by Chris Peterson » Tue Dec 16, 2014 2:56 pm
BennyH wrote:But with all these "star forming regions" I have read about here, I just wonder if anyone has actually seen a new star turned on, or just registered it through the last 100 years of astronomy.
Does anyone have a clue to what happens, or is it just guesswork?
Stars don't "turn on", so there's no event that can be observed. Star formation is a process. Initially, gas (and a little dust) comes together under its mutual gravitational attraction, heating as it collapses. As the density rises, so does the temperature, and as the temperature rises, so does the brightness. This is the protostar phase, and these objects shine as brightly as stars, and outside their dusty surrounds are generally not distinguishable from stars. The only difference between a protostar and a normal star is that the protostar is heated by gravitational collapse, not fusion. Although it is as hot at its surface as fully formed stars, the protostar isn't hot enough inside to sustain fusion.
Once the protostar has accreted all the available gas and dust around it (typically blowing much away), it is considered a pre-main-sequence star. At this point it looks like other stars in most respects, but is still heated by gravitational collapse. After a short period in this phase, the interior becomes hot enough for hydrogen fusion to begin, and the star enters the main sequence. That might be the point we would call "turning on", but it doesn't change the appearance of the star at all.
[quote="BennyH"]But with all these "star forming regions" I have read about here, I just wonder if anyone has actually seen a new star turned on, or just registered it through the last 100 years of astronomy.
Does anyone have a clue to what happens, or is it just guesswork?[/quote]
Stars don't "turn on", so there's no event that can be observed. Star formation is a process. Initially, gas (and a little dust) comes together under its mutual gravitational attraction, heating as it collapses. As the density rises, so does the temperature, and as the temperature rises, so does the brightness. This is the protostar phase, and these objects shine as brightly as stars, and outside their dusty surrounds are generally not distinguishable from stars. The only difference between a protostar and a normal star is that the protostar is heated by gravitational collapse, not fusion. Although it is as hot at its surface as fully formed stars, the protostar isn't hot enough inside to sustain fusion.
Once the protostar has accreted all the available gas and dust around it (typically blowing much away), it is considered a pre-main-sequence star. At this point it looks like other stars in most respects, but is still heated by gravitational collapse. After a short period in this phase, the interior becomes hot enough for hydrogen fusion to begin, and the star enters the main sequence. That might be the point we would call "turning on", but it doesn't change the appearance of the star at all.