# What are the minimum element requirements for a star?

What elements would be essential for a star to exist?

From what I understand our sun is made of mostly hydrogen and helium and most stars have small amounts of heavier elements like carbon, nitrogen, oxygen and iron.

But could a sun exist on a single element such as Hydrogen? Or on another gas for example oxygen or nitrogen? Or even rare gases such as Neon or Xenon?

Is it even possible for a star to exist without any hydrogen or helium?

• This is probably better placed in the Worldbuilding site. On astronomy we tend to consider such questions based on the real world, and what could actually happen in our universe. – James K Nov 16 '19 at 14:20
• My apologies because the question was about stars I (perhaps wrongly) assumed this was the correct place to ask the question. – Jake Graham Arnold Nov 16 '19 at 22:21
• I think this question is a great fit for the astronomy stackexchange. The world-building stackexchange is for fiction writers who might not even know much about science. This is an interesting scientific question which is aimed at understanding clearly the physics of stars. Scientists are always asking "what is possible under the laws of physics." – littleO Nov 17 '19 at 2:36

Yes, a star can exist that's entirely hydrogen. Hydrogen is the fuel that makes stars happen unless they're very large. A star made entirely of hydrogen, so long as it was massive enough, would be very similar to the stars we see. The "metalicity" which refers to non hydrogen-helium elements has some effect on the star's rate of fusion, density and spectroscopy but it doesn't have a major effect.

When the universe was very young and almost entirely hydrogen and helium in terms of regular matter or elements, star formation was difficult because those light gasses were difficult to clump together. The first stars were probably very large as a result, but once heavier elements began to be released in early supernovas, then star formation became easier because there were elements that could clump together and help smaller stars form. So the other elements play a key role in star formation, but a much smaller role in the life of the star.

Very large stars fuse heavier elements, but as I understand it, fusion of heavier elements is more explosive. Hydrogen is wonderful in the way that it's very unlikely to fuse, so even large stars take millions of years to fuse their hydrogen because something like 99.9999% of the time, when two hydrogens get close enough to fuse, they just return to being two hydrogens, and only 1 in a hundred million billion billion times (1 in 10^26), credit to @PM2Ring. It's only very rarely that the proton-proton collusion undergoes the fusion process and becomes a deuterium.

This very slow process doesn't happen with the heavier elements, so a star made entirely out of elements heavier than hydrogen and helium would begin it's life as a kind of white dwarf star, that is, the mass of a star but the size of a planet, and technically, it wouldn't be a star at all because it wouldn't be undergoing fusion and when the fusion process finally starts, it would be more like a type 1A supernova, happening very quickly and explosively.

The CNO process which creates some of the heavier elements in larger stars doesn't happen without hydrogen either, or, more accurately, free protons.

Basically, you can make stars out of pure hydrogen. Other elements would be made inside stars, but you can't make stars out of any other elements, unless you consider variations of a type-1a supernova a "star". I personally don't.

• Even if you managed to form a star out of pure hydrogen (not likely in our universe, where the Big Bang also produced helium), the first fusion reaction would result in it not being pure hydrogen any more. – user24157 Nov 16 '19 at 22:13
• Wow thanks! Forgive me, I didn't know that's how stars worked, regarding the hydrogen fusion. Do you know if H is unique in this regard or are there any others elements that would also fuse in this way? Oddly I'd not questioned why the sun lasts so long before... Thank you :) – Jake Graham Arnold Nov 16 '19 at 22:31
• The conversion ratio of a diproton to a deuteron in the solar core is much lower than 1 in a million, it's more like 1 in $10^{26}$. – PM 2Ring Nov 17 '19 at 11:51
• @JakeGrahamArnold It's not really the hydrogen, it's specifically the proton + proton fusion. Stars bigger than the Sun still fuse hydrogen, but not directly - the CNO cycle "catalyses" hydrogen fusion, because it no longer relies on the very unlikely event of two protons forming a diproton and one of those turning into a neutron through the weak nuclear force forming a deuteron in the process. In stars like our Sun, this is the limiting factor in the fusion rate - the rest of the fusion chain happens extremely quickly in comparison. This is also why we don't even try proton+proton fusion :) – Luaan Nov 17 '19 at 17:04
• @JakeGrahamArnold All elements smaller than iron will fuse in stars. This is why the elements past iron are less common; once you reach iron, there are fewer methods of getting other elements. (all elements heavier than lithium are exclusively formed in stars) – Hearth Nov 18 '19 at 0:49

Stars are formed from the thin gas in space, and the thin gas in space is made of roughly 3/4 Hydrogen and 1/4 Helium (+trace amounts of other elements). There's no way to get a star made of 100% Oxygen as there is no way to gather enough oxygen in one place to get sufficient mass to make a star. So all real stars will be made mostly of H and He, with small but significant amounts of other elements.

If it were, somehow, possible to do this then a star-massed ball of any of the lighter elements (lighter than iron), undergoing gravitational collapse, could start fusion in its core. Whether a particular nuclear reaction is energetically possible or statistically probable depends on the nuclear binding energy. In extreme conditions, silicon and other elements can photodisintegrate, which provides Helium that can then fuse with other nuclei, this can allow further reactions, even if the Si+Si is not directly possible. We don't know what such an object is like for sure, as one has never been observed. Even so-called "carbon stars" are made mostly of H and He. It is not possible for a star to form without H or He, because these are the principle components of the interstellar gas.

• Awesome, very cool. I'll take a look at 'carbon stars' I didn't realise most stars were made up of the same stuff as our sun, for some reason I assumed they'd all be made of different gasses and different ratios. Not sure where I got that idea from. Thank you. – Jake Graham Arnold Nov 16 '19 at 22:36
• There are various classes of hydrogen-deficient stars but even those likely have some hydrogen in them. – user24157 Nov 16 '19 at 22:39
• Does this mean that if someone was able to amass a star's worth of weight of pure manganese atoms in an empty region of space, a star would form from it (where the fusion from inside would counterbalance the gravitational contraction)? And if it was iron or something heavier instead, then fusion wouldn't ignite, and we would end up with a collapsed ball of some degenerate matter, hot for a relatively short time only because of compression? – vsz Nov 17 '19 at 10:38
• @vsz That sounds like a new question (either here or Physics). I don't think that manganese + manganese fusion is exothermic. The heavy element fusion reactions of stellar nucleosynthesis generally involve fusion with helium. See en.wikipedia.org/wiki/Silicon-burning_process and the various linked articles. – PM 2Ring Nov 17 '19 at 12:01
• @PM2Ring : I only picked manganese because it's just below iron, and iron is commonly referred to the point above which fusion no longer produces enough energy to be sustainable. – vsz Nov 17 '19 at 14:22

Formation of pure hydrogen stars is not likely to have happened in this universe, because Big Bang nucleosynthesis resulted in the production of helium and lithium (plus the unstable isotopes tritium and beryllium-7, which decay into helium-3 and lithium-7 respectively) in addition to hydrogen. It is therefore likely that the initial star-forming environments included these elements.

Even if you did somehow form a pure hydrogen star, a star is powered by nuclear fusion. This is the conversion of lighter elements into heavier ones, so the first fusion reaction would result in the star not being pure hydrogen any more.

As regards the question of stars without hydrogen, there are various categories of hydrogen-deficient stars, mostly the result of stellar evolution processes (some of these objects would be considered stellar remnants). Bear in mind that these are identified by their spectra which means that what is being probed is the outer layer of the star rather than the bulk composition. Most likely these objects are not entirely devoid of hydrogen.

Yes, it is possible for stars to exist that contain basically no hydrogen and helium - they are abundant and they are called white dwarfs. Rarer examples are the sdB and sdO stars that are composed almost entirely of helium.

However, white dwarf "stars" are not undergoing fusion processes - they are inert. This illustrates fundamental physics. There is usually considered to be a minimum mass for "starhood" - it is the minimum mass that will achieve a central temperature high enough to initiate fusion reactions that can supply the stellar luminosity.

For stars with a composition like the Sun, it is about 0.075 solar masses. For heavier elements it is higher because the Coulomb barrier to fusing heavier elements is higher and so higher core temperatures must be achieved.

For a star made of oxygen, the minimum mass would be something like 1.2 solar masses. Smaller objects made of pure oxygen, would contract till they were about the size of the Earth, and then stop contracting when they became supported by electron degeneracy pressure, and would never get hot enough to commence fusion. This is what we call a white dwarf.

Stars of various compositions are produced in nature at the centres of "normal" stars. "Normal stars" build up cores that are made predominantly of helium and then later of carbon, oxygen and ultimately iron. These cores are normally hidden underneath a hydrogen envelope, but mass loss or stripping by a companion can reveal these objects. An example would be the sdB and sdO stars. These are genuinely stars, in the sense that they are undergoing fusion reactions, turning the helium into carbon and oxygen. The minimum mass to do this is about 0.4 solar masses. Lower mass examples become almost pure helium white dwarfs.

The first stars after the big bang would have contained hydrogen and not much else. As they age heavier elements are created within. That's how elements are created. Well, up as far as iron at least. Beyond that you need a supernova to happen. This is all assuming that I'm remembering my A-level physics accurately. It's been a while.

• Given that baryonic matter in the Universe is about 25% helium by mass, or about 8% by number of atoms, it's incorrect to say "and not much else". Helium is inevitably present in all protostars. – Chappo Hasn't Forgotten Monica Nov 19 '19 at 2:01