M20     The Trifid Nebula in Sagittarius (NGC 6514)

        Object type:             Emission (H II region) nebula

        Distance from Earth:      3200 light years / 1000 parsecs

        Distance across Nebula:   12 light years / 3.7 parsecs


        The Trifid Nebula is a cloud of interstellar gas illuminated by a
        relatively new star (age 7 million years).  Three dark radial dust
        lanes divide the cloud, hence its name.  The light being emitted
        from the hydrogen gas is predominantly the red H-alpha spectral
        line, making the nebula appear pink.

        Pictured with the Trifid is a blue nebula surrounding star
        HD 164514.  This star is not hot enough to excite the hydrogen gas
        like the star within the Trifid, so all we see is starlight
        reflected from dust particles and cold gas.  These particles
        selectively scatter blue light, just as sunlight scattered in
        Earth's sky is blue.