Weizmann Institute scientists analyzed the composition of the elements emitted from a rare star, just before it exploded

Stellar winds blow from the surface of the stars, blowing the elements found in the outer layers into space. In stars like the one that exploded, these winds are so strong that they can sweep away every 10,000 years a mass equal to that of our Sun.

The discovery of a supernova - the explosion of a distant star - that happened a few months ago, prompted astrophysicists to observe the impressive spectacle with the help of telescopes around the world. The dramatic death of the star, of a rare type, whose mass is at least 10 times that of our Sun, reveals to scientists interesting details about the life of these fascinating celestial bodies, and helps complete details in the picture describing the formation of the heavy elements in the universe.

To understand the properties of the star that exploded, the scientists identified the mixture of elements that drifted from its surface just before the explosion began. Prof. Avishi Gal-Yam, from the Department of Particle Physics and Astrophysics at the Weizmann Institute of Science, explains that in order to identify the star, it is necessary to check whether the materials drifting from it into space contain large percentages of elements such as carbon, oxygen and nitrogen. These elements are created in a nuclear fusion process that provides the energy for the star. In our sun, for example, hydrogen atoms - which are the lightest - undergo fusion to form helium atoms, and that's where the process stops. However, in heavier and hotter stars, the fusion continues: helium atoms combine and form heavier and heavier atoms - up to iron atoms.

Scientists believe that stars of this type are made up of layers, similar to an onion. The heaviest elements, such as iron, are located in their core, while the lighter ones form the outer layers. Stellar winds blow from the surface of the stars, blowing the elements found in the outer layers into space.

In stars like the one that exploded, these winds are so strong that they can sweep away every 10,000 years a mass equal to that of our Sun. At a certain point during the life of the star, all the light hydrogen that makes up the outer layer of the star is swept away by the wind, and later the layers of helium, carbon, oxygen and nitrogen are also swept away from it.
A layer below the star's surface contains a mixture of hydrogen, helium and heavier elements.

Such a layer would have to be outer enough to contain even the light hydrogen, and still hot enough to provide the extreme temperatures required for nuclear fusion. Scientists are interested in this layer, because it is where the nitrogen is formed. Unlike carbon, which contains six protons (originating from the fusion of three helium atoms), or oxygen containing eight protons (originating from four helium atoms), the nitrogen atom contains an odd number of protons - seven. That is, it is formed by the fusion of atoms with an even number and atoms with an odd number of protons - for example, three helium atoms (two protons) and a hydrogen atom (one proton). Therefore, measuring the amounts of nitrogen may reveal what lies beneath the star's surface.

As the wind blows the star's outer layers into space, its core continues to produce and accumulate iron, until it is too heavy to be stable. At this point the core collapses in a sudden and violent movement, causing the outer layers of the star to be thrown into space - this is the supernova event we are watching.

The elements that were carried away by the stellar wind before the final explosion can only be detected in a very short window of time - one day after the supernova - because the strong radiation created in the explosion tears the electrons from the atoms. With the help of telescopes equipped with spectrographic equipment and tuned to observe the supernova, it is possible to identify the elements by measuring their spectrum - that is, the light emitted when electrons reconnect with the atoms from which they were torn. But these observations must be made quickly, before the rapidly dispersing remnants of the star after the explosion swallow up the last remnants of the stellar wind, erasing the last traces of the dying star.

The race to observe the spectrum of the young supernova has begun with the robotic telescopes at the "Palomar" observatory in California, which are part of a multinational project called iPTF, led by Prof. Shri Kulkarni from the California Institute of Technology. These telescopes are programmed to detect transient events, that is, sudden changes in the night sky that may be a new supernova, and alert the team members. On the other side of the earth, Dr. Yair Harkabi, who was then a research student in Prof. Gal-Yam's group, received the message. While the American scientists were sleeping, he evaluated the data, understood their meaning, and contacted Dr. Assaf Horesh, who was then a postdoctoral researcher at the California Institute of Technology in Pasadena (and has since joined the Weizmann Institute of Science). Dr. Horesh made spectroscopic observations using the Keck telescope located in Hawaii, west of the one in California, so he could extend the observation hours of the supernova after the morning had already risen in California. His quick reaction allowed him to record the emitted spectrum of the materials carried in the wind - only 15 hours after the explosion.

Examining the collected data revealed to Prof. Gal-Yam, Dr. Hakabi, Dr. Horesh and their colleagues that the interstellar wind around the exploded star did indeed contain large amounts of nitrogen, similar to the so-called Wolf-ray stars known from our galaxy. The findings of their research are published this week in the journal Nature. "Thanks to the ability to observe a supernova so soon after the explosion, we were able for the first time to measure the composition of the elements on the surface of a star, just before it exploded," explains Prof. Gal-Yam. Now, after the team of scientists has demonstrated that effective global organization and timed operation of telescopes around the world make it possible to collect data on fast events, he hopes that it will be possible to observe more "young" supernovae. Understanding how these stars live and die is important, according to him, not only because it opens a window to the way the universe works. "All the heavy elements in the universe - those whose mass is greater than that of helium - were created in the fusion furnace of large stars, and scattered in the universe in supernova explosions. Therefore, the origin of many scientific questions - in relation to the way different elements are created and their distribution in space - lies in those explosions that occur throughout the cosmos."