Black holes are one of nature's most wonderful and mysterious powers. At the same time, they are fundamental to our understanding of astrophysics. Not only are black holes the result of particularly massive stars that go supernova at the end of their lives, but they are also key to our understanding of general relativity and are thought to have played a role in cosmic evolution.
For this reason, astronomers have for many years been diligently trying to create a count of black holes in the Milky Way galaxy. However, new research shows that astronomers may have overlooked a whole class of black holes. This comes from a recent discovery in which a team of astronomers observed a black hole just above three solar masses, making it the smallest black hole ever discovered.
The study, "A Non-Interactive Black Hole with a Small Mass – a Giant Star Binary," appeared recently in the journal Science. The responsible team was led by astronomers from Ohio State University and included members from the Harvard-Smithsonian Center for Astrophysics, the Carnegie Institute of Science Observatories, the Center for Dark Cosmology, and numerous observatories and universities.
The discovery was particularly remarkable because it identified an object that astrophysicists did not previously know existed. As a result, scientists are forced to revise what they thought they knew about the black hole population in our galaxy. As explained by Todd Thompson, professor of astronomy at Ohio State University and lead author of the study: "
" We show this hint that there is another population out there that is yet to explore in the search for black holes. People try to understand supernova explosions, how supermassive black stars explode, how elements form supermassive stars. So if we could uncover a new population of black holes, that would tell us more about which stars explode, which ones don't, which ones form black holes that form neutron stars. This opens up a new field of research. "
Because of the influence they have on space and time, astronomers have long been looking for black holes and neutron stars. As they are also what the results are, when the stars die, they could also provide information about the life cycles of the stars and how elements are formed. To do this, astronomers must first determine where the black holes are in our galaxy, which requires them to know what to look for.
One way to find them is to search for binary systems where two stars are locked in orbit with each other due to their mutual gravity. When one of these stars dies, the intense gravitational pull it generates will begin to pull matter from the other star. This is evidenced by the heat and the X-rays that are emitted as the material from the star is accredited to its satellite in a black hole.
So far, all the black holes in our galaxy identified by astronomers have been between five and fifteen solar masses. In contrast, neutron stars are usually no larger than about 2.1 solar masses, since anything larger than 2.5 solar masses will collapse, forming a black hole. When LIGO and Virgo jointly detect gravitational waves caused by a black hole fusion, they are 31 and 25 solar masses, respectively.
This demonstrates that black holes can emerge beyond what astronomers consider normal range. As Thompson says:
"Everyone was immediately like 'wow' because it was such a grand thing. Not only because it proved that LIGO was working, but also because the tables were huge. Black holes of this size are a big deal – we have never seen them before. ”
This discovery inspired Thompson and his colleagues to consider the possibility of undetected objects residing between the largest neutron stars and the smallest black holes. To study this, they began to combine data from the Apache Point Observatory Galactic Observatory (APOGEE), an astronomical study that collects spectra of about 100,000 stars in the galaxy.
Thompson and his colleagues examine this spectrum for signs of change. This would mean whether a star can orbit around another object. In particular, if a star showed signs of Doppler shifting – where its spectra alternate between shifting to the blue end and then redder wavelengths – this would indicate that it may orbit an invisible satellite.
This method is one of the most effective and popular means of determining whether a star has an orbital system on planets. As the planets orbit a star, they exert a gravitational force on it that causes it to move back and forth. The same kind of change was used by Thompson and his colleagues to determine if any of APOGEE's stars could orbit in a black hole.
Started with Thompson narrowing the APOGEE data to 200 candidates, which proved to be the most interesting. He then gave the data to Tharindu Jayasinghe (a Fellow in Ohio), who then used data from the All-Sky Automated Survey for Supernovae (ASAS-SN) – which is managed by OSU and found over 1,000 superlatives – to collect thousands of images of each candidate.
This revealed a giant red star that seemed to orbit something much smaller than any known black hole, but much larger than all known neutron stars. After combining the results with additional data from the Tillinghast Echelle Spectrograph (TRES) and the Gaia satellite, they realized that they had found a black hole approximately 3.3 times the mass of the Sun.
This result not only confirms the existence of a new class of low-mass black hole, but also provides a new method for locating them. As Thompson explains:
"What we did here was inventing a new way to look for black holes, but we also potentially identified one of the first in a new class of low-mass black holes that astronomers didn't know before for. Masses tell us about their formation and evolution, and they tell us about their nature. ”
Further reading: OHS