Hotter still is the core. In the sun this is thought to be 10 - 20 million degrees. These temperatures are due to the pressure caused by gravity trying to compress the star. It has to be this temperature for fusion of hydrogen to take place. Once fusion starts in a young star this adds to the temperature to ensure that fusion continues.
Weather in Lisbon in September. The average maximum daytime temperature in Lisbon in September is a warm 27Â°C (81Â°F) with low heat & humidity. There are usually 9 hours of bright sunshine each day, which represents 68% of the 12 hours of daylight. The average sea temperature in and around Lisbon in September is 19Â°C (66Â°F). Expect 26mm (1.0 inches) of rainfall across the whole of September with 6 days with some rain falling. Maximum UV levels will be high (6 UV index) at midday when the skies are clear.
It depends the exact time you go, as usually September is hot in the beginning of the month and tends to gets a little bit colder at the end of the month. I would say that in the south (Algarve) the weather should be around 25Â°C, and still probably beach weather. In Lisbon it would probably be colder, probably around 20Â°C, so probably not the right weather to go to the beach. More info here: September Weather Averages for Algarve, Portugal, and September Weather Averages for Lisbon, Portugal.