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The Remains of our Stars

Background


"Gamma-ray bursts are the brightest explosions in the universe, allowing astronomers to observe intense gamma rays in short durations. Gamma-ray bursts are classified as either short or long, with long gamma-ray bursts being the result of massive stars dying out. They provide hidden clues about the evolution of the universe."

Gamma-ray bursts are the strongest and brightest explosions in the universe, thought to be generated during the formation of black holes. Gamma-ray bursts create as much energy in their brief lifetimes as the sun will in its whole 10-billion-year existence.


Since then, by building an ensemble of quick-response satellites and ground-based observatories that all focus on a gamma-ray burst as soon as it is discovered, researchers have learnt a great deal more about gamma-ray bursts. This network's data demonstrate that gamma-ray bursts occur in galaxies billions of light-years away and that the source of the burst emits an afterglow at less energetic wavelengths after the initial gamma-ray flare.



Gamma rays, as well as radio waves, optical lights, and X-rays, are all produced by gamma-ray bursts. The total explosion energy can be calculated by simply adding all of the emitted energy when the conversion efficiency, or ratio of explosion energy to emitted energy, is high. Measuring the emitted energy alone isn't enough when the conversion efficiency is low or unknown.


A group of astrophysicists has used light polarization to quantify the hidden energy of a gamma-ray burst. When an electromagnetic wave oscillates in only one direction, it is said to be polarized. Stars reflect polarized light even though they don't emit it. Polarization is a typical method to lessen glare from light sources moving in one direction. Sunglasses and light shields are two examples of these products.


The team was led by Dr Yuji Urata from the National Central University in Taiwan and MITOS Science CO., LTD and Professor Kenji Toma from Tohoku University’s Frontier Research Institute for Interdisciplinary Sciences (FRIS). Their work was published in the journal Nature Astronomy.



How Was The Energy Measured?



When an electromagnetic wave is polarized, it means that the oscillation of the wave flows in one direction. While light emitted from stars is not polarized, the reflection of that light is. Many commonplace items, including sunglasses and light shields, use polarization to reduce glare from light that travels in a single direction.


Polarimetry is the measurement of the degree of polarization. Polarimetry is not as simple as measuring an object's brightness in astrophysical observations of celestial objects like gamma-ray bursts. Yet, it provides insightful data about the materials properties of objects.





The team looked at a gamma-ray burst that occurred on December 21, 2019. They calculated the polarimetry of fast-fading emissions from GRB191221B using the Very Large Telescope of the European Southern Observatory and the Atacama Large Millimeter/submillimeter Array, some of the most powerful optical and radio telescopes in existence. The optical and radio polarizations were then successfully measured at the same time, and it was discovered that the radio polarization degree was much lower than the optical one.


“This difference in polarisation at the two wavelengths reveals detailed physical conditions of the gamma-ray burst’s emission region,” explained Toma. “In particular, it allowed us to measure the previously unmeasurable hidden energy.”


After taking into account the hidden energy, the researchers found that the total amount of energy was almost 3.5 times greater than they had projected. As the energy of the explosion correlates to the gravitational energy of the progenitor star, knowing this quantity has major consequences for estimating the masses of stars.


Toma said, “Knowing the measurements of the progenitor star’s true masses will help in understanding the universe’s evolutionary history. The first stars in the universe could be discovered if we can detect their long gamma-ray bursts.”


© iStock/sakkmesterke


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