HR 5171 is a yellow hypergiant star 1,300 times the size of the Sun, making it the largest yellow star, and one of the ten largest stars of any type, known to mankind. Or, so astronomers thought, until they examined new observations of the object made with the Very Large Telescope Interferometer in Chile.
Recent observations reveal HR5171 to be composed of two objects, each so close to each other, they are in contact. The newly-discovered companion to the main body has been designated HR5171B. This smaller object orbits around a common center of mass, located within the body of the more massive companion.
The stars are aligned in such a way that HR 5171B passes between its parent star and the Earth during each revolution. Astronomers, therefore, classify this as an eclipsing binary system. Together, the pair forms a composite body that resembles a giant peanut in space, rotating once every 1,300 days.
When investigators looked over data collected by observatories over the last six decades, they concluded the system has been rapidly changing over that period of time. The star has been growing larger, and cooler, during the last 40 years of observations. Due to the finite speed of light, the behavior astronomers see today from that object actually took place 12,000 years in the past.
To make these observations, astronomers combined a network of telescopes, using a process called interferometry. This provided researchers the same degree of details in their observations as if they were using a telescope more than 450 feet in diameter.
Astronomers only know of about a dozen yellow hypergiants in the Milky Way Galaxy. These super-massive stars are at a highly-unstable point in their lives, and change rapidly. As they mature, they expel gas, which expands outward from the star, encasing the stellar furnace. If enough stellar material is released, it can form a nebula around the body.
Betelgeuse, a red supergiant in the Constellation Orion, is one of the most famous of all these behemoth stars. Even that massive stellar body is only two-thirds of the size of the system comprising the double structure. If put in place of our Sun, the HR 5171 pair would swallow every planet from Mercury to Jupiter.
The less-massive of the duo has a higher surface temperature than its larger companion.
HR 5171 is so distant, when light from the observations left its source, the last ice age was still ending, agriculture was first being developed in Southeast Asia, and people were still living in caves. Despite this, this giant peanut in space can easily be seen as a point of light in binoculars. Look for it in southern skies, located in the constellation Centaurus.
