The M.A.S. Newsletter
Journal of the Mauritius Astronomical Society
April 99
The Next Meeting:
Friday 30th of April at 7hr30 pm at the St Esprit College, Quatre Bornes.
The photos of the partial
solar eclipse taken on the 16th February last will be on display at the club and members
are invited to address their request to the secretary with information about the number of
copies of each, format and paper (gloss or matt).
Shutting Out Starlight:
Astronomers have surmised the existence of about a dozen extrasolar planets by the periodic Doppler shift in the light received from the parent star. These shifting in the frequency of the light indicate a wobble in the star's position which can be accounted for by the pull of a massive orbiting planet on the star. Direct observations have proved impossible because the parent star, millions of times brighter than a planet, simply washes out the lesser image.
In a recent experiment, reported in Nature, astronomers have taken a snapshot of a dust cloud around Betelgeuse (the brighter shoulder star of Orion) without the use of a screen to blot out the star's light. The technique used is called nulling interferometry. The experiment was carried out on the Multiple Mirror Telescope on Mount Hopkins in Arizona. It used two mirrors mounted 5 metres apart on a rigid frame. When the target star is aligned along an exact right angle relative to the frame connecting the two mirrors, its light is cancelled out. The inverted light from one mirror interferes with the light from the other mirror, darkening both the core area of the star and the surrounding halo.
The experiment demonstrated
that the nulling interferometer could detect an object as little as 0.2 arc second from a
star. This distance is larger than the range for the method that looks for a star's wobble
but is far less than the minimum for any other direct technique that tries to blot out
starlight. Starting in 2003, the Large Binocular Telescope, which is under construction on
Mount Graham in Arizona, will use nulling interferometry and should be able to image
Jupiter-size planets close to nearby stars. Just over a decade from now, NASA plans to
launch a space based nulling interferometer, called the Terrestrial Planet Finder, that
may be able to spot extrasolar planets no bigger than Earth. If an Earth-size planet is
discovered, the observations at infrared wavelenghts will reveal whether it has an
atmosphere and whether that atmosphere contains the ozone, carbon dioxide and water that
suggest the presence of life!
The VLT:
The second of the four 8.2 m
diameter mirrors of the VLT has seen its first light. Unofficially named after the famous
Dalton brothers, Joe, William, Jack and Averell, the four telescopes have now received
their official names: Antu, Kueyen, Melipal and Yepun, the Mapuche indian words for the
Sun, Moon, the constellation of the 'Southern Cross' and the Sirius respectively.
The sky this month:
A thin crescent Moon is seen close to Mercury in the early hours on the 14th. The Moon which rises at 4hr18 am, is at an elevation of 22° above the eastern horizon at 6hr00 am and is 6% illuminated. Mercury is to be found at a distance of about 1° at '8 o'clock' from the Moon.
On the 27th, Venus is close to the open star cluster NGC 1746 in the Taurus. Best observed through binoculars or a telescope at low power at sunset above the western horizon.
Maximum of the Lyrids meteor shower in the early hours of the 22nd, ( count rate of 15 hr -1 ). The radiant is about 30° on the NNE horizon, i.e. in Lyra.
Mars is now a bright reddish
object at magnitude -1.47 on the 15th. It then rises shortly after sunset at 6hr38 pm. The
planet will be at opposition on the 24th of April. Mars being a small planet, only a high
power (about 150x or more) will reveal such surface details as the polar caps.
Mars:
Opposition is said to occur when the Sun, Earth and a planet are lined up. For Mars, the average interval between successive oppositions is 780 days (synodic period). The large eccentricity of the planet's orbit causes large variations in the synodic period (764 to 810 days). Also the distance separating the planets at opposition varies greatly. Mars comes within 56 million km at perihelic oppositions (apparent size of 25.1 arc-seconds) and 100 million km at aphelic oppositions (apparent size of 13.8 arc-seconds). The next perihelic opposition is due on 28th August 2003. This month's opposition will put Mars at a distance of 87 million km. Because of the eccentricity and inclination of Mars' orbit relative to our own orbit, the greatest approach will only take place on the 1st of May, i.e. one week after opposition!
Serge Florens, Secretary