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Details of Galaxy

A galaxy (from the Greek root γαλαξίας, meaning "milky", a reference to our own Milky Way) is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million (10⁷) stars up to giants with one trillion(10¹²) stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighbouring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

There are probably more than one hundred billion (10¹¹) galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic metre. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy, home of Earth and the solar system, appears to harbor at least one such object within its nucleus.

NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. Credit:Hubble Space TelescopeNASA/ESA.

Etymology

The word Galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας), or kyklos galaktikos, meaning "milky circle" for its appearance in the sky. In Greek mythology, Zeus placed his son born by a mortal woman, the infant Heracles, on Hera's breast as she was asleep, so that the baby would drink her divine milk and would thus become immortal. Hera woke up while breastfeeding, and realized she was nursing an unknown baby: she pushed the baby away and a jet of her milk sprayed the night sky, producing the faint band of light known as the Milky Way.^[11] It is thought this myth may have come from earlier Phoenician or even Egyptian myths, where Asherah and Nut (goddess) were portrayed as Goddesses of the Milky Way.^{[citation needed]}

In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our (Milky Way) galaxy, to distinguish it from the billions of other galaxies.

The term Milky Way first appeared in the English language in a poem by Chaucer.

When William Herschel constructed his catalog of deep sky objects, he used the name "spiral nebula" for certain objects such as M31. These would later be recognized as immense conglomerations of stars, once the true distance to these objects was appreciated, and they would be termed "Island universes". However, the word universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies

Observation history

The realization that we live in a "galaxy", and that there were in fact many other galaxies, parallel discoveries that were made about the Milky Way and other nebulae in the night sky.

The Milky Way

The Greek philosopher Democritus (450 BC-370 BC) was the first known person to propose that the bright band on the night sky known as the Milky Way might consist of distant stars.^{[citation needed]} Actual proof of this came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars.^[14] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Galaxy might be a rotating body of a huge number of stars, held together by gravitational forces akin to the solar system but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies.

The shape of the Milky Way Galaxy as deduced from star counts by William Herschel in 1785; the solar system was assumed near center.

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center.^{[citation needed]} Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.