Twenty years of observations led by this astrophysicist have proved the existence of a supermassive object at the heart of our galaxy

Reinhard Genzel: “The black hole at the center of the Milky Way has a mass equal to some four million suns”

  • TheDirector of the Max Planck Institute for Extraterrestrial Physics (Garching,Germany) and professor in the Physics Department at the University of California, Berkeley will visit the BBVA Foundation’s Madrid center on Wednesday, October 19 to take part in the astrophysics and cosmology lecture series.
  • For the moment, the black hole at the core of our galaxy is feeding on a mass equivalent to just “one asteroidper day.”
  • The BBVA Foundation lecture series Science of the Cosmos. Science in the Cosmos has welcomed world authorities in the most active areas of astrophysics, from the study of the origins of the Universe to ultra-exotic, high-energy phenomena like black holes or gamma ray bursts. Videos of the lectures can be viewed in full on

Madrid, October 17, 2016.-The center of our own galaxy, the Milky Way, is home to a black hole of a mass equivalent to around four million suns. It cannot be seen–its gravitational pull is so strong that it absorbs even light –but its discoverer, Reinhard Genzel, is clear that evidence gathered over more than twenty yearshas proved its existence “beyond any reasonable doubt”. Genzel, Director of the Max Planck Institute for Extraterrestrial Physics (Garching, Germany) and professor in the Physics Department at the University of California, Berkeley (USA) will explain his findingson Wednesday, October 19, as the latest speaker in the BBVA Foundation lecture series Science of the Cosmos. Science in the Cosmos.

The black hole at the center of the Milky Way is technically an astronomical radio source known as Sagittarius A-star,or Sgr A* for short, which appears intensely bright when observed with radio telescopes. It is, however, entirely opaque to visible-light telescopes, because the Galactic Center is full of obscuring dust. It was first discovered, using radio telescopes, in the mid-1970s, but it was not until the end of the last decade that its true nature was confirmed.

In his talk, titledGalaxies and Black Holes, Genzel will tell the story ofSgr A*, discussing what we know of it today and the implications of that knowledge. Among them, the fact that the Genzel team’s observations provide what he calls“the best evidence to date in astronomy” that these giant cosmic sinkholes lie at the center of most galaxies. “Only the detection of gravitational waves with LIGO at the start of this year brought more convincing proof of the existence of these strange objects predicted by Einstein's Theory of General Relativity,” he continues. What LIGO found, however, was the merger of two black holes of some thirty solar masses each, far smaller than the galactic objects of the type observed by Genze, weighing in at literally millions of suns.

Fifty stars point the way tothe black hole

It took over twenty years of precise observations to confirm that Sagittarius A-star was indeed a gigantic black hole. As Genzel relates, the existence of supermassive black holes was already being speculated on in the mid-1960s, after the discovery of a series of very bright but also distantsources, their enormous luminosity owing, it was supposed, to the energy released by matter falling towards an immensely large object. “However, direct proof of their existence,” Genzel points out, “would involve measuring[the gravitational potential] as close as possible to the hole itself.”

It was precisely thiskind of measurement that Genzel began working on in the late 1980s. He and his group developed instrumentation that combined superior precision with the ability to receive light from dust-filled regions, and installed it in the world’s most advanced telescopes. Now scientists were able to measure the orbits of numerous stars in the galactic core.

“We have orbits of about fifty stars very close to the black hole,” Genzel explains. The distance from the hole to the innermost of the stars is about one hundred times the distance Sun-Earth, virtually nothing in astronomical terms. And their distance from the event horizon marking the point of no return for any object entering the hole is around one thousand times Sun-Earth.

These stars’ orbits tell us a lot about the bodies they are circling.Hence Genzel’s team were able to estimate both the mass and size of the hole. “This technique doesn’t serve for distant quasars,” he observes, “but it can be applied to the center of the Milky Way, 24,000 light years away, and nearby galactic nuclei.”

In 2010 the group summarized its results in a paper which affirmed: “the Galactic Center is a unique astrophysical laboratory for studying in unparalleled detail the properties and evolution of gas and stars in a galactic nucleus (…) Evidence is presented, from the analysis of the orbits of more than two dozen stars and from the measurements of the size and motion of the central compact source, Sgr A* , that this radio source must be a massive black hole of about 4.4 million solar masses, beyond any reasonable doubt (…)”.

A black hole with a frugal appetite

What kind of environment can we expect in the vicinity of the galactic black hole? Is Sgr A* a Gargantua of insatiable appetite, like in the film Interstellar? On the contrary, says Genzel; right now, it is feeding itself on just “one asteroid’s mass per day.”

“TheGalactic Center is surrounded by a very dense cluster of stars, and there is also plenty of ionized gas swirling through the region, but relatively little of that matter is accreted onto the event horizon of the black hole,” he clarifies. “It is not impossible, but certainly very unlikely that the stars we see in orbit around the holewill experience a catastrophic event [being devoured by the hole].”

Researchers know that a black hole has “eaten” when they observe flaresof infrared lightandX-rays– a kind of radiation that shines despite the presence of dust. From time to time they also come across entirely unexpected phenomena. In recent years, they were, as Genzel says, “lucky” to observe a tiny, dusty, ionized gas cloud plunge through the inner region, so close to the event horizon that it was “spaghettified” by tidal forces, then come back out, “remarkably intact.” “The Galactic Center always has surprises that teach us more about the conditions and the physics operating in such a strange place.”

Bio notes

Reinhard Genzel is Director at the Max Planck Institute for Extraterrestrial Physics (Garching, Germany), and a scientific member of the Max Planck Society. He is also Full Professor in the Physics Department of the University of California, Berkeley (USA) and Honorary Professor at the Ludwig Maximilian University in Munich (Germany). His research focuses on massive black holes and star formation in galaxies, the Galactic Center, and galaxy formation and evolution, which he studies with infrared, submillimeter and millimeter spectroscopy and high resolution imaging, and with spatial interferometry. He has received numerous awards including, recently, the Tycho Brahe Prize of the European Astronomical Society (2012) and the Herschel Medal of the UK’s Royal Astronomical Society (2014), as well as several honorary doctorates.

Science of the Cosmos. Science in the Cosmos, forthcoming speakers

The next speakers in the series will be Werner Hofmann (Max Planck Institute for Nuclear Physics, Germany), who will talk about the future Cherenkov Telescope Array (CTA), part of it sited in the Canary Islands, and David Reitze, Executive Director of the LIGO Laboratory at Caltech and Professor of Physics at the University of Florida.

For more information, contact the BBVA Foundation Department of Communication and Institutional Relations (+34 91 374 5210 / 91 374 3139 / )

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