Question

Explain (in your own words) the shape of the Hayashi track for a pre-main sequence star, noting the
most important physical changes that occur in a star as it moves from being a collapsing cloud of
gas to being a hydrogen burning main sequence star. Explain how this varies for low and high mass
stars. What is the best wavelength range to detect pre-main sequence stars, explain why.

Please answer all sections as they are about the same concept.

Answer 1

When a protostar moves from stage 4 to 5 and 5 to 6, it follows hayaahi track.

Theae tracks are vertical line on HR diagram.

When a star is in stage 5 of it's evolution it's temp. at center is aprox 5 million kelvin

And surface temp. is aprox 4000kelvin

At this stage it doesn't have enough energy to start fusion. So it continuous to cotract under self gravity.

In it's way to stage 6

Due to it's contraction it's surface temperature increases rapidly so it's colour changes a little bit towards blue region but the luminosity drops significantly because while increasing temprature it keeps shrinking.

It's size get smaller and smaller so it's luminosity faces a big drop.

So ultimatly ut traces a nearly straight vertical line on HR diagram.

A sun like star takes aproximately 20 million years for a sun like star to move to stage 6.

At stage 6

Temperature at center is 10 million kelvin, which is sufficient enough to trigger a nuclear fusion.

At this stage the surface temp. is aprox 4500k.

In this stage contraction slows down because the fusion energy counters gravitational energy.

A sun like star stays 20 million years in this stage.

Then comes stage 7, in which the fusion energy completely balances gravitational energy.

Now it is a main sequence star.

It stays billions of years in this stage untill all of the feul gets burned.

And there is not enough gravity to fuse further and the kelvin boltzmann contraction stops due to electron degeneracy pressure.

This processes vary with the masses of star...

The time taken to move from one stage to another decreases with increasing mass and time increases with decreasing mass.

Pre main sequence stars get formed in denser regions so wavelenths longer than optical are best to observe them, Since they are not hot enough to be observed in X ray or UV regions and not cool enough to be observed in very long wavelenths.