View Full Version here: : Question On Star Formation.

05-01-2011, 10:37 AM
Hello everyone in the Science Forum :hi:

I have a question which I've had trouble finding the answer to, despite plenty of googling and searching books...

How long does it take for a star to form. I realise there will be different time frames depending on the mass of the star.

And am I correct... do protoplanetary discs around stars take around 250 million years to form planets :question: I read this once and I'm going from memory.

Many thanks for your help, and have a wonderful day :).

05-01-2011, 01:18 PM
Hi Suzy;

Good question, as the answer is not straight-forward.

From a quick read-up, (good to refresh my memory on this, also), the approximate durations, (by the various phases which have so far been observed), seem to be as follows:

Protostar – large mass that forms by contraction of a giant molecular cloud. For say a one solar-mass soon-to-be 'star', this phase can last for about 100,000 years (if all goes undisturbed).

T Tauri Star less than 2 solar masses but larger radii than a main sequence star. These are still considered to be pre-main sequence stars. They are more luminous than a main sequence star (because of their larger size), and the surface temperatures are similar to main sequence stars, their central core temperatures however, are too low for hydrogen fusion. They contract to a main sequence star in about 100 million years.

FU Orionis star (less than 2 solar masses)
These guys erupt suddenly as the accretion matter collapses suddenly into the young, forming T Tauri star. The ramp-up phase of these eruptions can be typically approx one year (or can be much longer). The eruption itself can last for decades. The average star can undergo approx 10 to 20 eruptions over its lifetime (before it goes main sequence).
These stars are defined by the eruption(s), and the visible spectrum it creates.

Herbig Ae/Be stars (2 to 8 solar masses).
These are not hydrogen burning yet. They are embedded in gas-dust envelopes. A star of this type can be in this phase for up to about 10 million years.

Greater than 8 Solar Masses in the pre-main sequence are not observed because evolve very quickly. When they become visible, the hydrogen is already burning and they are thus in the main sequence.

(The above is a quick collection of info from various sources).

Hope this helps to fill in a few of the blanks.

PS: I should clarify a little more: even though the above are called 'stars', they aren't really 'stars' until they reach the main sequence which is when they 'turn on' and start hydrogen fusion.

07-01-2011, 12:37 AM
Craig, many thanks for going to all that trouble for me, I'm very grateful. I see now why I was having so much trouble finding this answer - I didn't know what T-Tauri stars were about.
I have the Cambridge Encyclopedia of Stars which is a very comprehensive book, but as educational as it is, right now at this stage, it is far too advanced for me in many areas. But I will slowly come to understand the info in it in time to come no doubt. Thanks to your help, the last couple of days I have been reading up on Protostars and T-Tauri stars. I'm glad you added that clarification at the end of your post, as the biggest part of my question was after how long do they "turn on their light". So, another thank you.

What I'm understanding so far is this (say, a star the same size as ours):

- A prostar spends 10-15 million years at this stage.

Then, the star goes on to be a T Tauri star, which spends around 100 million years in this stage before it goes on to be a main sequence star.

Then, once on the main sequence, they stay there for around 10 billion years.

Like you, I had to source the information on these different stages from different sources :rolleyes: (with thanks to you giving me what to search for).

So, am I on the right track? :question:

07-01-2011, 11:04 AM
Hi Suzy;

Bear in mind, there's a lot of variability out there.

Also, They still haven't figured it all out, yet. We tend to forget that the technology used to observe all the stars necessary to develop credible models, has really only recently become available. When I started reading up on this, I thought all this was already known. I think the difficulty you experienced in attempting to find a straight-forward answer, indicates that the story about star evolution is still in its infancy.

I mean - look at Ron's 'Trillions of Earths' thread, and there was another .. (forgotten what it was called) .. which showed that there are a lot more stars, (& exoplanets), out there than was previously thought. If there's a lot more stars, then the conditions of their formation may be more varied than we think, also.

I guess, theoretically, fusion starts in an instant, once you have the right conditions for it the question is really how long does it take for the conditions to occur, for stellar fusion. (Same thinking would also seem to apply for proto-planet formation)


09-01-2011, 06:18 PM
You hit the nail on the head, Craig. Well said.
It's difficult trying to learn this stuff when theories are constantly changing and variations appear from different sources.
I understand a lot of this stuff is based on theories and information is always changing, but it's very hard trying to learn even the most basic of things. :shrug:

A couple of days ago, I was reading in one of my astro mags about that super massive star that was recently discovered in the LMC (around 320 solar masses). What they think now (as per this article) it could be in fact a double star.

09-01-2011, 06:34 PM
Well .. y'know, the theory, as such, may not change so much .. but the observation technologies are revealing much more, in much shorter timeframes than most of us would have imagined. This then may result in 'challenges' to the standing theory, but one of the virtues of Science is that established theory is usually built on very sound physics and observations, so 'theory' is usually built upon, rather than demolished and rebuilt.

.. Some folk may prefer the demolition technique, but sometimes babies are actually worth more than the bathwater !! (Chuckle, chuckle ;) ).

Sounds like an interesting topic for a new thread there, Suzy !


09-01-2011, 09:05 PM
It's extremely pleasing to see you guys discussing stellar "formation" and not "birth"!


10-01-2011, 04:31 PM
The life cycles of stars especially in their formative stages varies greatly depending on the initial mass of the cloud which is undergoing contraction. A typical star like the Sun will spend anywhere from 20-30 million years contracting down to ZAMS (Zero Age Main Sequence). The protostar stage doesn't last all that long for solar mass stars and larger. Once contraction of the initial mass begins, it can occur very rapidly....10,000 to 100,000 years from the initial cloud down to the protostar. It's the next stage in the evolution of the star that takes the longest. The T-Tauri stage, which occurs in MS stars of 2 solar masses and less can take upto 100 million years and more, for the little M class stars. For a star of around 1 solar mass, the stage lasts 20-30 million years. Herbig Haro object are usually stars between 2-10 solar masses (B and A classes usually) and these object contract in periods from 100000 to 10 million years, depending on their initial mass. The larger of these stars have usually ignited core burning by the time they become visible and are actually still contracting down to their final radius as MS stars. Large stars, over 10 solar masses, form every quickly. A large 50-100 solar mass O class star will have contracted from gas cloud to ZAMS star in 50000 or less and have already ignited core burning long before they stop contracting to their final radius. Actually, their formation is not quite understood as there are problems with some of the kinematics of the contraction and the physics of the enormous energies involved. By rights, they should come close to not forming at all.

10-01-2011, 05:01 PM
That's science:)

Some of the scientist want it (R136a) to be a binary star as they feel uncomfortable with the fact that it's so massive, but the evidence for it being binary is very sketchy to begin with. Massive stars like R136a give off a large amount of X rays when they're binaries due to the interaction between the stellar winds of each of the stars. However, with R136a, the X ray flux is normal for a single star of it's size. It can be masked, however, by large circumstellar clouds of gas and dust given off by the stars themselves, so it could still be a binary. But there's no indication this is the case. These massive stars form in exceptional circumstances and I think what needs to be done is a revision of the theory to account for them. The Eddington Limit needs to be revised and extended for these massive stars.

Let's look at it in this way....even if it is a binary, one or both of the stars present will be pushing the currently accepted Eddington Limit of around 150 solar masses. You might as well say it's a multiple system of, say, 4 or more stars, but that would be observationally easy to detect as the X ray flux coming from such a system would be crazy. Plus the system's orbital motions would be seen in the periodic changes in that flux. In any case, the stars would still be very massive...well over 40 solar masses each. Those scientist proposing a binary nature have to account for all the observations of the system and its characteristics and then be able to model them with multiple stars being present. Then it all has to be agreed upon by the other scientist.

10-01-2011, 05:12 PM
Thanks so much Carl for explaining all that, I'm very grateful :).

And Carl .... good to have you back!!! :2thumbs::jump::jump2:
I missed your presence :computer:

10-01-2011, 05:25 PM
Thank you, Suzy :)

11-01-2011, 05:13 PM
Suzy....I was born a " Star " .......just ask my Mum.......:rofl::rofl::rofl:.