Milky Way’s Structure

FIELDS OF STUDY: Astronomy; Observational Astronomy

ABSTRACT: The Milky Way is the galaxy that contains Earth’s solar system. It is a barred spiral galaxy, which means it consists of a flat, bar-shaped central disk surrounded by spiraling arms. The Milky Way is home to hundreds of billions of stars. Its disk measures roughly one hundred thousand light-years across.

The Makeup of the Milky Way

The Milky Way is the galaxy that includes Earth’s solar system. The Milky Way is a barred spiral galaxy, according to the Hubble classification system. This system, also known as the Hubble sequence, is used to categorize galaxies according to their shape. The four main categories are spiral, elliptical, lenticular (lens-shaped), and irregular. Roughly 77 percent of all observed galaxies are spiral galaxies.

The Milky Way is an old galaxy, believed to have formed about 13.2 billion years ago. Earth’s solar system was created only 4.6 billion years ago. The Milky Way most likely contains between two hundred billion and four hundred billion stars.

Attributes of the Milky Way

At the center of the Milky Way is its bulge. The bulge is so named because it is a large, somewhat spheroid mass of stars and gas that bulges above and below the plane of the galactic disk. The Milky Way’s bulge is estimated to contain about ten billion stars. Most of these stars are older, generally seven billion years or more, although there are some younger stars as well. The bulge of the Milky Way is approximately ten thousand light-years across and is roughly peanut-shaped.

Observations have revealed that in the center of the Milky Way’s bulge is an object of incredible mass, about 4.5 million times the mass of the sun. This object, named Sagittarius A*, is generally thought to be a supermassive black hole. Scientists believe that most, if not all, large galaxies have a supermassive black hole at their center, around which the entire galaxy rotates. NASA’s Chandra X-Ray Observatory has recorded near-daily x-ray flares from Sagittarius A*. In January 2015, it detected an x-ray flare about four hundred times brighter than usual, the largest such flare to date.

The disk of the Milky Way is the part of the galaxy that holds most of the gas and dust. This gas and dust is also known as the interstellar medium, because it is matter that exists in the space between the stars. The Milky Way’s disk is approximately one hundred thousand light-years across. The galaxy’s spiral arms are attached to the disk. Scientists had long debated whether the Milky Way had two main spiral arms or four. A study published in late 2013 confirmed that there are in fact four main arms, plus a number of smaller arms called spurs. The arms of a spiral galaxy are what give it its pinwheel-like shape.

In many spiral galaxies, including the Milky Way, the disk is made up of two distinct regions: the thin disk and the thick disk. The Milky Way’s thin disk contains about 80 percent of all normal (non-dark) matter in the galaxy. Due to the presence of the interstellar medium, it is a site of active star formation. The stars in the thin disk range in age from newborn to ten billion years old. The thick disk is made up of older stars and contains no interstellar medium. The majority of stars in the thick disk are older than ten billion years.

The stellar halo around the Milky Way and other spiral galaxies is a sphere made up of old stars, globular clusters (groups of stars), dust, and gas. This halo is believed to be surrounded by the dark halo, a huge, invisible area made up of dark matter. Dark matter is matter that has never been directly observed but is believed to exist due to its gravitational effects. It may account for as much as 84.5 percent of all matter in the universe. Scientists believe that the dark halo may be ten times as massive as the rest of the Milky Way.

Human Knowledge of the Milky Way

People have studied the sky and pondered the stars throughout all of human history. For much of this time, people did not understand the nature of the Milky Way or its relationship with Earth. During the twentieth century, however, humanity’s knowledge of galaxies changed dramatically.

Previously, many scientists had believed that Earth’s sun was near the center of the Milky Way. In the late 1910s, American astronomer Harlow Shapley (1885–1972) calculated that the sun was actually more than fifty thousand light-years from the galactic center. While this was an overestimate—the true distance is around twenty-eight thousand light-years—it was a fundamental shift in scientists’ understanding of the galaxy. Then, in 1924, American astronomer Edwin Hubble (1889–1953) confirmed that what was then known as the Andromeda Nebula was in fact the Andromeda Galaxy, as it was too distant to be part of the Milky Way. These two discoveries transformed the way people saw the galaxy and the universe. By the early twenty-first century, scientists were able to calculate that at least 170 billion galaxies, and possibly more than twice that number, existed in the known universe.

Launched in 2013, the European Space Agency (ESA)'s Gaia 7.5-foot (2.3 meters) satellite contains two telescopes. They have produced the most complete star catalog to date. The catalog contains high-precision measurements of nearly 2 billion stars. The telescopes will remain operation until 2025 when they run out of fuel.

Importance of Studying the Milky Way

Scientists study the Milky Way for a number of reasons. A better understanding of the galaxy could reveal more information about the origins of the universe. It could also shed new light on dark matter, dark energy, and other phenomena that scientists do not fully understand.

PRINCIPAL TERMS

• bulge: a large, relatively dense group of stars at the center of a spiral galaxy.
• dark halo: a hypothetical massive cloud of dark matter that surrounds a galaxy’s disk.
• galaxy: a huge collection of stars, gas, dust, and dark matter that is held together by gravity.
• Hubble classification: a galactic classification system based on shape, invented by Edwin Hubble; also called the Hubble sequence.
• spiral galaxy: a galaxy consisting of a flat central disk surrounded by spiraling arms.
• stellar halo: a spherical mass of stars, star clusters, dust, and gas that surrounds certain types of galaxies.
• thick disk: the part of a galactic disk that contains older stars, typically more than ten billion years old.
• thin disk: the part of a galactic disk that contains younger stars, typically less than ten billion years old, as well as the dust and gas from which new stars are formed.

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