Galaxies are some of the most massive objects in the universe. Fortunately for us, they
only appear in a few different varieties, making them easy to recognize and theorize
about. Almost all of them are spirals, elliptical,
or irregular of some sort. However, a few oddballs fit into none of these
categories, and one galaxy stands alone. If you look closely at the serpent constellation
slithering over the northern sky, you might see a galaxy within a galaxy within a galaxy.
In 1950, astronomer Arthur Hoag discovered this cosmic object known as Hoag’s object,
and 70 years later, scientists are still trying to piece together this galactic mystery.
This fascinating object is a small, faint, 16th-magnitude ring surrounding a ball-like
core, and it was once thought to be a planetary nebula, a nearby puff of gas ejected from
a single old-aged star. Hoag also proposed that this was an “Einstein
Ring” from a distant quasar, which was a far more exotic description.
In this scenario, the light from the quasar is warped into a halo due to space-warping
caused by a vast foreground spherical galaxy that it seems to surround.
However, later spectroscopic studies refuted this theory because the golden core ball and
the blue ring had the same redshift, indicating a massive rush-away speed of 7,916 miles (12,740
kilometers) per second, proving they’re both the same distance away from Earth.
Hoag also considered the possibility that this was a strange galaxy. And, as if to prove
that making enough guesses increases your chances of succeeding, his final prediction
was correct. In 2002, the Hubble Space Telescope’s ultra-sharp
optics revealed Hoag’s object, as it is now known, to be a flawless blue ring of stars,
dust, and gas, complete with knotted clumps that are unresolved star clusters.
In simple words, yes, this is a galaxy. The only issue is that Hoag’s object does not
follow the usual spiral pattern, with arms wending inward to the older, yellower stars
that make up nearly every galaxy’s nucleus. Instead, the nucleus floats in space by itself.
A whopping 70,000 light-years away from it, separated by near-nothingness, hovers that
ring of billions of stars, planets, and who-knows-what-else. Hoag’s object is 600 million light-years
away from us in the constellation Serpens Caput.
Its yellow inner core is roughly the same size as our Milky Way’s center, measuring
24,000 light-years across. The outer ring of Hoag’s object is similar
in size to our galaxy’s diameter, with a width of 120,000 light-years. It’s almost
as if a malicious alien starship vacuumed up the galaxy’s middle regions but surprisingly
spared its center and outermost regions. Certain “ring galaxies” resemble Hoag’s
object because their peripheries are substantially brighter than their interiors.
However, none have a ring that surrounds emptiness, which in turn surrounds a massive yellow ball
of suns. Aside from its strange appearance, the real
puzzle is how such a configuration could have ever formed. Regrettably, idea after idea
has been proposed and then dismissed. Other “classic” ring galaxies are thought
to have formed when an invading companion infiltrated and tidally destroyed a giant
disk-shaped spiral galaxy. Collisions of this type result in either wild
distortions, such as M82 or the Antennae Galaxies, or mergers into new elliptical shapes like
M87. According to computer simulations, collisions
can also cause density waves, which can sometimes result in a dazzling outer ring. But what
about a ring encircling nothing? Some believe Hoag’s Object formed 2 to 3
billion years ago when a small galaxy passed through a larger one like a cannonball. However,
most astronomers reject this theory due to the lack of candidates in the area, as well
as the slow spin of the core relative to the motions of the ring.
Some scientists believe that a barred galaxy can undergo extreme instability, resulting
in this shape, although others argue that if this were the case, the core would be disk-shaped
rather than the ball-like shape seen currently. Nothing is entirely functional.
A few exceedingly rare so-called Hoag-type galaxies can be found with difficulty, but
they all have distinctive features, such as spiral structure, or a nucleus that has a
bar, or is elliptical or disk-shaped. Hoag’s object is unique in the universe.
As if to taunt further, nature offers a final surprise for us.
Despite its great rarity, a Hoag-like ring galaxy emerges within Hoag’s own ring. Hoag’s
object is so far away that its apparent width is only a quarter of an arcminute, which is
comparable to Mercury’s minuscule diameter. Hoag’s object remains a mystery wrapped
in a riddle inside an enigma.