A Yellow Hypergiant with a Close Companion

Yellow hypergiants (YHGs) are extremely rare stars. Only several of them have been identified in the galaxy so far. These stellar oddities are stars that are at a particularly unstable and rapidly changing phase of stellar evolution. They are known to expel prodigious amounts of material, creating complex circumstellar environments. An international team of collaborators using ESO’s Very Large Telescope Interferometer (VLTI) has found that the star HR 5171 is the largest YHG known.

HR 5171 measures more than 1,300 times the Sun’s diameter, and it is one of the biggest and rarest stars in the galaxy. If placed in the solar system, its visible surface would extend all the way to between the orbits of Jupiter and Saturn. HR 5171 is located about 12,000 light years away and it shines with roughly a million times the Sun’s luminosity.

Artist’s impression of the yellow hypergiant star HR 5171. Image credit: ESO.

Observations using the VLTI also show that HR 5171 has a smaller companion star going around it every 1,300 days. The companion star is close enough that it is actually in contact with the main star, resulting in what known as a contact binary. The presence of the companion star distorts the overall shape of HR 5171, causing the observed flux to modulate by 17 ± 5 percent.

Despite its smaller size, the companion star is expected to have a significant influence on the subsequent evolution of HR 5171. Piror to the recent observations by the VLTI, observations of HR 5171 over the past 40 years have shown that the star is cooling as it enlarges. Future observations of HR 5171 will provide a fascinating insight into the evolution of these rare stellar behemoths.

Reference:
Chesneau et al. (2014), “The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase”, arXiv:1401.2628 [astro-ph.SR]