Sometimes a project becomes remarkable. Some years ago we started thinking about what to do with the James Webb Space Telescope, whenever it would become available. An idea was developed and we worked out what kind of data would be needed. It turned out to be feasible, so we proposed it.
That is harder than it sounds. JWST has ‘proposal deadlines’: about once a year people can send in their proposals. They have to be complete with a strong science case saying what would be learned from it and why that is important to current science. A detailed explanation is needed of what has been done before, what are the open questions, and how your observations will address those. The science case alone can be many pages. The observations need to be fully defined: which targets, which instruments, which filters or gratings, how long each, and of course why each of these. You calculate the overheads (how long will the telescope take to slew and acquire the target, how long does it take to change filters, what kind of calibration is needed, etc. A team is formed who can carry out the analysis. Hit the ‘submit’ button and wait.
JWST will get 10 times as many requests as it can accommodate. Panels of scientists read and judge the applications and pick the 5-10% best ones. After some months, the announcements arrive, with 90% or more of the applicants left disappointed.
We were lucky. The panels liked our idea and it would not take too much time to carry out – that means, typically 10-20 hours although some observations are shorter or longer. We did further preparations, submitted the final observational setups and waited again. Now JWST was launched and moved out to its location beyond the moon, far from Earth. It took 6 months before everything was working, all problems sorted out, and the ‘features’ of the instruments were understood. Science observations began in July 2022. It so turned out that our observations were among the very first to be scheduled. That was pure luck as at any time the telescope can only look at a part of the sky. If your target is not in there, you will have to wait until the Earth and Sun have moved out of the way. There was nothing for to do: the observations were set from the control centres by qualified people after extensive checking. We were not allowed anywhere near the telescope. After, the data us checked for quality and any mishaps, and if ok passed on to the proposers. For us, some of the data was taken in the first week of July, and a second set later in August.
The data turned out to be spectacular. But understanding all the idiosyncrasies took a long time. The Space Telescope institute developed software to calibrate the data and remove all problems. That took time. By January we were happy with the data and started work. It got busy. If you are wondering why so few posts here recently, I was (still am) a JWST captive.
The target of our observations was the Ring Nebula.It is a small object in the northern sky, discovered in the 18th century by Charles Messier and a few weeks later by Antoine Darquier, possibly because Messier told him. Darquier wrote about it and is therefore often credited with the discovery, but it has a number in the Messier catalogue (M57, to be precise). Neither was interested in it: they were hunting comets and it so happened one was near this little nebula. Darquier compared to a ‘fading planet’. The name stuck, and we still call it a ‘planetary nebula’
We now know some 3000 planetary nebulae in our Milky Way galaxy. They range in size from pinpricks looking like stars to nebulae appearing as large as the Moon. The collage below was made by David Frew, Ivan Bojicic and Quentin Parker at the University of Hong Kong. It shows the variety of shapes and sizes, scaled to their proper physical size in space. The range of structures is amazing. Messier and Darquier knew nothing about this. All they saw through their small telescope was a small cloud. The ring shape is though recognizable through even a small telescope. Using a telescope the size of JWST is definitely overkill.
So what are they? Clearly planetary nebulae have nothing to do with planets. It turns out, they are stars just after their death. The nebulae almost always have a faint star dead centre. That star is the cause of the nebula. The star is dead. It is still hot (very hot) and luminous, but that is from recent days. It has used up all its hydrogen and even helium and can generate no more energy. At the very end, in a moment of madness, sorry in a well-defined process of physics, it ejected its remaining envelope with all of its left-over fuel. Now it is sitting there, half the star it used to be, and is beginning a phase of cooling and fading that will last for the rest of the life of the Universe. Why so hot? That is because you are looking at the core of the star, a Fukushima without the shell. Why still luminous? That is because stars at the very end become very bright. Our Sun will become 6000 times brighter than it is now. Why so faint? That is because it is hot. All the energy is coming out in the far ultraviolet which we can’t see. But the energy it is radiating away cannot be replaced. It is on its final journey to nowhere.
We are studying those shells, the nebulae that the star ejected. Where do all those amazing structures come from? Stars are round, so why these nebulae? The solar wind does not look like that. How does the material in the nebulae change? We see different colours throughout – why? And finally, how and why does the star eject so much mass? They only ever do it at the very end.
The images of the Ring Nebulae are fantastic. They have higher resolution than the Hubble Space telescope (at least some of the images we have do) and show very different aspects.
This is the Hubble image of the Ring Nebula. It shows the vibrant colours, but also details of the structure. The colours come from different elements. The nebula is ionized by the star and becomes a plasma. (Think the inside of lightning.) The hottest gas is closest to the star and is bright in oxygen which shines blue-green. Further out the gas is a bit cooler and there it shines mainly in nitrogen and hydrogen, which looks red. Of course we can make the images any colour we want, but we often (not always) try to stay close to the natural colours.
Around the ring you can see several fainter, broken rings, forming a halo. In the main ring you can see a few dark clumps, dense enough that they absorb light from behind them. And in the centre one can see that dying star.
The JWST images show the nebula differently, different from how we have ever seen it before.
So what do we see? Can’t tell you. We have more images that are not yet public. But some details are obvious. The ring is very broken here. (It isn’t really a ring but a torus that we see almost pole-on.) Clumps are everywhere. I counted 25,000 clumps. We used a software program designed to find clumps – it counted 17,000. I think my number is better. We estimate that half of the gas in the ring is in those clumps. We have ideas on how the clumps formed. They are so dense that the gas inside is shielded from the hot star and has started to form molecules. Some have started to form short tails, becoming like planet-sized comets. The halo shows a wealth of structure. One of these is a series of hundred of spikes or rays, pointing directly away from the star. They seem illuminated by light coming through holes in the shell. But we don’t see the holes.
What does it mean for us? Well, it is good entertainment, I guess. But there is a point of personal interest. Some of the gas in the shell becomes cool enough to condense. About 1% consists of condensable materials, such as silicon and iron. They form small dust grains. Those dust grains travel with the gas and in some 10,000 years will become part of interstellar space. And perhaps a few hundred million years later, they find themselves, much changed but still with a core that came from the planetary nebula, in a region where new stars form. Now they have an important role to play. For while the gas forms the star, the dust clumps together and grows bigger. And bigger. Eventually, it forms planets. Our Earth formed in this way. And there is more. The material ejected by the star contained products of its nuclear burning. When helium fuses, it produces carbon. Much of the carbon in space comes from here, from stars like the one forming the Ring Nebula. The Earth managed to capture a thin cover of carbon. Add some water and you get hydrocarbons. Add nitrogen, and you get the building blocks of life. Eventually, you get us, complete with a nice planet to live on and a nice star right next door to keep us warm.
This is the process we are studying, an ever richer cycle of matter in space where stars form, die and new stars form enriched by the debris, now with a consortium of planets. One day they too will die and add their ashes to the Universe, renewing the cycle. It has been called (thanks, Xander) a galactic ecology.
And this is what we are studying. That dead star in the Ring Nebula is leaving us a legacy. And by studying the Ring Nebula perhaps we can understand this ecology a little better. I am only a part of a team (and everything was awesome). Other people did much of the work and deserve the credit. But it has been a rewarding experience.
Albert, August 2023