Was the Sun hotter or cooler long time ago ?

mca

(問題源自 Jonathan Lau，藍色字是天文學會教育組的回應)

----- Original Message -----
From: "Jonathan Lau" To: <[email protected]>
Sent: Wednesday, November 11, 2009 9:53 PM
Subject: Properties of our Sun long time ago


I have a question regarding the properites of the our Sun, not at present, but long time ago (say 1 or more billion years ago).

According to any scientific hypothesis, theories, was our Sun hotter than it is now, or cooler ?

I've heard that our Sun probably formed from a intersetellar-cloud / protostar cloud. (I hope I've got my terminology right, as my knowledge on Astronomy is close to zero).

But would it be possible that there is a period between the protostar cloud period and now that its temperature (both core and surface) is lower than it is now ? Or, on the other hand, must Sun's temperature remain higher (or at least as high) as it is now ?  If it can be lower, by how much would this possibly be ?

I would also like to hear additional viewpoints from IAU (Internatonal Astronomy Union) and different astrophysists (Sun experts) around the world as I understand that different astrophysists may have different point of view on this question……

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Reply:

Current theory states that the Sun was cooler in its early stage; its temperature was about 5700 K three billion years ago versus 5780 K at present . This is based on the astrophysics law that  L = 4 pi‧R^2‧B‧T^4,  where L is the solar luminosity (energy radiated per sec by the Sun), R is the solar radius, B is Stefan-Boltzmann constant, and T is the star’s effective temperature.  Details refer to this figure：



You can pay Nature magazine to buy this pape
http://www.nature.com/nature/jou ... bs/nature05072.html

Or you use Google to search the weblinks about “solar luminosity”.

There are also papers from real experts in the book “Encyclopedia of the Solar System” (a very thick book available from Amazon.com). There are further references listed in the papers. Write to the authors if you could.