Astronomers found convincing evidence that a black hole was found in a young cluster of stars. This is the first time such a thing has been discovered. This is a significant discovery that will allow astronomers to better understand the nature of young black holes and how they change over time.
NGC 1850 is the name of the cluster. It's beautiful because it lies about 156,000 light-years away in the Large Magellanic Cloud. This dwarf galaxy orbits our galaxy, the Milky Way. It is young and less than 100,000,000 years old, which according to an astronomer is a baby. There are hundreds of thousands of stars within the cluster.
Infrared images of the Large Magellanic Cloud (a dwarf galaxy orbiting the Milky Way), show it. Credit: ESO/VMC survey
The MUSE (Multi Unit Spectroscopic Explorer), a camera mounted on Chile's Very Large Telescope, can capture spectra from thousands of stars simultaneously. Many spectrographs are limited to only one or a few stars. A spectrum is the light coming from a source, broken down into thousands or hundreds of individual wavelengths (or colors). You can learn a lot about an object by looking at its composition and speed. Also, you can see how fast it is moving towards or away from Earth using the Doppler shift.
The unusual spectrum was found from a star located 15 light years away, near the cluster's centre. Even though only one star can be seen, the spectrum clearly shows that it orbits around another object approximately every 5 days and is carrying mass. At times, the star's speed is more than 300 km per second (720,000 km/hour), which is incredible fast. Although the star has a mass five times that of the Sun, it is being torn about at an incredible speed by some invisible companion.
A black hole is the only thing that can keep a star from being impacted by so much gravitational force: it's impossible for a black hole to remain hidden. But how big is a black hole?
Combined Very Large Telescope/Hubble Space Telescope image showing the star cluster NGC 1850 and nebula NGC1850. This is located in the Milky Way satellite galaxies the Large Magellanic Cloud. Photo by ESO, NASA/ESA/R. Gilmozzi/S. Casertano, J. Schmidt
This can be determined by the speed and mass the star that orbits it. It all depends on how they orbit. If we see the orbit edge-on, we get maximum Doppler shift. But if it is face-on, we don't see any shift. We need to know the angle of their orbit in order to determine the mass of the black holes.
There's a way to make this happen. The star that is "normal" is in decline is the "normal star". As it runs out of hydrogen fuel, it starts to fuse into helium. Stars reach this stage when they begin to expand and eventually become red giants. Although this star is not yet at the red giant stage, it is growing.
It would normally expand as a circle, but the black hole is close by. Although the physics of the star is complicated, it forms a teardrop shape with the narrow part pointing towards the black hole. The star's brightness is also affected by this distortion, which makes it appear brighter or dimmer depending on the angle. Observations of this cluster were made with OGLE (the Optical Gravitational Lensing Experiment), which has measured the brightness of the star many times. It changes its brightness as it orbits around a black hole. This is dependent on the star's shape and the angle of its orbit. You can find that angle by playing with a little geometry.
Illustration of a black hole orbiting around a normal star. Its intense gravity is causing star to change shape. Photo by ESO/M. Kornmesser
They could also determine the orbital angle, which is critical for determining the black hole mass. They calculate that the black hole has a mass of 11.1 times that of the Sun.
This is a significant result! It can be difficult to find masses of black holes. If they are actively eating material, their mass can sometimes be calculated by the heat of the material before it falls in. This is a rare occurrence, as most black holes remain quiet. Many of them are located in binary star systems. However, it can be difficult to determine the true mass if the angle isn't known. Because the NGC1850 cluster is so young, we are only seeing the black hole at the beginning of its "life." They are usually visible when they are much older. Their behavior changes over time: they grow in mass, affect their star orbits, change their environment. Astronomers have a great starting point for understanding how these objects change.
In this instance, the star should slowly feed material to the black holes as it expands. The material emits X-rays when it falls in and can be seen as X-ray data. This marginal detection is visible at the star's position. Although it's weak, the beauty of this is that the black hole mass can be used to calibrate future black hole measurements.
Just the other day, I wrote about a heap of binary black hole merger detections using gravitational waves. Many of these detections come from larger black holes and it is not known how they get to that size. Astronomers will be able to better analyze gravitational wave data if they have a mass measurement for the one in NGC1850.
This is an impressive amount of science for an object that we cannot even see. Although the black hole is small (it's only 65 km across), it doesn't necessarily mean that it is weak. This, together with its mass, is what gives the black hole its power and explains why it can be detected anywhere in intergalactic space.
