[thedoc (OP)]

Robin Freestun  asked the Naked Scientists:
   
Hi

It seems to me that the Sun must have been quite a bit larger at the time of the dinosaurs than it is now.

What would the temperatures have been then?

How long HAS. Earth been habitable?

Best Regards
Robin Freestun
Melbourne, Australia

What do you think?

[RD]

Q. Would dinosaurs have experienced a hotter, larger Sun? 

A. No ... https://en.wikipedia.org/wiki/Faint_young_sun_paradox#Early_solar_output
         
 https://en.wikipedia.org/wiki/File:Solar_evolution_%28English%29.svg

... How long HAS. Earth been habitable?

Habitable by what ?, bacteria ?, plants ?, things that breathe oxygen ? ... https://en.wikipedia.org/wiki/Great_Oxygenation_Event

[Soul Surfer]

No and in some ways yes. 

For all the time that life has existed on the earth the sun has very slowly been getting brighter and it will continue to do this because it is on the main sequence where it energy output is determined by the rate of energy generation within the core by neucleosynthesis. 

However in the very early stages of its life the sun was shrinking during its gravitational collapse it was much larger and cooler but radiated more energy so the earth would have been very much hotter.  This is the reason why the inner planers are rocky planets because they were well fried and most of the light elements driven off of them.  This phase was quite short and the earth itself was still forming and there were were lots of collisions and crater forming.

There is also a phase when the star has collapsed close to its final size but the nuclear reactions in the core have not fully stabilised it is then considerably fainter and less luminous than it is at what is called the zero age main sequence line.

These pre main sequence aspects of stellar evolution are not often described in popular science but are well known and examples are observed in serious astronomical circles.

The final phases as the main nuclear fuel burning shell gets larger and the core generates more energy are better known.  Counterintuitively as the star has to radiate more energy it gets bigger and cooler because the radiated energy goes up as the square of the radius but falls as the fourth power of the surface temperature an the star becomes a red giant.  Eventually it just wafts away a significant part of its mass as a planetary nebula leaving the core as a hot but tiny white dwarf star about the size of a planet which just slowly cools down over a vast length of time

[CliffordK]

Keep in mind that if you talk about the time of the dinosaurs, that actually was relatively late in Earth's history.

http://en.wikipedia.org/wiki/Geologic_time_scale



Almost everything that we would consider recognizable with life on Earth happened in the last billion or so years, with the dinosaurs in the most recent quarter-billion years. 

Much of early earth was dominated by bacteria and single cellular life.

[acecharly]

In the time of the dinosaurs the atmosphere was in a different ratio of gases to now which may also have affected the temptrature and possibly the size of the sun.