How can we tell if a star is made of matter or anti-matter? For example, is there any difference spectroscopically? Or could we tell from cosmic rays?

If half of the stars were made of matter and half of anti-matter, then they would be in balance.

After the Big Bang, the slightest of imbalances in the distribution of matter and anti-matter would, I think, grow more pronounced as a local surplus of matter would persist if bombarded with equal amounts of matter and anti-matter. Over time, that matter would gravitationally come together and locally grow more dense. Gravitation thus would segregate matter and anti-matter as most of space became empty. Thus perhaps half the stars that we see might be made of anti-matter. For the photons they produce would be the same, yes? And the gravitational force is the same.

In particular, I wish to know approximately what is the density of mass in the space midway between our solar system and Alpha Centauri? What would be the nature and magnitude of energy released there if matter from our side was colliding with anti-matter from Alpha Centauri's side? Would we be able to detect that? Please, I would like to see a calculation!


An antimatter star would release an antimatter solar wind (antiprotons and positrons mainly). As this antimatter solar wind collides with the ordinary-matter solar wind from the ordinary-matter stars surrounding it, it produces a gamma-ray source with a very specific spectrum corresponding to matter-antimatter annihilation where these two winds collide. The fact that we don't see any such nearby gamma-ray source is strong evidence that there are no antimatter stars anywhere near us (or, in fact, basically anywhere - gamma-ray telescopes are quite sensitive to such a signature).

  • $\begingroup$ Thank you, @probably_someone Please could you express that with a calculation? $\endgroup$ – Andrius Kulikauskas Oct 4 '19 at 16:04
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    $\begingroup$ @AndriusKulikauskas what quantity should be calculated? $\endgroup$ – hobbs Oct 5 '19 at 2:35
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    $\begingroup$ What do we detect when stellar winds collide? How do we know they do? $\endgroup$ – Kevin Krumwiede Oct 5 '19 at 7:23
  • $\begingroup$ @hobbs what I will try to do (or others might do) is to focus on collisions of hydrogen and antihydrogen. In the sparsest region of space, what is the density and what is the average velocity, what is the average energy of the particles, and what are the gamma rays produced? Imagine a plane midway between our sun and Alpha Centauri which separates them, that is, which is normal to the geodesic connecting them. In that plane, how many collisions would be occuring per square meter per second? How much energy would that be in a square light year per second? What would that look like to us? $\endgroup$ – Andrius Kulikauskas Oct 6 '19 at 20:18

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