Posted on by Fraser Cain

Spiral Galaxy

Spiral galaxy M101. Image credit: Hubble

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When you think of a galaxy, you’re probably thinking of a spiral galaxy. You know, with the central bulge and grand sweeping arms that spiral outward from the center. In fact, our own Milky Way is a spiral galaxy, and there are many others out there in the Universe. But have you ever thought about how they form in such a beautiful shape?

A spiral galaxy is shaped like a flat disk with a thicker bulge in the center. Bright spiral arms start from the center and then coil outward like a pinwheel. All spiral galaxies rotate, but very slowly; our own Milky Way completes a single revolution once every 250 million years or so.

The spiral arms are actually density waves that move around the disk of the spiral galaxy. As the density wave passes over a region, masses are pulled together, and you get bright pockets of star formation. Then the density wave moves on, and encourages another region to begin star formation.

The central bulge at the center of a spiral galaxy contains older stars, similar to an elliptical galaxy. And at the very center, there’s always a supermassive black hole containing millions of times the mass of the Sun.

Spiral galaxies are also surrounded by a vast spheroidal halo of stars. These stars might not have formed in the galaxy, but were stolen through successive mergers with other galaxies. This galactic halo also contains many globular star clusters.

Astronomers think that spiral galaxies are slowly built over time through the merger of smaller galaxies. As these tiny galaxies came together, their total momentum set the merged galaxy spinning. This spin flattened out the galaxy and set the spiral arms in motion.

We have written many articles about the galaxies for Universe Today. Here’s an article with twin spiral galaxies interacting, and here’s spiral galaxy NGC 2403.

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.

Posted on by Fraser Cain

Galaxies

Galaxies
Spiral galaxy NGC 3982. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Hello, is there anyone out there? With all of the galaxies in the Universe, is it possible that there is cognizant life somewhere else? Well, I can not answer that question for you, but I can offer you a great deal of information about galaxies so that you can make your own informed decision on the matter.

Our’s is a spiral galaxy. A spiral galaxy is shaped like a flat disk with a thicker bulge in the center. Bright spiral arms start from the center and then coil outward like a pinwheel. All spirals rotate very slowly. The Milky Way completes a single revolution once every 250 million years.

Most galaxies are billions of years old. The youngest known galaxy is 1 Zwicky 18. At an estimated age of a mere 500 million years, it is a babe in diapers compared to the Milky Way at 10-14 billion years, which is the average age of the known galaxies.

Scientists think that galaxy formation was led by dark matter. This invisible material clumped together and it attracted regular mass with its gravity, channeling material together into larger and larger collections. This process of matter accretion led to the first proto-galaxies.

The Canis Major Dwarf Galaxy is the closest galaxy to ours. It is actually contained within the Milky Way and only 42,000 light years form the galactic core.

In the links below you will find thousands of facts, figures, and images that will help you understand many things about galaxies in general and some specific types. Enjoy your reading.

Posted on by Fraser Cain

Comments: The Crackdown

I’m sure you all knew this was coming. I enabled comments on Universe Today about 6 months ago, and it was great to have everyone’s feedback. But now people are abusing the system, being rude to each other and advertising their personal theories, so it’s time to crack down. I’m going to implement some new policies that should keep things cleaner. This is a shortened version, inspired by the powerful (but fair) rules over at the Bad Astronomy/Universe Today forum.

The writers and I will be deleting any posts that break the following rules:

  • Be nice – Don’t abuse other readers, the writers, or the Universe in general. Don’t swear, make racist or sexist comments, etc. I think you know when you’re being rude. Stop it.
  • Be brief – Don’t write a long rambling comment that’s longer than the original article.
  • Don’t advertise – Don’t use the site to promote your product, service, or your own website. If you’d like to promote your stuff, buy advertising.
  • Don’t promote your personal, alternative physics theories – This is the same as advertising. You’ve got an idea and you want the world to know about it, then start up your own website, and blog away, but don’t do it here.

I’m also going to make commenters register with the website shortly, so you have to create an account to be able to comment.

Posted on by Fraser Cain

Volcanic Mountain

View north into the summit crater of Redoubt volcano where recent eruptions have removed a significant portion of the glacial ice. A remnant shelf of ice remains on the west (right) side of crater, and in this view, fumaroles are rising from near the ice/wall-rock contact. Image Creator: Payne, Allison

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Feel the ground. It’s nice and cool, right? Well, dig down a few kilometers and things really heat up. Once you’re down more than 30 km, and temperatures can reach more than 1,000 degrees C; that’s hot enough to melt rock. The melted rock is called magma, and it collects into vast chambers beneath the Earth’s surface. The molten rock is less dense than the surrounding rock and so it “floats” upwards through cracks and faults. When the magma finds its way to the surface, it erupts as lava, rock, ash and volcanic gases; this is a volcanic mountain.

A volcanic mountain starts out as a simple crack in the Earth called a volcanic vent. Magma erupts out of the ground as lava flows, clouds of ash, and explosions of rock. This material falls back to Earth around the vent, and piles up around it. Over time (and sometimes quite quickly) a volcanic mountain builds up, with the familiar cone shape.

There are different kinds of volcanic mountains. Cinder cone mountains are made up of material blasted out that rains back down. They don’t usually grow too large. Shield volcanoes are built up by many lava flows of low viscosity lava (low viscosity means that it flows more easily). The lava can flow for dozens of kilometers, and the volcano can be very wide. A stratovolcano or composite volcano is made up of many layers of ash, rock and hardened lava. Some of the largest, most impressive volcanoes in the world are stratovolcanoes (think about Mount Fuji or Rainier).

And we don’t just have volcanic mountains here on Earth. The largest mountain in the Solar System is Olympus Mons on Mars. This enormous shield volcano has grown to more than 21 km tall. There are also active volcanoes on Jupiter’s moon Io.

We have written many articles about volcanoes for Universe Today. Here’s an article about the biggest volcano in the Solar System, and here’s an article about different types of volcanoes.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Posted on by Fraser Cain

Dome Mountains

Half dome mountain. Credit: Mila Zinkova

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The interior of the Earth is hot enough to melt rock, and that’s just what happens. Melted rock squeezes together into vast pools of magma beneath the ground. Since it’s less dense than the surrounding rock, it makes its way upward to the surface. If the magma reaches the surface you get a volcano; with the ash, and the lava and the explosions. But if the magma pushes up but doesn’t actually crack through the surface, you can get a dome mountain.

Dome mountains don’t usually get as high as folded mountains because the force of the magma underneath doesn’t push hard enough. Over a long period, the magma cools to become cold, hard rock. The result is a dome-shaped mountain.

Over long periods of time, erosion wipes away the outer layers of the mountain, exposing the dome-shaped cooled magma of harder rock.

An example of a dome-shaped mountain is Half Dome in the Sierra Nevada range in California. It’s made of granite, and was once a large blob of magma pushed up through the Earth. Granite is much harder than other rock, and so it doesn’t erode as easily as the rest of the mountain. The softer layers of sedimentary rock were washed away, leaving the hard granite dome.

Other dome mountains aren’t so easy to spot. You need satellite images to see the circular shape in the Earth’s surface.

We have written many articles about the Earth for Universe Today. Here’s an article about how satellites can measure the movement of the Earth after an earthquake.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Posted on by Fraser Cain

Fault-Block Mountains

Diagram of a fault-block mountain range.

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Fault-block mountains are formed by the movement of large crustal blocks when forces in the Earth’s crust pull it apart. Some parts of the Earth are pushed upward and others collapse down.

To understand a fault-block mountain, or sometimes referred as a “fault mountain”, you need to understand what a fault is. Faults are simply cracks in the Earth’s crust. The surface of the Earth can move along these faults, and displace rock layers on either side. Wherever you have movement along the faults, you can get earthquakes, and over long periods of time mountains form under the intense pressure.

Large blocks of rock along the sides of these faults can be uplifted and tilted sideways by this incredible force. And then, on the opposite sides of the faults, the ground tilts downwards forming a depression. This depression gets filled in and leveled by the erosion of the mountains above.

The Sierra Nevada mountains in California are an example of a fault-block mountain range.

We have written many articles about the Earth for Universe Today. Here’s an article that shows how satellites can calculate the movement of the Earth during earthquakes.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Posted on by Fraser Cain

Fold Mountains

Mount Everest from Kalapatthar. Photo: Pavel Novak

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Some of the most dramatic mountains in the world are fold mountains. These are created when two of the Earth’s tectonic plates crash together – like in a head-on car crash. The edges of the two plates buckle and fold, and the peaks of these folds are mountains. Entire mountain ranges, thousands of kilometers long, are created during these slow motion collisions between tectonic plates.

Some famous examples of fold mountains are the Himalayan mountains in Asia and the Rocky Mountains in North America. Consider the fact that the Earth’s tectonic plates are moving very slowly, just a few centimeters every year. These folding collisions play out in incredibly slow motion, taking millions of years. The Indian subcontinent crashed into Asia 24 million years ago, and since then it has built up the Himalayan mountains – the tallest mountains in the world. In fact, the Himalayans are still growing.

Want to make your own folded mountain range? Take two flat strips of modeling clay and put them side to side. Then slowly push one strip into the other and you’ll see how one or both will crumple up under the pressure. You’ll make your own mini mountain range.

We have written many articles about mountains for Universe Today. Here’s an article about different types of mountains.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Posted on by Fraser Cain

Types of Mountains

Mount Everest from Kalapatthar. Photo: Pavel Novak

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One feature of the Earth that you can’t miss are its mountains. But did you know there are different types of mountains? The different mountain types are formed in different ways, through tectonic plates crunching into each other, or sliding past one another, or even from magma coming up out of the Earth. The mountains are different in their appearance, and in their formation. Let’s take a look at each of the major mountain types.

Fold Mountains
The most common type of mountain in the world are called fold mountains. When you see vast mountain ranges stretching on for thousands of kilometers, those are fold mountains. Fold mountains are formed when two of the Earth’s tectonic plates collide head on; like two cars crashing together. The edges of each tectonic plate crumple and buckle, and these create the mountains. Some examples of fold mountain ranges include the Rocky Mountains in North America, and the Himalayan Mountains in Asia.

Fault-Block Mountains
Fault-block mountains (or just “block mountain“) are created when faults or cracks in the Earth’s crust force materials upward. So instead of folding, like the plate collision we get with fold mountains, block mountains break up into chunks and move up or down. Fault-block mountains usually have a steep front side and then a sloping back side. Examples of fault-block mountains include the Sierra Nevada mountains.

Dome Mountains
Dome mountains are created when a large amount of magma pushes up from below the Earth’s crust, but it never actually reaches the surface and erupts. And then, before it can erupt, the source of the magma goes away and the pushed up rock cools and hardens into a dome shape. Since the dome is higher than its surroundings, erosion works from the top creating a circular mountain range.

Volcanic Mountains
Here’s a fairly familiar kind of mountain. Volcanic mountains are created when magma from beneath the Earth makes its way to the surface. When does get the surface, the magma erupts as lava, ash, rock and volcanic gases. This material builds up around the volcanic vent, building up a mountain. Some of the largest mountains in the world were created this way, including Mauna Loa and Mauna Kea on the Big Island of Hawaii. Other familiar volcanoes are Mt. Fuji in Japan and Mt. Rainier in the US.

Plateau Mountains
Plateau mountains are actually formed by the Earth’s internal activity; instead, they’re revealed by erosion. They’re created when running water carves deep channels into a region, creating mountains. Over billions of years, the rivers can cut deep into a plateau and make tall mountains. Plateau mountains are usually found near folded mountains.

We have written many articles about mountains for Universe Today. Here’s an article about a massive mountain range seen on Titan, and the search for a mountain of eternal sunlight on the Moon.

Here are more article about mountains:

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/List_of_mountain_types
http://www.wvgs.wvnet.edu/www/geology/geolf001.htm
http://library.thinkquest.org/05aug/00184/Mountain%20Ranges%20Page.htm

Posted on by Fraser Cain

Pelean Eruption

Mount Pelee

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Pelean eruptions, or Nuee Ardente eruptions occur when a large quantity of gas, dust, ash and lava fragments are blown out of a volcano’s central crater. This material falls back, and then travels down the side of the volcano at tremendous speeds – faster than 150 km/hour.

These eruptions are also known as pyroclastic flows, and they’re one of the most dangerous kinds of eruptions that volcanoes can do. Material blasted out in a Pelean eruption can tear through populated areas, killing thousands of people.

Pelean eruptions got their name from Mont Pelee, the volcano that caused tremendous destruction on Martinique, Lesser Antilles in 1902. The Pelean eruption and following pyroclastic flows killed more than 30,000 people in the worst volcanic disaster of the 20th century. The town of St. Pierre was effectively wiped off the map by a series of powerful eruptions.

We have written many articles about volcanoes for Universe Today. Here’s an article about Plinian eruptions, and here’s an article about Strombolian eruptions.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Posted on by Fraser Cain

Volcano Dangers

View north into the summit crater of Redoubt volcano where recent eruptions have removed a significant portion of the glacial ice. A remnant shelf of ice remains on the west (right) side of crater, and in this view, fumaroles are rising from near the ice/wall-rock contact. Image Creator: Payne, Allison

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Volcanoes make bad neighbors. Between 1900 and 1986, volcanoes have killed an average of 845 people every year. And volcanoes have so many ways to kill you, from the hot lava flows and clogging ash to the rock bombs and toxic fumes. Let’s take a look at dangerous volcanoes, and their associated volcanic dangers.

One of the most familiar aspects of a volcanic eruption are the lava flows. You might be surprised to know that lava flows are actually one of the least dangerous ways that volcanoes can try to kill you. Lava flows rarely move faster than walking speed, so you can easily outrun and avoid them. Buildings, roads and trees aren’t so lucky; however, and can be destroyed by the crushing weight and burning temperature of a lava flow.

Poisonous volcanic gases are a danger from volcanoes too. During an eruption, volcanoes can release vast amounts of water vapor, carbon dioxide and sulfur dioxide. If you encounter a cloud of pure carbon dioxide, you can suffocate without air. Other volcanic gases are poisonous and people have even been killed by acidic gases (ouch).

During an explosive eruption, volcanic ash is hurled up to 45 km in the air. Several cubic kilometers of ash can rain down around the volcano, covering everything in a thick layer of ash. It might look a bit like snow, but it’s rock, and very heavy. Just a few centimeters of volcanic ash is heavy enough to collapse buildings and kill crops.

You also have to watch out for rocks hurled out of volcanoes during an eruption. These volcanic bombs can be meters across and be hurled hundreds of meters and even kilometers away from the volcanic vent. Imagine a rock the size of a house falling from the sky.

But the volcano danger that kills more people every year is known as a pyroclastic flow. In some eruptions, hot rock and gas flow down the side of the volcano at speeds greater than 700 km/hour. A wall of material as hot as 1,000 degrees C plunges down the side of the volcano and can travel hundreds of kilometers away from the vent, destroying anything in its path. This is what destroyed the ancient Roman town of Pompeii.

Volcanoes are beautiful sights, but they have their dangers too, so be careful.

We have written many articles about volcanoes for Universe Today. Here’s an article about lightning around Redoubt volcano in Alaska.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

References:
http://www.geo.mtu.edu/volcanoes/hazards/primer/
http://www.appstate.edu/~abbottrn/vlcns/