[Note: NASA/ESA SOHO image]

For years, astronomers have wondered whether stars similar to the Sun
go through periodic cycles of enhanced X-ray activity, like those often
causing troubles to telephone and power lines here on Earth. ESA’s
X-ray observatory XMM-Newton has now revealed for the first time a
cyclic behaviour in the X-ray radiation emitted by a star similar to
the Sun. This discovery may help scientists to understand how stars
affect the development of life on their planets.

Since the time Galileo discovered sunspots, in 1610, astronomers have
measured their number, size and location on the disc of the Sun.
Sunspots are relatively cooler areas on the Sun that are observed as
dark patches. Their number rises and falls with the level of activity
of the Sun in a cycle of about 11 years.

When the Sun is very active, large-scale phenomena take place, such as
the flares and coronal mass ejections observed by the ESA/NASA solar
observatory SOHO. These events release a large amount of energy and
charged particles that hit the Earth and can cause powerful magnetic
storms, affecting radio communications, power distribution lines and
even our weather and climate.

During the solar cycle, the X-ray emission from the Sun varies by a
large amount (about a factor of 100) and is strongest when the cycle is
at its peak and the surface of the Sun is covered by the largest number
of spots.

ESA’s X-ray observatory, XMM-Newton, has now shown for the first time
that this cyclic X-ray behaviour is common to other stars as well. A
team of astronomers, led by Fabio Favata, from ESA’s European Space
Research and Technology Centre, The Netherlands, has monitored a small
number of solar-type stars since the beginning of the XMM-Newton
mission in 2000. The X-ray brightness of HD 81809, a star located 90
light years away in the constellation Hydra (the water snake), has
varied by more than 10 times over the past two and a half years,
reaching a well defined peak in mid 2002.

The star has shown the characteristic X-ray modulation (brightening and
dimming) typical of the solar cycle. “This is the first clear sign of a
cyclic pattern in the X-ray emission of stars other than the Sun,” said
Favata. Furthermore, the data show that these variations are
synchronised with the starspot cycle. If HD 81809 behaves like the Sun,
its X-ray brightness can vary by a factor of one hundred over a few
years. “We might well have caught HD 81809 at the beginning of an X-ray
activity cycle,” added Favata.

The existence of starspot cycles on other stars had already been
established long ago, thanks to observations that began in the 1950’s.
However, scientists did not know whether the X-ray radiation would also
vary with the number of starspots. ESA’s XMM-Newton has now shown that
this is indeed the case and that this cyclic X-ray pattern is not
typical of the Sun alone. “This suggests that our Sun’s behaviour is
probably nothing exceptional,” said Favata.

Besides its interest for scientists, the Sun’s cyclical behaviour can
have an influence on everyone on Earth. Our climate is known to be
significantly affected by the high-energy radiation emitted by the Sun.
For instance, a temporary disappearance of the solar cycle in the 18th
century corresponded with an exceptionally cold period on Earth.
Similarly, in the early phases of the lifetime of a planet, this high
energy radiation has a strong influence on the conditions of the
atmosphere, and thus potentially on the development of life.

Finding out whether the Sun’s X-ray cycle is common among other
solar-type stars, and in particular among those hosting potential rocky
planets, can give scientists much needed clues on whether and where
other forms of life might exist outside the Solar System. At the same
time, understanding how typical and long-lasting is the solar behaviour
will tell us more about the evolution of the climate on Earth.

Further observations of HD 81809 and other similar stars are already
planned with XMM-Newton. They will allow astronomers to study whether
the large modulations in X-ray brightness observed in the Sun are
indeed the norm for stars of its type. Understanding how other
solar-like stars behave in general will give scientists better insight
into the past and future of our own Sun.

Figure captions

Figure 1

http://esamultimedia.esa.int/images/spcs/xmmnewton/xmm20040511a.tiff

The Sun as observed by the ESA/NASA SOHO observatory in extreme
ultraviolet light. The Sun’s appearance changed dramatically as it
approached the solar maximum, which it reached in 2000. New XMM-Newton
observations suggest that this behaviour may be typical of stars like
the Sun, such as HD 81809 in the constellation Hydra.

Figure 2

http://www.esa.int/export/esaSC/SEMNFTWLDMD_extreme_1.html

This sequence of images of the Sun, obtained by the ESA/NASA SOHO
observatory, shows the development of the most powerful X-ray flare
ever observed. The flare took place on 4 November 2003. The associated
coronal mass ejection, coming out of the Sun at a speed of 8,2 million
km/h, hit the Earth several hours later. New XMM-Newton observations
suggest that this behaviour may be typical of stars like the Sun, such
as HD 81809 in the constellation Hydra.

Note to editors

The results described here were published in the April issue of the
scientific journal “Astronomy and Astrophysics” (Vol. 418, p. L13).
The authors of the paper are F. Favata, G. Micela, S. Baliunas, J.
Schmitt, M. Guedel, F. Harnden Jr., S. Sciortino and R. Stern.

A reprint of the paper can be found at:

http://arxiv.org/abs/astro-ph/0403142

The complete text and the figures of this release can be also found at:

http://www.esa.int/science/media

More about XMM-Newton

XMM-Newton can detect more X-ray sources than any previous satellite
and is helping to solve many cosmic mysteries of the violent Universe,
from black holes to the formation of galaxies. It was launched on 10
December 1999, using an Ariane-5 rocket from French Guiana. It is
expected to return data for a decade. XMM-Newton’s high-tech design
uses over 170 wafer-thin cylindrical mirrors spread over three
telescopes. Its orbit takes it almost a third of the way to the Moon,
so that astronomers can enjoy long, uninterrupted views of celestial
objects.

More information on XMM-Newton can be found at:

http://www.esa.int/esaSC/SEMM8IGHZTD_1_spk.html

More about SOHO

SOHO is a project of international cooperation between ESA and NASA to
study the Sun, from its deep core to the outer corona, and the solar
wind. Fourteen European countries, led by the European Space Agency and
prime contractor Astrium (formerly Matra-Marconi), built the SOHO
spacecraft. It carries twelve instruments (nine European-led and three
American-led) and was launched by an NASA’s Atlas II-AS rocket on 2
December 1995. Mission operations are coordinated at NASA’s Goddard
Space Flight Centre. The spacecraft was designed for a two-year-mission
but its spectacular success has led to two extensions of the mission,
the first until 2003, and then again until March 2007.

More information on SOHO can be found at:

http://www.esa.int/esaSC/SEMJFH374OD_1_spk.html

For further information please contact

Dr. Fabio Favata
European Space Agency
Astrophysics Division
Tel: +31 71 565 4665
Email: ffavata @ rssd.esa.int

Dr. Norbert Schartel
European Space Agency
XMM-Newton Project Scientist
Tel: +34 91 813 1184
Email: nscharte @ xmm.vilspa.esa.es

Dr. Guido De Marchi
European Space Agency
Science Communication Office
Tel: +31 71 565 3273
Email: ibruckne @ esa.int