The only major difference between the sun and the stars we see at night is that the sun is close to us, which is advantageous, provided you enjoy being alive.
Astronomers like this too, but have another reason to rejoice in the sun’s proximity: it allows us to see it as a disk. The sun is of course three-dimensional. But from a distance, we see it as a filled circle in the sky, which means we can study its surface in detail, revealing its sunspots, faculae, granules and other amazing features.
The stars in the night sky are a little further away; the closest to us, Proxima Centauri, is approximately 280,000 times farther away than the sun! This makes it appear that much smaller through a telescope – infinitely smaller, in fact, appearing only as a point of light. When an object appears this way, we say it is unresolved; when it is apparently large enough to present a real form, then it is solved.
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Is it possible to see other stars in the same way we see our sun, resolved in all (or at least part) of its splendor?
Well, technically, yes. But in practice, it’s difficult.
The visual acuity of a telescope depends on the size of its light-collecting aperture, which is usually a mirror or lens. If we do the math, many stars appear large enough in the sky to be detected by our largest telescopes. But there remains a problem: our turbulent atmosphere smudges the details of astronomical objects.
This sets a sort of limit to the smallest details you can see for objects in the sky. But clever techniques can get around this limitation, including adaptive optics, which quickly reshapes a telescope mirror to counter the movement of the air above. Another solution is speckle imaging, which uses extremely short exposure sequences to freeze this same movement. In the 1970s, astronomers used a variation of this technique to obtain sharp images of several large nearby stars, including Antares in Scorpius and everyone’s favorite nascent supernova, Betelgeuse in Orion. Be careful, even though they are physically large stars, they are so far away that they appear little, less than 0.00002 degrees wide, or about the same size as an American neighborhood would appear at a distance of 100 kilometers. For comparison, the sun is half a degree, more than 30,000 times larger.
As clever as these techniques are, they still face the most fundamental obstacle: the size of the aperture defining a telescope’s resolution. Building even larger ground-based telescopes would be useful, but would offer diminishing returns: at a certain size – around the size we already have today – the task becomes prohibitively difficult and expensive.
But there is another technique that can bypass even this limitation! This is called interferometry, and it depends on the fact that light is a wave.
An interferometric view of the red giant star π1 Gruis, seen by the PIONIER instrument on ESO’s Very Large Telescope. The resolved image reveals the convective cells that make up the surface of this immense star. Each cell covers more than a quarter of the star’s diameter and is about 120 million kilometers across.
Technically, light is an oscillation of electric and magnetic fields, but it still acts, in most cases, exactly like a wave. A beam of light has peaks and valleys, and when two beams pass through each other, they can create interference. The crests and troughs add up, sometimes forming higher crests and lower troughs or sometimes canceling each other out.
You are probably already familiar with this phenomenon, which also works for other types of waves. If you sit in a tub full of water and move back and forth in a rhythmic manner, you create waves that go up and down the length of the tub. When the crests of two waves intersect, they can become so high that they throw water out of the bathtub. Congratulations! You did some complex physics at bath time.
Starlight can also behave this way. Typically, interference is not as simple as pairs of interacting peaks or valleys; Starlight has many wavelengths, and the pattern it forms in any telescope is quite complex. But this structure, called interference or fringe pattern, encodes information about its stellar source, including its size, shape and brightness distribution (i.e. which parts of it are brighter or darker than others).
Here is the very Clever part: If you have two telescopes separated by a certain distance, light from both can be sent to a device that adds them to create interference patterns that can be analyzed, decoded and then used to create an image of the object that maps its details. But critically, the resolution of these telescopes is defined by their separation, not their size. Two modest telescopes 100 meters apart could, in principle, see as much detail as a telescope as wide as a football field!
This technique is called interferometry. Astronomers demonstrated this with radio telescopes in the 1940s and 1950s, and it is now commonplace in radio observations. Interferometry, however, becomes more difficult as the wavelength of light shortens. The “optical” wavelengths of visible light, for example, are much shorter than those of radio, so combining them is much more complicated. However, over the years, optical interferometry has developed with great success.
One of the world’s largest telescopes, the Very Large Telescope (VLT), consists of four 8.2-meter telescopes (along with four smaller telescopes) that cover a distance of more than 100 meters, giving them phenomenal resolution. But even this is not the largest: the Center for High Angular Resolution Astronomy (CHARA) network includes six one-meter telescopes separated by up to 330 meters. CHARA has a resolution greater than a millionth of a degree, more than enough to see the features of a decent sample of stars. In fact, most of the resolved images of stars we have come from CHARA.
Very high-resolution images of stars have revealed many surprising – and frankly strange – structures. The VLT scrutinized the red giant π1 Gruis and discovered that there were huge bubbles of hot gas rising from its interior. CHARA observed the bright star Altaïr and found that it had a distinctly ovoid shape due to its very rapid rotation. CHARA observations of the massive hypergiant RW Cephei showed that its shape was irregular and changing, indicating that it blew a huge starlight-choking dust cloud in 2022, as Betelgeuse did in 2019.
As for Betelgeuse itself, it has been the focus of interferometers on numerous occasions. Its size has changed over the years and its surface has proven to be complex, agitated by enormous bubbles of hot gas like those of π.1 Gruis. Massive red supergiants such as Betelgeuse create much of the dust we see scattered throughout the galaxy, but the mechanism is not well understood. Interferometric observations can help astronomers study how this happens.
The resolution of optical interferometry is limited only by our engineering and the speed at which computers can process the data. No one can imagine how big such a virtual telescope can reach. In fact, the Event Horizon Telescope, which connects radio telescopes around the world to take images of the magnetic fields around the Milky Way’s central black hole, is actually as big as Earth! As our technology advances and improves, we could yet see the faces of many more stars and learn from them as we did from our own sun.
