Sub-millimetre observations of one of the most luminous red giant stars have recently revealed several simple molecules such as HCN, NaCl, and a phosphorus-containing radical PN. Similar molecules are known to exist in large quantities in molecular clouds such as the Orion nebula where they are protected from ultraviolet starlight by copious quantities of dust. An analogous shielding mechanism seems to occur in the atmosphere of evolved, red giant and supergiant stars, which have cool extended atmospheres at temperatures of a few thousand Kelvin.The star VY Canis Majoris is an extreme example of the red supergiant family, with a luminosity of 500,000 times the Sun's and an estimated mass of 25 times that of the Sun. For comparison, the well-known red supergiant Betelgeuse has a luminosity of 15,000 times the Sun's and a mass of perhaps 10 solar masses.Seventeen species have been found in the oxygen-rich environment, including CS, HCO+ and HNC as well as the sulphur-containing compounds SO and SO2. The shapes of the lines suggest that SiO dominates in a roughly spherical wind-like outflow, while SO and SO2 are abundant in the collimated jet-like outflows. VY CMa has a strong stellar wind, losing an entire solar mass of material every 5000 years.The discovery was made with a new detector destined for the Atacama Large Millimetre Array (ALMA), an array of telescopes built in the Atacama desert in Chile for observing short-wave radio and sub-mm wavebands. It was fitted to the 10-metre dish at Arizona Radio Observatory for these measurements, and contains a superconducting-isolating-superconducting (SIS) receiver to achieve quantum-limited sensitivity levels. ALMA will operate in the range 30 to 1000 GHz (0.3 to 9.6 millimetres) and will be ideal for observing star formation and interstellar chemistry.
Links
AAVSO article on VY Canis Majoris
I recently happened upon a report discussing an unusual binary star system in the Magellanic Clouds, which are prominent in the southern sky. In fact, the Large Magellanic Cloud is brighter than the Orion nebula despite being 100 times more distant, and is a tremendous star-forming region containing many massive stars, novae, and a famous supernova known as SN1987A. The binary, with the unexciting name of LH54-425, is made up two young, hot blue stars with 62 and 37 times the mass of our Sun in a close orbit. In fact, the stars revolve once around each other every 2.25 days and are almost in contact.
Due to the disparity in their masses, co-discoverer George Sonneborn of NASA's Goddard Spaceflight Center believes that in the not-too-distant future (that is, within a few million years!) the two stars may merge together, creating an even more massive star. The result may appear similar to the hyper-luminous eta Carinae (about which more in a later post) which is probably the most luminous star in our own Milky Way galaxy. Astronomers do not know exactly when or how such stars merge, but it probably occurs roughly once per century in the Milky Way for massive stars, and much more frequently in stars of lower mass.
Hot on the heels of this news from the LMC (Large Magellanic Cloud), astronomers at the telescope have been able to weigh a star about 7000 light-years away in the star cluster NGC3603 (see picture). The result: a truly enormous 114 times the mass of the Sun! This star is also a member of a binary system; its partner is a comparative lightweight at 84 solar masses! Whether these stars will eventually merge is anyone's guess; it is quite possible that the giant star will lose so much gas it will never grow large enough to absorb its companion.
Links
Star duo on New Scientist Space
Super stars create new Sun
