谁知道有关自然，天文或宇宙或外星人的英文权威网站

http://www.nasa.gov/ 美国国家宇航局，这个应该是最权威的

http://www.heavens-above.com/

www.noao.edu，美国国家光学天文台

紫金山天文台的英文网站
http://www.pmo.ac.cn/English/index.htm

since you are perparing an english speech, an english website may be what you are looking for.

NASA and Discovery's Channel are good places to find whatever you're seeking, and Wikipedia as well. Their websites containts a lot of information about the nature and the universe. Just google them and you will find their urls.

中国天文网：http://astronomy.uua.cn/
中国UFO网：http://www.ufocn.org/

www.nasa.gov
aas.org