This image shows the hidden spiral arms that were discovered around the
galaxy called NGC 4625 (top) by the ultraviolet eyes of NASA's Galaxy
Evolution Explorer. An armless companion galaxy called NGC 4618 is
pictured below.
Though the lengthy spiral arms are nearly invisible when viewed in
optical light, they glow brightly in ultraviolet. This is because they
are bustling with hot, newborn stars that radiate primarily ultraviolet
light.
The youthful arms are also very long, stretching out to a distance four
times the size of the galaxy's core. They are part of the largest
ultraviolet galactic disk discovered so far.
Located 31 million light-years away in the constellation Canes Venatici,
NGC 4625 is the closest galaxy ever seen with such a young halo of arms.
It is slightly smaller than our Milky Way, both in size and mass. However,
the fact that this galaxy's disk is forming stars very actively suggests
that it might evolve into a more massive and mature galaxy resembling our
own.
Astronomers do not know why NGC 4618 lacks arms but speculate that it may
have triggered the development of arms in NGC 4625.
