                        Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                               2020 January 31

                         Goldilocks Zones and Stars
                Infographic Credit: NASA ESA, Z. Levy (STScI)

   Explanation: The Goldilocks zone is the habitable zone around a star
   where it's not too hot and not too cold for liquid water to exist on
   the surface of orbiting planets. This intriguing infographic includes
   relative sizes of those zones for yellow G stars like the Sun, along
   with orange K dwarf stars and red M dwarf stars, both cooler and
   fainter than the Sun. M stars (top) have small, close-in Goldilocks
   zones. They are also seen to live long (100 billion years or so) and
   are very abundant, making up about 73 percent of the stars in the Milky
   Way. Still, they have very active magnetic fields and may produce too
   much radiation harmful to life, with an estimated X-ray irradiance 400
   times the quiet Sun. Sun-like G stars (bottom) have large Goldilocks
   zones and are relatively calm, with low amounts of harmful radiation.
   But they only account for 6 percent of Milky Way stars and are much
   shorter lived. In the search for habitable planets, K dwarf stars could
   be just right, though. Not too rare they have 40 billion year
   lifetimes, much longer than the Sun. With a relatively wide habitable
   zone they produce only modest amounts of harmful radiation. These
   Goldilocks stars account for about 13 percent of the stars of the Milky
   Way.

                   Tomorrow's picture: Apollo 14 Earthrise
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                      A service of: ASD at NASA / GSFC
                             & Michigan Tech. U.

