When heavier stars run out of hydrogen to fuse into helium, the fusion reactions that keep the stars from imploding due to their own gravity become more difficult (as they infeasibly fuse helium into heavier elements) and eventually stop happening. At this stage, they blow away their outermost layer of gases and collapse into neutron stars (If the parent star is heavy enough, the neutron star collapses into a black hole).
The neutron star is an extremely dense, rapidly rotating body composed mostly of neutrons and ridden with powerful magnetic fields. These magnetic fields accelerate particles on and around some neutron stars and eject them in beams from the poles. Because the star is spinning, these beams periodically point toward and away from Earth, making them look like flashing points of light in the night sky.
For this reason, such neutron stars are called pulsars, and pulsars are used as ‘cosmic candlesticks’, relatively fixed points of light that astronomers use to gauge distances in the universe. Pulsars can remain stable for 10-100 million years, making them reliable on par with atomic clocks when it comes to keeping time as well.
The keys to their relevance for human observations are the stability and distinctness of the beams. Astronomers would use any other natural object like pulsars if only they emitted radiation that was long-lasting and distinguishable from the other light in the universe. Now, they might have a new kind of candidates starting with the ‘Eye of Sauron’.
That’s the common name of the galaxy NGC 4151, located about 40 million light-years from Earth. A group of Danish astrophysicists have measured the distance between the supermassive black hole at the heart of this galaxy and Earth by studying how it is heating up gas clouds and a ring of dust that surround it.
The clouds are heated as they’re compressed by the black hole’s intense gravitational pull. In the process, they emit ultraviolet radiation. The UV radiation then heats up a ring of dust orbiting the black hole at a large distance, and in turn the ring emits infrared radiation. Effectively, thanks to the thermal cascade, there are two concentric ‘zones’ of radiation around the singularity.
Astronomers from the Niels Bohr Institute at the University of Copenhagen used the twin Keck Telescopes in Hawaii and this effect to their advantage when trying to measure how far the black hole is from Earth. Darach Watson, an associate professor at the institute, explained,
Using telescopes on Earth, we [measured] the time delay between the ultraviolet light from the black hole and the subsequent infrared radiation emitted from the dust cloud.
Keeping in mind the speed of light, Watson’s team calculated the delay to be 30 light-days, corresponding to a distance of about 777 million km between the cloud of irradiated gas and the ring of dust.
If this weren’t cool enough, the astronomers then used a technique from 19th century (a.k.a. high school) optics to measure the distance between the black hole itself and Earth.
The most powerful astronomical telescopes are not built to observe electromagnetic radiation at all wavelengths because their resolution depends on the wavelength of the radiation they’re observing. Specifically, a telescope with a fixed lens diameter will have lower angular resolution (which is good) when observing radiation of lower wavelengths. So each of the 10-meter-wide Keck Telescopes will have an angular resolution of 8.75 arc-seconds when observing infrared emissions but 1.6 arc-seconds when observing UV light – an increase in resolution by 5.4-times.
But what makes Keck much better is a technique called interferometry. The two telescopes are separated by 85 meters, which makes their collective effective lens diameter 85 meters. The resultant interference pattern due to the difference in the time at which light reaches each of the lenses is then corrected for by computers, giving rise to an image of the object as if it were observed by an 85-meter-wide telescope.
Using interferometry, Watson and his colleagues were able to measure the diameter of the entire dust ring. As a result, they had two fixed distances in the night sky: the distance between the ring and the cloud of gas, and the width of the ring. The only thing left to find out the black hole’s distance from Earth was simple trigonometry, and a simple trigonometric calculation later, the astronomers had their answer: 62 million light-years.
Clouds of gas and rings of dust are common around supermassive black holes, which often reside at the center of large galaxies (the one at the Milky Way’s center is called Sagittarius A*). This means the ‘Eye of Sauron’ needn’t be an uncommon occurrence and could instead join pulsars in holding up candles in space’s dark for astronomers.
And coolness wasn’t the only outcome of the Niels Bohr Institute group’s experiment. Their work heralds a long-sought element of precision missing until now in measuring the masses of black holes. As Watson explained, again,
The calculations of the mass of the supermassive black holes at the heart of galaxies depends on two main factors: the rotational speed of the stars in the galaxy and how far it is from the black hole to the stars. The rotational speed can be observed and the distance from the black hole out to the rotating disc of stars can now be calculated precisely using the new method.
Watson & co. were able to find that the ‘Eye of Sauron’ was 40% heavier than expected.
So, not just coolness…
… but also awesomeness.