Can you make a star from other gases, and how long would they last?

Question

In my Conworld, the inhabitants have discovered a type of star they cannot identify. I would like it to be a star made not from just hydrogen and helium, like our sun and countless others. But my question here is: Can a star be made of other materials? I am mainly interested in:

• Oxygen
• Nitrogen
• Neon
• Argon
• Radon

If these are not available, then tell me which, if any, can be used to make a star. I would prefer noble gases, and maybe other, non-diatomic gases like methane (like an inflated Neptune would be cool!). I understand that some of these may fuse to become heavier elements, which cause stars to explode. On top of this question, I would like to ask; If these are possible, then how long would they last? this one is fully optional, but if you can answer it, please do.

Answer 1

Fusion in stars generates energy only when the binding energies of the resulting (fused) nucleus is less than the combined binding energies of the "ingredients". Broadly speaking this means that a star made up of elements up to and including Iron generates more energy from fusion than it uses up from gravitational pressure.

Once you go beyond Iron (with a few exceptional cases) the energy the star generates is not enough to prevent gravitational collapse. The details are somewhat more complex, but you'd need to get into pretty heavyweight (and long) physics to understand them and the net result won't be much different.

So while you can in principle manage Oxygen, Nitrogen, Neon and Argon (all before Iron in the periodic table), Radon is not going to work.

You should, however, note that their no natural process (I can imagine) could generate a large enough and concentrated enough amount of these heavier elements without a great deal more Hydrogen and Helium being in existence at the same time.

But even when a star does "run out of fuel", that typically does not mean it has run out of Hydrogen. Most of the hydrogen is outside the core where fusion takes place and even when a star "explodes" after the collapse it will be mostly scattering "left over" Hydrogen and not heavier elements. The next generation of stars will be created by collapse of the resulting nebula and these will be made up mostly of Hydrogen.

I would then say that there is a minute possibility that these type of stars could form. I could never say it's impossible, but it's extremely unlikely in the universe as we see it today.

If these are possible, then how long would they last?

In general lighter stars last longer than heavier ones. In main-sequence stars red dwarfs will outlast all other stars by many orders of magnitude. The Sun might last a total of maybe 12 billion years, whereas a small red dwarf could last trillions of years.

I don't have any specific links to how long stars of your type could last beyond that. Think billions of years for stars as marge as the Sun, but much, much long for stars near the threshold for this type of star to exist at all. Smaller is better for longer life is the (very rough) rule of thumb.

The actual mass threshold for these types of fusion is harder to know. Red dwarfs are hydrogen fusers (like our Sun) and require a minimum mass of about 0.07 Solar masses, but stars fusing heavier elements require more mass to ignite such self sustaining fusion (but do not require as much mass to sustain it because temperatures increase in the core after ignition). A guess would be 0.15 to 0.5 Solar masses to for these "heavy element" stars to ignite.

Answer 2

White dwarf stars may already fit your criteria. They're fairly common, extremely long-lived (trillions of years), and, most importantly for your purposes, made mostly of carbon and oxygen. Larger-mass white dwarfs can also contain large amounts of neon and magnesium. Unfortunately, elements heavier than that aren't possible in this case, since if the star gets any more massive, it will become a neutron star instead (at which point the notion of the star being made of elements at all breaks down).

If white dwarfs are too mainstream for your purposes, the amusingly-titled paper Some Stars are Totally Metal suggests that turbulence in stellar nebulae can cause heavy-element debris to cluster in sufficiently high densities to ignite stars. It suggests that about 1 in every 10,000 stars forms this way. This might be your only way to see high quantities of something like calcium in a realistic star (noble gases would still be very uncommon, since they wouldn't form heavier dust particles in the first place).

This is a relatively new concept, so I'm not aware of a lot of theory that has tried to precisely map out the concentrations of various elements in these 'metal stars.' However, the article suggests that carbon will be especially abundant, though there's still going to be a lot of hydrogen and helium in there. In addition, the paper predicts that these stars will most likely only last on the order of a few million years before collapsing into white dwarfs.

Answer 3

From Professor Barbara Ryden:

For instance, consider the stages in the life of a 25 Msun star:

• Hydrogen fusion lasts 7 million years
• Helium fusion lasts 500,000 years
• Carbon fusion lasts 600 years
• Neon fusion lasts 1 year
• Oxygen fusion lasts 6 months
• Silicon fusion lasts 1 day

Answer 4

What can a star be made of?

A star's composition is limited by the elements that exist in significant quantities in the universe. These include primordial elements - hydrogen, helium and lithium - as well as heavier elements formed through nucleosynthesis in stars, supernovae and certain rare processes like cosmic ray spallation. This narrows down our options considerably; hydrogen, helium, oxygen and carbon are the four most abundant elements, by mass, in the interstellar medium. Radon, to use one of your examples, simply doesn't exist in significant amounts.

We also can't have molecules like ammonia (to use your example) as part of a fusion pathway. At the high temperatures at which fusion takes place (well over $\sim10^6$ Kelvin), molecules aren't even able to form; the metals that do exist in stars, like titanium oxide, are only found in the cool stellar atmospheres of the least massive stars. Even molecular hydrogen can't survive in the core of a cool star, let alone a star hot enough to fuse heavy elements.

We're even further restricted in our choice of elements because not all fusion reactions are exothermic, or energy-releasing. Famously, iron (and nickel) fusion consumes more energy than it releases, although it still occurs inside the most massive stars for very brief periods of time. We need our star to be supported by exothermic nuclear reactions. At typical temperatures in a star's core ($\sim10^6\text{-}10^9$ Kelvin), several types of processes dominate in different regimes. The elements typically fused are carbon, neon, oxygen and silicon. Other elements aren't going to be able to release energy in significant amounts through realistic fusion pathways.

Interlude: The alpha particle problem

One issue here is that many of these reactions either produce or consume hydrogen or helium. For example, one of the main oxygen-burning processes produces silicon and helium: $${}^{16}\text{O}+{}^{16}\text{O}\to{}^{28}\text{Si}+{}^4\text{He}$$ In fact, helium nuclei (also known as alpha particles) play key roles in the fusion of many of these heavy elements, including the production of nickel and iron. This means that you do need some helium in your star for fusion to be significant.

How can you form a heavy-metal star?

Your best bet is to try to make a star out of one of the lightest stable, easily-fusable metals: carbon. It's a reasonably common elements that's produced regularly by massive stars, and the interstellar medium is enriched with it by supernovae. Furthermore, carbon fusion can happen at temperatures just under $10^9$ Kelvin - easier to attain that the conditions required to fuse neon, oxygen or silicon.

At low carbon abundances, when helium is present, the dominant pathway is $${}^{12}\text{C}+{}^4\text{He}\to{}^{16}\text{O}+\gamma$$ where an oxygen nucleus and a photon are produced. However, when carbon is much more common, a different net reaction occurs: $${}^{12}\text{C}+{}^{12}\text{C}\to{}^{20}\text{Ne}+{}^4\text{He}$$ creating neon and an alpha particle. This is the reaction most likely to happen in your star.

Before trying to form a star devoid of hydrogen and helium, I think it's instructive, for a start, to look at extreme helium stars, part of a broader class of hydrogen-deficient stars that includes R Corona Borealis (R CrB) variables and AM Canum Venaticorum (AM CVn) stars. These are all stars with essentially no hydrogen; instead, they're dominated by helium envelopes and cores of heavy metals. Extreme helium stars, in general, form through one of two types of processes:

• Double-degenerate mergers, where two white dwarfs merge and the resulting product is hot enough to undergo fusion. For instance, the most likely model for the formation of R CrB variables and some extreme helium stars comes from the collision of a $0.6M_{\odot}$ carbon-oxygen white dwarf and a $0.3M_{\odot}$ helium white dwarf.
• Shell fusion processes, such as a late thermal pulse or a dramatic shell flash, that involve the rapid conversion of hydrogen into helium, leaving behind a highly hydrogen-deficient star. Obviously, this requires a progenitor with non-zero hydrogen abundances, but the result clearly has negligible hydrogen.

You might be wondering why I bring these up; after all, the products still have helium. However, it seems reasonable that analogous processes could happen that yield stars deficient in helium, too. Let's look at what happens if we consider the double-degenerate model - with a twist. If both of our white dwarfs are carbon-oxygen white dwarfs, deficient in helium, there's the possibility that a merger could form a star that is now purely heavy metals.

The problem is that to produce the white dwarfs required for the collision, you need progenitor stars of intermediate masses (say, $\sim5M_{\odot}$). Massive progenitors may yield massive white dwarfs, and so these carbon-oxygen white dwarfs could be $\sim0.6\text{-}0.7M_{\odot}$, meaning that the resulting product will be near the Chandrasekhar limit and thus highly unstable. It shouldn't be a surprise that white dwarf mergers are now being studied as the progenitors of many Type Ia supernovae.

Answer 5

Natural stars start out as hydrogen (protons and electrons), since that is basically what the universe is made of. When the star is sufficiently big and hot, protons fuse to make deuterium; once there is enough deuterium, some deuterium nuclei will fuse to make helium; and so on, all the way up to uranium and beyond (in exponentially tinier amounts).

Whether fusion is sustainable for a given element is a different question. The rate of fusion for a given element is correlated with pressure and temperature, and inversely correlated with atomic number. For a star to be stable, it has to release enough energy from fusion to compensate for the heat it's constantly radiating into space. Achieving this equilibrium with heavier fuels requires hotter, denser stars, and with elements heavier than iron it cannot be achieved at all, since fusing those nuclei results in a net loss of thermal energy. However:

1. a star can be in an unsustainable transition state for many years before it collapses; and
2. heavier nuclei do still undergo fusion, even if it doesn't contribute to keeping the star alive. Were this not the case, we wouldn't have naturally-occurring uranium, or gold, or zinc.

Even healthy stars contain some amount of heavy elements (though I believe most heavy-element production is thought to occur in dying stars). So that in itself would not be surprising. What would be surprising is if the proportions of different elements implied that the star was not all hydrogen to begin with.

In nature, the only way a star is burning a high proportion of second-row elements is if it is enormously heavy and on the point of becoming a supernova. But if you could figure out a way to start with just lithium or beryllium, I would guess it is possible to set that up so that the star lives for millions of years at least. But I would guess that if it was all-oxygen or all-neon, that would be less like a "dense normal star" and more like a "skinny supernova" that would quickly collapse.

(NB molecules, like methane or O₂, are not meaningful in this context; at star temperatures, chemical bonds don't exist, it's all just atomic nuclei and electrons).