For more than a decade it has been widely accepted that the center of the Milky Way is not, in fact, nougaty caramel, but a supermassive black hole. Due to the nature of the region surrounding the black hole, data was difficult to come by for many years. That is, until a series of celestial events provided confirmation of the black hole in our backyard. The mysteries surrounding the supermassive object have been steadily unraveled in recent years; however, the center of our galaxy is still a provocative and enigmatic place. The latest conundrum confronting astrophysicists is the presence of a “glow” emanating from the center of the Milky Way.

How do we know it’s a black hole?

Despite the consensus that our galaxy likely orbits a black hole, astronomers had struggled to conclusively prove its existence for quite some time — they had been unable to directly observe the hole itself. In fact, beyond pointing out the twelve stars orbiting the massive, invisible object, it is nearly impossible to observe a thing that allows hardly anything to escape its gluttonous maw. Key word: hardly.

When the black hole, Sagittarius A* (pronounced Sagittarius A-star), consumes objects — from stars and gas clouds to asteroids — it releases high energy x-ray flares that can penetrate the high-density gas cloud that usually obscures the hole from view — a celestial burp of sorts. Much of what is known about Sagittarius A* is obtained from studying these flares, but even these are sometimes difficult to deal with. Take, for example, an instance when the mysterious object called G2 in local orbit around the black hole lost its stable orbit and began its death spiral into the black hole. Scientists were on the edges of their seats in anticipation. This could be the opportunity they had been waiting for… until it didn’t happen. G2 was presumed to be a gas cloud until the black hole did not siphon off the gas as expected, but rather let the object pass intact, leading several scientists to believe that it was not a gas cloud but a star.

Our black hole has been starving for approximately the last ten thousand years after another star went supernova close to the ring of super-heated gas that normally orbits a black hole and blew all of that gas away. Thus, data have been few and far between. Despite failing to consume G2, Sagittarius A* has still been snacking every so often.


The location of gamma-rays relative to the Milky Way.


What about that mysterious glow?

Despite having solved the mystery of the supermassive object present at the center of our galaxy, many secrets remain, the latest of which is the mysterious “glow” from the middle of the Milky Way. NASA’s Fermi Gamma-ray Space Telescope recorded the presence of far more high spectrum gamma-ray radiation than known light sources can account for. Theories about the causes of the excess light range from the presence of pulsars too dim to be observed to the presence of dark matter at the center of the galaxy.

Many scientists have noted that the radiation aligns especially well with predictions for dark matter annihilation. Dark matter accounts for approximately 27% of the mass of the universe, while regular matter accounts for less than 5%; however, dark matter is still one of the least understood phenomena in astrophysics. One theory posits that dark matter is composed of weakly interacting massive particles, or WIMPs, that occasionally interact with matter particles in so-called “annihilation” events. When the matter and antimatter components explode, the energy released is in the gamma radiation spectrum and would therefore account for the gamma signatures from the center of the Milky Way.

However, two recent studies hint that pulsars rather than dark matter may be responsible for the excess light. Pulsars are rapidly spinning neutron stars that emit gamma light from their poles, creating a lighthouse-like effect. Since they are so dim relative to average stars (like our sun) it is difficult to observe them at great distances. The pulsar theory is supported by inconsistencies in the gamma radiation distribution. This inconsistent distribution points to radiation from many individual objects rather than the smooth distribution that a cloud of dark matter would produce. However, there is still not enough evidence to determine which phenomenon is responsible for the strange light at the center of the galaxy.

It seems some mysteries are just meant to stay that way — at least for now.

About The Author

Madelyn Broome

Madelyn was the 2018 Editor-in-Chief of Innovation, and a former writer and editor for the Space/Physics section. Her piece "Where's the Water?" won the 2019 Gregory T. Pope Prize for Science Writing. She is passionate about science communication and about making science engaging and accessible for people of all ages - though she especially enjoys working to ignite excitement for the sciences in young girls and other underrepresented communities in STEM. When she's not trying to share her enthusiasm for the sciences, she can usually be found exploring, practicing mixed martial arts, archery, lifting, playing soccer, or just generally trying to make up for the dessert she just ate.

  • Gesamtzusammenhang

    Given that this article discusses radiation and the black hole in the Milky Way’s center, I’m very surprised that there’s no mention of Hawking Radiation.
    For those that read the article and have questions on “dark matter,” take comfort in the fact that it is a controversial hypothesis designed to conveniently fill the gap in our understanding of black holes and quantum gravitational effects at a black hole’s event horizon.

    A few notes on Hawking Radiation:

    -Stephen Hawking demonstrated in 1974 what others had suspected and later confirmed: that black holes emit the electromagnetic radiation of a hot body.

    -Black holes were thought to not emit anything, hence their name, but Hawking acknowledged that this is a clear violation of Thermodynamic entropy. Matter/energy entering a black hole generate heat. Heat is a property that requires energy emission. As black holes exponentially rise in temperature, its gravity emits particles at the event horizon.

    -Though controversial at the time, this theory is now accepted because it does not violate the laws of Thermodynamics; even if quantum gravity is not wholly understood. The frequency of a photon that enters a black hole’s event horizon will approach infinity. The particle falls into the black hole, but the wavelength (towards infinity) is shorter than the Planck scale – where the classical laws of gravity do not apply. It is thought that quantum tunneling, superposition, and entanglement facilitate the process by which black holes emit radiation.

    -Indeed; as radiation leaves the event horizon of a black hole, the black hole’s mass decreases. It absorbs everything at a cost: an exponential temperature rise that involves radiation emission. Over time, the black hole radiates so much that it undergoes an “evaporation” of gamma rays. Its ultimate demise is marked by a massive burst of gamma rays.

    Dark matter might exist (for other reasons), but the calculations are most certainly incorrect. Even Hawking is aware that we don’t know the distribution or frequency of black holes in the galaxy. We also don’t know how many primordial black holes have burst into gamma rays – leaving a gamma ray cloud roiled into the galaxy’s gravitational spin. But one thing that is very probable, is that a great chunk of that “excess” radiation is due to the growth and death of black holes. The activities of Sagittarius A*’s supermassive black hole aren’t an exception. In fact, the gamma rays may be the best example yet of Hawking Radiation.