The following text is for questions 11 and 12

    The brightest star visible from any part of Earth is Sirius in the constellation Canis Major, the Greater Dog. Sirius is sometimes called the Dog Star. Most people in the Northern hemisphere notice Sirius in the southeast - south - or southwest on evenings from winter to mid-spring. February evenings are a good time to see it. It's also fun to spot Sirius as it arises in the east before dawn on late summer mornings.

    Although white to blue white in colour, Sirius might be called a rainbow star, as it often flickers with many colours. The brightness, twinkling and colour changes sometimes cause people to report Sirius as a UFO. In fact, these changes are simply what happen when such a bright star as Sirius shines through the blanket of earth's atmosphere. The varying thickness and temperature of Earth's air affect starlight, especially when we're seeing the star low in the sky. The position of Sirius is RA: 06h 45m 08.9s, dec:-16° 42' 58°.

Source: http://earthsky.org/brightest-starslsirius-thebrightest-star

Why is Sirius often called a rainbow star?