Universe Today

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A recently discovered type of star called ultracool subdwarfs have unusual and wild orbits unlike any ever encountered, with one such star having an orbit that loops outside and then back into our galaxy. Astronomers have now been able to clarify the origins of these unusual, faint stars that may have come from other galaxies.

“The orbits we calculated for these object are far more diverse than we have originally thought,” said Adam Burgasser of MIT. “One of the stars we studied has an orbit that appears to take it outside the Milky Way galaxy, and it may even have an extra-galactic origin.” Burgasser and colleague John Bochanski of MIT presented their findings on June 9, in a press conference at the American Astronomical Society’s meeting in Pasadena, California.

Ultracool subdwarfs were first recognized as a unique class of stars in 2003, and are distinguished by their low temperatures (“ultracool”) and low concentrations of elements other than hydrogen and helium (“subdwarf”). They sit at the bottom end of the size range for stars, and some are so small that they are closer to the planet-like objects called brown dwarfs. Only a few dozen ultracool subdwarfs are known today, as they are both very faint — up to 10,000 times fainter than the Sun — and extremely rare.
While most stars travel in circular orbits around the center of the Milky Way, ultracool subdwarfs and eccentric and very fast orbits. They appear to be traveling at very high speeds, up to 500 km/s, or over a million miles per hour.

“If there are interstellar cops out there, these stars would surely lose their driver’s licenses,” said Burgasser.

Burgasser’s team of astronomers assembled measurements of the positions, distances and motions of roughly two dozen of these rare stars. Robyn Sanderson, co-author and MIT graduate student, then used these measurements to calculate the orbits of the subdwarfs using a numerical code developed to study galaxy collisions. Despite doing similar calculations for other types of low-mass stars, “these orbits were like nothing I’d ever seen before,” said Sanderson.

Watch this movie of the projected “wild ride” of one of these ultracool subdwarfs (great music!)

Sanderson’s calculations showed an unexpected diversity in the ultracool subdwarf orbits. Some plunge deep into the center of the Milky Way on eccentric, comet?like tracks; others make slow, swooping loops far beyond the Sun’s orbit. Unlike the majority of nearby stars, most of the ultracool subdwarfs spend a great deal of time thousands of light?years above or below the disk of the Milky Way.

“Someone living on a planet around one of these subdwarfs would have an incredible nighttime view of a beautiful spiral galaxy — our Milky Way — spread across the sky,” Burgasser speculated.

Sanderson’s orbit calculations confirm that all of the ultracool subdwarfs are part of the Milky Way’s halo, a widely dispersed population of stars that likely formed in the Milky Way’s distant past. However, one of the subdwarfs, a star named 2MASS 1227?0447 in the constellation Virgo, has an orbit indicating that it might have a very different lineage, possibly extragalactic.

“Our calculations show that this subdwarf travels up to 200,000 light years away from the center of the Galaxy, almost 10 times farther than the Sun,” said Bochanski. This is farther than many of the Milky Way’s nearest galactic neighbors, suggesting that this particular subdwarf may have originated somewhere else.

“Based on the size of its one billion?year orbit and direction of motion, we speculate that 2MASS 1227?0447 might have come from another, smaller galaxy that at some point got too close to the Milky Way and was ripped apart by gravitational forces,” said Bochanksi.

Astronomers have previously identified streams of stars in the Milky Way originating from neighboring galaxies, but all have been distant, massive, red giant stars. The ultracool subdwarf identified by Burgasser and his team is the first nearby, low?mass star to be found on such a trajectory. “If we can identify what stream this star is associated with, or which dwarf galaxy it came from, we could learn more about the types of stars that have built up the Milky Way’s halo over the past 10 billion years,” said Burgasser.

Sources: AAS, MIT (see more images and animations)

The red supergiant star Betelgeuse is undoubtedly enormous. But it’s shrinking, and astronomers aren’t sure why.

Researchers at the University of California at Berkeley have been monitoring the star by aiming the Infrared Spatial Interferometer, atop Mt. Wilson in Southern California, toward the star’s home in the constellation Orion. Since 1993, the Betelgeuse star (pictured in a NASA image at left) has shrunk in diameter by more than 15 percent.

Betelgeuse is so big that in our solar system it would reach to the orbit of Jupiter. Its radius is about five astronomical units, or five times the radius of Earth’s orbit. Its measured shrinkage means the star’s radius has shrunk by a distance equal to the orbit of Venus.

“To see this change is very striking,” said Charles Townes, a UC Berkeley professor emeritus of physics. “We will be watching it carefully over the next few years to see if it will keep contracting or will go back up in size.”

Townes and his colleague, Edward Wishnow, a research physicist at UC Berkeley, presented their findings at a press conference on Tuesday during the Pasadena meeting of the American Astronomical Society. The results also appeared June 1 in The Astrophysical Journal Letters.

Despite Betelgeuse’s diminished size, Wishnow pointed out that its visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers, has shown no significant dimming over the past 15 years.

The ISI has been focusing on Betelgeuse for more than 15 years in an attempt to learn more about these giant massive stars and to discern features on the star’s surface, Wishnow said. He speculated that giant convection cells on the star’s surface might affect the measurements. Like convection granules on the Sun, the cells are so large that they bulge out from the surface. Townes and a former graduate student observed a bright spot on the surface of Betelgeuse in recent years, although at the moment, the star appears spherically symmetrical.

“But we do not know why the star is shrinking,” Wishnow said. “Considering all that we know about galaxies and the distant universe, there are still lots of things we don’t know about stars, including what happens as red giants near the ends of their lives.”

Betelgeuse was the first star ever to have its size measured, and even today is one of only a handful of stars that appears through the Hubble Space Telescope as a disk rather than a point of light. In 1921, Francis G. Pease and Albert Michelson used optical interferometry to estimate its diameter was equivalent to the orbit of Mars. Last year, new measurements of the distance to Betelgeuse raised it from 430 light-years to 640, which increased the star’s diameter from about 3.7 to about 5.5 AU.

“Since the 1921 measurement, its size has been re-measured by many different interferometer systems over a range of wavelengths where the diameter measured varies by about 30 percent,” Wishnow said. “At a given wavelength, however, the star has not varied in size much beyond the measurement uncertainties.”

The measurements cannot be compared anyway, because the star’s size depends on the wavelength of light used to measure it, Townes said. This is because the tenuous gas in the outer regions of the star emits light as well as absorbs it, which makes it difficult to determine the edge of the star.

The Infrared Spatial Interferometer, which Townes and his colleagues first built in the early 1990s, sidesteps these confounding emission and absorption lines by observing in the mid-infrared with a narrow bandwidth that can be tuned between spectral lines. The technique of stellar interferometry is highlighted in the June 2009 issue of Physics Today magazine.

Townes, who turns 94 in July, plans to continue monitoring Betelgeuse in hopes of finding a pattern in the changing diameter, and to improve the ISI’s capabilities by adding a spectrometer to the interferometer.

“Whenever you look at things with more precision, you are going to find some surprises,” he said, “and uncover very fundamental and important new things.”

Sources: AAS and UC Berkeley. The paper is available here.

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The Hawaiian Island chain is a group of islands formed by the constant eruption of a volcanic hotspot underneath the Earth’s tectonic plates. The slow movement of the Pacific plate carries the islands away from the volcano hotspot, so the volcanoes go dormant and eventually extinct. The tallest volcano and the largest volcano in the world are located in Hawaii. Even the most active volcano in the world can be found in Hawaii. If you want to see volcanoes, it’s the place to go.

Hawaii Volcanoes

• Mauna Loa – This active shield volcano is the second tallest volcano in the world, but it’s thbiggest volcano in the world. It has erupted within the last century.
• Hualalai – The third most active volcano in Hawaii.
• Kohala – the oldest of the 5 shield volcanoes that make up the Big Island of Hawaii.
• Kilauea – An active volcano on the eastern side of the Island of Hawaii. It’s in an almost constant state of eruption.
• Mauna Kea – The tallest volcano in the world, located on the Big Island of Hawaii.

We have written many article about volcanoes for Universe Today. Here’s some information about shield volcanoes, the kind found in Hawaii.

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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Scoria is a kind of rock produced by volcanic activity. Like pumice, it forms when lava which is rich in gas cools quickly. It forms when molten rock is rising in a volcanic pipe, the decreasing pressure allows the gas to expand out (like opening a can of soda releases carbon dioxide).

Scoria rocks can either form inside the volcano, and be ejected out in eruptions. Or it can erupt as lava that cools quickly before the gas bubbles can escape the rapidly cooling rock. Scoria is similar to pumice, in that it has bubbles of gas trapped within it, but the bubbles are much smaller. Unlike pumice, scoria doesn’t usually float in water.

Another name for scoria is cinder, and it’s the primary component of cinder cones. These are relatively small volcanoes that appear suddenly, built up to a maximum height of a few hundred meters and then go extinct. The cone builds up from the scoria, rock and ash ejected from the volcano, which rains back down around it.

While pumice ranges in color from white to black, scoria is darker in color, ranging from dark brown, to black to red. The Easter Island statues were carved out of volcanic rock, and the red stones on top were carved out of a different type of scoria rock prized for its red color.

We have written many articles about volcanoes and rocks for Universe Today. Here’s an article about obsidian, a type of volcanic glass produced when lava cools quickly. And here’s an article about basalt, rocks formed from cooled lava.

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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A computer simulation of the dark nebula Barnard 68 suggests the cloud will collapse into a brand new star relatively soon… at least on an astronomical time scale.

Astrophysicist João Alves, director of the Calar Alto Observatory in Spain, and his colleague Andreas Bürkert from  the University of Munich, believe the dark cloud Barnard 68 will inevitably collapse and give rise to the new star, according to an article published recently in the April 2009 issue of  The Astrophysical Journal.

Barnard 68 (B68) is a dark nebula about 400 light years away in the constellation of Ophiuchus. Such nebulae are interstellar clouds of dust and gas located within the Milky Way which block out the light of the stars and other objects behind them.

Most astronomers believe stars form from giant gas clouds which collapse under their own gravity until high density and temperatures lead to nuclear fusion.  Although many details of the process are still not understood, the new study may be able to shed some light on this.

Alves and Bürkert suggest the collision of two gas clouds could be the mechanism that activates the birth of a star. They suggest Barnard 68 is already in an initial unstable state and that it will collapse “soon” – within some 200,000 years.

Images show B68 is a cold gas cloud with a mass equivalent to that of two suns.  But there’s a smaller cloud just 1/10 as massive getting close enough to collide with the larger cloud.

In order to prove their theory, the two astrophysicists have simulated the scenario in a supercomputer at the University of Munich. They modelled two globules separated by one light year, with masses and speeds similar to those of Barnard 68 and its “small” companion. By using a numerical algorithm, the researchers showed how these two virtual gas clouds evolved over time.

The results showed that the smaller globule penetrated the larger one after around 1.7 million years at a speed of 370 metres per second. The model also showed that the stability of the initial situation declined over time. At the moment when the two globules merged, enormous densities were generated, making the system collapse and creating the ideal conditions for the formation of a star.

The researchers varied the physical parameters of the globules until they worked out the circumstances in which the merger of two gas clouds will lead to their subsequent collapse. According to Bürkert and Alves’ calculations, a new star system will form from B68 within 200,000 years.

Source: FECYT – Spanish Foundation for Science and Technology

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Almost immediately after it was discovered last November by 14-year-old Caroline Moore of the Puckett Observatory Supernova Search Team, professional astronomers knew supernova SN2008ha was a strange one.  The spectra of the blast showed no signs of hydrogen, which meant it must be a Type Ia supernova caused by the explosion of a white dwarf accreting matter in a binary star system.  But if so, why was it some 50 times fainter than other supernova of its type?

Now in a controversial new paper in the journal Nature, astronomers from Queen’s University Belfast have proposed a new explanation of this supernova.  The researchers, led by Dr. Stefano Valenti, suggest that even though the explosion contained no hydrogen, SN2008ha could be a Type II supernova, the kind caused by the core collapse of a massive star.

Valenti and his colleagues argue that, despite the lack of hydrogen, the spectrum of SN2008ha more closely resembles Type II supernovae.  They cite the lack of emission lines from ionized silicon as as evidence of why SN2008ha is not a Type Ia.  And they cite other supernovae that exhibited similar characteristics, which he says might be less extreme examples of hydrogen-deficient Type II supernovae.

“SN2008ha is the most extreme example of a group of supernovae that show similar properties”, said Dr. Valenti. Up until now the community had thought that they were from the explosion of white dwarfs, which we call type  Ia supernovae. But we think SN2008ha doesn’t quite fit this picture and appears physically related to massive stars”.

But if SN2008ha is a Type II supernova, where did the hydrogen go?  The answer might be mass loss.  Some stars are so massive and luminous that they lose their outer hydrogen layers in strong outflowing stellar winds.  And because they’re so massive, their cores collapse into a black hole without transfering energy to the outer layers of the star, which may explain the low luminosity of the explosion.

“The implications are quite important. If this is a massive star explosion, then it is the first one that might fit the theoretical models of massive stars that lose their outer layers through their huge luminosity pressure and then, perhaps, collapse to black holes with a whimper”, said Dr. Valenti.

Professor Stephen Smartt from Queen’s added “This is still quite controversial, we have put this idea forward and it certainly needs to be taken seriously.

Dr. Valenti’s team is keen to use new deep, time resolved surveys of the Universe to find more of these and test their ideas. One such experiment is the first of the Pan-STARRS telescopes that has started surveying the sky in the last month.

Source:  Queen’s University Belfast

Original Paper:  Nature