Why was a Black Hole In Our Own Galaxy Not Photographed First?
The Science Behind Photographing a Black Hole
On May 12, 2022, the second-ever photo of a black hole was taken. This time astronomers photographed Sagittarius A, a black hole much closer to earth, at the center of our milky way galaxy. Not too long ago, 4 years back the discovery of the first black hole Messier 87 (M87) created news waves among the scientific community. The question arises as to why M87 was the first photographed black hole, though it is two thousand times farther than Sagittarius A. In this article, we will explore the challenges of photographing back holes, how Sagittarius A was discovered, and the science behind black hole photography and their color scheme.
Challenges of Photographing Sagittarius A vs M87 Black Hole
Photographing Sagittarius A is the equivalent of capturing a picture of a donut on the moon, from the earth. If the sky was represented as 360 degrees, then Sagittarius A would only occupy 0.00000001 degrees.
Although Sagittarius A is much closer to earth at 25,640 light-years compared to the M87 black hole at 55 million light-years away, Sagittarius A is much smaller (about 27 million miles in diameter) than the Messier 87 black hole which is about 24 billion miles in diameter. The M87 black hole’s diameter is about a thousand times bigger than that of Sagittarius A. However, from Earth, these two black holes appear roughly the same size. In comparison, the moon and the sun look roughly the same size from earth and this is why we can see total solar eclipses where the moon covers the sun. So if both M87 and Sagittarius appear roughly the same size from the earth, why was the M87 photographed first?
When photographing the M87, radio telescopes pointed up and away from the center of our galaxy which makes images a lot more clear. When photographing Sagittarius A, radio telescopes pointed to the center of our galaxy.
The huge amount of dust and ionized gas in the center of our galaxy obstructs signals heading towards these telescopes. Also from earth’s perspective, M87 doesn’t move as much as Sagittarius A. This means the image of M87 will have less motion blur than the image of Sagittarius A. Due to the fact that there are dust clouds and other factors obstructing the light waves from Sagittarius A reaching the earth, astronomers decided to observe the black hole using infrared waves and radio waves. These can better penetrate through the debris, which would allow us to observe the black hole better. Though black hole activity was observed at both Sagittarius A and M87, scientists first photographed M87 for the above-mentioned reasons.
How was Sagittarius A discovered?
Based on the motion of a large number of planets and stars at the center of our galaxy, scientists hypothesized that a black hole existed. By observing the stars’ color schemes, the intensity of light, rotational patterns, and the speeds around a central invisible point, scientists made a prediction that there must be a black hole at the center of our galaxy.
Interferometry — Science behind photographing a black hole
Sagittarius A was photographed using the event horizon telescope which is a series of radio telescopes placed around the world. I explained more about this in my blog about the first picture of the black hole. A radio telescope captures information from the lower end of the electromagnetic spectrum like infrared and radio waves. This is different from an optical telescope that captures visible light. The radio telescopes that astronomers use look a lot like satellite dishes because they serve a similar purpose which is to focus incoming radio waves to one point.
The image created by two radio telescopes is a series of fringes as shown in the picture below. When two telescopes are closer together, they will create wider fringes whereas two radio telescopes that are farther apart, will create narrower fringes. This is very similar to the double-slit experiment, where the placement of the telescopes resembles the placement of the slits. Each pair of telescopes part of capturing Sagittarius A will give a different set of fringes.
Combining all the possible pairs of the fringes from the 8 telescopes at all the points in the earth’s rotations will give you a raw black and white image that resembles the shape of the black hole. This is a process known as interferometry which is shown in the animation below.
Why is the black hole image colored the way it is?
The image from the telescopes is a black and white image. Astronomers had to use a certain color scheme to represent the intensity of the light. They could have made the color scheme: rainbow, blue to red, green to yellow, or a black to red color scheme. They chose the black to red color scheme for the black hole image. This black background color scheme has been a default color scheme for wavelengths longer than visible light, which is located on the lower end of the electromagnetic spectrum.
Final Thoughts
As we unravel the mysteries of the universe one after the other, we see how waves and optics play an important role in observing, understanding, and making breakthrough discoveries in Astrophysics and Cosmology. With two black hole photographs under our belt, let’s see what the future has in store for us.
