A red supergiant star has expelled the largest cloud of gas and dust ever seen in the process of being blown off one of these stellar behemoths. The vast size and intricacy of the cloud suggests that there could be a hidden group of stars that are contributing to the growth of the cloud.
In a false-color image taken by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the parts colored blue are expanding towards us, and the parts colored red are traveling in the opposite direction. The cloud stretches up to 1.4 light-years across, centered on the star, known as DFK 52. To give an idea how large it is, if DFK 52 were as far from us as another well-known supergiant is, the star Betelgeuse in the constellation of Orion the Hunter, somewhere between 550 and 700 light-years away, then the cloud around it would appear as large in the night sky as roughly one third of the full moon in the sky.
The star in question, DFK 52, is quite similar to another well-known red supergiant, Betelgeuse. It is located in an extreme cluster of stars called Stephenson 2 that is home to at least 25 other red supergiants and found 18,900 light-years away in our Milky Way galaxy.
Once these stars ran out of hydrogen in their core for nuclear fusion reactions, the loss of energy meant that their cores began to contract under gravity, raising the temperatures there to the point that the stars could turn to fusing helium to produce energy instead. This leads to the star’s outer layers heating up too, enough to start fusing hydrogen in them instead, producing extra energy that causes the star’s outer layers to expand.
As these stars expanded and grew diffuse at their edges, their surface temperatures cool and they shine red. In many ways, this is similar to how our sun will expand into a red giant towards the end of its life in five billion years’ time. The difference is that these stars have masses between 10 and 40 times the mass of our sun, so their expansion is more extreme. They became red supergiants, the most enormous stars in existence. Betelgeuse, for instance, is a red supergiant that is somewhere between 640 and 764 times the radius of our sun! (Its exact size is tricky to ascertain, partly because its outer limits are so diffuse, and partly because the distance to Betelgeuse is not precisely known.)
Eventually, the red supergiants of Stephenson 2 will all go boom in a supernova, but before then they undergo periods of mass loss, belching out clouds of gas that cool and condense into dust that hangs around just on the edge of a red supergiant’s system.
On the face of things, DFK 2 looks unremarkable as far as red supergiants go. Its luminosity is about 20,000 times brighter than our sun, but that’s typical, albeit a bit on the low-end, for red supergiants. Its mass is between 10 and 15 times the mass of our sun; again, typical for a red supergiant.
But where DFK 2 is extraordinary is in how it is losing mass. When astronomers led by Mark Siebert of the Chalmers University of Technology in Gothenburg, Sweden, observed Stephenson 2 with ALMA, they found that DFK 52 was surrounded by a cloud of material three to four times larger than any ever seen to have been ejected by a red supergiant.
The velocity with which the material is racing away from the star is also interesting, in that it seems to have changed. Siebert’s team estimates that about 4,000 years ago, DFK 52 underwent a titanic outburst in which it unleashed most of this mass, blowing it away at a velocity of 27 kilometers per second (60,370 miles per hour). Astronomers call this a ‘superwind’, which isn’t a wind in the conventional size like on Earth, but a sleet of radiation and charged particles. There is evidence that other red supergiants switch on their superwind in the run-up to going supernova. However, DFK 52 seems to have taken a different option, switching off its superwind instead for a slow breeze of 10 kilometers per second (22,300 miles per hour), which is slower than the winds emanating from other well-known red supergiants such as Betelgeuse and Antares in the constellation of Scorpius, the Scorpion.
Even so, Siebert’s team conservatively estimates that in the past 4,000 years DFK 52 has lost as much material as makes up our sun. Mass loss from red supergiants is not well understood, but DFK 52 seems to be an anomaly even among what is currently known. Why is its mass loss so extreme, and the structure of its circumstellar cloud so complex?
Generally, there is a dichotomy among red supergiants. The more luminous ones have more extreme and asymmetric mass loss, while the less luminous ones have slower and more spherical mass loss. DKF 52 is less luminous than the most energetic red supergiants by a factor of 10, but has experienced mass loss in excess of the more luminous red supergiants. What’s going on?
Siebert and his colleague’s best guess is that there is more than one star at the heart of DFK 52, so tightly bound that we cannot see them from our perspective on earth. Other red supergiants in binary or multiple star systems display equatorial rings in their circumstellar material driven by the orbital interactions of a companion star.
Partial equatorial rings are visible in DFK 52’s circumstellar material, but other evidence of a binary, such as bipolar symmetry in the ejected material, is lacking. Furthermore, the complexity of the structure of the circumstellar material will also take some disentangling. Nonetheless, a second or even a third star would provide the extra gravitational energy required to first drive such mass loss, and then to contribute to a superwind that blows the ejecta to such great size.
That said, a companion star was recently discovered around Betelgeuse, but why that newly found star does not have the same effect on Betelgeuse as on DFK 52 is unclear.
DFK 52 is more than just an astronomical curiosity. It will one day explode as a supernova, and understanding the behavior of “its dramatic mass loss warrants extensive follow-up” to better understand how and why red supergiants lose mass and then explode.
Siebert’s team published a study of DFK 52 in Astronomy & Astrophysics.
