Hello!
There is a very good reason why we have not sent astronauts to the Sun! The Moon is made of rock, much like our own planet Earth, so the Moon has a nice solid surface we can stand on and plant a flag in. However, the Sun is made of a hot, ionized gas called a plasma and does not have a solid surface that you can walk on or plant a flag on. Trying to stand on the Sun would be like trying to stand on the surface of water, except that because the Sun is so hot you would be instantly vaporized before you have time to sink.
The Sun is the same round ball of glowing, hot ionized gas or plasma during both the day and the night, and in the winter and summer. The Sun is never silvery and filled with craters. During the day, we see the hot, glowing Sun in the sky. The Sun doesn't change at night, it is just hidden on the other side of the Earth where we cannot see it. The silvery, crater-filled object you see in the sky at night is the Moon. You can sometimes see the Moon and the Sun in the sky at the same time during the day, but you will never see the Sun in the sky after sunset at night when it is dark outside.
Kris