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Some stellar formation theories suggest that stars are formed by shock waves from trigger events such as supernovae.

This excerpt from Star Formation basically gives the background to my question:

Many astronomers regard the passage of a shock wave through interstellar matter as the triggering mechanism needed to initiate star formation in a galaxy. Calculations show that when a shock wave encounters an interstellar cloud, it races around the thinner exterior of the cloud more rapidly than it can penetrate its thicker interior. Thus, shock waves do not blast a cloud from only one direction. They effectively squeeze it from many directions.

Two questions:

  1. If the Sun (and Solar System) was formed in this way, rather than a collapse of a cloud of dust and gas, (by itself, through gravity alone), do we have any idea of what event caused it, for example, do we know where a possible ex-supernovae is located, or has it long since vanished by now?

  2. Stars are often formed in groups. I wonder do we have an age for the nearest stars (using the H-R diagram) or any evidence that they may have been part of the same gas and dust cloud that formed our Sun?

I do appreciate that:

  1. Astronomical observations are difficult in many circumstances, e.g. obscuration by dust or gas often prevents an accurate picture of objects located many light years from Earth.

  2. The Sun was formed 4,500 million years ago, this fact may make finding evidence of a possible trigger event extremely difficult to discover.

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    $\begingroup$ Small point, but when you say "The sun was formed 4,500 million years ago, so we may be looking back at least that distance" - that's a little unclear. If that means 4.5 billion light years - that would be the maximum distance, and it's unlikely to have traveled anywhere remotely close to that far. $\endgroup$
    – userLTK
    Commented Jun 18, 2015 at 22:02
  • $\begingroup$ Valid point, its not just a little unclear, it's totally wrong, I did not think it through, thank you for that, I will edit the post $\endgroup$
    – user81619
    Commented Jun 18, 2015 at 22:19
  • $\begingroup$ Not withstanding that wikipedia is not the best of the best of the best, if the cited page was a wikipedia page, that "many astronomers" claim would quickly be flagged as "citation needed". $\endgroup$
    – David Hammen
    Commented Jun 19, 2015 at 9:28

2 Answers 2

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You're right that the Sun being 4.5 billion years old makes observations difficult. The Sun goes around the Galaxy about once every 225 million years, so since the Sun formed it has gone around the Galaxy perhaps 20 times. The trouble is that the Galaxy is not like the Solar System: stars don't go around on nice nearly circular orbits, everything is a bit jumbled. To give you an idea, here's an estimate of where the stars that are currently in the Solar neighbourhood have been during the last orbit of the Sun around the Galaxy:

animation of some MW stellar orbits for stars now near the Sun

As you can see, they span over 80,000 light years (that's basically the full width of the Milky Way) just 1 orbit ago, so a supernova remnant that was near the Sun 20 orbits ago could be virtually anywhere. We can't measure the age of the Sun or supernova remnants accurately enough to help either (as in being able to say "aha! a remnant with exactly the age of the Sun!").

It's similarly difficult to find stars that may have formed near the Sun. About the best we can do is to look for stars with similar age and chemical composition to the Sun, but at 4.5 billion years old, the accuracy of stellar ages isn't terribly good. Asking more broadly what star formation conditions were like in the Milky Way when the Sun formed is also a difficult question and a topic of current research, see for instance this reasonably current review.

A potentially more fruitful approach is to look for Sun-like stars that are forming now and see what conditions look like for their formation - potential triggering mechanisms, how many stars formed in a group, etc. But this only helps inform what the formation of the Sun would have looked like - it's likely the Milky Way was a significantly different beast 4.5 billion years ago in terms of gas supply, morphology, ISM conditions, etc.

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    $\begingroup$ Thanks for your time and great answer Kyle, I did (of course) completely forget about galaxy rotation: the sillier the question, the more you remember the answer though. I take your point about looking at stellar nurseries and using extrapolation for possible triggers. Thanks again $\endgroup$
    – user81619
    Commented Jun 18, 2015 at 22:00
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Good answer from Kyle. I will just add that there is a great deal of effort going into trying to discover "solar twins". These are stars with such similar parameters (including age inferred from the HR diagram or asteroseismology, which can be good to about 10% in the best cases) and photospheric compositions to the Sun, that it is thought likely they must have formed from the same gas cloud. Nothing conclusive has emerged from these studies.

It is widely believed that the Sun probably formed as part of a cluster of 1000s of stars (e.g. Adams 2010), so such searches are not futile. However, as Kyle points out, 20 orbits is plenty of "scrambling" time and it is still unclear exactly how chemically homogeneous you would expect the stars born from a star forming cloud to be.

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    $\begingroup$ That really brings it home how large those initial proto-star clouds must be. I though I was overdoing it even stretching it to the nearest star:) With a cloud that size it's hard (for me at least), to believe it needs a shockwave to condense, (or that there is not a significant amount of heavier elements in the cloud), but I'm guessing the precollapse gas density must be very, very low. $\endgroup$
    – user81619
    Commented Jun 18, 2015 at 23:37
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    $\begingroup$ @AcidJazz The densest nebulae rival the best laboratory vacuums on Earth. As for the shockwave, clearly some stars (if nothing else the first star in the universe) formed without a supernova trigger, so it's more a matter of whether or not something gives gravity a shove in the right direction. $\endgroup$
    – user10851
    Commented Jun 19, 2015 at 3:22
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    $\begingroup$ @ChrisWhite. Thanks Chris, I will push my luck here but I can ask it as a question if you would prefer: do we know if it's the radiation or the following particles in the shockwaves that have the greatest effect on condensing the gas cloud? I'm guessing lots more photons but far more particle total momentum. Both combined I suppose, I did not appreciate that the shockwaves could spread "around" the gas cloud to any extent. Thanks. $\endgroup$
    – user81619
    Commented Jun 19, 2015 at 3:55
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    $\begingroup$ @AcidJazz Stars may also simply form because clouds "bump into each other". The whole issue of "triggering" is very open at the moment. There is very little direct evidence, but lots of circumstantial evidence. For instance you might see evidence of star formation on the rim of a cloud being shaped by a supernova remnant, but you don't know whether those stars would have formed anyway. $\endgroup$
    – ProfRob
    Commented Jun 19, 2015 at 8:08
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    $\begingroup$ Re "chemically homogeneous stars": The stars formed within a single stellar nursery will have had different initial masses, affecting their evolution, even if the chemical composition of each was originally identical. A greater mass means the star will use up its hydrogen faster, so this would complicate the search for stars chemically similar to our sun. However, if we did positively identify our sun's siblings they could provide a glimpse of our sun's past and future, depending on their size. $\endgroup$
    – CJ Dennis
    Commented Jun 19, 2015 at 12:04