White dwarfs, heavy elements

The supernova SN 2005E belongs to a relatively new and rare type of exploding stars. The scientists believe that the relatively weak light of these supernovae makes it difficult to detect them, but in fact they are quite numerous, and have a significant contribution to the creation of certain elements in the universe - in particular calcium and titanium

Not all explosions are equal. Stars that die in an explosion are usually divided into one of two groups: young, giant stars that collapse under their enormous weight and throw their outer layers everywhere, and older stars, with a mass similar to that of our Sun, that undergo a thermonuclear explosion. But the star explosion known as SN 2005E, recorded in January 2005, does not fit into either of these two groups. This is according to an analysis performed by Dr. Avishai Gal-Yam and the post-doctoral researcher Dr. Hagai Binyamin Peretz from the Department of Particle Physics and Astrophysics at the Weizmann Institute of Science, and their research partners.

The scientists found that the explosion ejected only a small amount of material - roughly equal to a third of the mass of our sun, and that it occurred in a single galaxy in an area where no stars are formed. In the explosion of a giant star, a much larger amount of matter is emitted, and naturally it takes place in the area where stars are born (this is because the giant stars are characterized by a short life, and do not move away from their place of birth). On the other hand, the event also does not fit the pattern of a typical thermonuclear explosion: it emitted too little radiation, and the material scattered from it included elements that are not typical of other thermonuclear explosions. A spectroscopic analysis of the dispersed material showed that it contained five to ten times more calcium than those found in the explosions of other stars. In addition, it seems that a relatively high proportion of a radioactive isotope of titanium is also found there. In an article published in the scientific journal Nature, Dr. Peretz and Dr. Gal-Yam reported that the unusual explosion corresponds to a model in which a compact star of the type called a white dwarf "steals" A thick layer of helium from a neighboring star. According to this model, the star undergoes a special type of thermonuclear explosion, which destroys the helium but does not destroy the rest of the white dwarf. On the other hand, in a common type of supernova called Ia, which also involves a white dwarf (containing mainly carbon and oxygen), the star explodes into pieces after "stealing" material from its host.

These findings led the scientists to the conclusion that the supernova SN 2005E belongs to a relatively new and rare type of exploding stars. The scientists believe that the relatively weak light of these supernovae makes it difficult to detect them, but in fact they are quite numerous, and have a significant contribution to the creation of certain elements in the universe - in particular calcium and titanium.

Dwarf galaxies, giant stars

What happens when a giant star - hundreds of times bigger than our sun - explodes? A theory in this regard was developed years ago, but the first observation of such an explosion was made only recently, by a team of scientists from Israel, Germany, the United States, England and China, led by Dr. Avishi Gal-Yam from the Weizmann Institute of Science. The team followed such an explosion (supernova) for a year and a half, and found that the observation corresponds to the predictions arising from the theory of the explosion of stars that are 150 times or more the mass of our Sun. These findings, which may expand our understanding regarding natural limits on the size of stars, as well as regarding the processes of creating the elements in the universe, were recently published in the scientific journal Nature.

"The secret lies in balance," says Dr. Avishi Gal-Yam from the Department of Particle Physics and Astrophysics at the Weizmann Institute of Science. "During the life of the star, a balance is maintained between the force of gravity, which pulls its material in, and the heat created by the nuclear reaction in its center, which pushes the material out. In the supernovae we know, those of stars 100-10 times larger than our Sun, the nuclear reaction begins with the fusion of hydrogen into helium, as in our Sun. But in stars where hydrogen runs out, nuclear fusion of heavier elements continues, until the star's core becomes iron. At this point, since iron atoms do not fuse easily, the nuclear reaction ends - and the balance is broken. In the absence of a force pushing outward, gravity takes over, and the star's mass collapses inward. During the collapse, a lot of energy is released, causing an explosion, and the star throws its outer layers into the vastness of the universe."

But the physical process occurring in a supergiant star is different. In these stars, photons (particles of light) are created so energetic that they may merge with each other and become pairs of particles: electrons and their opposite particles, positrons. That is, particles with mass (electrons and positrons) are created from the photons, which have no mass, which draws energy from the star. Again, the balance is broken, but this time the star collapses at a stage where its core is made of oxygen, not iron. The hot, compressed oxygen explodes in a rapid thermonuclear reaction that destroys the center of the star completely, leaving behind only glowing stardust. "Models of 'couple supernovae' were calculated decades ago, for example by Prof. Zalman Barkat and Prof. Gideon Ravavi from the Hebrew University, and Prof. Giora Shabiv from the Technion," says Dr. Gal-Yam, "but no one knew whether huge explosions These really happen in nature. The new supernova we discovered fits these models."

By analyzing the information they gathered from the new supernova, the scientists estimated the size of the star, and found that its mass is about 200 times greater than the mass of the Sun. This result is particularly interesting, because until now many scientists believed that in our part of the universe there are no stars whose size exceeds about 150 solar masses, and it is possible that there is a certain physical constraint that limits the extent of the stars. The findings that emerge from the research of Dr. Gal-Yam and his research partners imply that supergiant stars are indeed rare, but they do exist. It is even possible that even larger stars - up to 1,000 times the size of the Sun - existed in the young universe. "This is the first time we have been able to analyze observations of such a huge exploding star," says Dr. Paolo Mazzali of the Max Planck Institute for Astrophysics in Germany, who led the theoretical study of the explosion. "We were able to measure the amounts of new elements created in this explosion, including newly formed radioactive nickel, whose mass is more than five times the mass of the Sun. Such explosions may be important 'factories' for the production of heavy metals in the universe." The observed giant supernova is in a tiny galaxy - only a hundredth of the size of our galaxy. Scientists believe that such dwarf galaxies may - for various reasons - be home to such giant stars. Dr. Gal-Yam: "The discovery and analysis of this unique explosion gave us new insights into the maximum dimensions that such massive stars can reach, and into the way in which these giants contribute to the composition of the elements in our universe. We hope to expand our understanding even further when we find new examples of such stars. To that end, we recently started conducting new surveys in large and unknown regions of the universe."