Astronomy questions from sixth graders, Part 4

Note: There is a special note to teachers at the bottom of this post. If you're a teacher, please be sure to check it out! My friend Tina is a teacher at the Saegert Sixth Grade Center in Austin, Texas. She asked her sixth grade students to send me questions they had about astronomy, and I answer them on camera. There were so many I had to split this into five parts! I'll be posting one part every day, first thing in the morning. To catch you up, check out Part 1, Part 2, and Part 3. Below is Part 4. The questions asked in Part 4 are: 1) Is there another planet past Pluto or inside Mercury's orbit? 2) Why can't we live without the Sun? 3) How do stars form? 4) Have the constellations changed? 5) Why is space black? Why doesn't the Sun light it up? 6) Do galaxies move around in space and do they collide? 7) What is the biggest galaxy? SPECIAL NOTE TO TEACHERS: Many schools block access to YouTube. There is another video hosting platform called TeacherTube, which is designed to be used in schools. I've uploaded this video (Part 4) to my channel there, where you can access it in your school (note: the video is in higher-resolution on YouTube). If you do, please let me know! I'd love to know what the students thought of the video -- warts and all.