U.S. astronomers see first magnetic reversal of star other than Sun

William Atkins
Tuesday, 26 February 2008 20:44

Science - Space

Astronomers have found the first evidence of a star changing its magnetic field. The Sun is known to reverse its north and south magnetic poles, but astronomers had never seen another star do it. Now they have seen it in tau Bootis.            

Tau Bootis is a star that is about 51 light-years away from Earth.

One light-year is the amount of distance that light travels in one Earth-year while in the vacuum of space. Light travels at the “speed of light,” which is approximately 186,000 miles (300,000 kilometers) per second. (Comparing this to a speed of a car, which may travel 67 miles per hour, light travels over 670 million miles in one hour.)

Such a discovery should help gain more knowledge on the Sun’s eleven-year magnetic cycle, which dramatically affects the climate of the Earth. Every eleven years or so the Sun reverses its magnetic field, making its north magnetic pole its south magnetic pole, and changing its south pole to its north pole.

When this happens, the number and intensity of sunspots on the Sun reaches its maximum, which sends more charged particles toward the Earth causing disruptions in our power systems, earth-orbiting satellites, and causing possible health problems to astronauts in space.

Astronomers think that star tau Bootis changes its magnetic field because of a large planet that is orbiting very close to it.

The planet is in an approximate 3.3-day orbit about tau Bootis. As a comparison, the planet Earth orbits the Sun once in about 365 days, and the planet Mercury, the solar system’s closest planet to the Sun, orbits the Sun once every 88 days.

In addition, the planet around tau Bootis is about 6.5 times heavier than Jupiter, the largest planet in our solar system. It is 2.5 times more massive than all of the other planets—combined—in the solar system. It is over 317 times more massive than the Earth—with a mass of about 1.9 x 10²⁷ kilograms.

So, the large planet’s gravity has a very big affect on tau Bootis. Scientists think that the large gravitation pull of the planet causes the star to frequently reverse its magnetic field.

One group of astronomers studying tau Bootis and other similar stars is Andrew Collier Cameron and fellow colleagues at the University of St. Andrews (Scotland). They used telescopes in Hawaii and France to measure the polarization of light from tau Bootis. Their research showed than some time between 2006 and 2007 tau Bootis reversed its magnetic field.

Their research also points to the possibility that tau Bootis reverses its magnetic field more frequently than the Sun.

The report of their discovery, the first of its kind, will appear in a future issue of the Monthly Notices of the Royal Astronomical Society.

Astronomer Marina Romanova (Cornell University, United States) stated in the article “Stellar Switch” in the magazine Science News (February 23, 2008, subscription required) that, the frequent magnetic field reversals of stars “makes it very attractive to monitor tau Bootis and possibly other similar stars with high differential rotation, which may improve our understanding of the generation and dynamics of magnetic fields in stars.”