GALAXY
A system of stars or a collection of stars and interstellar material in the universe. Such
assemblages are common, and many of them are so massive that they hold hundreds of billions
of stars. A vast variety of galaxies, from dim, diffuse dwarf objects to brilliant spiral-shaped giants,
have been created by nature. Almost all galaxies seem to have formed shortly after the universe
started, and they are everywhere in space, even at the farthest reaches of the universe that can
be seen by the most advanced telescopes.
The majority of galaxies are found in clusters, some of which are further organized into
larger clusters that span hundreds of millions of light-years. These so-called superclusters are
separated by nearly empty voids, which makes the overall structure of the universe resemble a
network of sheets and chains of galaxies (a light-year is a distance traveled by light in one year at
a velocity of 300,000 km/sec, or 650,000,000 miles per hour).
Galaxies have different shapes from one another, with variations stemming from how the
systems were created and later evolved. Galaxies are incredibly diverse in terms of their internal
structure and the amount of activity that can be seen. Some of these are the locations of active
star formation, complete with the accompanying luminous gas, dust clouds, and molecular
complexes. Others, in contrast, are dormant and no longer produce new stars.
The greatest noticeable activity in galaxies may be seen in their nucleus, where evidence
suggests that supermassive objects—likely black holes—lurk often. These central black holes,
which are now seen forming in galaxies at great distances (and, consequently, because of the
time it takes for light to travel to Earth, at times in the far distant past), appear to have formed
several billion years ago. They are now observed as brilliant objects known as quasars.
Before the early 20th century, galaxies were not even thought to exist. But since then,
galaxies have been one of the main areas of astronomy study. This article surveys the significant
advancements and successes in the study of galaxies. Discussion topics include the evolution of
galaxies and quasars and the external galaxies (i.e., those outside the Milky Way Galaxy, the
neighborhood galaxy to which the Sun and Earth belong). Visit Milky Way Galaxy to learn more
about the Milky Way Galaxy. See star and nebula for more details on the elements that make up
galaxies.
HISTORICAL ANALYSIS OF GALAXY RESEARCH
One of the biggest in the history of astronomy is the debate about the true nature of what
was originally known as spiral nebulae. The size of the universe was a contentious issue that hung
in the balance. Were we constrained to a single, small stellar system that existed in isolation in
deep space, or was our Milky Way Galaxy merely one of the millions of galaxies that spanned
space, extending beyond the vast distances peered by our most powerful telescopes? Our current
worldview has evolved in large part as a result of how this question came to be and how it was
answered.
The issue with the Magellanic Clouds
The Magellanic Clouds, two patchy irregular objects visible in the Southern Hemisphere
skies, are currently understood to be the closest external galaxies. Most astronomers who believed
the Magellanic Clouds to be parts of the Milky Way Galaxy system that were isolated from it for
years were unable to investigate them due to their location. Moreover, the irregular shapes of the
objects and their abundance of hot blue stars, star clusters, and gas clouds did make them resemble
the southern Milky Way Galaxy (both Magellanic Clouds are too far south to be viewed from most
northern latitudes).
The Andromeda Nebula's Novae
The Andromeda Nebula, also known as M31, is the brightest nearby galaxy in the northern
sky, but its early discovery was hampered by a regrettable misidentification. Near the center of
M31 in 1885, a previously invisible star became bright and nearly visible without a telescope.
Astronomers concluded that it had to be a nova, or "new star," one of a class of transient stars that
are relatively common in populated regions of the Milky Way Galaxy as it slowly faded away
again. It was argued that if this were the case, M31 cannot be very far away—certainly not outside
the neighborhood star system—given its extraordinary brightness.
This alleged nova, designated S Andromeda in accordance with the terminology used to
describe stars of variable brightness, was a compelling argument in favor of the theory that nebulae
are nearby objects in the Milky Way Galaxy.
After the 254-cm (100-inch) telescope on Mount Wilson was finished in 1917, a new set
of pictures was taken that showed even fainter objects. Milton L. Humason, who was Edwin P.
Hubble's assistant at the time and one of the truly brilliant astronomers of the day, discovered
several novae in M31. Hubble eventually investigated 63 of these stars, and his discoveries turned
out to be one of the conclusive answers to the debate.