The mysterious, invisible object that keeps our galaxy in motion

The universe is a space that prompts questions; questions such as what exactly happens at the centre of a galaxy? Andrea Ghez, winner of the 2020 Nobel Prize in Physics, has dedicated herself to this very question throughout her career as astronomer and astrophysicist. Could it actually be possible that at the core of every galaxy lies a supermassive black hole that gets more massive the more massive a galaxy is? These are the types of questions discussed and researched in astronomy.

Although the scientific community acknowledges the existence of black holes, their nature remains a mystery. Basic research is yet to come up with any kind of logical explanation for how ¡°supermassive black holes¡± were formed, what they are made of, and what part they play in the emergence and evolution of entire galaxies. According to one common theory, they may have been created by the fusion of what were once stellar black holes. The latter emerge when massive stars collapse. Of course, this assumption has yet to be verified or refuted on the basis of tangible measurement data.

From a possibility to certainty

In the case of our galaxy, the Milky Way, it is now generally accepted, both theoretically and empirically, that a supermassive black hole exists at its centre. Its name is Sagittarius A*. Reinhard Genzel, Director of the Max Planck Institute for Extraterrestrial Physics, and Andrea Ghez, Professor of Physics & Astronomy at the University of California, Los Angeles, have provided the most conclusive empirical evidence of its existence to date. Next week, Ghez will talk about the journey ¡°from a possibility to a certainty of a supermassive black hole¡± as part of the Paul Bernays Lectures 2021. She will also discuss the extent to which lessons learned from the Milky Way might broaden our knowledge about other galaxies.

Ghez shared the 2020 Nobel Prize in Physics with Reinhard Genzel and Roger Penrose: Penrose for discovering that the formation of black holes is a robust prediction of the general theory of relativity; Genzel and Ghez for discovering a supermassive compact object at the centre of our galaxy. From a physical perspective, black holes are actually immensely compact objects with an extremely high mass and a force of gravity so strong that they pull in anything that finds its way into their entry area, known as the event horizon. Nothing can escape from a black hole, neither stars nor suns, radiation nor information ¨C not even light. The fact that they ¡°swallow up¡± light and remain invisible to humans is what gives black holes their name.

While Penrose received his Nobel Prize for providing mathematical evidence, Genzel and Ghez were honoured for having refined measurement technology and instruments to such a degree that they were able to use state-of-the-art, high-resolution imaging and optical techniques to indirectly prove that the essentially invisible and extremely heavy object at the heart of the Milky Way is a black hole.

¡°Over the course of many years, Ghez has resolutely investigated what happens at the centre of our galaxy. Her exceptional talent for building instruments has enabled her to continuously develop and decisively improve the measurement technology required to observe black holes,¡± says Philippe Jetzer, astrophysicist and professor at the University of Zurich, who will introduce Ghez¡¯s research at the Bernays Lectures. Jetzer¡¯s focus areas include gravitational waves and general relativity. He is conducting research into the formation of supermassive black holes in connection with the planned ESA research satellite ¡°LISA¡±. ETH Zurich researchers are involved in the project, which is expected to provide new measurement data on gravitational waves from space.

Sharper images expose the black hole

By contrast, Ghez is focused on measurement technologies that can be used to observe stars and the space between them as well as galaxies and their centres from the Earth. She usually captures her images using the 10-metre telescopes located at the Keck Observatory in Hawaii. With their conventional imaging techniques, Earth-based telescope images are at a disadvantage compared to images taken from space: the turbulence in the Earth¡¯s atmosphere causes the images to become distorted and blurred.

Ghez has been refining imaging and optical techniques as well as infrared astronomy detectors since the 1990s: the telescopes¡¯ blurring effects can now be reduced and corrected in real time. The resulting high-resolution images are so sharp that stars and astronomical objects are clearly recognisable (see photograph).

Using these improved imaging techniques, Ghez ¨C as well as Genzel ¨C was able to identify that there are thousands of stars circling around one and the same invisible object at the centre of the Milky Way. What¡¯s more, this central object must be very small because stars follow their orbits undisturbed without colliding with it. The high speed of the stars ¨C up to four percent of the speed of light ¨C also indicates that this object is extremely heavy and has a bearing on the movement of the stars with its enormous gravitational pull. In actual fact, the mass of this object corresponds to around four-million times that of our sun. Based on these findings and on alternative explanations that can be ruled out, a supermassive black hole appears to be the most plausible and compelling explanation for what this object is.

Andrea Ghez is currently looking into ways of expanding her approach. Compared with other galaxies, the black hole in the Milky Way is relatively calm. If movement within a galaxy such as the Milky Way that appears rather inactive are controlled by a black hole at its centre, such objects could well be found at the centre of every galaxy.

Ghez¡¯s participation in the Paul Bernays Lectures 2021 is a fitting continuation of the ¡°Women in Science and Space¡± lecture series held back in the spring, which saw female physicists present their research on the topic of space at the invitation of ETH Rector Sarah Springman (for the recordings see the links below).