Universe Today

Image credit: ESA

New observations from the European Space Agency’s Ulysses spacecraft show that galactic dust in the Milky Way is passing through our solar system more than normal. The Sun’s magnetic field normally forms a barrier around our solar system that forces dust to go around us, but the Sun has reached the high point of its 11-year cycle, and the magnetic field is highly disordered – so the interstellar dust is coming through the solar system more directly. Although it has no direct effects on the planets, the dust impacts asteroids and comets producing more fragments, and may increase the amount of material that rains down on the Earth.

Since early 1992 Ulysses has been monitoring the stream of stardust flowing through our Solar System. The stardust is embedded in the local galactic cloud through which the Sun is moving at a speed of 26 kilometres every second. As a result of this relative motion, a single dust grain takes twenty years to traverse the Solar System. Observations by the DUST experiment on board Ulysses have shown that the stream of stardust is highly affected by the Sun’s magnetic field.

In the 1990s, this field, which is drawn out deep into space by the out-flowing solar wind, kept most of the stardust out. The most recent data, collected up to the end of 2002, shows that this magnetic shield has lost its protective power during the recent solar maximum. In an upcoming publication in the Journal of Geophysical Research ESA scientist Markus Landgraf and his co-workers from the Max-Planck-Institute in Heidelberg report that about three times more stardust is now able to enter the Solar System.

The reason for the weakening of the Sun’s magnetic shield is the increased solar activity, which leads to a highly disordered field configuration. In the mid-1990s, during the last solar minimum, the Sun’s magnetic field resembled a dipole field with well-defined magnetic poles (North positive, South negative), very much like the Earth. Unlike Earth, however, the Sun reverses its magnetic polarity every 11 years. The reversal always occurs during solar maximum. That’s when the magnetic field is highly disordered, allowing more interstellar dust to enter the Solar System. It is interesting to note that in the reversed configuration after the recent solar maximum (North negative, South positive), the interstellar dust is even channelled more efficiently towards the inner Solar System. So we can expect even more interstellar dust from 2005 onwards, once the changes become fully effective.

While grains of stardust are very small, about one hundredth the diameter of a human hair, they do not directly influence the planets of the Solar System. However, the dust particles move very fast, and produce large numbers of fragments when they impact asteroids or comets. It is therefore conceivable that an increase in the amount of interstellar dust in the Solar System will create more cosmic dust by collisions with asteroids and comets. We know from the measurements by high-flying aircraft that 40 000 tonnes dust from asteroids and comets enters the Earth’s atmosphere each year. It is possible that the increase of stardust in the Solar System will influence the amount of extraterrestrial material that rains down to Earth.

Original Source: ESA News Release

Image credit: Chandra

Many of the stars that we see in globular star clusters are actually binary stars, formed when two stars get caught in each other’s gravity. But new research from the Chandra X-Ray Observatory shows that there are many more binary objects which are stars orbiting a neutron star or white dwarf. Chandra can detect the unique x-ray signature that a neutron star gives off, which is invisible in an optical telescope. The research seems to indicate that these neutron star binaries form much more commonly found in globular clusters than in other parts of a galaxy.

NASA’s Chandra X-ray Observatory has confirmed that close encounters between stars form X-ray emitting, double-star systems in dense globular star clusters. These X-ray binaries have a different birth process than their cousins outside globular clusters, and should have a profound influence on the cluster’s evolution.

A team of scientists led by David Pooley of the Massachusetts Institute of Technology in Cambridge took advantage of Chandra’s unique ability to precisely locate and resolve individual sources to determine the number of X-ray sources in 12 globular clusters in our Galaxy. Most of the sources are binary systems containing a collapsed star such as a neutron star or a white dwarf star that is pulling matter off a normal, Sun-like companion star.

“We found that the number of X-ray binaries is closely correlated with the rate of encounters between stars in the clusters,” said Pooley. “Our conclusion is that the binaries are formed as a consequence of these encounters. It is a case of nurture not nature.”

A similar study led by Craig Heinke of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. confirmed this conclusion, and showed that roughly 10 percent of these X-ray binary systems contain neutron stars. Most of these neutron stars are usually quiet, spending less than 10% of their time actively feeding from their companion.

A globular cluster is a spherical collection of hundreds of thousands or even millions of stars buzzing around each other in a gravitationally-bound stellar beehive that is about a hundred light years in diameter. The stars in a globular cluster are often only about a tenth of a light year apart. For comparison, the nearest star to the Sun, Proxima Centauri, is 4.2 light years away.

With so many stars moving so close together, interactions between stars occur frequently in globular clusters. The stars, while rarely colliding, do get close enough to form binary star systems or cause binary stars to exchange partners in intricate dances. The data suggest that X-ray binary systems are formed in dense clusters known as globular clusters about once a day somewhere in the universe.

Observations by NASA’s Uhuru X-ray satellite in the 1970’s showed that globular clusters seemed to contain a disproportionately large number of X-ray binary sources compared to the Galaxy as a whole. Normally only one in a billion stars is a member of an X-ray binary system containing a neutron star, whereas in globular clusters, the fraction is more like one in a million.

The present research confirms earlier suggestions that the chance of forming an X-ray binary system is dramatically increased by the congestion in a globular cluster. Under these conditions two processes, known as three-star exchange collisions, and tidal captures, can lead to a thousandfold increase in the number of X-ray sources in globular clusters.

In an exchange collision, a lone neutron star encounters a pair of ordinary stars. The intense gravity of the neutron star can induce the most massive ordinary star to “change partners,” and pair up with the neutron star while ejecting the lighter star.

A neutron star could also make a grazing collision with a single normal star, and the intense gravity of the neutron star could distort the gravity of the normal star in the process. The energy lost in the distortion, could prevent the normal star from escaping from the neutron star, leading to what is called tidal capture.

“In addition to solving a long-standing mystery, Chandra data offer an opportunity for a deeper understanding of globular cluster evolution,” said Heinke. “For example, the energy released in the formation of close binary systems could keep the central parts of the cluster from collapsing to form a massive black hole.”

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Original Source: Chandra News Release