Explanation:
In the 1920s, examining photographic plates from the
Mt. Wilson Observatory's
100 inch telescope,
Edwin Hubble determined the distance to the
Andromeda Nebula,
decisively demonstrating the existence of other galaxies far beyond
the Milky Way.
His notations are evident on the historic plate image
inset at the lower right, shown in context with ground based
and Hubble Space Telescope images of the region made
nearly 90 years later.
By intercomparing different plates, Hubble
searched for novae, stars which underwent a
sudden increase in brightness.
He found several on this plate and marked them with an "N".
Later, discovering that the one near the upper right corner (marked by
lines) was actually a type of
variable star known as
a cepheid,
he crossed out the "N" and wrote "VAR!".
Thanks to the work of Harvard
astronomer Henrietta Leavitt, cepheids,
regularly varying pulsating stars, could be used as standard candle
distance indicators.
Identifying such a star allowed Hubble to show
that Andromeda was not a small cluster of stars and gas within our own
galaxy, but a large galaxy in its own right at a substantial
distance from the Milky Way.
Hubble's
discovery is responsible for establishing our modern concept of a
Universe filled with galaxies.
What is it?:
Help identify an unusual circular cloud seen today over Italy
