Astronomy

Astronomers Have Been Watching a Supernova’s Debris Cloud Expand for Decades with Hubble

Twenty thousand years ago, a star in the constellation Cygnus went supernova. Like all supernovae, the explosion released a staggering amount of energy. The explosion sent a powerful shockwave into the surrounding space at half a million miles per hour, and it shows no signs of slowing down.

For twenty years, the Hubble Space Telescope has been watching some of the action.

This supernova remnant (SNR) is called Cygnus Loop, and it’s about 2600 light-years away. It’s one of the largest SNRs astronomers know of, at 120 light-years in diameter, and it’s still expanding. Cygnus Loop contains numerous knots of nebulosity and also contains the well-known Veil Nebula, which is the visible portion of the Loop.

This is a UV image of the Cygnus Loop SNR captured by GALEX, an orbiting UV telescope that was decommissioned in 2013. Image Credit: By Elphion – This file was derived from Cygnus Loop Nebula.jpg, CC0, https://commons.wikimedia.org/w/index.php?curid=22859583

Some new research based on Hubble’s observation of the Cygnus Loop was published in The Astrophysical Journal in May. It’s based on a northeastern section of the Loop that the Hubble has observed over the last two decades. It’s titled “Third Epoch HST Imaging of a Nonradiative Shock in the Cygnus Loop Supernova Remnant.” The lead author is Ravi Sankrit, an astronomer at the Space Telescope Science Institute.

“The Hubble images are spectacular when you look at them in detail.”

Ravi Sankrit, Space Telescope Science Institute.

“Hubble is the only way that we can actually watch what’s happening at the edge of the bubble with such clarity,” said Sankrit. “The Hubble images are spectacular when you look at them in detail. They’re telling us about the density differences encountered by the supernova shocks as they propagate through space and the turbulence in the regions behind these shocks.”

The Hubble watched a thin strip on the outside edge of the Cygnus Loop to analyze the SNR’s expanding shockwave. Image Credit: NASA, ESA, STScI

At the heart of the study is a Balmer-line filament, the thin wisp of gas that appears reddish-orange in the image. Balmer lines are spectral line emissions from hydrogen atoms. The lines indicate different energy states as electrons transition from one level to another due to ionization. The SNR’s shock wave heats the otherwise invisible neutral hydrogen to a million degrees F as it passes through. This heating is followed by cooling, and the electrons in the hydrogen change state, releasing photons. The specific energy level of the photon makes it appear red to our eyes. The primary Balmer line is called H-alpha, and it’s a visible deep-red spectral line that’s conspicuous throughout the Universe. Many nebulae are red or partially red because of the H-alpha line, and it indicates ionized hydrogen.

The Hubble observed this part of the Cygnus Loop in three epochs that spanned 22 years. Three of the epochs observed the H-alpha, but only two observed another spectral line called OIII, which is doubly-ionized oxygen. It’s another common spectral line in astronomy because nebulae contain concentrated levels of OIII. It’s also caused by ionization and an electron changing energy states and releasing a photon. In OIII’s case, the photon’s energy level makes it appear blue. The OIII is further behind the shock wave and is visible in the images in its characteristic blue.

But the colours alone don’t tell astronomers how the gas in the Cygnus Loop is moving. This is where the Doppler effect comes in. By measuring the Doppler shift in the light from H-alpha and OIII, astronomers can measure the radial velocity of the gas as it expands outward from the supernova remnant.

Overall, the gas is moving at over half a million miles per hour, but there are variations. This produces a “rippled sheet” morphology.

“Those wiggles arise as the shock wave encounters more or less dense material in the interstellar medium.”

William Blair, study co-author, Johns Hopkins University.
This figure from the study illustrates the rippled sheet morphology in the filament. Each labelled red box is a projection region, with the numbers at the top of each representing proper motions. Each blue arrow indicates the leading edges of shocks at different locations in the filament. The image shows that the shock fronts have retained their overall structure so far without changes. This means that so far, the filament is moving through space without turbulence. Image Credit: Sankrit et al. 2023.

“You’re seeing ripples in the sheet that is being seen edge-on, so it looks like twisted ribbons of light,” said co-author William Blair of Johns Hopkins University in Maryland. “Those wiggles arise as the shock wave encounters more or less dense material in the interstellar medium.”

“When we pointed Hubble at the Cygnus Loop, we knew that this was the leading edge of a shock front, which we wanted to study. When we got the initial picture and saw this incredible, delicate ribbon of light, well, that was a bonus. We didn’t know it was going to resolve that kind of structure,” said Blair.

The Cygnus Loop’s overall shape is shell-like. But on the SNR’s perimeter, there are notable places where the outgoing shock wave forms knots. These are shocks, and there are two types: radiative and non-radiative. Radiative ones radiate energy, and non-radiative ones don’t. A third type, transitional shocks, is transitioning from non-radiative to radiative. Observing and mapping all three types in the study region sheds light on how SNRs behave as their shock waves travel through space and interact with other matter.

This image from the research shows different OIII emissions. The one on the left is faint and is near the filament’s upper left end. The one on the right is one of the knots in the filament at the bottom right end. It’s one of the transition shocks, where emissions are turning on as the shock becomes radiative. Image Credit: Sankrit et al. 2023.

“The shock front has been moving smoothly into the surrounding medium over a 20-year period, with no measurable deceleration and no drastic changes in filament morphology or brightness,” the authors write. That will eventually change as the shock wave’s energy diminishes, and it meets more regions of higher and lower density.

Even though the Cygnus Loop shockwave is travelling at half a million miles per hour, it’s still relatively slow compared to other SNRs. The velocity varies at different locations, and Cygnus Loop’s velocity variations, density knots, and other features all paint a picture of an SNR and how it behaves over time. Astronomers monitor all these features so they can not only understand the Cygnus Loop better but other SNRs as well.

The main question about the Cygnus Loop regards its nature. Is the pre-shock medium a cavity wall or an interstellar cloud? Unfortunately, there’s no way to be sure yet. But if the Hubble keeps watching for another 20 years, it might answer that question.

Evan Gough

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