[1] APOD/NASA: https://apod.nasa.gov/apod/ap140119.html
    D. Elmegreen: http://faculty.vassar.edu/elmegree/Site/Welcome.html
    The Hubble Heritage Team: http://heritage.stsci.edu/1999/41/bio/bio_elmegreen.html
[2] APOD/NASA: https://apod.nasa.gov/apod/ap060824.html
[3] APOD: https://apod.nasa.gov/apod/ap160212.html
    SXS Project: http://www.black-holes.org/