Morgan and others investigated the distribution in distance of luminous
early-type stars along the Milky Way, and the first vestiges of spiral arms became discernible.
To the eye,
early-type stars like Vega appear pale bluish, while late types like Betelgeuse appear reddish.
The remaining sources include photospheres and winds of the massive and hot
early-type stars, cataclysmic variables, active galactic nuclei (AGNs), and such unique objects as the Vela supernova remnant, Cygnus Loop, and Jupiter-Io torus.
As shown in the plot at left, the brightness contrast is roughly [10.sup.-7] to [10.sup.-9] for Earth-like planets around M dwarf stars such as Proxima Centauri or Barnard's star; about [10.sup.-10] for those around Sun-like stars such as Alpha Centauri, Tau Ceti, or Epsilon Eridani; and about [10.sup.-11] for those around intensely bright,
early-type stars such as Procyon and Altair.