Milky Way Galaxy

The Milky Way is our home galaxy; it contains our Solar System with the Sun and all of its planets. It got the name ‘Milky Way’ from the fact that it appears as a hazy band in the night sky, a somewhat ‘milky’ appearance with individual stars being largely indistinguishable without some kind of optical aid.

The Milky Way is classified as a barred spiral galaxy – a spiral galaxy with a central bar-like structure of stars across the central bulge – which is about 150000 – 200000 light-years in diameter containing somewhere in the range of 100 – 400 billion stars and more than 100 billion planets conservatively. This range of stars is imprecise since we ourselves are located within the plane of the galaxy and therefore are not able to make accurate objective measurements.

Our Solar System is located approximately 26500 light-years from the Galactic Center of the Milky Way Galaxy in the inner region of the Orion Arm(the full name is the Orion-Cygnus Arm), a minor spiral arm about 10000 light-years long and 3500 light-years wide. This location has sometimes been jokingly referred to as the boondocks of the Milky Way Galaxy.

Our galaxy also contains a number of smaller satellite galaxies, two of which are visible to the unaided eye, the Large and Small Magellanic Clouds – the name ‘clouds’ coming from the fact that they appear as hazy whiteish clouds of indistinguishable stars in the night sky of the Southern Hemisphere, although they can be seen close to the horizon of very low latitudes of the Northern Hemisphere.

The Milky Way Galaxy is currently believed to have begun as one or several asymmetrical overdensities in the overall mass distribution of the Universe soon after the occurrence of the Big Bang. Some of these overdensities later became globular clusters in the Milky Way; these clusters contain some of the oldest stars in the Milky Way.

Over a period of billions of years as more and more matter coalesced the huge sphere of matter and gas which would eventually become the Milky Way started spinning – the spinning became more rapid and the conservation of angular momentum gradually caused this huge sphere of matter and interstellar medium to collapse into the disk shape that we observe today and which our own Solar System lies in.

Globular clusters in the Milky Way contain some of the oldest stars and can, therefore, help us with our estimate of how old the Milky Way is. For example, the age of the globular cluster M4 has been estimated at somewhere around 12.7 billion years old.

In November 2018 astronomers discovered what turns out to be one of the oldest stars in the Universe, 2MASS J18082002−5104378, which is estimated to be about 13.53 billion years old and made up almost entirely of materials released from the Big Bang. The fact that this star is in the Milky Way Galaxy increases estimates of the age of the galaxy by around 3 billion years over what was previously thought. This means that the Milky Way may, in fact, be at least 13.53 billion years old, only slightly older than the Big Bang itself, and perhaps one of the oldest galaxies in the Universe.

The Special Theory Of Relativity does not provide any preferred inertial frame of reference but the speed of the Milky Way Galaxy can be estimated with respect to other cosmological frames of reference. One example of this is called the Hubble Flow, which is the speed of various clusters of galaxies due to the expansion of the Universe. In regards to this particular frame of reference, astronomers believe the Milky Way is moving at a velocity of about 1.4 million miles per hour in the direction of the Great Attractor – see the ‘Environment’ section below.

The Milky Way has two major satellite galaxies, the Large and Small Magellanic Clouds, along with a number of smaller orbiting tiny galaxies and star clusters.

The Milky Way Galaxy and the Andromeda Galaxy are giant spiral galaxies, along with the smaller spiral galaxy Triangulum, in what is known as the Local Group of galaxies – a group of about 50 closely bound galaxies. The Andromeda Galaxy is the largest, the Milky Way Galaxy is the second largest, and the Triangulum Galaxy is the third largest in the Local Group; the rest are smaller dwarf galaxies.

Surrounding this Local Group of galaxies is a huge region of empty intergalactic space known as the Local Void. The Local Group of galaxies and the Local Void are part of a gigantic cluster of galaxies(over 100 groups of galaxies) known as the Virgo Supercluster, about 110 million light-years in diameter and believed to contain over 1300 galaxies.

Since the late 1980s it has been known that all matter within a radius of about 163 million light-years, this includes the Local Group and the Virgo Supercluster, has been moving in the direction of the Norma Cluster to something called the Great Attractor at the bulk speed of about 1.34 million miles per hour. This mysterious Great Attractor is now understood to be the center of mass of an even larger structure of galaxy clusters which is now called Laniakea. Laniakea is believed to include the Local Group, the Virgo Supercluster, the Hydra-Centaurus Supercluster, the Pavo-Indus Supercluster, and the Fornax Group.

Galileo Galilei was the first to resolve this hazy band of light into individual stars with his primitive telescope around the year 1610. All through the decades and centuries from this time until about the early 1920s the vast majority of astronomers thought that the stars in the Milky Way Galaxy represented the entire Universe. In other words our galaxy, the Milky Way was the only galaxy in the Universe, there were no others – it effectively WAS the whole Universe.

On April 26th, 1920 the Shapley-Curtis Debate, also known in astronomy as the Great Debate, was held in the Smithsonian Museum of Natural History between astronomers Harlow Shapley and Heber Curtis. This debate was about the nature of various spiral nebulae which had been observed and how big the Universe was. Shapley thought that these spiral nebulae were relatively small and were very close to the Milky Way Galaxy, even within the outskirts. Curtis believed that these spiral nebulae were actually galaxies which were independent of the Milky Way and were located at very great distances away.

Following this debate, the great astronomer Edwin Hubble made observations which indicated that our Milky Way Galaxy is just one of many different galaxies in the Universe, all of which are independent of each other. This confirmed the position of Heber Curtis, effectively making him the winner of the debate on this major point.

The Milky Way Galaxy is observable from Earths night sky as a whiteish hazy band of light about 30 degrees wide arcing from horizon to horizon. Although all of the stars visible without optical aid in the night sky are from the Milky Way – with the possible exceptions of the Andromeda Galaxy and the Triangulum Galaxy(just barely visible), the term ‘Milky Way’ only refers to this hazy band of light.

This hazy band of light, the Milky Way, has a relatively low level of brightness and so can be difficult to see if there is any light pollution(such as city lights) or moonlight in the sky. Clear dark moonless night skies in the country away from the city are some of the best places to observe the Milky Way. In order to be visible, the sky needs to be darker than a magnitude of 20.2 per square arcsecond. If the faintest celestial body that is visible has at least a magnitude of 5.1, the Milky Way should be visible; it will be easily visible if this limiting magnitude factor is around 6. Something important to note is that about one-third of the Earth’s population can not see the Milky Way because of light pollution!

Another important fact to know about observing the Milky Way is that the galactic plane is inclined about 60 degrees in relation to the plane of the Earths orbit, the ecliptic. This means that because of this high inclination the arc of the Milky Way can appear anywhere from relatively low in the night sky, close to the horizon, or relatively high, close to the zenith depending on the time of the night and year.

From the surface of the Earth, the Milky Way spans a wide swath of the night sky which includes 30 constellations. The Milky Way is brightest in the region of the constellation Sagittarius – it is in this direction that the galactic center of the Milky Way lies.

The Milky Way is the second largest galaxy in the Local Group(the Andromeda Galaxy is the largest), a cluster of over 54 galaxies – most of them dwarf sized, and has an estimated total mass of about 1.5 trillion times that of the Sun according to the most recent research results. For the sake of comparison with other galaxies, the smallest galaxies have a total mass of only about 1 billion times that of the Sun, while the largest ones can have a total mass of 30 trillion times that of the Sun.

It’s really very difficult to understand how large the Milky Way Galaxy is, but here is a comparison to put its gigantic size in perspective; let’s dramatically lower the scale of size and say that our Solar System out to Neptune is the size of a quarter – then the Milky Way would be the size of the United States from coast to coast!

As mentioned before, the Milky Way is about 150000 to 200000 light-years across with a range of 100 to 400 billion stars. There is also a filament of stars above and below the mostly flat galactic plane(there is a bulge in the central core) which encircle the Milky Way with a diameter of around 150-180000 light-years.

The observable mass of the Milky Way Galaxy consists of mostly stars and interstellar gas and dust – this interstellar gas is composed of a mass of roughly 90 percent hydrogen and 10 percent helium. The interstellar gas itself is believed to make up about 10 to 15 percent of the total observable mass of the galaxy, while the interstellar dust probably makes up about 1 percent of the total observable mass.

However, it is now believed that approximately 90 percent of the total mass of the Milky Way is in the form of dark matter, an unseen and largely undetectable(except through the observation of gravitational effects upon it) type of matter. It is not definitive that dark matter even exists since it is largely contingent upon the Standard Model of the Universe being correct, though at the present time the vast majority of scientists believe this to be the case. They conjecture that there may be a dark matter halo around the galaxy extending uniformly to a distance of 320000 to 330000 light-years.

This section basically describes the various objects inside the Milky Way Galaxy. When we are talking about these objects inside the galaxy we mean by default visible matter, not the mysterious and unseen dark matter which is now believed to constitute around 90 percent of the matter in the Milky Way Galaxy.

As mentioned previously there are somewhere in the range of 100 to 400 billion stars in the Milky Way galaxy. By comparison, the Andromeda Galaxy is estimated to have around one trillion stars. The exact number of stars in the Milky Way escapes us because of the facts that we are inside the galaxy so making measurements are very difficult to make and very low mass stars such as certain types of dwarf stars, neutron stars, and other types are very difficult if not impossible to detect. This includes billions of white dwarf stars and neutron stars and perhaps over one hundred million black holes.

Making observations and measurements in our local region of the galaxy, say within a thousand light-years or so, are one thing. But as distances increase across the void of interstellar space these more difficult objects become virtually impossible to detect with our current technology so we have to make some estimates based on our observations closer by, within a few thousand light-years.

The disk of stars in the Milky Way has no sharp boundary beyond which there are no stars, rather there is a gradual decrease in the number of stars as one moves away from the galactic center where the concentration of stars is the highest, probably because of the gravitational effects of a giant black hole at the center of the Milky Way Galaxy. An interesting fact is that beyond the distance of about 40000 light-years from the galactic core of the Milky Way the concentration of stars drops off much more rapidly, thus the decrease in the concentration of stars as the radius from the center of the galaxy increases is not linear.

Between the stars in the Milky Way is the existence of something called the interstellar medium. This is basically all the matter and radiation which fills in the space between the stars in the galaxy. The interstellar medium includes interstellar gas in ionic, atomic, and molecular form as well as dust, cosmic rays, and other types of electromagnetic radiation – this radiation between stars is called the interstellar radiation field. This all blends in smoothly in the space between the stars.

Current observations indicate that there may well be as many planets in the Milky Way as there are stars, giving an estimated range of 100 to 400 billion planets. Again, we are not able to get the exact number; these are our best estimates based on observations and measurements with our current state of technology.

Of this very large number of planets that we believe exist, recent observations indicate that there could be as many as 40 billion Earth-size planets orbiting in the habitable zone of stars, including over 10 billion around Sun-like stars and at least 30 billion around red dwarf stars. In fact, it is now thought that the number of these Earth-sized planets may even be more numerous than gas giant sized planets.

Huge numbers of so-called rogue planets may also exist. These are planets not bound to the gravitational field of any star system and are roaming free in the void of interstellar space. Without any heat from a parent star, their temperatures probably approach close to absolute zero. It is now thought that there could be as many of these rogue planets drifting deep in interstellar space as there are gravitationally bound planets in various star systems.

There are of course various other objects not included in the above categories such as exocomets, asteroids, meteors, and other debris.

The Milky Way is a large spiral-shaped galaxy with a bar-shaped galactic core area which is surrounded by a disk of stars along with interstellar gas and dust. It has four somewhat loosely bound spiral arms, the nature of which has not been completely determined. Enveloping the entire galaxy is a spherical halo of stars and star clusters called the galactic halo. This spherical halo extends out to the Large and Small Magellanic Clouds, two smaller satellite galaxies of the Milky Way – there are around 59 smaller galaxies within about 1.4 million light years of the Milky Way though not all of them are in orbit and are thus not all satellites. The next sections will go into more detail about the structure of the Milky Way.

As the name implies, the Milky Way has 4 galactic quadrants – it is divided into four circular areas called simply 1st galactic quadrant, 2nd galactic quadrant, 3rd galactic quadrant, and 4th galactic quadrant.

The galactic core of the Milky Way also called the galactic center, is the central region of the Milky Way which is characterized by a pseudobulge, a somewhat bulge shaped but not fully formed central area, transversed by a ‘bar’ structure of stars. This bulge has a radius of roughly 1630 light-years and is surrounded by a compact region of mostly older stars in a spherical shape out to a radius of about 10000 light-years.

Our Sun is approximately 25000 to 28000 light-years away from the galactic center. This galactic center would be the brightest feature of the Milky Way Galaxy if it were viewed from the Andromeda Galaxy.

The galactic bar structure transverses the central region, the galactic center, of the Milky Way Galaxy as already mentioned. But recent observations seem to indicate that there are actually two bars in this galactic center transversing the central bulge.

Galactic bar A is the primary and much more distinct bar in the galactic center, while galactic bar B is less distinct and somewhat difficult to discern – it is nestled within galactic bar A. Estimates of the radius of this bar structure ranges from 3000 to 16000 light-years; unfortunately precise measurements are not possible at this time. It also appears that this bar structure may be surrounded by a ring of stars about 16300 light-years in diameter, although again this is only an estimate.

For years there has been uncertainty about the spiral arm structure of the Milky Way caused in large part because we live inside the Milky Way Galaxy in a small and somewhat remote region and so cannot view it from a distance outside the galaxy. The Earth is located about two-thirds of the way out from the galactic center, and since the Milky Way is permeated with interstellar dust clouds, many stars are obscured making estimates even more difficult.

Thanks to NASA’s Wide-Field Infrared Survey Explorer(WISE) a new way of mapping our galaxy has shown that there are 4 major spiral arms in the Milky Way Galaxy. These 4 spiral arms are named Perseus and Scutum-Centaurus, the two more prominent arms with the highest concentration of stars, and Sagittarius and Outer, the two less prominent arms with a lower concentration of stars but with more interstellar gas. WISE has also confirmed a number of star clusters embedded within these arms.

All of the stars, interstellar gas, and other objects rotate around the center of the Milky Way Galaxy differentially. This means that objects in the Milky Way rotate at different velocities at different latitudes and depths which is characteristic of a nonsolid object – here nonsolid is in the context of the entire Milky Way as a whole.

The rotational period of the Milky Way is about 240 million years. Moving away from the central bulge the typical orbital speed of the various stars is roughly 470000 miles per hour, the orbital period of the typical star being directly proportional to the length of the path traveled. The orbital speeds do not decrease as one moves away from the galactic center as would be expected from Newtons Law of Universal Gravitation; instead, these speeds stay somewhat flat moving away from the galactic center towards the outer regions of the galactic disk.

The visible amount of mass in the Milky Way Galaxy is insufficient to be able to explain this discrepancy; this is some very strong evidence for the existence of some form of invisible matter – dark matter. In fact, to explain this sufficiently about 90 percent of the total matter in the Milky Way would have to be dark matter. It should be mentioned here that there are some scientists who believe that these orbital velocities could be explained with some modifications to the Law of Universal Gravitation.

The galactic halo is the extended and roughly spherical component of the Milky Way Galaxy which extends outward from the main body of the galaxy. There are three main components of the galactic halo as follows:

The stellar halo is the field of stars and globular clusters which surrounds the Milky Way Galaxy in a giant spherical shape. Only about one percent of the total mass of the Milky Way is located in the stellar halo, thus it has a much lower level of luminosity than the main body of the galaxy.

The galactic corona is simply the distribution of gas extending far away from the galactic core and which is detectable using X-ray spectroscopy.

The dark matter halo component of the galactic halo is a theoretical one which is hypothesized to exist throughout the galaxy and extending far beyond any of its visible components, the mass of which is far greater than the visible matter in the galaxy – roughly 90 percent of the total mass of the Milky Way is believed to be dark matter, assuming the Standard Model of the universe is correct.

Very little is currently known about dark matter since its only means of interaction with visible matter is through gravitational attraction. The hypothetical existence of dark matter is an intense area of research with important implications in cosmology as well as the formation and evolution of galaxies.

Sagittarius A is the name given to a powerful radio source at the center of the Milky Way Galaxy which is believed to be caused by a gigantic black hole over 4 million times as massive as the Sun. It is located in the constellation Sagittarius and is hidden from optical wavelength views because of large clouds of interstellar dust in the spiral arms of the galaxy.

The three components of Sagittarius A consist of Sagittarius A East – a supernova remnant, Sagittarius A West – the spiral structure, and a very powerful and compact radio source at the center of the spiral structure. The matter in the Milky Way Galaxy orbit around this central galactic core.

The Sun is close to the inner rim of the Orion Arm, a minor spiral arm of the Milky Way, in the Local Interstellar Cloud(the Local Fluff) of the Local Bubble(a void in the interstellar medium of the Orion Arm), which is in turn in the Gould Belt which is a partial ring of stars about 3000 light-years across – this all lies at a distance of about 26400 light-years from the galactic center.

This article would not be complete without pondering the possibility of some kind of extraterrestrial life in the Milky Way Galaxy, perhaps very highly advanced and more intelligent than ourselves. And the answer is definitely yes, there is advanced, intelligent life in the Milky Way Galaxy – the planet Earth near the Orion Arm, in the boondocks of the galaxy. But what of other intelligent life in the galaxy besides Earth?

We have not detected anything yet which could definitely be identified as any type of extraterrestrial life, although we are actively searching for it directly through programs like SETI – Search for Extraterrestrial Intelligence, and indirectly through our discovery of more and more Earth-like planets in the habitable zones of their parent stars.

At the present time, this search as not produced positive results, although it really seems just a matter of time until it does so. After all, it is now believed that there could be as many as 500 million habitable planets are in the Milky Way. The only thing more astonishing than finding extraterrestrial life would be coming to the conclusion that we are all alone in the Milky Way Galaxy and the Universe.

Of course, there are those who believe that highly intelligent life is actually visiting our planet, the Earth, but any kind of proof is at best inconclusive. We will leave finding any definitive evidence of extraterrestrial life to the scientists, the other discussion is best reserved for other articles.