Free Astronomy Magazine March-April 2016

STELLAR EVOLUTION ciety meeting held on 4-8 January 2016 in Kissimmee, Flor- ida, during which Khan announced the discovery of five Eta twins, two of which in M83 and the other three equally distributed in the other galaxies. The spectrophotometric properties of these objects mirror those of Eta Carinae: they appear relatively dim in visible light and in the near-infrared (Hubble images), but they show them- selves luminous at the mid-infrared wave- lengths (Spitzer images), where their emis- sion shows the type of “flattening out” expected in the presence of an envelope of dust with temperatures between 400 and 600 Kelvin. The 5 Eta twins are obviously too far away to be able to observe them in detail as they are smaller than one pixel across and their image appears therefore “spread” over a wider area than the actual one, and this in environments crowded with stars. Such limitation does not allow researchers to find the answers to basic issues that could shed light on what caused the Great Eruption of Eta Carinae. In particular, we do not yet know if the 5 new objects are single or binary stars, if they are going through the same evolutionary stage of Eta Carinae, if the surrounding material was released in a single, massive eruption or it is the sum of several minor events oc- curred in different eras. In order to have exhaustive answers we will have to wait for the arrival on the scene of the NASA’s James Webb Space Telescope (JWST), whose launch is scheduled for the end of 2018. With its 6.5 meters diameter mirror and the Mid-Infrared Instrument (MIRI), the JWST will provide images of Eta twins with an angular resolution 10 times great- er than that of Spitzer and it will also be much more sensitive than the latter to wavelengths in which these objects shine brightest. Precise estimates of the number of Eta twins, frequency of their eruptions and the recurrence of binary systems among them will provide astronomers a fuller picture about the influence that these extreme objects have on the physical-chemical evo- lution of galaxies. T he side dia- gram shows some peculiar el- ements detected in the near-infra- red of Eta Cari- nae’s spectrum. With JWST this type of observa- tions will be able to be carried out also on Eta twins. [NASA, ESA and the Hubble SM4 ERO Team] Below, the Eta twins location in the galaxies in which they were discovered. [R. Khan et al./NASA] n