# Tag Info

44

The latter. To astronomers, a metal is any element that is not hydrogen or helium, because these elements together constitute most of the elements in the Universe, by far. This means that, in many circumstances all other elements can be neglected, at least to first order. By mass, H and He account for some 74% and 24% in the present-day Universe, ...

43

The straightforward answer is, "Yes, we are made of star stuff." Some of it will be from the interior of collapsing stars, some will be from supernovas, some from normal everyday fusion, and some from other processes. The answers from @HDE226868 and @RobJeffries on this question on where heavier elements come from gives good background, including this ...

38

I think that your thought process is flawed in that you assume that by drastically increasing the temperature you are guaranteed to get heavy elements. As odd as this may sound, this isn't the case (especially during the Big Bang Nucleosynthesis (BBN)) for a few reasons. In fact, if you took a hydrogen-only star and made it go supernova, you wouldn't get ...

28

The final stages of nucleosynthesis are a statistical equilibrium process. At the same time as nuclei are being built up, photodisintegration is breaking them down. The temperatures required to produce zinc by fusion are high enough that the radiation field is energetic enough to break it up. So there is some present in the mix, but nowhere near as much as ...

28

The Sun is currently turning hydrogen into helium. There are no other nuclear reactions taking place at any significant rate in the Sun. The Sun will not start to make heavier elements until it reaches the tip of the red giant branch in about 7 billion years time. The elements heavier than helium that are present in the Sun were almost all made inside other ...

19

Sagan's quote is half-correct. While some of these elements are created during or immediately prior to a supernova of some sort, others are either partially or entirely fused during normal stellar nucleosynthesis. Nitrogen falls into the latter category, whereas calcium and iron have one foot in each. On the whole, though, calling these elements "...

19

Does a star fuse helium to beryllium on the main sequence? Stars don't fuse helium to beryllium except as a very, very short intermediate step toward carbon. Helium-helium fusion to form beryllium is endothermic: It consumes energy. To make matters worse, the beryllium-8 that results has an extremely short half-life, less than $10^{-16}$ seconds. Helium ...

16

Short answer: Without tunnelling, stars like the Sun would never reach nuclear fusion temperatures; stars less massive than around $5M_{\odot}$ would become "hydrogen white dwarfs" supported by electron degeneracy pressure. More massive objects would contract to around a tenth of a solar radius and commence nuclear fusion. They would be hotter than "normal" ...

16

As you are suspecting, the sun burns in a different sense, not by chemical reaction with oxygen. Atoms consist of a tiny, heavy nucleus, surrounded by an almost empty space, populated by electrons. Burning by chemical reaction with oxygen doesn't change the nucleus of atoms, but takes place in the hull of atoms: Atoms may assemble to form molecules; ...

14

Do you mean, "Can we in principle", or just, "Can we..." ? The answer to the latter is no. We have no technology to create black holes. It was a remote possibility that was considered when creating the high energy densities in the LHC, but in practice the energies aren't high enough. Can we in principle do it? The answer is yes. If you can squash enough ...

10

What defines the main sequence? Main sequence stars are characterized by hydrogen fusion in their cores, either through the proton-proton chain (for lower-mass stars) or the CNO cycle (for stars more than about 1.5 times the Sun's mass). Outside the core, no significant fusion takes place; the outer layers are involved in radiative or convective energy ...

10

There are plenty of misleading statements in Wikipedia and elsewhere on the internet about nucleosynthesis (I am busy searching to see if I have said something similar in the past!) The reason that the alpha chain does not proceed significantly beyond $^{56}$Ni is that in order to overcome the Coulomb barrier the temperatures need to be so high that the iron-...

9

The material (heavier than helium) that makes up the solar system was made in millions, if not hundreds of millions of stars that lived and died in the ~7 billion years between the formation of the Galaxy and the birth of the Sun. This material has been thoroughly mixed in the interstellar medium and so the heavier elements arise from countless individual ...

7

The slowest reaction rate in the pp chain determines how quickly hydrogen can "burn" in the core of a sun-like star. That rate-determining step is actually the fusion of two protons to form deuterium via the diproton and a weak interaction decay. The fusion of lithium, whereby it fuses with a proton and then splits into two Helium nuclei is actually part of ...

6

It would be good if you referenced your sources, because you may be misunderstanding them. We'd be able to see what they actually say, and help you understand them. Nucleosynthesis of iron does not use more energy than it produces. It is, however often referred to as the heaviest element created in fusion that results in more energy produced than consumed. ...

5

I think it's an interesting question. The trick would be a sustained fission reaction, faster than half life, but slower than a chain reaction. A chain reaction could hardly be considered "star like" - it would just explode. Lets say you had a planet sized object, maybe 100 parts Iron, basically inert, to 1 part Uranium , which would generate heat ...

5

Kind of a quick answer, if you don't mind. Can we compress any object to create black Holes? The pressure at the bottom of the ocean is enormous, it would kill a person in a fraction of a second, but it's tiny compared to the pressure in the center of the earth, and that's tiny compared to the pressure in the center of Jupiter, and that's tiny compared ...

5

As can be seen in this question: What effects besides "mass defect" cause the alpha ladder beyond iron-56/nickel-56 to be endothermic? It is not so straightforward to explain why fusion stops at iron (actually it stops at 56Ni and then radioactive decay produces 56Fe afterwards - i.e. 56Ni is the final fusion product, not 56Fe). Adding alpha ...

5

I don't have detailed model calculations to hand and I'm not sure they have been done for the small range of metallicity you mention. However, in handwaving terms. (1) Mass and composition are the most important variables. Helium abundance, in addition to metallicity, could play a role. Rotation could be a very second order effect. Very rapid rotation would ...

4

My stellar astrophysics text, Francis LeBlanc's An Introduction to Stellar Astrophysics, gives the following quantities for the stages of burning in a $25M_{\odot}$ star (citing models by Arnould & Samyn 2001). This is somewhat less massive than a typical main sequence O5-type star, and with higher masses come higher core temperatures and shorter ...

3

There's a rough equation based on a star's mass but the age and metalicity of the star are factors too. As large stars grow older they grow hotter on the inside and as they grow hotter, they fuse hydrogen faster, but they also become less massive over their lifetimes and have less and less hydrogen over time, so there's two opposing factors. A rough ...

3

I highly recommend Nucleosynthesis and Chemical Evolution of Galaxies by Bernard Pagel. It contains the basics of nuclear reactions andstellar evolution, chapters on big bang nucleosynthesis and light element production, as well as covering the broad swathe of stellar nucleoynthesis and how these link together into predicting the chemical evolution of ...

3

My question is 2-fold:   1. Is the primordial amounts of $^3$He significant or insignificant compared to what stars produce and eject by coronal mass ejections or blowing up into nebulas, and ... The $^3$He composition of CMEs can vary significantly, see: "Unusual composition of the solar wind in the 2-3 May 1998 CME observed with SWICS on ACE" (...

3

The elements you are talking about are created by the r-process - the rapid neutron capture onto heavy elements that can occur in neutron-rich environments. The r-process is probably responsible to some extent for all the most neutron rich (i.e. more neutrons than protons) nuclei with atomic masses above that of iron and is probably responsible for all ...

3

You are referring to pyconuclear reactions - these are reactions that are initiated even when the temperature is effectively zero. They are caused by zeropoint oscillations of particles that are trapped in a deep potential well and hence are purely a quantum mechanical effect. This is far from just of theoretical interest. It may be that pyconuclear ...

3

The issue is not whether the dense core can fuse, but what fusion processes can occur on its surface. Remember that novas happen when gas accumulate on the surface of white dwarf stars: the high temperature and density is enough to trigger runaway fusion. Hence this method, where you to magically make a dense core appear inside a gas giant or somehow drop it ...

2

It is very unlikely with the normal fission process for most of the elements. They can be divided into 2 groups: slow reaction and fast reactions. The elements with slow reactions do not generate enough energy in a short enough time to be able to heat sufficiently to provide light. The elements with fast reactions would disappear before enough accumulated ...

2

The sun is a big ball of hydrogen atoms, that get compressed together due to a huge gravitational force. We start off with 4 H atoms: P-e, P-e, P-e, P-e (note: P=proton, e=electron) Now, when a proton (P) and electron (e) fuse together, they simply combine into a neutron (note: neutron=N). So, if two of those P-e pairs fuse into a neutron, we now have: N,...

2

The main sources of neutrons for the s-process (can be found in under a minute on wikipedia and in this case is quite reliable) are carbon 13 and neon 22. These undergo reactions with alpha particles yielding oxygen and magnesium respectively plus a free neutron. The first reaction takes place in low-mass (1-8$M_{\odot}$) stars whilst they are in the AGB ...

2

I have yet to find good information regarding the s-process, so I'll just talk about the r-process here. The key difference between the two is the conditions under which they take place. The r-process takes place during supernovae, specifically those due to gravitational collapse. In a collapsing star, many of the protons and electrons are squeezed tightly ...

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