Stellar Formation

How did astronomers come to this theory on stellar formation?

by Ju Boo Lim - Saturday, 19 October 2019, 5:54 PM

I read with great interest the formation of young clusters of stars  from collapsing clouds

As the cloud accretes through gravitational forces they form prestellar core, and as  more and more hydrogen (I suppose) collapsed, they form protostars

The hydrogen of these prostar core begins to heat up, fuse, and form stars, and they begin to change hydrogen into helium through nuclear fusion.

The radiation from these newly formed stars began to push materials away from them so that  they become isolated stars

I hope I got this scenario right from the text I read in this course 

I also understand that the formation of clusters of stars from the accretion of surrounding clouds may take tens of thousands of years.

This raises one question I have in mind.

 How did astronomers know this entire event took place  if it takes such a long time for stars to form from the surrounding clouds and dusts?

In Science we can only make a conclusion based on two factors.

First, we make an inference based on a theoretical assumption that relies on already known facts such as gravity can pull matter together, and that matter crushing together can heat up. This is perhaps basic physics.

Second, we can also  base a conclusion by direct observation and measurements

But since what I read in this Unit 5 that it takes some tens of thousands of years for clusters of stars to form, there is no way for present day astronomers, or for any astronomer in the past to be able to  live for such a long time  to observe the entire scenarios to make this conclusion?

I believe even in their entire life span, of  let's say 100 years, and even if they observe the same nebulae night after night for a 100 years, they would still not be able to see any stellar formation within the nebulae or even any changes or movement in the nebulous clouds.  Am I right?

Thus my question is, did astronomers  actually see or have measured the contraction of the prestellar clouds and saw the birth of young stars within their entire  life time?

I am of course not   questioning the hypothesis of stellar formation from what I read here. The theory  can be dead right. After all, I am not an astronomer or an astrophysicist, else I would not be taking this course to learn something..

I am just being curious after what I read in the text,  and I am only asking a question like a student.

Let me give an example.

Take for instance the Crab Nebula in Taurus A (catalogues as  M1, NGC 1952)

We know it is a very bright supernova remnant first recorded by the Chinese astronomer in 1054, and later by the English astronomer John Bevis in 1731.

It lies about kiloparsecs (6,500 ly) away  from Earth and has a diameter of 3.4 parsecs (11 ly) with an apparent diameter of some 7 arcminutes

We know it is expanding at a rate of about 1,500 kilometres per second (930 mi/s), or 0.5% of the speed of light.

I think we can measure that  in our lifetime  for us to conclude that this  nebula is expanding due to a massive supernova explosion, and not contacting into a star.

I guess astronomers can make the measurements of  this expansion within a few years  since stellar distances can easily be measured by parallax method, or if too far away using Cepheid variables by their insintric  variations of luminosity over time.

Further distances like the Crab Nebula, their distances can be measured by the luminosity of their supernova explosions. This they have done within their life time, and hence they can conclude the Crab Nebula is a supernova explosion and is expanding at the rate they have measured.

 So we can conclude the Crab Nebula was a super massive star that exploded tens of thousands of years ago by intra-polating its explosion rate over many years

But how do we measure stellar formation by just measuring the size of nebulae surrounding clusters of young stars

Did the astronomers actually saw it shrinking, collapsing and matter accreating  into stars?

That is my question.  It is so puzzling and fascinating to me

Maybe Dr Grant Miller can help us answer this question

Thanks for reading

Jb lim