Article written by: Jorick Vink, Astronomer at the Armagh Observatory and Planetarium
Have you ever been worried about the impact of an asteroid wiping out human life? Well, I have some disturbing news for you: there is another possibility involving the explosion of a massive star that gives rise to a gamma-ray burst (called by astronomers simply a GRB), when the star ends its life producing a black hole. Stars like our Sun spin slowly, with just one rotation each month. By contrast, massive stars rotate much more rapidly, reaching rotation rates of once a day. This rapid rotation is thought to have dramatic consequences for their evolution and ultimate demise, which may involve the production of a gamma-ray burst, the most intense type of cosmic explosion since the Big Bang.
Rotating massive stars
The evolution of massive stars is thought to be the result of a complex interplay between different physical processes, most notably mass loss and rotation.
Whilst the importance of mass loss was already established in the 1970s, the role of rotation was only fully appreciated in the 2000s. When a star rotates, the pole becomes hotter than the stellar equator, enabling a rather complex circulation in the stellar interior (see Figure 1).
During the star’s life, nuclear processed material is transported from the star’s core to the outer layers, thereby enriching the surface with elements such as nitrogen. During later evolutionary phases, the combination of mass loss and rotation also leads to the transfer of carbon and oxygen to the surface, before the stellar core collapses, sometimes producing a supernova, sometimes a GRB, and sometimes both at the same time.
The collapsar model for gamma-ray bursts
From the 1960s onwards, GRBs were discovered appearing from all cosmic directions. However, an explanation for their origin was still to be found. A massive breakthrough occurred in 1998 when a European team led by graduate student Titus Galama discovered that the unusual supernova 1998bw was associated with GRB 980425. This was convincing evidence that long (those lasting longer than two seconds) GRBs were associated with the deaths of massive stars.The most popular explanation for the long GRB phenomenon is that of the collapsar model by Stan Woosley (1993), where a rapidly rotating massive core collapses, forming a disc around a black hole. In this process, some of the gas is ejected in the shape of two jets at very high (relativistic) speeds, which are aligned with the rotation axis of the dying star (see Figure 2).
These jets are thought to involve an opening angle of just a few degrees, and only when these jets happen to be directed towards Earth are we able to detect the event as a GRB.
The gamma-ray burst puzzle
One of the persistent problems with the collapsar model was that it not only required the star to have a high rotation speed initially, but that the star needs to maintain this rapid rotation until the very end of its life. The reason this is such a challenge is that one of the most characteristic other features of massive stars, their strong mass loss, is expected to remove the ability to rotate (this is related to the physics of angular momentum).
Most stellar evolution models show that as a result of mass loss, massive stars not only remove 90% of their initial mass when reaching their final (“Wolf–Rayet”) phase, but as a result of mass loss, the stars are expected to come to a complete standstill!
We would therefore not anticipate massive stars in our own Milky Way to produce a GRB when they expire. The question is what do we expect from massive stars in galaxies that are more metal-free (i.e. with a far lower proportion of heavy elements than typically found in stars in our own Galaxy), and have a composition that is more characteristic of the early Universe? To address this issue, we need to explore the underlying physical origin of massive star mass loss.
Iron iron iron
Dynamical simulations of gas and radiation show that mass loss from massive stars is caused by metal lines, and more specifically by iron, despite the fact that iron is such a rare element. In the Milky Way, for each and every iron atom there are more than 2,500 hydrogen atoms, and iron becomes even scarcer in galaxies
with lower metal content, such as the nearby Magellanic Clouds. Owing to the highly complex atomic structure of iron with millions of line transitions, this makes iron an extremely efficient absorber of radiation in the atmospheres of massive stars where the mass loss is set in motion. Up to 2005, most stellar modellers had assumed that due to the overwhelming presence of carbon in the atmospheres of these stars in their final Wolf-Rayet phase, it should also be the element carbon that causes the mass loss, rather than the few iron atoms, which were basically thought to be negligible.
This assumption also implied that massive stars in low-metal galaxies would have undergone an amount of mass loss equally severe as massive stars in our Galaxy, basically removing all the rotation from the star. It was for this very reason that there was no satisfactory explanation for the GRB puzzle.
Nevertheless, GRBs were found to arise predominately in low-metal galaxies, characteristic of conditions in the early Universe. It is interesting to note that the most distant object in our Universe known today is indeed a GRB, estimated to have resulted from the collapse of a massive star only some 500 million years after the Big Bang.
That GRBs occur in low-metal galaxies may imply that they were common at earlier times in the Universe when the gas between the stars was less enriched. In 2005, the author performed a study of Wolf–Rayet mass loss in low-metal galaxies, discovering that although carbon is the most common element in Wolf–Rayet atmospheres, it is the much more complex iron element that causes the mass loss.
In other words, massvie stars that are simply born with fewer iron atoms will lose less mass by the time they reach the ends of their lives. The striking implication is that objects formed in the early Universe and in other low-metal galaxies can maintain their rapid rotation towards collapse, enabling a GRB event. We now think GRBs exclusively occur in low-metal galaxies, and that it is the iron in our own Milky Way that prevents GRBs from happening too close for comfort.
Next time you see rust on your car, don’t get angry – it is iron that keeps us safe!