Unveiling the origins of elements and the evolution of the universe through X-ray astronomy

Observing supernovae exceeding millions of degrees Celsius using satellites

Immediately after the birth of the universe, or the Big Bang, only elements like hydrogen, helium, and a small amount of lithium existed in the universe. Oxygen, water, and iron were not available, and obviously life could not exist. Iron and oxygen around us today are produced in the interiors of fixed stars that shine on their own like the Sun, or through nuclear fusion reactions during stellar explosions.

At the end of their lives, massive stars explode, shining hundreds of millions of times brighter than the Sun. This state is called a supernova, and the elements produced in the interiors of the stars or during the explosions are scattered into the surrounding space, enriching the universe and giving birth to various stars. In other words, by examining the remnants of supernovae, we can learn what elements were created and dispersed by the stars and their explosions.

In the first place, a star’s internal structure is not visible. We can only obtain chemical information from what is on the surface, so we have no choice but to make predictions by combining complex theories. However, by examining the remnants, we can observe the internal information dispersed by the star, which allows us to extract information about its state just before it died and how it ended. The elements scattered by a supernova can be examined in great detail by observing X-rays. This is the focus of my field, X-ray astronomy, in which I primarily observe supernova remnants.

I believe that the general image of astronomy is to make observations with telescopes or observatories on the ground. These instruments observe visible light, which corresponds to the wavelengths our eyes can see. However, light outside the visible spectrum cannot be seen by the naked eye. Therefore, an instrument suitable for each wavelength is needed. Furthermore, X-rays from space cannot reach the Earth’s surface because its atmosphere absorbs and scatters them. As a result, we need to launch specially developed satellites to observe X-rays in space.

So, why do we observe X-rays? Light is broadly classified into the following categories, in order of longest wavelength (i.e., lowest energy): radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. Simply put, low-energy light comes from cold objects and high-energy light comes from hot objects, and the temperature of objects emitting X-rays is extremely high, ranging from millions to hundreds of millions of degrees Celsius. Since a supernova explosion disperses material at incredibly high temperatures and with immense energy, observing X-rays enables us to gather a wealth of information.

A supernova is generally observed as a point source. This is because it has not expanded much immediately after the explosion. It gradually gets darker over a year or two after the explosion, but after several hundred years, the remnants have spread out considerably, making it easier to see information about the shape of the supernova. By examining this, we can deduce how the explosion occurred and which elements were dispersed. In other words, it is like performing a judicial autopsy on a star.

Observing rare elements with a satellite that boasts over 20 times the performance of conventional models

While astronomy is a discipline that has been around since B.C., X-ray astronomy is a relatively new field, having started around the 1960s. Initially, it was believed that no star could emit X-rays. The surface temperature of the Sun is about 6,000 degrees C, and it primarily emits visible light. Even the largest stars, despite their high energy, only reach temperatures of a few tens of thousands of degrees Celsius. To emit X-rays, a star must reach temperatures exceeding a million degrees Celsius, which is an entirely different magnitude.

However, at that time, it was becoming evident that the Sun also emitted X-rays. When an experiment was conducted to observe these X-rays in detail, an extremely bright X-ray source was discovered in a completely different part of space. This discovery sparked the development of X-ray astronomy, which involved launching satellites to observe X-rays. Ricardo Giacconi, a pioneer in this field, was awarded the Nobel Prize in Physics in 2002 for his discovery of cosmic X-ray sources.

I myself traveled to the United States from July 2018 to December 2020 to conduct satellite development and observational research at NASA’s Goddard Space Flight Center. The satellite I was involved in developing at that time was the XRISM satellite. This satellite has an extremely high capability to measure light energy, boasting over 20 times the performance of conventional observation satellites. With this enhanced capability to measure energy with high precision, we will be able to observe even more detailed structures and identify rare elements that were previously unobservable.

Among the elements we are focusing on are the lighter and less abundant elements, such as phosphorus, chlorine, and potassium. Although we have theoretical predictions about which stars produced these elements, we have not yet been able to observe them. If we can observe what was produced, where, and in what quantities, we can validate these theories. Studying such rare elements will help us understand how fixed stars evolve and explode, which is also a complex problem in physics. How many and what types of stars have been born and exploded in the universe? We will be able to understand the origin of the elements in our bodies and around us, which could be the seeds of life, and trace the history of life in the universe.

The XRISM satellite was launched in September of last year, and we are now in the phase of confirming that its capabilities are performing as expected. Moving forward, we will collaborate with researchers worldwide to analyze the data, with the expectation of gradually publishing our findings over the next year. Furthermore, open-call observations will be conducted, so the satellite will become a new weapon for astronomers worldwide, functioning as a space observatory in the future.

X-ray astronomy has potential applications in space exploration and medicine

X-ray astronomy is a field closely tied to the space industry, as it involves the development of observation satellites. Various researchers are involved, including universities that have already developed small satellites and begun space observations, and there are expectations of the technology’s applications.

For example, some are considering using our astronomical instruments for resource exploration on the Moon, while others are exploring how advancements in X-ray observation technology (already used in both space and medical X-rays) could advance healthcare. While we are working to create the best possible products for observing the universe, there is a good chance that this technology will also benefit other areas of society and industries. It is fascinating how the same technology, though initially developed for specific purposes, can find applications across a diverse range of fields.

In terms of space exploration, I believe that within the next 10 to 20 years, humanity will begin making strides toward the Moon and Mars. From an astronomical standpoint, the Sun is destined to die in about 5 billion years, which means humanity will ultimately face extinction unless we move out of the solar system. For society and humanity to continue progressing, the development of astronomy and, more broadly, all fields of space science will be essential.

Humanity still does not know where and in what quantities the elements essential for life were created. How were they supplied, how did they create the solar system and how were they incorporated into the Earth, including the history from the birth of the universe to the present? These are very challenging problems because they require integrating knowledge from many different fields. However, if experts in each field collaborate, we should be able to get closer to a solution.

* The information contained herein is current as of August 2024.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.

Information noted in the articles and videos, such as positions and affiliations, are current at the time of production.