Cosmic timeline 14
The Milky Way
The Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies. The galaxy photographed by the Hubble Space Telescope is NGC 1300 and is a barred spiral galaxy about 61 million light-years away in the constellation Eridanus. The galaxy is about 110,000 light-years across; just slightly larger than our own galaxy, so gives us an idea of what our galaxy would like like from another galaxy. It is part of the Eridanus Cluster of galaxies. The Milky Way is one of billions of galaxies in the observable universe. Its name is a translation of the Latin Via Lactea, in turn translated from the Greek Γαλαξίας (Galaxias), referring to the pale band of light formed by stars in the galactic plane as seen from Earth. The English phrase is a translation from Ancient Greek Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy. In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles. Our galaxy of stars is estimated to have been formed between 6.5 and 10.1 billion years ago. This is the age of the thin disc of stars, gas and dust that is primordial structure of the galaxy. The Milky Way formed between 6 and 7 billion years after the big bang. From the briefest moments of time imaginable after the big bang to the present, is some 13.5 billion years, so our galaxy begins to form and structure roughly half way in time between the big bang and now.
Nucleocosmochronology, also known as cosmochronology, is a relatively new technique used to determine timescales for astrophysical objects and events. Nucleocosmochronology has already been successfully employed to determine the age of the Sun (4.57±0.02 Ga, i.e., 4.57×109 years) and of the Galactic thin disk (8.3±1.8 Ga), among others. It has also been used to estimate the age of the Milky Way itself.
Types of galaxy
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage.
Hubble’s scheme divides galaxies into 3 broad classes based on their visual appearance.
Elliptical galaxies, like the giant elliptical galaxy ESO 325-G004, have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Most elliptical galaxies are composed of older, low-mass stars. They are surrounded by large numbers of globular clusters. Elliptical galaxies probably make up 10–15% of galaxies in the local Universe but are not the dominant type of galaxy in the universe overall. They are usually found close to the centers of galaxy clusters and are less common in the early Universe.
Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol S. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Lenticular galaxies (designated S0), like the Spindle Galaxy (NGC 5866), a lenticular galaxy in the Draco constellation, also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity, because they have used up or lost most of their interstellar matter. As a result, they consist mainly of aging stars (like elliptical galaxies). The dust in most lenticular galaxies is generally found only near the nucleus and generally follows the light profile of the galaxies’ bulges. Because of their ill-defined spiral arms, if they are inclined face-on it is often difficult to distinguish between them and elliptical galaxies.
An irregular galaxy is a galaxy that does not have a regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull. Irregular galaxies also contain abundant amounts of gas and dust.
There are two major Hubble types of irregular galaxies:
An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. de Vaucouleurs subtypes this into galaxies that have some spiral structure Sm, and those that do not Im.
An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence.
A third classification of irregular galaxies are the dwarf irregulars, labelled as dI or dIrrs. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe.
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy. Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.
The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter. Hence interactions between these galaxies are relatively frequent, and play an important role in their evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.
The Antennae Galaxies are undergoing a collision that will result in their eventual merger.
Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, so they are known as a starburst galaxies. Should they continue to do so, however, they would consume their reserve of gas in a time frame lower than the lifespan of the galaxy. Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the universe, and, at present, still contribute an estimated 15% to the total star production rate.
Within a billion years of a galaxy’s formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added. During this early epoch, galaxies undergo a major burst of star formation.
I Zwicky 18 (lower left) resembles a newly formed galaxy
During the following two billion years, the accumulated matter settles into a galactic disc. A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life. This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.