Or "How natural processes make stars from stuff (which is not the same as stars coming from nothing)"
Gidday folk,
It’s been an ongoing problem scientists have been wanting to solve. Just as natural processes make clouds, snow, and rain, so how do similar kinds of processes make stars?
In a broad scope, a theory has been around for over a hundred years, namely that stars are born via a process of gravitational collapse. And for a long time there has been some evidence to support the idea.
Over the past decades, thanks to ever sophisticated instrumentation and data processing abilities, scientists have been able to address questions regarding the nitty-gritty of the process. However, one observation has always been somewhat elusive, namely the infall of material from the surrounding gas and dust cloud, toward the protostar itself.
For a while now, and somewhat to everyone's surprise, paradoxical outflows from collapsing stars have been observed. Inflows have been another matter altogether. There has been evidence of these processes but it has never been conclusive.
Now, thanks to even better technology, particularly satellite instrumentation, the situation is beginning to change.
The following paper reports on some key observations regarding this:-
Waterfalls around protostars: Infall motions towards Class 0/I envelopes as probed by water
The paper is technical and hence a slog, but enough of it is understandable by a layperson such as myself to get something from it. For those of you who have a better understanding of physics, particularly of the physics as it relates to this kind of study, well hopefully you will get even more from it.
Some points are:-
1) To go from the interstellar medium to a gas cloud, the density must increase from around 0.001 ions (atoms) per cc to around 10,000 ions per cc. To go from the gas cloud to a very dense clumps within a cloud, the density must increase form around 10,000 ions per cc to 1,000,000 ions per cc. But to get from the dense clump to the star, the density has to go from 1,000,000 ions per cc to 1,000,000,000,000,000,000 ions per cc. Ouch!!!
2) The formation of the protostar by gravitational collapse is what is thought to bring about this massive increase in density.
3)A gas cloud is rotating and as matter moves in towards the central dense globule (the future star), it picks up rotational velocity and eventually enough of this gas forms a flattened disk which orbits the star. It is thought that gas then falls from this disk into the future star. This disk may be stable or it may be unstable.
4) So, in general, there are three major components - a large external enveloping cloud from which matter falls onto a flat accretion disk which orbits a central dense globule which later becomes the star. Matter falls from this accretion disk onto the globule and when the globules density is high enough, it becomes hot enough to be a star.
5) Another component, already mentioned, is an outflow - a strong beam of material escaping as a jet from each pole of the cocooned protostar.
6) Various molecules are used to follow the motions of gas within the cocoon that encloses the protostar. With respect to infall, the molecules are CS, HCO+, N2H+ etc.
7) Infall gives a specific kind of signature called an “asymmetric line profile” which has a bit more blue than red shifted light in the profile. The signature can be modified by foreground gas clouds and by outflow from the protostar itself.
8) However, infall has, unlike outflow, been hard to verify. (I suspect because infall occurs inside the cocoon, while outflow actually breaks out of and moves well beyond the protostar cocoon).
9) The authors of this study decided that H2O (water) would make a better tracer.
10) Because of the particular water spectral line being examined, they had to use satellite observations, owing to the fact that the earth’s atmosphere absorbed that particular wavelength, making earth bound observations impractical.
11) The observations covered a region from 1,000 AU out to 11,000 AU from the central protostar. (1AU = 93,000,000 miles).
12) Data from a model of protostar formation was matched against observations. The best fit model was for infall over the whole of the gas shell surrounding the protostar and its accretion disk.
13) They discuss the case of IRAS4A using it as their representative model.
14) Infall exceeded outflow by one to three orders of magnitude.
15) Infall seemed to be slowing at the accretion disk, suggesting that the accretion disk was increasing in mass and would therefore become unstable.
Anyway, the paper might make something of an interesting read for those with appropriate knowledge. Otherwise it can be an informative paper just to scan over, while paying attention to the introductory sections and the conclusion.
Warning
For those who are as afraid of stars being able to form naturally as they are afraid of extra solar planets, best not to open this at all.
Gidday folk,
It’s been an ongoing problem scientists have been wanting to solve. Just as natural processes make clouds, snow, and rain, so how do similar kinds of processes make stars?
In a broad scope, a theory has been around for over a hundred years, namely that stars are born via a process of gravitational collapse. And for a long time there has been some evidence to support the idea.
Over the past decades, thanks to ever sophisticated instrumentation and data processing abilities, scientists have been able to address questions regarding the nitty-gritty of the process. However, one observation has always been somewhat elusive, namely the infall of material from the surrounding gas and dust cloud, toward the protostar itself.
For a while now, and somewhat to everyone's surprise, paradoxical outflows from collapsing stars have been observed. Inflows have been another matter altogether. There has been evidence of these processes but it has never been conclusive.
Now, thanks to even better technology, particularly satellite instrumentation, the situation is beginning to change.
The following paper reports on some key observations regarding this:-
Waterfalls around protostars: Infall motions towards Class 0/I envelopes as probed by water
The paper is technical and hence a slog, but enough of it is understandable by a layperson such as myself to get something from it. For those of you who have a better understanding of physics, particularly of the physics as it relates to this kind of study, well hopefully you will get even more from it.
Some points are:-
1) To go from the interstellar medium to a gas cloud, the density must increase from around 0.001 ions (atoms) per cc to around 10,000 ions per cc. To go from the gas cloud to a very dense clumps within a cloud, the density must increase form around 10,000 ions per cc to 1,000,000 ions per cc. But to get from the dense clump to the star, the density has to go from 1,000,000 ions per cc to 1,000,000,000,000,000,000 ions per cc. Ouch!!!
2) The formation of the protostar by gravitational collapse is what is thought to bring about this massive increase in density.
3)A gas cloud is rotating and as matter moves in towards the central dense globule (the future star), it picks up rotational velocity and eventually enough of this gas forms a flattened disk which orbits the star. It is thought that gas then falls from this disk into the future star. This disk may be stable or it may be unstable.
4) So, in general, there are three major components - a large external enveloping cloud from which matter falls onto a flat accretion disk which orbits a central dense globule which later becomes the star. Matter falls from this accretion disk onto the globule and when the globules density is high enough, it becomes hot enough to be a star.
5) Another component, already mentioned, is an outflow - a strong beam of material escaping as a jet from each pole of the cocooned protostar.
6) Various molecules are used to follow the motions of gas within the cocoon that encloses the protostar. With respect to infall, the molecules are CS, HCO+, N2H+ etc.
7) Infall gives a specific kind of signature called an “asymmetric line profile” which has a bit more blue than red shifted light in the profile. The signature can be modified by foreground gas clouds and by outflow from the protostar itself.
8) However, infall has, unlike outflow, been hard to verify. (I suspect because infall occurs inside the cocoon, while outflow actually breaks out of and moves well beyond the protostar cocoon).
9) The authors of this study decided that H2O (water) would make a better tracer.
10) Because of the particular water spectral line being examined, they had to use satellite observations, owing to the fact that the earth’s atmosphere absorbed that particular wavelength, making earth bound observations impractical.
11) The observations covered a region from 1,000 AU out to 11,000 AU from the central protostar. (1AU = 93,000,000 miles).
12) Data from a model of protostar formation was matched against observations. The best fit model was for infall over the whole of the gas shell surrounding the protostar and its accretion disk.
13) They discuss the case of IRAS4A using it as their representative model.
14) Infall exceeded outflow by one to three orders of magnitude.
15) Infall seemed to be slowing at the accretion disk, suggesting that the accretion disk was increasing in mass and would therefore become unstable.
Anyway, the paper might make something of an interesting read for those with appropriate knowledge. Otherwise it can be an informative paper just to scan over, while paying attention to the introductory sections and the conclusion.
Warning
For those who are as afraid of stars being able to form naturally as they are afraid of extra solar planets, best not to open this at all.
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