理工学部、建築・環境学部教養学会主催ミニ講演会
第５７回理科系学生のための公開英語講演会
Exploring the Dynamic High-Energy Universe
激動の高エネルギー宇宙への旅
講師：理工学部、数理・物理コース
中嶋 大
On June 15, 2023, the 57th session of the English Lecture Meeting for Science Major Students was held under the sponsorship of the Academic Society of Faculty of Liberal Arts, inviting Doctor Hiroshi Nakajima of the College of Science and Technology to give a lecture with the above title, which was the 4th lecture for this event given by the lecturer.
Dr. Nakajima started this lecture with the following words and emphasized the importance of learning English for science major students: “Especially for the freshmen, it is quite important to expand your interests through this kind of events because there are many “seeds” of interests written or spoken in English.”
As quoted from Wikipedia, “Astrophysics is the branch of astronomy that employs the principles of physics and chemistry to ascertain the nature of the astronomical objects”. In the field of astrophysics, Dr. Nakajima has been particularly engaged in the observations of such celestial objects as “neutron stars,” “black holes,” and “the remains of supernovas,” which are 中性子星、ブラックホール、超新星残骸, in Japanese, respectively.
In our everyday life, we don’t stop and think about the universe, do we? Usually, people tend to regard it as an extremely silent and stable world, where stars and galaxies are shining with constant brightness. However, when watched through the eyes of high-energy lights such as X-rays or gamma-rays, it shows quite a different profile of its own; for example, you would realize how dynamic and volatile it is when gases are swallowed by a black hole with glaring explosions.
Among many issues concerning the latest results of observational research on high-energy phenomena in the universe, Dr. Nakajima focused on several general topics on black holes such as what the black hole is or how it is created. Of course, the lecturer referred to the two pieces of breaking news that an international team of researchers have recently succeeded in taking the images of a black hole in a galaxy called Messier 87 in April 2019 and of another huge black hole called Sagittarius A* (射手座A＊) in 2022. The former is located very far from us, i.e., 55 million light years (5500万光年) away from the earth; the latter was found at the center of our own Milky Way Galaxy.
Dr. Nakajima referred to the epoch-making method adopted for taking the images of those black holes: A group of observatories in several places in the world (called the Event Horizon Telescope (EHT) Collaboration), united to form a worldwide network of radio telescopes, which collaboratively produced images of black holes.
The lecturer also explained how significant in the history of astrophysics the discoveries of black holes are because they confirm Sir Roger Penrose’s (Noble Laureate of Physics) claim that the formation of black holes is what is predicted by the general theory of relativity.
Finally, Dr. Nakajima touched upon two recent projects in the development of which he has been engaged as a member, i.e., the development of state-of-the-art observational instruments based on the latest result of inquiry and the XRISM (X-ray Imaging and Spectroscopy Mission).
In the subsequent Q and A period, the audience asked eight questions, to all of which the lecturer gave detailed answers. The following are two of the questions and answers exchanged on that occasion:
Questions and Answers
Q. Is there a neutron star which does not become a black hole? Another question I have is what the turning point is where it is decided whether a neutron star will become a black hole or not.
ブラックホールにならない中性子星もありますか？また、その分岐点は何ですか？
A. The key parameter that determines if a neutron star becomes a black hole or not is its mass (M_NS). It is theoretically predicted that if M_NS exceeds Tolman–Oppenheimer–Volkoff limit (or TOV limit), the neutron star will collapse to form a black hole. The limit is in the range from 2.01 to 2.17 M_⊙, placed with the observation of GW170817. When M_NS becomes larger than the value, the quantum degeneracy pressure of neutrons cannot overcome the strong gravity of itself.
Then how does M_NS change? Accretion of matters onto the neutron star is the most common possibility where M_NS can be increased. In a specific type of mass accretion, its rate will be 0.2 M_⊙/yr.
Q. My question is about Betelgeuse. I heard rumors that Betelgeuse will explode as a supernova. What would happen if it should ever explode?
私はベテルギウスが超新星爆発するという噂を聞きました。もしベテルギウスが爆発したらどう影響するのか？
A. For sure, Betelgeuse will be a supernova in the (astronomically) near future! On the other hand, the distance from the Earth to Betelgeuse is approximately 640 light years. Therefore, there will not be any practical influence on our life.
The following figure shows the expected brightness of the supernova as a function of time since explosion. It will be brighter than Venus for several months after the ignition.
