaxies for Universe Today. Here’s an article about a companion dwarf galaxy that’s almost invisible.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

There’s a list out there somewhere of the most extreme things in the Universe. Blazars must certainly be on that list. Astronomers used to think that blazars were variable stars, but strangely, they didn’t change in brightness in any predictable way. But then in the 1970s astronomers realized that these objects were actually millions of light-years away. They were outside our galaxy, and yet they were so bright they outshone all the rest of the stars in their galaxy.

So what is a blazar? Simply put, it’s the core of an active galaxy, where the galaxy is oriented face on, so a relativistic jet blasting out of the galaxy is oriented directly towards the Earth.

All large galaxies seem to contain supermassive black holes. There are times when these black holes are actively feeding on infalling material. In fact, so much material tries to get into the black hole that it backs up into an accretion disk around the center of the galaxy. The gravitational pressure is so extreme that the material heats up to millions of degrees and becomes like a star, emitting a tremendous amount of radiation. The rapidly spinning black hole generates a powerful magnetic field that whips up the material into jets that blast above and below the black hole. Material caught in these jets is accelerated nearly to the speed of light and fired out for hundreds of thousands of light-years.

When we see a blazar, we’re looking at an actively feeding galaxy face on. Furthermore, one of the relativistic jets is oriented so that it’s pointed directly towards us, and we can see the radiation emitted by both the black hole and the jet.

Even though these blazars can be as far as 9 billion light-years away, they’re still detectable by Earth-based instruments. Now that’s bright.

We have written many articles about galaxies for Universe Today. Here’s an article about a recent blazar observation.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

Everything in the Universe seems to be part of something bigger. Our Earth is part of the Solar System, the Solar System is part of the Milky Way, and even our Milky Way is part of the Local Group. The Local Group is part of the Virgo Cluster. But there is an end to this, the largest structures in the Universe are the superclusters, measuring hundreds of millions of light-years across and containing millions of galaxies.

Our own Milky Way is part of the Virgo Supercluster. This giant formation fills a volume of space 110 million light-years across and contains at least 100 galaxy groups and clusters. And you might be amazed to know that the Virgo Supercluster is just one of millions of superclusters in the observable Universe.

A typical supercluster contains 10¹⁵ times the mass of the Sun; that’s a quadrillion solar masses. It contains all the galaxy groups and galaxy clusters that seem to be associated with one another through mutual gravitational attraction. Astronomers have estimated that there are 130 superclusters located within 1.3 billion light-years of the Milky Way. Some example superclusters include Hydra-Centaurus, Perseus, and Cetus. Superclusters are typically named after the constellation they’re found in.

Superclusters show that our Universe is not evenly distributed. Instead, the large scale structure of the Universe is these giant superclusters connected together in long filaments. Seen from far enough away, the Universe would look foamy in texture, with superclusters strung out in filaments surrounding vast voids.

We have written many articles about galaxies for Universe Today. Here’s an article about a supercluster ruled by the pull of dark matter.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

Astronomers in the southern hemisphere are lucky enough to have a clear view of the Large and Small Magellanic Clouds. Those of us in the northern hemisphere are totally out of luck. The Large Magellanic Cloud is a dwarf galaxy located about 160,000 light years away. In fact, it’s the third closest galaxy after the Sagittarius Dwarf and the Canis Major Dwarf Galaxies.

The Large Magellanic Cloud is only about 1/10th the mass of the Milky Way, containing a mere 10 billion stars worth of mass. This makes it the 4th most massive galaxy in our Local Group of galaxies, after Andromeda, the Milky Way and the Triangulum Galaxies.

It’s considered an irregular galaxy, without the grand spiral shape that we see with other galaxies, but it does have a prominent central bar. It’s possible that the Large Magellanic Cloud was once a spiral galaxy like the Milky Way, but a near pass with our galaxy or another distorted its shape, wiping away the spiral formation.

You can see the Large Magellanic Cloud with the unaided eye; no telescope is necessary. It’s visible as a faint cloud in the night sky, right on the border between the constellations of Dorado and Mensa. With a good pair of binoculars, you can see it much better; and it’s even bigger and brighter in a small telescope.

The Large Magellanic Cloud has large pockets of gas and dust, and it’s undergoing furious star formation. In fact, some of the largest, most active star forming regions ever observed are in the LMC. Astronomers have found 60 globular clusters, 400 planetary nebulae, and 700 open clusters, with hundreds of thousands of giant and supergiant stars.

In 1987, a supernova detonated in the Large Magellanic Cloud – the brightest supernova seen in 300 years. For a brief time, the supernova was visible with the unaided eye. The supernova remnant is still being studied as it continues to evolve and expand.

The Large Magellanic Cloud is named after the explorer Ferdinand Magellan, who completed the first circumnavigation of the Earth between 1519-22, and saw the clouds as part of his travels.

We have written many articles about galaxies for Universe Today. Here’s an article the describes how the LMC is moving past the Milky Way too quickly to have been captured by our gravity.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

References:
NASA APOD
NASA: The Nearest Galaxies
Keele University
Wikipedia

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If you live near the equator or in the Earth’s southern hemisphere and you watch the skies at night, you’re familiar with the Large and Small Magellanic Clouds. These are smaller galaxies nearby the Milky Way, and so they’re close enough and bright enough to see with the unaided eye. Let’s take a look at the Small Magellanic Cloud.

The Small Magellanic Cloud is a dwarf galaxy located about 200,000 light years from the Milky Way, making it one of our closest neighbors. At a magnitude of 2.7, it’s easily visible with the unaided eye from a dark location. Since it’s a galaxy, the Small Magellanic Cloud looks a bit like a detached piece of the Milky Way, over in the constellation of Tucana. Astronomers think that the SMC was once a barred spiral galaxy that was disrupted by the gravity of the Milky Way. It no longer has the familiar spiral arms, but it does still seem to have a central bar structure.

The Magellanic Clouds have been seen by people in the southern hemisphere for thousands of years, but they were made famous by the voyage of Ferdinand Magellan between 1519-22. The clouds were later observed by William Herschel with a 6.1 meter telescope at the Cape of Good Hope. This was a powerful enough telescope to reveal clusters and nebula inside the galaxy.

Astronomers once thought that the Small Magellanic Cloud was a satellite galaxy around the Milky Way, trapped in orbit by our gravity. More recent velocity calculations have thrown that theory on its head though. The Small Magellanic Cloud is moving fast enough that it can’t be captured by our gravity, and must be just passing us by.

We have written many articles about galaxies for Universe Today. Here’s an article about a supernova blowing bubbles in the Small Magellanic Cloud.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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As you probably know, galaxies like our Milky Way are made up of billions of stars. But how did we get from the first particles of hydrogen and helium left over from the Big Bang to the beautiful spiral galaxy structures we see today? What was the process of galaxy formation?

Shortly after the Big Bang, the Universe was entirely hydrogen and helium with a few other trace elements like lithium. Thanks to tiny fluctuations in density of this material, it started to clump together into vast clouds of gas with increasing density. Astronomers think that the process of galaxy formation was really led by dark matter, which outnumbers regular matter. This invisible material was also clumped together, and it attracted regular mass with its gravity, channeling material together into larger and larger collections. And so, the first proto-galaxies were formed.

Within these proto-galaxies, clumps of material gathered together, and eventually created star forming regions, and within these regions the first stars began to form. These stars lived short violent lives, and seeded the next generations of stars with the material created in their powerful supernovae. These first proto-galaxies were gravitationally attracted to one other, and merged together into larger and larger structures, eventually becoming the large spiral galaxies we know today.

But the process of galaxy formation is still going on today. Our Milky Way is expected to collide with the Andromeda Galaxy in the next few billion years, and created an even larger elliptical galaxy. We can see examples of these largest galaxies elsewhere in the Universe.

We have written many articles about galaxies for Universe Today. Here’s an article about new theories in galaxy formation.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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You’ve probably heard that our Sun is located in the Milky Way galaxy. But what is a galaxy anyway? The simple answer is that a galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter).

We know the Milky Way pretty well, so let’s consider it as a good example of a galaxy. The Milky Way is a spiral galaxy. It has a bright central core with a high density of stars, and then a flattened disk surrounding it – like a spinning record. Two spiral arms start just outside the core, and then spiral outward like a pinwheel to the outer edges of the galaxy. The Milky Way measures about 100,000 light-years across, and is thought to contain 200-400 billion stars.

But the stars we can see are just a tiny fraction of the complete galaxy. It’s also surrounded by a vast halo of dark matter. This material is invisible, and doesn’t interact with regular matter or give off any kind of radiation that we can detect. But astronomers can measure its effects because it does exert a gravitational force on other matter. In fact, the Milky Way is made up of mostly dark matter. The stars account for about 580 billion solar masses, and the dark matter could be another 6 trillion solar masses.

Our Milky Way is just an example of a galaxy, though. There is another type of galaxy called elliptical, and they’re even more common. The smallest galaxies in the Universe, the ultra-compact dwarf galaxies are only a little larger than a globular star cluster. But then the largest galaxies in the Universe also have this elliptical (egg-like) shape. A good example is the galaxy M87. It’s thought to have 2.7 trillion stars.

Stars are collected together into galaxies. Galaxies are collected together into groups of galaxies, and these groups are collected into clusters. The largest structures in the Universe are galaxy superclusters, which contain millions of galaxies and can measure hundreds of millions of light-years across.

We have written many articles about galaxies for Universe Today. Here’s an article about new research about the Milky Way.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

References:
NASA Imagine the Universe
NOAA.org
SEDS.org

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We live on the Earth. That’s just one planet orbiting a typical main sequence star called the Sun. And the Sun is a member of a pretty typical spiral galaxy called the Milky Way. But there are many other galaxies out there. Some are larger and older than the Milky Way, and others are smaller and younger. And some galaxies just defy description entirely. Let’s take a look at the different kinds of galaxies astronomers have discovered.

As I mentioned above, we live in a spiral galaxy, measuring about 100,000 light-years across. Spiral galaxies are all about the same size as the Milky Way. Some are a little larger, and some are a little smaller, but they all have that beautiful spiral shape. Examples of other galaxies with the spiral shape include Andromeda galaxy and the Whirlpool Galaxy.

Another kind of galaxies are the elliptical galaxies. These can be small dwarf galaxies with a fraction of the size and mass of the Milky Way. The smallest known galaxies, the ultra-compact dwarf galaxies have this elliptical (or egg like) shape. But the largest galaxies in the Universe are also elliptical. The massive galaxy M87, with a diameter of 120,000 light years is thought to contain several trillion times the mass of the Sun.

Other galaxies just defy classification entirely. The irregular galaxies don’t seem to have any spiral or elliptical shape. They might have started out with a more recognizable shape, but after a few devastating gravitational interactions with other galaxies, and they got completely reshaped into a unique structure.

The closest other galaxy to the Milky Way is the Canis Major Dwarf Galaxy, located only 25,000 light-years away from Earth.

We have written many articles about galaxies for Universe Today. Here’s an article about the Andromeda Galaxy, and here’s an article about the Whirlpool Galaxy.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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The Universe is built up by various structures. Stars are collected together into galaxies, galaxies are collected into galaxy groups, and galaxy groups are collected together into galaxy clusters.

For example, our own Milky Way is part of the Local Group of galaxies (not exactly an original name…). The Local Group contains about 50 galaxies; mostly smaller dwarf galaxies. The Milky Way, Andromeda and the Triangulum Galaxy are the three large spiral galaxies in the Local Group. It contains about 1 trillion times the mass of the Sun.

At an even larger scale, the Local Group is part of a galaxy cluster called Virgo Cluster which contains up to 2000 galaxies. The center of the Virgo Cluster is located about 59 million light years away in the constellation of Virgo. It has a mass of about 1.5 quadrillion suns. One of the largest, brightest galaxies in the Virgo Cluster is the giant elliptical galaxy M87.

And so, galaxy clusters are made up of galaxy groups. They’re large collections of thousands of galaxies held together by mutual gravitational attraction. A typical galaxy cluster contains 50 to 1000 galaxies, and has a total mass between 100 trillion and 1 quadrillion solar masses, stretching across a distance of 30 million light-years.

The galaxies and galaxy groups in a galaxy cluster are buzzing around a mutual center of gravity like bees around a hive. But here’s the thing. The individual galaxies are moving too quickly to be held by the mutual gravity of just the other members in their cluster. That’s because galaxy clusters are just a part of an even larger structure called a supercluster.

Superclusters really are the largest structures in the Universe, with millions of members and stretching across distances greater than 100 million light-years.

We have written many articles about galaxies for Universe Today. Here’s an article about a spectacular, and disturbed galaxy cluster. And here’s an article about a new galaxy cluster discovered by Galaxy Zoo.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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The Whirlpool Galaxy is also known as Messier 51a, and it’s one of the most familiar galaxies. If you’ve seen a picture of a galaxy captured by the Hubble Space Telescope, chances are you were looking at the Whirlpool Galaxy. That’s because this galaxy, located about 23 million light-years away in the constellation Canes Venatici, is aligned almost perfect face on. We have beautiful view of the Whirlpool Galaxy’s entire structure, from its spiral arms to its dense galactic core.

The Whirlpool Galaxy is fascinating for another reason as well. It has a companion galaxy to one side called NGC 5195. The two galaxies interact through gravity, and this gives astronomers a chance to study what happens when galaxies collide.

Astronomers have calculated that the Whirlpool Galaxy measures about 38,000 light-years across, with a mass of about 160 million times the mass of the Sun. This makes the galaxy smaller and less massive than our own Milky Way.

You can see the Whirlpool galaxy with a good pair of binoculars, or a small backyard telescope; although, you’ll want a bigger telescope to see the spiral structure and detect the companion galaxy NGC 5195. To find the Whirlpool Galaxy, located the easternmost star in the Big Dipper. Then go about 3.5 degrees to the southeast. On a dark night you should be able to see a fuzzy spot where the galaxy is.

Astronomers think that NGC 5195 first passed through the main disk of the Whirlpool Galaxy about 500 to 600 million years ago, and then made another disk crossing about 50 to 100 million years ago.

We have written many articles about galaxies for Universe Today. Here’s a link to a photograph of the Whirlpool Galaxy captured by amateur astronomer Robert Gendler.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.