Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: IAMREALITY on 20/07/2016 22:59:46

Title: How come stars won't form if the dust cloud is too hot?
Post by: IAMREALITY on 20/07/2016 22:59:46

I was reading an article yesterday about their having found some evidence to support the direct collapse theory; that dust clouds directly collapsed to form supermassive black holes prior to galaxy formation etc.  And one thing the piqued my interest in the article was it saying that the temperature was so hot that the hydrogen and helium couldn't collapse into stars, but instead directly collapsed into black holes.  But I thought the heat generated by gravity and the particles bumping into each other or whatever is how stars collapse to start the fusion process to begin with?  So why wouldn't this occur if things were too hot?? 

My apologies if a stupid question.  But definitely something I hadn't prior known.

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: IAMREALITY on 21/07/2016 15:41:49

Oh c'mon, I know the members here gotta know the answer no? lol

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: chiralSPO on 21/07/2016 17:08:18

This is new to me as well. Can you post a link to or reference for this article?

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: IAMREALITY on 21/07/2016 17:24:07

I'll see what I can find.  It was an article I got through my smartnews app on my mobile, which isn't a browser so doesn't have a space to copy the web address.

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: IAMREALITY on 21/07/2016 17:37:10

Quote from: chiralSPO on 21/07/2016 17:08:18

This is new to me as well. Can you post a link to or reference for this article?

Here's the relevant quote from the article:

Quote

“Begin with a “primordial cloud of hydrogen and helium, suffused in a sea of ultraviolet radiation,” Bromm said. “You crunch this cloud in the gravitational field of a dark-matter halo. Normally, the cloud would be able to cool, and fragment to form stars. However, the ultraviolet photons keep the gas hot, thus suppressing any star formation. These are the desired, near-miraculous conditions: collapse without fragmentation! As the gas gets more and more compact, eventually you have the conditions for a massive black hole.”

http://www.universetoday.com/129806/indications-early-forming-direct-collapse-black-hole-seen/

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: evan_au on 22/07/2016 12:28:38

Quote from: IAMREALITY

I thought the heat generated by gravity and the particles bumping into each other or whatever is how stars collapse to start the fusion process to begin with?

It is thought that today, stars are formed in "stellar nurseries (https://en.wikipedia.org/wiki/Star_formation)", molecular gas clouds that are a temperature close to absolute zero - perhaps 10K.

Because the gas here is so cold, eddies of gas can condense to form denser aggregations that become protostars and protoplanetary disks. It is thought that this condensation takes millions of years, because collisions of the gas particles heat them up, causing the gas to expand - which opposes the gravitational condensation.

As soon as the star ignites, the heat of radiation and stellar winds blows away most of the protoplanetary disk that hasn't already condensed into planets.

However, in the early universe was still hot after the big bang (ie the CMBR was in the ultraviolet range, not microwaves), so there were no cool clouds of dust that could condense to form protostars and protoplanets; in fact for a considerable time, all atoms were an ionized plasma.

However, the universe was also much smaller and denser soon after the big bang. So if you had enough mass in a small enough volume, you might have eddies that become dense enough to form a black hole, without reaching the extreme densities needed to start hydrogen fusion.

The Jeans Length (https://en.wikipedia.org/wiki/Jeans_instability#Jeans.27_length) is an estimate of the size of a gas cloud that could collapse. The size of the cloud is proportional to the square root of the temperature (all other things being equal). So in the early universe, you would need a truly massive cloud.

In this early era, fusion was hard to start, because the universe consisted of Hydrogen & Helium (and a miniscule amount of lithium). There were none of the Carbon, Nitrogen and Oxygen atoms that we see in stellar nurseries today; The CNO Cycle (https://en.wikipedia.org/wiki/CNO_cycle) acts as a catalyst to kick-start hydrogen fusion. So this is a barrier to star formation in the early universe.

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: jeffreyH on 22/07/2016 14:36:47

I think I might just put away my textbooks and read Evans posts instead.

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: IAMREALITY on 22/07/2016 18:28:41

Quote from: jeffreyH on 22/07/2016 14:36:47

I think I might just put away my textbooks and read Evans posts instead.

Hear hear to that!  I'm awed by both his knowledge and ability to impart it on others...

Title: Re: How come stars won't form if the dust cloud is too hot?
Post by: PmbPhy on 22/07/2016 22:30:37

Evan - You are a true treasure to this forum. We're very fortunate to have you as a member!  :)