Free Astronomy Magazine November-December 2015

SPACE CHRONICLES T his video begins with a ground- based view of the night sky, before zooming in on the Twin Jet Nebula, a bipolar planetary nebula, as the NASA/ESA Hubble Space Telescope sees it. [NASA & ESA] has already ejected its outer layers of gas into space, whereas its part- ner is further evolved, and is a small white dwarf. The characteristic shape of the wings of the Twin Jet Nebula is most likely caused by the motion of the two central stars around each other. It is believed that a white dwarf orbits its partner star and thus the ejected gas from the dying star is pulled into two lobes rather than expanding as a uniform sphere. However, astrono- mers are still debating whether all bipolar nebulae are created by bi- nary stars. Meanwhile the nebula’s wings are still growing and, by mea- suring their expansion, astronomers have calculated that the nebula was created only 1200 years ago. Within the wings, starting from the star system and extending horizon- tally outwards like veins are two faint blue patches. Although these may seem subtle in comparison to the nebula’s rainbow colours, these are actually violent twin jets stream- ing out into space, at speeds in ex- cess of one million kilometres per hour. This is a phenomenon that is another consequence of the binary system at the heart of the nebula. These jets slowly change their orien- tation, precessing across the lobes as they are pulled by the wayward gravity of the binary system. The two stars at the heart of the nebula circle one another roughly every 100 years. This rotation not only creates the wings of the butter- fly and the two jets, it also allows the white dwarf to strip gas from its larger companion, which then forms a large disc of material around the stars, extending out as far as 15 times the orbit of Pluto! Even though this disc is of incredible size, it is much too small to be seen on the image taken by Hubble. An earlier image of the Twin Jet Nebula using data gathered by Hubble’s Wide Field Planetary Camera 2 was re- leased in 1997. This newer version in- corporates more recent observations from the telescope’s Space Telescope Imaging Spectrograph (STIS). n