Because the Sun continues to 'burn' hydrogen into helium in its core, the core slowly collapses and heats up, causing the outer layers of the Sun to grow larger. This has been going on since soon after the Sun was formed 4.5 billion years ago. It is a very gradual process, and in the last 4 billion years, the Sun has barely grown by perhaps 20 percent at most. It will not grow by much more than another factor of a few for the next 6 billion years, but at that distant time, it will make a rapid transition to a red giant phase and its outer surface will expand by several hundred times to perhaps the orbit of Venus. Astronomers have searched for short term changes in the radius of the Sun, but have not been able to find much reliable evidence that the sun's diameter is changing, at least over times as short as the solar cycle. Longer timescales are hard to study historically because it is very hard to measure the diameter of the sun to 100-meter resolution at the distance of the Earth, and to do so over many decades! If the radius of the Sun were to double in a time as short as 1 billion years, you would need to be able to detect a 65 meter per year change which corresponds to an angular change in its radius of 0.0002 arcseconds! This is a factor of at least 100 times smaller that what long term accuracy one could hope to acheive. Perhaps the most sensitive test is the change in the brightness of the sun itself, and since the temperature of the earth during the last few million years has not systematically changed by more than a few degrees AT MOST...ignoring ice ages...this restricts any change in the radius of the Sun from historical records to less than a few percent or 10,000 kilometers. This is not a very interesting constraint, but at least it excluses any potential 'catastrophic' solar radius increases!!
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.