When a new star evolves from a cool cloud of gas to a hot star, what happens to its spectrum of electromag rad

Question

Does its length increase or decrease? stay the same or does not rely on any temperature?

Answer 1

The spectrum shifts towards the blue, i.e. higher frequencies. When it is just a cloud of gas, will it have any radiation? Not likely unless there are a lot of charged particles and their acceleration is causing emission of radiation. As the cloud starts condensing, gravitational attraction starts accelerating the particles towards the centre and ionisation may produce ions which may emit radiation. Later the inreasing density and the temperature (due to a sort of adiabatic compression) may cause a thermonuclear reaction to start causing the star to form.But even before that stage is reached, the proto-star will be hot and will be emitting thermal radiation.

A good question and may need more extensive research for a thorough answer. Look at Red Giants etc.

Answer 2

Good question. Let's begin with a dense cloud of gas that is on the verge of collapse, then move through the stages of collapse.

The cloud will start off relatively cool, at around 3000 degrees centigrade. It will emit blackbody radiation at this temperature. You'll also observe spectral lines with a distinctive profile: there will be emission at the rest wavelength (from the gas surrounding the cloud) and absorbtion at a slight redshift (due to gas falling onto the cloud).

The cloud will begin by collapsing isothermally. This means that its temperature will stay constant, while its radius decreases. Thus its overall luminosity will diminish (L proportional r^2), but its spectrum will still be of a 3000 degree blackbody. When in this stage, the star is said to be on the "Hayashi track".

The isothermal collapse ends when the star's interior becomes so hot that radiation can more easily escape, causing the surface temperature to increase. Now it begins a stage of slower collapse, while the temperature steadily rises. As the temperature rises, the blackbody spectrum will shift blueward. Interestingly, the radius is shrinking at roughly the rate necessary to counteract the increased luminosity from the higher temperature -- thus the total luminosity is roughly constant. As the gas cloud around it rarifies, the line profiles will become purely absorbtion. When in this stage, the star is said to be on the "Henyey track".

Finally, the star will settle onto the Main Sequence with a temperature of roughly 6000 degrees (for around solar mass stars) when hydrogen fusion is ignited in its core. It will achieve an equilibrium which will last roughly 10 billion years.

I've referenced a graduate-level textbook and two Wikipedia articles. If you don't have a strong physics background, then you'll find the Wikipedia articles helpful.