The Milky Way Galaxy

The Milky Way Galaxy

Commonly referred to as just the Milky Way, or sometimes simply as the Galaxy, is the home galaxy of the Solar System, and of Earth. Milky Way is a spiral galaxy. It contains 100 to 400 billion stars and is estimated to have at least 50 billion planets, 500 million of which could be located in the habitable zone of their parent star, like the Earth. New data suggests there may be up to twice as many free-floating planets in the Milky Way as there are stars. The Milky Way is part of the Local Group of galaxies and is one of around 100 to 200 billion galaxies in the observable universe.

The Solar System is located in the Milky Way galaxy halfway out from the centre, on the inner edge of the Orion Cygnus Arm. The Sun orbits around the centre of the galaxy in a galactic year once every 225-250 million Earth years.

All the stars that the eye can distinguish in the night sky are part of the Milky Way galaxy. The Milky Way has a relatively low surface brightness due to the interstellar medium that fills the galactic disk, which prevents us from seeing the bright galactic centre. It is thus difficult to see from any urban or suburban location suffering from light pollution.

The fact that the band divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane.

The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (ly) in diameter, and is considered to be, on average, about 1,000 ly thick. It is estimated to contain at least 100 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass, or dwarf stars, which are hard to detect, especially more than 300 ly from the Sun, and so current estimates of the total number remain highly uncertain, though often speculated to be around 250 billion. This can be compared to the one trillion (10^12) stars of the neighbouring Andromeda Galaxy.

As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a coin, the Milky Way would be the size of;  A room ? No. A Theatre? No. A stadium ? No. A village? No. A city? No. Srilanka ? No.

Then ?

Approximately the size of Australia or more than twice the area of India!

The galactic centre harbours a compact object of very large mass as determined by the motion of material around the centre. The intense radio source named Sagittarius A*, thought to mark the centre of the Milky Way, is newly confirmed to be a super-massive black hole. Most galaxies are believed to have super-massive black holes at their centres.

As is typical for many galaxies, the distribution of mass in the Milky Way galaxy is such that the orbital speed of most stars in the galaxy does not depend strongly on its distance from the centre. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies. This can be likened to a moving traffic jam on a highway, the cars are all moving, but there is always a region of slow-moving cars.

The collection of stars rises close to perpendicular to the plane of the spiral arms of the galaxy. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years away.

The Sun's orbit around the galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. It takes the Solar System about 225-250 million years to complete one orbit of the galaxy (a galactic year), so it is thought to have completed 20-25 orbits during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the centre of the galaxy is approximately 220 km/s.

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster.

Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri.

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.

In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein's special theory of relativity, which declares that there is no "preferred" inertial frame of reference in space with which to compare the object's motion. (Motion must always be specified with respect to another object.) This must be kept in mind when discussing the galaxy's motion.

Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the local co-moving frame of reference that moves with the Hubble flow. If the galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Milky Way is thought to be moving in the direction of the Great Attractor. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda galaxy) is part of a supercluster called the Local Supercluster, centred near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, the velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.

There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The English phrase is a translation from Ancient Greek Galaxias, which is derived from the word for milk (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.

In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (River Ganges of the heavens). The milky way is held to be sacred in the Hindu scriptures known as the Puranas, and the Ganges and the Milky Way are considered to be terrestrial-celestial analogs of each other. However, the term Kshira (milk) is also used as an alternative name for the milky way in Hindu texts.

As Aristotle (384-322 BC) informs us in Meteorologica, the Greek philosophers Anaxagoras (500-428 BC) and Democritus (450-370 BC) proposed the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (c. 495-570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.

The Arabian astronomer, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere."

Actual proof of the Milky Way consisting of many stars 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. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Milky Way 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" themselves, similar to our own. Kant referred to both our galaxy and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.

The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

Giant Frisbee. The Milky Way is a flattened spiral galaxy that spins. You might imagine it flying through space like a giant Frisbee having a diameter of 100,000 light years across at a speed of more than a million miles an hour.  Just a toy of a giant who likes fun like a kid?