When does a protostar become a star? (star birth)

Nobody knows anything at all about stars, not even the one we have right here. For instance, everybody assumes that it is powered by fusion. That implies the inside should be hotter than the outside, and the reverse is true. All theories that you ever hear about are based on the Hertzprung-Russel diagram, which is not accepted as valid by all scientists.

Pardon???

You cannot dismiss an entire area of science like that, simply on the basis of possible flaws in the theories.

The Sun is a pretty run-of-the-mill star of modest size, but an enormous amount is known about it and other stars, even if much of that knowledge has to be derived mathematically and from nuclear physics research. 

Its atmosphere is certainly far hotter than its surface - from memory about 2 000 000 ºC compared to a mere 6 000ºC,  but its core is much, much hotter still. The only way a star can produce so much energy is by nuclear fusion, but even if it started the other way round, by fission starting with uranium, the enclosed core would still be much hotter than the radiating surface.

Whether or not the Hertzprung-Russel diagram eventually proves incorrect or at least needs revising, does not mean "nobody knows anything at all about stars": that's plain daft. The sizes, radiations and chemistries of stars can be analysed to quite high details; and the more data are collected and analysed, the greater the total knowledge and the more refined the theories can become.

To answer the original question itself, a pretty good stellar "birthday" would be its nuclear ignition. I don't know if that is used as the standard, but prior to that it is simply a massive ball of very hot gas, and some poor little stars are not sufficiently hot and dense to trigger fusion.   This post was edited by Durdle at October 4, 2016 5:51 PM MDT

Yes, science does proceed by assumptions and theories, but each is rejected only when shown no longer to fit the observed or experimental evidence. You cannot reject science simply because it does not give 100% unassailably true answers to every question; any more than you can fall into the trap you describe of taking a theory as true when it is still awaiting revision, refinement or indeed replacement.


The principle you lay down in your opening sentence is that because we do not know for certain everything about a subject then we know nothing at all about it. Uncertainty in science encourages curiosity, study and revision; denial is mere stagnation and ignorance.

A classic example, and nearer to my own scientific interests, is the history of the discovery of Continental Drift. Those who first postulated that the continents move were initially dismissed by their contemporary geologists, but the latter could not properly account for mountain-building, volcanoes and earthquakes. When great advances in ocean-floor studies produced new, vital evidence, then everything started to fall into place. If the geologists had simply given up and admitted "no-one knows how mountains form" the science would have stagnated at that point and the actual processes never discovered. 

If everyone had taken that "let's-give-up-because-it's-harder-then-we-thought"  stance we would not be able to sit here discussing it by computer. We would not even have electricity! In fact if we go back about 400 years, we'd now be little more advanced than the Church authorities of the time who stuck to Aristotle's geocentric (and anthropocentric!) universe model as true because they did not know and far worse, refused to want to know, any better.