Are Hubble Images Real? Part III: What Makes Them Real?

TLDR: Yes, Hubble images are real. This series of posts is dedicated to the scrutiny of Hubble imagery and a broader discussion of the veracity of astronomical imagery. In this post, we’ll look at a few examples of how Hubble extends beyond human vision to illuminate our understanding of the universe.

So far in this series of posts, we’ve taken a close look at Hubble’s optical system, and the path of light through the telescope into its detectors. This allowed us to explore the different kinds of image artifacts that image processors deal with when processing data from Hubble. We’ve also explored the history of astrophotography placing Hubble within the larger historical context of astronomical imagery—demonstrating a clear path to Hubble through the advancements of both the process of photography and the development of more sophisticated telescopes. Now, I would like to take a step back and consider what Hubble reveals to us about the universe. Why are these fantastical landscapes that Hubble reveals actually real, and what does that mean? What does Hubble reveal to us about the nature and limitations of human vision? Why do we tend to become preoccupied with asking the question “Is that what it would look like if I could go there in a spaceship and see it with my eyes”? But, before we dive into those deep questions of the macrocosm, let us first consider something a little closer to home, the microcosm!

Macro to Micro

I would simply like to point out that we often take for granted a fantastical realm that exists right in front of our faces at all times. Just as advancements in telescope and detector technology have uncovered more and more secrets of the universe, concurrent advancements in microscopy, diagnostic medical imaging, and even particle physics have similarly uncovered a rich and dynamic world that we are completely oblivious to, yet exists all around and within each of us. Over the course of the past year, in countless news articles, we’ve become intimately familiar with the look of a certain blobby sphere with spikes sticking out of it. Indeed, this tiny troublemaker is the reason life has been turned upside down for so many of us. But how do we know what the COVID-19 virus looks like? Do we ever stop to wonder, is that what it would look like if I could somehow shrink myself down to the size of a bacterium and float by a coronavirus particle? It almost sounds ridiculous to consider! Yet, just as with Hubble, we rely on extremely sensitive instruments designed to reveal the microscopic world to us, to extend our vision into the realm of impossibly tiny particles—and things can certainly get a lot smaller than the COVID-19 virus. 

Figure 1: The Relative Size of Particles—note the coronavirus particle third from the left. Credit: Visual Capitalist

The COVID-19 virus particle is on average about 0.1–0.5 micrometers in diameter, which is about a factor of 100 times smaller than the width of a human hair and well beyond visibility to the naked eye. A micrometer (or micron) is just a unit of measure that is equal to one millionth of a meter (and a meter is just over three feet). For perspective, a helium atom is about 1,000 times smaller than the small end of the coronavirus at about 0.1 nanometers, or 0.0001 micrometers. That size measurement of the helium atom is based on the electron cloud swirling around the nucleus. The atomic nucleus is actually 100,000 times smaller than the electron cloud (Russell 2008)! And yet, if you’ve ever had the pleasure of observing a cloud chamber, you can, for a fleeting moment, see on a macroscopic scale, the paths of subatomic particles as they whiz by us in all directions at all times. Of course, we’re not actually seeing the particles themselves, but just their effect on a very specific kind of environment designed to reveal them indirectly.

Figure 2: Radioactivity of a Thorite mineral seen in a cloud chamber. Credit: Cloudylabs via Wikipedia

This does raise an interesting question of how we can “see” atoms and molecules at all. To see something requires the interaction of light with the object in question. Light reflects off of different surfaces and our visual system interprets the reflected light as color, intensity, and shape. At a certain scale size however, it is no longer possible to use light to see things. If the thing you’re trying to see is smaller than the shortest wavelength of light that you’re using to illuminate it, you cannot see it. This is exactly why the electron microscope was invented, essentially replacing photons with electrons at much smaller wavelengths—allowing for high-resolution photography of extremely tiny things (Smith 2019). As with Hubble, this demonstrates human ingenuity, devising new tools to extend human vision beyond its natural limitations.

Is That What It Would Look Like if …

But, what are the limitations of human vision, and what does that mean in the context of astronomy and astrophotography? Our eyes are very well adapted to the environment in which we live, bathed in the light of the sun which emits the bulk of its radiation in the relatively small, visible portion of the electromagnetic spectrum. Our eyes are equipped with photoreceptive (light-sensitive) cells known as rods and cones, with the color-sensitive cones clustered around the focal point of the retina and the luminosity-sensitive rods scattered throughout the retina. The rods enable us to see in low-light situations in which there are not enough photons around to trigger the cone cells. They give us that extra bit of visual power to be able to see basic shapes in shades of gray, and track movement in a dark environment (Mukamal 2017).

Their lack of color sensitivity does not translate well to naked-eye astronomy however. If you’ve ever had the opportunity to observe deep space objects such as distant nebulae, or galaxies under dark skies with a telescope, you may have noticed that they’re barely visible in the eyepiece. You almost have to look off to one side to even see the object (a result of having fewer rod cells near the focal point of your eye) and it is very difficult to see any color. The light coming through the eyepiece just barely activates your rod cells, and isn’t strong enough to trigger your cone cells. Even with very powerful telescopes this is true. It’s only when we combine the power of the telescope with the light-gathering capabilities of extremely sensitive detectors that we can collect enough light of different colors to produce a stunning image revealing faint details and colors that our eyes simply cannot see.

Figure 3: Here are two views of the same scene, looking at the core of the Milky Way from photos taken by myself at Joshua Tree National Park. On the left is a single exposure of 20 seconds, which fairly closely matches my memory of how this scene looked to my eyes—the brilliant core of the Milky Way barely registering to my eyes, or on the screen. On the right is a combination of 15 exposures for a total of 300 seconds (5 minutes) exposure time. Combining multiple exposures allows the photographer to reduce noise and bring out more detail and color in the faintest areas of an image. It also helps to have the darkest skies possible, thanks to miles of uninhabited desert!

Learning from Light

Getting back to the title of this particular post, what makes these images real? How does Hubble see colors, and what do those colors mean? Unlike our eyes with their specialized light-sensitive cells that respond to color, Hubble’s detectors see everything in shades of gray, which is why the raw data from Hubble are all black and white images. The detectors are actually sensitive to light of all visible wavelengths and even slightly beyond both in infrared and ultra-violet (see Figure 4). As we learned in part I of this series, there is a physical mechanism in Hubble’s cameras which places a filter in front of the detector to allow only certain wavelengths of light to pass through and be detected. Astronomers use these filters to create images that isolate different wavelengths of light. There are wide-band, medium-band, and narrow-band filters to isolate large swaths of the electromagnetic spectrum, or very targeted, specific wavelengths.

Figure 4: Hubble’s instruments and their spectral sensitivity. Credit: NASA/STScI

For example, nebulae which are large concentrations of gas and dust, are often imaged using narrow-band filters to isolate the light emitted by specific elements like Hydrogen or Oxygen. Figure 5 shows an example of just such an image. When a Hydrogen atom is excited by nearby radiation (in most cases usually due to very nearby hot stars shining on the gas), the single electron in the atom may become excited enough to leave the atom, in a process known as ionization. Ionized Hydrogen atoms carry a positive charge and want to return to a neutral state by capturing a free-floating, negatively charged electron. When that happens, a photon is emitted by the atom at a very specific wavelength (around 656 nanometers) in the red portion of the spectrum (Cosmos: The Swinburne Astronomy Online Encyclopedia).

Figure 5: Messier 8—the Lagoon Nebula—a Hubble image showing ionized gases in different colors. Credit: NASA/ESA/STScI

This same process occurs with other atoms, with the same results except that the emitted photons will have different wavelengths. Narrow-band filters are designed to capture that very specific light. To create the color image, we combine images made in several different filters and chromatically assign colors to them, meaning that the longest wavelengths are assigned to red, the medium wavelengths green, and the shortest wavelengths blue. The color image then provides astronomers with a detailed view and deep insight into the physical processes which carve out the ethereal beauty of a distant nebula. The image, while beautiful, is also filled with information. 

One last thing to consider as we wrap up this series is the distance scale of the objects that Hubble observes.  These objects are unfathomably large and distant, and with that insight in mind, it quickly becomes apparent that the question of what these objects would look like if we could somehow fly to them becomes less and less relevant. It is somewhat analogous to looking at a large mountain from a distance. Imagine using a telephoto lens to snap a great photo of a large mountain chain from very far away, and then proceeding to approach the mountain to climb it. As you approach, the scale of the mountain continues to grow and grow, and eventually overwhelms your senses. When closer to the mountain, the views surrounding you will undoubtedly significantly differ from what you saw from far away through the lens.  So it is with the objects we observe with Hubble, even within our own galaxy. Take the famous image, Eagle Nebula Pillars of Creation, for example (see Figure 6). The tallest tower on the left side of that image is about 4 light-years tall, about 24 trillion miles, a scale so immense that it boggles the mind! If you could somehow get closer to this object, it would eventually lose all sense of its tower-like appearance from our vantage point many light-years away. This doesn’t take anything away from these amazing images, but it’s worthwhile to keep this perspective in mind.

Figure 6: Messier 16—the Eagle Nebula—Pillars of Creation. Credit: NASA/ESA/STScI

Are Hubble Images Real?

Are Hubble images real? Absolutely and without question, beyond the shadow of a doubt, yes! Hubble is perhaps one of humanity’s greatest and most prolific scientific endeavors. Its thirty-one years in orbit have reshaped our understanding of the universe and our place within it. Later this year, the James Webb Space Telescope will launch and much of what we’ve discussed in this blog series will apply to Webb images as well. Webb’s ability to focus infrared light will allow us to peer deeper into the very early, very distant universe, revealing the first galaxies and providing insight into their formation soon after the Big Bang, aka the Horrendous Space Kablooie. Its exquisite sensitivity will also open new windows into distant worlds around other stars, answering questions about their atmospheres and habitability. All along the way, we here at Illuminated Universe will be translating the cosmic light captured by Webb into a new generation of space imagery, as the universe will continue to unveil its beauty for all to see and ponder. 

References: 

  1. Russell, R. (2008) Atomic Nucleus https://www.windows2universe.org/physical_science/physics/atom_particle/atomic_nucleus.html&edu=high
  2. Smith, Y. (2019) History of the Electron Microscope https://www.news-medical.net/life-sciences/History-of-the-Electron-Microscope.aspx
  3. Mukamal, R. (2017) How Humans See In Color https://www.aao.org/eye-health/tips-prevention/how-humans-see-in-color

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