Andromeda Galaxy

The Andromeda Galaxy is a large spiral galaxy about 2.5 million light years from Earth. Named for its celestial location in the constellation of Andromeda, it is the closest major galaxy to the Milky Way—the home galaxy of our solar system. Andromeda is one of the most distant objects visible to the human eye without magnification, appearing as a fuzzy smudge of light about the diameter of a full moon. The galaxy, which is also known according to the classification M31, is larger than the Milky Way, and it was able to grow to its size by "consuming" several smaller neighboring galaxies. According to scientists, Andromeda is approaching the Milky Way. Several billion years in the future, the two galaxies will collide, merging to form a giant elliptical galaxy.

Background

Most astronomers believe the universe was born about 13.8 billion years ago in a massive explosion called the big bang. The early universe was so hot that it took almost 400,000 years for space to cool enough to allow matter to form. The first galaxies came into being about 200 to 300 million years after the big bang, though astronomers are unsure about how they formed. One theory suggests that galaxies took shape when massive clouds of gas and dust collapsed and coalesced under gravitational forces, igniting the first stars. Another theory holds that the young universe was full of clumps of newly formed matter that stuck together, absorbing more clumps to form galaxies.

Astronomers classify galaxies into three basic types. The most commonly observed are spiral galaxies, which have a central rotating disk surrounded by spiral arms. Elliptical galaxies are oval or spherical shaped, and they are typically made up of older stars. These galaxies are believed to be formed by collisions between other galaxies. Elliptical galaxies are more numerous throughout the universe, but many remain unseen because of their lower brightness. The third type of galaxy is an irregular galaxy, which as the name suggests does not have any recognizable structure.

Overview

The Andromeda Galaxy is believed to be at least 10 billion years old. The first recorded observation of the galaxy was made in 964 CE by Persian astronomer Abd al-Rahman al-Sufi, who described it as a "small cloud." Andromeda was first observed with a telescope in 1612; however, early astronomers classified it as a nebula, a cloud of gas and dust within the Milky Way. In the eighteenth century, French astronomer Charles Messier began cataloging a group of more than one hundred "nebulous" objects in the night sky. As the thirty-first object he identified, Messier classified the Andromeda Nebula—as he called it—as Messier object 31, or M31. It was not until the early twentieth century that American astronomer Edwin Hubble was able to calculate its distance at 2.5 million light years away, placing it well outside the Milky Way.

The Andromeda Galaxy is located in the constellation of Andromeda, named after the princess in Greek mythology who was saved from a sea monster by the hero Perseus. The galaxy is visible in the Northern Hemisphere for most of the year for at least part of the night. It is most prominent in the night sky from early autumn through December. It is the third brightest Messier object, and it appears as a faint cloud-like blur to the left of the Great Square of the constellation Pegasus. It can also be seen by looking south of the W-shaped constellation of Cassiopeia. To the naked eye or with the aid of binoculars, the Andromeda Galaxy is slightly larger than the diameter of the full moon. Larger telescopes reveal it to be about six times the width of the moon. It has two bright companion dwarf galaxies—M32 and M110—which can be seen through binoculars.

The Andromeda Galaxy is a spiral galaxy with at least two spiral arms about 220,000 light years across—more than twice the diameter of the Milky Way. Its spiral arms were formed by gravitational interaction with M32 and M110. Andromeda has a mass equivalent to about 1.23 billion times that of the Sun. It is the largest galaxy in a cluster known as the Local Group, which includes the Milky Way, the Triangulum Galaxy (M33), and more than forty other smaller galaxies. The galaxy contains about 1 trillion stars compared to the 200 to 400 billion in the Milky Way. While several smaller irregular galaxies lie closer, the Andromeda Galaxy is the closest large galaxy to the Milky Way. In 2009, scientists observed an astronomical phenomenon affecting a star in Andromeda, suggesting the presence of a large planet about six to seven times the size of Jupiter orbiting the star.

At least one supermassive black hole—the remains of a collapsed star so large its gravity will not allow light to escape—lies at the heart of Andromeda, while surrounding it are fourteen dwarf galaxies. Like most large galaxies, Andromeda has achieved its size by cannibalizing its smaller neighbors. The galaxy has absorbed numerous smaller galaxies over its lifetime, and it has siphoned off interstellar matter from a close encounter with its satellite galaxy M32. Some astronomers believe Andromeda will eventually merge with and absorb the neighboring Triangulum Galaxy, so called because of its location in the constellation Triangulum.

Moving toward the Milky Way at about 250,000 miles per hour (402,336 kilometers per hour), the Andromeda Galaxy is destined to collide with our galaxy in about four billion years. While galaxy collisions and mergers are common, the vast distances between stars prevent them and their solar systems from colliding with one another. In some cases, gravity from the collision can alter stellar orbits around the galactic core and force a star into a new orbit. Astronomers estimate the collision between Andromeda and the Milky Way will last about two billion years, after which the galaxies' stars will be scattered and reorganized by gravity to form a giant elliptical or disk-shaped galaxy.

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