Universe Today

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A now-you-see-it, now-you don’t accretion disk (white and blue in the artist’s rendering at left) has tipped astronomers to the birth of a superfast, “millisecond” pulsar that was happening right before their eyes — er, their radio telescopes.

The new finding confirms the long-suspected evolutionary connection between a neutron star and a millisecond pulsar: they are two life stages of the same object.

Anne Archibald, of McGill University in Montreal, Canada and her colleagues announced their discovery in the May 21 online issue of the journal Science.

Pulsars are superdense neutron stars, the remnants left after massive stars have exploded as supernovae. Their powerful magnetic fields generate lighthouse-like beams of light and radio waves that sweep around as the star rotates, and are detectable as pulses on Earth.

Some, dubbed millisecond pulsars, rotate hundreds of times a second. Astronomers believe the fast rotation is caused by a companion star dumping material onto the neutron star and spinning it up.

The material from the companion would form a flat, spinning disk around the neutron star, and during this period, the radio waves characteristic of a pulsar would not be seen coming from the system. As the amount of matter falling onto the neutron star decreased and stopped, the radio waves could emerge, and the object would be recognized as a pulsar.

This sequence of events is apparently what happened with a binary-star system some 4000 light-years from Earth, in the constellation of Sextans just south of Leo. The millisecond pulsar in this system, called J1023, was discovered by the National Science Foundation’s Robert C. Byrd Green Bank Telescope (GBT) in West Virginia in 2007 in a survey led by astronomers at West Virginia University and the National Radio Astronomy Observatory.

The astronomers then found that the object had been detected by National Science Foundation’s Very Large Array radio telescope in New Mexico, during a large sky survey in 1998, and had been observed in visible light by the Sloan Digital Sky Survey in 1999, revealing a Sun-like star.

When observed again in 2000, the object had changed dramatically, showing evidence for a rotating disk of material, called an accretion disk, surrounding the neutron star. By May of 2002, the evidence for this disk had disappeared.

“This strange behavior puzzled astronomers, and there were several different theories for what the object could be,” said Ingrid Stairs of the University of British Columbia.

The 2007 GBT observations showed that the object is a millisecond pulsar, spinning 592 times per second.

“No other millisecond pulsar has ever shown evidence for an accretion disk,” Archibald said. “We know that another type of binary-star system, called a low-mass X-ray binary (LMXB), also contains a fast-spinning neutron star and an accretion disk, but these don’t emit radio waves. We’ve thought that LMXBs probably are in the process of getting spun up, and will later emit radio waves as a pulsar. This object appears to be the ‘missing link’ connecting the two types of
systems.”

The scientists have studied J1023 in detail with the GBT, with the Westerbork radio telescope in the Netherlands, with the Arecibo radio telescope in Puerto Rico, and with the Parkes radio telescope in Australia. Their results indicate that the neutron star’s companion has less than half the Sun’s mass, and orbits the neutron star once every four hours and 45 minutes.

Image caption: Material from distended “normal” star. right, streams onto accretion disk (white and blue) surrounding neutron star, left. Credit: Bill Saxton, NRAO/AUI/NSF

Source: National Radio Astronomy Observatory. Animations are here and here. Warning: that last one may cause dizziness.

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A volcano caldera is a depression in the ground created by the collapse of land after a volcanic eruption. In some cases, the caldera is created slowly, when the ground sinks down after a magma chamber is emptied. In other situations, the magma explodes violently, and the caldera is the deep pit in the ground that remains after the volcano blows its top off.

A caldera forms when the magma chamber beneath a volcano is emptied in a large eruption. If the eruption happens quickly enough, the empty chamber beneath isn’t strong enough to support the weight of the volcano and it collapses down. This can happen in a single cataclysmic event, or over the course of several eruptions. Volcano calderas can be hundreds and even thousands of square kilometers in area.

There are many famous examples of volcano calderas. Crater Lake in Oregon was created when a volcano detonated around 10,000 years ago. Over time water filled up the nearly 597 meter deep caldera, making it the deepest lake in the United States, and the second deepest lake in North America (Great Slave Lake is the deepest). Another example of a volcanic caldera is the Yellowstone Caldera, which last erupted 640,000 years ago. It released 1,000 cubic kilometers of rock, covering much of North America in two meters of debris.

There are other examples of non-explosive volcano calderas. For example, the volcano Kilauea on the Big Island of Hawaii has a magma chamber that’s emptied by large lava flows, and not explosive eruptions. This causes the ground to collapse down after the lava eruptions, sinking many meters into the ground.

Volcano calderas have been seen on many other places in the Solar System. Although there aren’t active volcanoes on Mars today, regions of Mars were active billions of years ago, and there are many regions with large calderas. Jupiter’s moon Io is an example of a place with active calderas being created right now.

We have written many articles about volcanoes for Universe Today. Here’s an article about types of eruptions, and here’s an article about volcanic craters (not to be confused with calderas).

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://vulcan.wr.usgs.gov/Volcanoes/CraterLake/Locale/framework.html
http://volcanoes.usgs.gov/yvo/about/faq/faqhistory.php
http://hvo.wr.usgs.gov/gallery/kilauea/caldera/main.html

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Some craters are formed by a meteor impact, when a space rock strikes the Earth. And other craters are volcanic in origin. A volcano crater is a circular depression around a volcanic vent. This is where the lava, ash and rock erupt out of a volcano. In most situations, the volcano crater is located at the top of the volcano.

Think of a classic cone-shaped volcano, with steep sides and a slightly flattened top. If you could climb up to the top of the volcano and peer over the edge, you would look down into the volcano crater. And when the volcano does erupt, the material comes out of this volcano crater.

But not all volcano craters are found at the top of the volcano. They can also appear on the flanks of large volcanoes, when side vents reach the surface and erupt. These are known as flank craters, while the crater on top is called a summit crater. The volcano crater will often fill up with water and create a crater lake.

One kind of eruption creates a crater without a volcanic cinder cone at all: a phreatic eruption. This is a situation where magma rises through water-saturated rocks and causes steam to build up under the surface. The pressure from all the steam causes the rock to explode outward and create a volcanic crater. These can be strange to discover as they often appear in plains, far away from any other obvious volcanoes.

When the ground depresses down because a volcano’s magma chamber has been emptied, this is actually known as a caldera, and not a volcano crater.

We have written many articles about volcanoes for Universe Today. Here’s an article about types of volcanoes, and here’s an article about cinder cone 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.

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The Pacific Ring of Fire is a region that surrounds the Earth’s Pacific Ocean, and is known for its volcanoes and earthquake activity. The Ring of Fire has a total of 452 volcanoes, and has 75% of the Earth’s active and dormant volcanoes. The whole Ring of Fire stretches for 40,000 km in length.

The Ring of Fire is created by the Earth’s plate tectonics. The Earth’s crust is broken up into plates which float on top of the mantle. When these plates come together, you can get volcanoes and earthquakes. The eastern side of the Ring of Fire has the Nazca Plate and the Cocos Plate being subducted (going underneath) the South American Plate. And in the North, the Pacific Plate and the Juan de Fuca Plate are being subducted underneath the North American Plate. The Pacific plate is also subducting underneath the Kamchatka Peninsula and Japan.

Because of all this subduction, there are many cracks in the Earth’s crust where magma can reach the surface and erupt as volcanoes. There are volcanoes in Chile, Mexico, the United States, Canada, Russia, Japan, the Philippines, Indonesia, New Zealand and Antarctica.

Some of the most famous volcanoes on Earth are in the Ring of Fire. A famous volcano, Mount St. Helens, which erupted in 1980 is a good example, or Mount Rainier in Washington State, or Mount Shasta in California. The recent eruption of Mount Redoubt in Alaska is part of the Ring of Fire. And so are Mount Pinatubo in the Philippines and Mount Fuji in Japan.

The Ring of Fire has produced more than just volcanoes. It has also created islands and mountain chains around the Pacific Ocean. The Aleutian Islands in Alaska are all volcanic, and Japan is part of the subduction of the Pacific Plate going underneath the Eurasian Plate.

I personally live in the Ring of Fire, so I’m aware that there are dangers to living in this part of the world. There can be powerful earthquakes, huge tsunami, and new volcanic eruptions. Every year the Ring of Fire volcanoes demonstrate just how active they still are.

We have written many articles all about volcanoes for Universe Today. Here’s an article about different types of volcanoes, and here’s an article about different types of 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.

Reference:
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html