What is the Milky Way? It’s our home galaxy

View at EarthSky Community Photos. | Amr Abdulwahab captured this image of the Milky Way on July 8, 2022. Amr wrote: “Sahara el Beyda, the White Desert Protected Area, is a national park in Egypt, first established as a protected area in 2002. It is located in the Farafra depression, 28 miles (45 km) north of the town of Qsar El Farafra. Part of the park is in the Farafra Oasis (New Valley Governorate). The park is the site of large white chalk rock formations, created through erosion by wind and sand.” Thank you, Amr!

Do you think of the Milky Way as a starry band across a dark night sky? Or do you think of it as a great spiral galaxy in space? Both are correct. Both refer to our home galaxy, our local island in the vast ocean of the universe, composed of hundreds of billions of stars, one of which is our sun.

Long ago, it was possible for everybody in the world to see a dark, star-strewn sky when they looked heavenward at night. In those ancient times, humans looked to the starry sky and saw a ghostly band of light arcing from horizon to horizon. This graceful arc of light moved across the sky with the seasons. The most casual sky-watchers could notice that darkness obscured parts of the band, which we now know to be vast clouds of dust.

Myths of the Milky Way

Myths and legends grew up in different cultures around this mysterious apparition in the heavens. Each culture explained this band of light in the sky according to its own beliefs. To the ancient Armenians, it was straw strewn across the sky by the god Vahagn. In eastern Asia, it was the Silvery River of Heaven. The Finns and Estonians saw it as the Pathway of the Birds.

Meanwhile, because ancient Greek and Roman legends and myths came to dominate western culture, it was their interpretations that were passed down to a majority of languages. Both the Greeks and the Romans saw the starry band as a river of milk. The Greek myth said it was milk from the breast of the goddess Hera, divine wife of Zeus. The Romans saw the river of light as milk from their goddess Ops.

Thus it was bequeathed the name by which, today, we know that ghostly arc stretching across the sky: the Milky Way.

View at EarthSky Community Photos. | William Mathe captured this image on August 15, 2020. William wrote: “I hiked up to the top of Rocky Mountain National Park in Colorado … just below 12,000 feet (3,700 m). Was greeted with a raging forest fire about 10 miles (16 km) to the west … hung around long enough to get a couple of snaps of the Milky Way. You can see the brown clouds of smoke hanging in the valley below the rock outcrop on which I was perched.” Thank you, William!

Observing the river of stars

When you are standing under a completely dark, starry sky, away from light pollution, the Milky Way appears like a cloud across the cosmos. But that cloud betrays no clue as to what it actually is. Until the invention of the telescope, no human could have known the nature of the Milky Way.

Just point even a small telescope anywhere along its length and you will be rewarded with a beautiful sight. What appears as a cloud to the unaided eye resolves into countless stars. Their distance and close relative proximity to each other do not permit us to pick them out individually with just our eyes.

It’s the same way a raincloud looks solid in the sky but actually consists of countless water droplets. The stars of the Milky Way merge together into a single band of light. But through a telescope, we see the Milky Way for what it truly is: a spiral arm of our galaxy.

What is the Milky Way?

Thus we arrive at the second answer to the question of what the Milky Way is. To astronomers, it is the name given to the entire galaxy we live in, not just the part of it we see in the sky. If this seems confusing, we must acknowledge the need for our galaxy to have a name.

Many other galaxies are designated by catalog numbers rather than names, for example the New General Catalogue. First published in 1888, it merely assigns a sequential number to each. More recent catalog numbers contain information of far more use to astronomers, for example, the galaxy’s location on the sky and during which survey it was discovered. Moreover, a galaxy may appear in more than one catalog and thus possess more than one designation. For example, the galaxy NGC 2470 is also known as 2MFGC 6271.

Other galaxies, particularly those brighter and closer, received names from astronomers of the 17th and 18th centuries. The names reflected their appearance: the Pinwheel, the Sombrero, the Sunflower, the Cartwheel, the cigar and so forth. These names came long before any systematic sky surveys with numerical labeling systems.

In time, the galaxies with descriptive labels received catalog numbers as well. Yet, our own galaxy does not appear in any index of galaxies. So, it needed a name for astronomers to refer to it by. Thus we call it the Milky Way instead of the galaxy or our galaxy. That name refers to both that river of light across the sky, which is part of our galaxy, and the galaxy as a whole. When not using the name, astronomers refer to it with a capital G (the Galaxy), and all other galaxies with a lowercase g.

Where is the sun in our galaxy?

Our solar system lies about 2/3 of the way out from the galactic center. We’re 26,000 light-years from the center, or 153,000 trillion miles (246,000 trillion km).

When we look toward the edge of the galaxy, we see the Orion-Cygnus Arm (or the Orion spur). The solar system is just on the inner edge of this spiral arm.

Or we can look toward the center of the galaxy, in the direction of Sagittarius. Vast clouds of dark gas hide the galactic center from us. Only in recent decades have astronomers pierced that dusty fog with infrared telescopes. A study of around 100 stars at the galactic center revealed that those giant clouds of dark dust were hiding a monster: a black hole. This black hole – known as Sagittarius A* – has a mass four million times that of our sun.

In this artist’s concept of the Milky Way, you can see the sun’s location below the central bar, at the inward side of the Orion Arm (called by its slightly dated name, the Orion Spur). The Orion Arm lies between the Sagittarius Arm and the Perseus Arm. Image via NASA/ JPL/ ESO/ R. Hurt/ Wikimedia Commons.

The stats on our galaxy

Our Milky Way galaxy is one of billions in the universe. We do not know exactly how many galaxies exist: a modern estimate vastly increases previous counts to as many as 2 trillion.

The Milky Way is approximately 100,000 light-years across, or 600,000 trillion miles (950,000 trillion km). We do not know its exact age, but we assume it came into being in the very early universe along with most other galaxies: within perhaps a billion years after the Big Bang. Estimates of how many stars live within the Milky Way vary quite considerably, but it seems to be somewhere between 100 billion and double that figure.

Why is there so much variance? Simply because it is so difficult to count the number of stars in the galaxy from our vantage point here on Earth. Imagine being in a banquet room full of people and trying to count everyone without being able to move around the room. From where you are standing, all you can do is make an estimate because people close to you block the view of those farther away. Neither can you see what size and shape the room is. The mass of people hides the edges of the room. It’s exactly the same from our position in the galaxy.

The Milky Way as seen in different wavelengths of light. The most familiar view is optical (or visible) light, which is the 3rd image from the bottom. In optical light, gas clouds darken our view of much of the galaxy. But look in the same direction in infrared light, and you can see through the clouds (4th, 5th and 6th image from the bottom). Read more about these images. Image via NASA.

Seeing the city of stars

It is this inability to see the structure of the Milky Way from our location inside it that meant for most of human history we did not even recognize that we live inside a galaxy in the first place. Indeed, we did not even realize what a galaxy is: a vast city of stars, separated from others by even vaster distances.

Without telescopes, we couldn’t see most of the other galaxies in the sky. The unaided eye can only see three of them: from the Northern Hemisphere we can see the Andromeda galaxy. Also known as M31, the Andromeda galaxy lies some two million light-years from us. In fact, it’s the farthest object we can see with our eyes alone, under dark skies. The skies in the Southern Hemisphere also have the Small and Large Magellanic Clouds, two amorphous dwarf galaxies orbiting our own. They are far larger and brighter in the sky than M31 simply because they are much closer to us.

The Large and Small Magellanic Clouds over Paranal in Chile. These are satellite galaxies of the Milky Way that you can only see from the Southern Hemisphere. Image via the European Southern Observatory.

Other galaxies in the universe

Until the 1910s, astronomers had not observationally confirmed the existence of other galaxies. Astronomers long believed that those fuzzy patches of light they saw through their telescopes were nebulae, vast clouds of gas and dust in our own galaxy.

But the concept of other galaxies was born earlier, in the early and mid-18th century. Swedish philosopher and scientist Emanuel Swedenborg and English astronomer Thomas Wright apparently conceived the idea independently of each other. Building upon the work of Wright, German philosopher Immanuel Kant referred to galaxies as island universes. The first observational evidence came in 1912 by American astronomer Vesto Slipher, who found that the spectra of the “nebulae” he measured were redshifted and thus much further away than astronomers previously thought.

Edwin Hubble and distant galaxies

And then came Edwin Hubble. Through years of painstaking work at the Mount Wilson Observatory in California, he confirmed in the 1920s that we do not live in a unique location. Our galaxy is just one of perhaps trillions.

Hubble came to this realization by studying a type of star known as a Cepheid variable, which pulsates with a regular periodicity. The intrinsic brightness of a Cepheid variable is directly related to its pulsation period: by measuring how long it takes for the star to brighten, fade and brighten again you can calculate how bright it is, that is to say, how much light it emits. Consequently, by observing how bright it appears from the Earth, you can calculate its distance.

It’s like seeing distant car headlights at night and estimating how far away the car is from how bright its lights appear. You can judge the distance of the car because you know all car headlights have about the same brightness.

An example of a Cepheid variable star is RS Puppis. It varies in brightness by almost a factor of 5 every 40 days. Image via NASA/ ESA/ Wikimedia Commons.

Cepheid variables in Andromeda

One of Edwin Hubble’s great achievements was the discovery of Cepheid variables in M31, the Andromeda galaxy. Hubble repeatedly photographed Andromeda with the Hooker Telescope. Eventually, he found stars that changed in brightness over a regular period. Performing the calculations, Hubble realized that M31 is not astronomically close to us at all. It’s 2 million light-years away, and it’s a galaxy like our own.

Hubble, for whom this discovery must have been a monumental shock, surmised that our galaxy was no different from M31 and the others he observed. Thus, he relegated us to a position of lesser importance in the universe. This was as big a revelation and diminution of our position in the universe. It was like when we learned that Earth is not the center of the universe.

We do not live in a special or privileged location. The universe does not have any vantage points which are superior to others. Wherever you are in the universe and you look up at the stars, you will see the same thing. Your constellations may be different, but no matter in which direction you look, you see galaxies rushing away from you in all directions as the universe expands, carrying the galaxies along with it.

Until the work by Slipher and Hubble (and others), we did not know the universe was expanding. It took a surprisingly long time for the astronomical community to accept this fact. Even Albert Einstein did not believe it, introducing an arbitrary correction into his calculations on relativity to achieve a static, non-expanding universe. However, Einstein later called this correction his greatest error.

The Milky Way from a distance

So, what does the Milky Way would look like from the outside? How many spiral arms there are? How big is the galaxy and how many stars does it hold? These were questions still unanswered in the 1920s. It took most of the 20th century after Hubble’s discoveries to piece together those answers through a combination of painstaking work with both Earth- and space-based telescopes.

So, if you could travel outside our galaxy, what would it look like? A standard analogy compares it to two fried eggs stuck together back-to-back. The yolk of the egg is known as the Galactic Bulge, a huge globe of stars at the center extending above and below the plane of the galaxy.

Astronomers now think the Milky Way has four spiral arms winding out from its center like the arms of a Catherine wheel. But these arms do not actually meet at the center. A few years ago astronomers discovered that the Milky Way is a barred spiral galaxy. This means a “bar” of stars runs across its center, and the spiral arms extend from either end. Barred spiral galaxies are not uncommon in the universe. But we do not yet understand how that central bar forms.

This Hubble image shows galaxy NGC 7773, an example of a barred spiral galaxy that may be similar to the Milky Way. Its bulge has a bar-shaped structure, extending to the inner parts of the galaxy’s spiral arms. Astronomers believe a bar in the center of a galaxy is a sign of galaxy maturity. Younger spiral galaxies do not feature barred central structures as often as older spirals do. Image via ESA/ Hubble/ NASA/ J. Walsh.

New discoveries in the Milky Way

Only a few years ago, astronomers made another major discovery. The Milky Way is not a flat disk of stars but has a kink running across it like an extended S. Something has warped the disk. At the moment, the finger points at the gravitational influence of the astronomically close Sagittarius dwarf galaxy. It’s one of perhaps twenty small galaxies that orbit the Milky Way, like moths around a flame. As the Sagittarius galaxy slowly orbits around us, its gravity has pulled on our galaxy’s stars, eventually creating the warp.

Other objects are also bound to the Milky Way. A halo of globular clusters surrounds our galaxy. Globular clusters are concentrations of stars that look like fuzzy golf balls. They contain perhaps a million or so extremely ancient stars.

Discoveries about the Milky Way continue. The study of its nature and origin is accelerating as new astronomical tools become available, such as the European Space Agency’s orbiting Gaia telescope. Gaia is making a three-dimensional map of our galaxy’s stars with exquisite and quite unprecedented accuracy. Read more about Gaia’s 3rd data release.

It’s an extremely exciting time for the study of our galaxy. It is all a far cry from when, thousands of years ago, our ancestors ascribed fantastic beasts and gods to that mysterious band of light they saw as they stood in awe under the starry sky.

Bottom line: Learn about our galaxy, the Milky Way. We discuss the origin of the name, its structure, and the history of how our knowledge has developed over the centuries and continues to develop today.

Milky Way Galaxy

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While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.

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Professor Emeritus, Department of Astronomy, University of Washington, Seattle. Author of The Andromeda Galaxy; Higher then Everest: An Adventurer’s Guide to the Solar System; and others.

Encyclopaedia Britannica’s editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.

Recent News

The Milky Way Galaxy takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth.

The first reliable measurement of the size of the Milky Way Galaxy was made in 1917 by American astronomer Harlow Shapley. Assuming that the globular clusters outlined the Galaxy, he determined that it has a diameter of about 100,000 light-years. His values have held up remarkably well over the years.

The solar system is about 30,000 light-years from the centre of the Milky Way Galaxy. The Galaxy itself is thought to be about 100,000 light-years in diameter.

The Milky Way Galaxy viewed at night from Tuolumne Meadows, Yosemite National Park, California. (more)

Milky Way Galaxy, large spiral system consisting of several hundred billion stars, one of which is the Sun. It takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth. Although Earth lies well within the Milky Way Galaxy (sometimes simply called the Galaxy), astronomers do not have as complete an understanding of its nature as they do of some external star systems. A thick layer of interstellar dust obscures much of the Galaxy from scrutiny by optical telescopes, and astronomers can determine its large-scale structure only with the aid of radio and infrared telescopes, which can detect the forms of radiation that penetrate the obscuring matter.

This article discusses the structure, properties, and component parts of the Milky Way Galaxy. For a full-length discussion of the cosmic universe of which the Galaxy is only a small part, see cosmology. For the star system within the Galaxy that is the home of Earth, see solar system.

Major components of the Galaxy

Star clusters and stellar associations

Although most stars in the Galaxy exist either as single stars like the Sun or as double stars, there are many conspicuous groups and clusters of stars that contain tens to thousands of members. These objects can be subdivided into three types: globular clusters, open clusters, and stellar associations. They differ primarily in age and in the number of member stars.

Globular clusters

Globular cluster M80 (also known as NGC 6093) in an optical image taken by the Hubble Space Telescope. M80 is located 28,000 light-years from Earth and contains hundreds of thousands of stars. (more)

The largest and most massive star clusters are the globular clusters, so called because of their roughly spherical appearance. The Galaxy contains more than 150 globular clusters (the exact number is uncertain because of obscuration by dust in the Milky Way band, which probably prevents some globular clusters from being seen). They are arranged in a nearly spherical halo around the Milky Way, with relatively few toward the galactic plane but a heavy concentration toward the centre. The radial distribution, when plotted as a function of distance from the galactic centre, fits a mathematical expression of a form identical to the one describing the star distribution in elliptical galaxies.

Globular clusters are extremely luminous objects. Their mean luminosity is the equivalent of approximately 25,000 Suns. The most luminous are 50 times brighter. The masses of globular clusters, measured by determining the dispersion in the velocities of individual stars, range from a few thousand to more than 1,000,000 solar masses. The clusters are very large, with diameters measuring from 10 to as much as 300 light-years. Most globular clusters are highly concentrated at their centres, having stellar distributions that resemble isothermal gas spheres with a cutoff that corresponds to the tidal effects of the Galaxy. A precise model of star distribution within a cluster can be derived from stellar dynamics, which takes into account the kinds of orbits that stars have in the cluster, encounters between these member stars, and the effects of exterior influences. The American astronomer Ivan R. King, for instance, derived dynamical models that fit observed stellar distributions very closely. He finds that a cluster’s structure can be described in terms of two numbers: (1) the core radius, which measures the degree of concentration at the centre, and (2) the tidal radius, which measures the cutoff of star densities at the edge of the cluster.

A key distinguishing feature of globular clusters in the Galaxy is their uniformly old age. Determined by comparing the stellar population of globular clusters with stellar evolutionary models, the ages of all those so far measured range from 11 billion to 13 billion years. They are the oldest objects in the Galaxy and so must have been among the first formed. That this was the case is also indicated by the fact that the globular clusters tend to have much smaller amounts of heavy elements than do the stars in the plane of the Galaxy, e.g., the Sun. Composed of stars belonging to the extreme Population II (see below Stars and stellar populations), as well as the high-latitude halo stars, these nearly spherical assemblages apparently formed before the material of the Galaxy flattened into the present thin disk. As their component stars evolved, they gave up some of their gas to interstellar space. This gas was enriched in the heavy elements (i.e., elements heavier than helium) produced in stars during the later stages of their evolution, so that the interstellar gas in the Galaxy is continually being changed. Hydrogen and helium have always been the major constituents, but heavy elements have gradually grown in importance. The present interstellar gas contains elements heavier than helium at a level of about 2 percent by mass, while the globular clusters contain as little as 0.02 percent of the same elements.

Open clusters

Clusters smaller and less massive than the globular clusters are found in the plane of the Galaxy intermixed with the majority of the system’s stars, including the Sun. These objects are the open clusters, so called because they generally have a more open, loose appearance than typical globular clusters.

Open clusters are distributed in the Galaxy very similarly to young stars. They are highly concentrated along the plane of the Galaxy and slowly decrease in number outward from its centre. The large-scale distribution of these clusters cannot be learned directly because their existence in the Milky Way plane means that dust obscures those that are more than a few thousand light-years from the Sun. By analogy with open clusters in external galaxies similar to the Galaxy, it is surmised that they follow the general distribution of integrated light in the Galaxy, except that there are probably fewer of them in the central areas. There is some evidence that the younger open clusters are more densely concentrated in the Galaxy’s spiral arms, at least in the neighbourhood of the Sun where these arms can be discerned.

The brightest open clusters are considerably fainter than the brightest globular clusters. The peak absolute luminosity appears to be about 50,000 times the luminosity of the Sun, but the largest percentage of known open clusters has a brightness equivalent to 500 solar luminosities. Masses can be determined from the dispersion in the measured velocities of individual stellar members of clusters. Most open clusters have small masses on the order of 50 solar masses. Their total populations of stars are small, ranging from tens to a few thousand.

Open clusters have diameters of only 2 or 3 to about 20 light-years, with the majority being less than 5 light-years across. In structure they look very different from globular clusters, though they can be understood in terms of similar dynamical models. The most important structural difference is their small total mass and relative looseness, which result from their comparatively large core radii. These two features have disastrous consequences as far as their ultimate fate is concerned, because open clusters are not sufficiently gravitationally bound to be able to withstand the disruptive tidal effects in the Galaxy (see star cluster: Open clusters). Judging from the sample of open clusters within 3,000 light-years of the Sun, only half of them can withstand such tidal forces for more than 200 million years, and a mere 2 percent have life expectancies as high as 1 billion years.

Measured ages of open clusters agree with the conclusions that have been reached about their life expectancies. They tend to be young objects; only a few are known to exceed 1 billion years in age. Most are younger than 200 million years, and some are 1 or 2 million years old. Ages of open clusters are determined by comparing their stellar membership with theoretical models of stellar evolution. Because all the stars in a cluster have very nearly the same age and chemical composition, the differences between the member stars are entirely the result of their different masses. As time progresses after the formation of a cluster, the massive stars, which evolve the fastest, gradually disappear from the cluster, becoming white dwarf stars or other underluminous stellar remnants. Theoretical models of clusters show how this effect changes the stellar content with time, and direct comparisons with real clusters give reliable ages for them. To make this comparison, astronomers use a diagram (the colour-magnitude diagram) that plots the temperatures of the stars against their luminosities. Colour-magnitude diagrams have been obtained for more than 1,000 open clusters, and ages are thus known for this large sample.

Because open clusters are mostly young objects, they have chemical compositions that correspond to the enriched environment from which they formed. Most of them are like the Sun in their abundance of the heavy elements, and some are even richer. For instance, the Hyades, which compose one of the nearest clusters, have almost twice the abundance of heavy elements as the Sun. It became possible in the 1990s to discover very young open clusters that previously had been entirely hidden in deep, dusty regions. Using infrared array detectors, astronomers found that many molecular clouds contained very young groups of stars that had just formed and, in some cases, were still forming.

Stellar associations

Even younger than open clusters, stellar associations are very loose groupings of young stars that share a common place and time of origin but that are not generally tied closely enough together gravitationally to form a stable cluster. Stellar associations are limited strictly to the plane of the Galaxy and appear only in regions of the system where star formation is occurring, notably in the spiral arms. They are very luminous objects. The brightest are even brighter than the brightest globular clusters, but this is not because they contain more stars; instead it is the result of the fact that their constituent stars are very much brighter than the stars constituting globular clusters. The most luminous stars in stellar associations are very young stars of spectral types O and B. They have absolute luminosities as bright as any star in the Galaxy—on the order of one million times the luminosity of the Sun. Such stars have very short lifetimes, only lasting a few million years. With luminous stars of this type there need not be very many to make up a highly luminous and conspicuous grouping. The total masses of stellar associations amount to only a few hundred solar masses, with the population of stars being in the hundreds or, in a few cases, thousands.

The sizes of stellar associations are large; the average diameter of those in the Galaxy is about 250 light-years. They are so large and loosely structured that their self-gravitation is insufficient to hold them together, and in a matter of a few million years the members disperse into surrounding space, becoming separate and unconnected stars in the galactic field.

Moving groups

Bright nebulosity in the Pleiades (M45, NGC 1432), distance 490 light-years. Cluster stars provide the light, and surrounding clouds of dust reflect and scatter the rays from the stars. (more)

These objects are organizations of stars that share common measurable motions. Sometimes these do not form a noticeable cluster. This definition allows the term to be applied to a range of objects from the nearest gravitationally bound clusters to groups of widely spread stars with no apparent gravitational identity, which are discovered only by searching the catalogs for stars of common motion. Among the best known of the moving groups is the Hyades in the constellation Taurus. Also known as the Taurus moving cluster or the Taurus stream, this system comprises the relatively dense Hyades cluster along with a few very distant members. It contains a total of about 350 stars, including several white dwarfs. Its centre lies about 150 light-years away. Other notable moving stellar groups include the Ursa Major, Scorpius-Centaurus, and Pleiades groups. Besides these remote organizations, investigators have observed what appear to be groups of high-velocity stars near the Sun. One of these, called the Groombridge 1830 group, consists of a number of subdwarfs and the star RR Lyrae, after which the RR Lyrae variables were named.

Recent advances in the study of moving groups have had an impact on the investigation of the kinematic history of stars and on the absolute calibration of the distance scale of the Galaxy. Moving groups have proved particularly useful with respect to the latter because their commonality of motion enables astronomers to determine accurately (for the nearer examples) the distance of each individual member. Together with nearby parallax stars, moving-group parallaxes provide the basis for the galactic distance scale. Astronomers have found the Hyades moving cluster well suited for their purpose: it is close enough to permit the reliable application of the method, and it has enough members for deducing an accurate age.

One of the basic problems of using moving groups for distance determination is the selection of members. In the case of the Hyades, this has been done very carefully but not without considerable dispute. The members of a moving group (and its actual existence) are established by the degree to which their motions define a common convergent point in the sky. One technique is to determine the coordinates of the poles of the great circles defined by the proper motions and positions of individual stars. The positions of the poles will define a great circle, and one of its poles will be the convergent point for the moving group. Membership of stars can be established by criteria applied to the distances of proper-motion poles of individual stars from the mean great circle. The reliability of the existence of the group itself can be measured by the dispersion of the great circle points about their mean.

As radial velocities will not have been used for the preliminary selection of members, they can be subsequently examined to eliminate further nonmembers. The final list of members should contain only a very few nonmembers—either those that appear to agree with the group motion because of observational errors or those that happen to share the group’s motion at the present time but are not related to the group historically.

The distances of individual stars in a moving group may be determined if their radial velocities and proper motions are known (see below Stellar motions) and if the exact position of the radiant is determined. If the angular distance of a star from the radiant is λ and if the velocity of the cluster as a whole with respect to the Sun is V, then the radial velocity of the star, V_r, is V_r= V cos λ. The transverse (or tangential) velocity, T, is given by T = V sin λ = 4.74 μ/p where p is the star’s parallax in arc seconds. Thus, the parallax of a star is given by p = 4.74 μ cot λ/V_r.

The key to achieving reliable distances by this method is to locate the convergent point of the group as accurately as possible. The various techniques used (e.g., Charlier’s method) are capable of high accuracy, provided that the measurements themselves are free of systematic errors. For the Taurus moving group, for example, it has been estimated that the accuracy for the best-observed stars is on the order of 3 percent in the parallax, discounting any errors due to systematic problems in the proper motions. Accuracies of this order were not possible by other means until the space-based telescope Hipparcos was able to measure highly precise stellar parallaxes for thousands of individual stars.

Emission nebulae

Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula’s light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope. (more)

A conspicuous component of the Galaxy is the collection of large, bright, diffuse gaseous objects generally called nebulae. The brightest of these cloudlike objects are the emission nebulae, large complexes of interstellar gas and stars in which the gas exists in an ionized and excited state (with the electrons of the atoms excited to a higher than normal energy level). This condition is produced by the strong ultraviolet light emitted from the very luminous, hot stars embedded in the gas. Because emission nebulae consist almost entirely of ionized hydrogen, they are usually referred to as H II regions.

H II regions are found in the plane of the Galaxy intermixed with young stars, stellar associations, and the youngest of the open clusters. They are areas where very massive stars have recently formed, and many contain the uncondensed gas, dust, and molecular complexes commonly associated with ongoing star formation. The H II regions are concentrated in the spiral arms of the Galaxy, though some exist between the arms. Many of them are found at intermediate distances from the centre of the Milky Way Galaxy, with the largest number occurring at a distance of 10,000 light-years. This latter fact can be ascertained even though the H II regions cannot be seen clearly beyond a few thousand light-years from the Sun. They emit radio radiation of a characteristic type, with a thermal spectrum that indicates that their temperatures are about 10,000 kelvins. This thermal radio radiation enables astronomers to map the distribution of H II regions in distant parts of the Galaxy.

The largest and brightest H II regions in the Galaxy rival the brightest star clusters in total luminosity. Even though most of the visible radiation is concentrated in a few discrete emission lines, the total apparent brightness of the brightest is the equivalent of tens of thousands of solar luminosities. These H II regions are also remarkable in size, having diameters of about 1,000 light-years. More typically, common H II regions such as the Orion Nebula are about 50 light-years across. They contain gas that has a total mass ranging from one or two solar masses up to several thousand. H II regions consist primarily of hydrogen, but they also contain measurable amounts of other gases. Helium is second in abundance, and large amounts of carbon, nitrogen, and oxygen occur as well. Preliminary evidence indicates that the ratio of the abundance of the heavier elements among the detected gases to hydrogen decreases outward from the centre of the Galaxy, a tendency that has been observed in other spiral galaxies.

Planetary nebulae

Composite picture of the Cat’s Eye Nebula (NGC 6543), combining three images taken by the Hubble Space Telescope. This planetary nebula has an unusually complicated structure, with concentric shells (seen as bright rings), jets (the projections at upper left and lower right), and a number of details that suggest complex interactions of shock waves. (more)

The gaseous clouds known as planetary nebulae are only superficially similar to other types of nebulae. So called because the smaller varieties almost resemble planetary disks when viewed through a telescope, planetary nebulae represent a stage at the end of the stellar life cycle rather than one at the beginning. The distribution of such nebulae in the Galaxy is different from that of H II regions. Planetary nebulae belong to an intermediate population and are found throughout the disk and the inner halo. There are more than 1,000 known planetary nebulae in the Galaxy, but more might be overlooked because of obscuration in the Milky Way region.

Supernova remnants

The Crab Nebula, which was formed by a supernova explosion recorded in 1054. This image was made by combining two dozen exposures from the Hubble Space Telescope. (more)

Another type of nebulous object found in the Galaxy is the remnant of the gas blown out from an exploding star that forms a supernova. Occasionally these objects look something like planetary nebulae, as in the case of the Crab Nebula, but they differ from the latter in three ways: (1) the total mass of their gas (they involve a larger mass, essentially all the mass of the exploding star), (2) their kinematics (they are expanding with higher velocities), and (3) their lifetimes (they last for a shorter time as visible nebulae). The best-known supernova remnants are those resulting from three historically observed supernovae: that of 1054, which made the Crab Nebula its remnant; that of 1572, called Tycho’s Nova; and that of 1604, called Kepler’s Nova. These objects and the many others like them in the Galaxy are detected at radio wavelengths. They release radio energy in a nearly flat spectrum because of the emission of radiation by charged particles moving spirally at nearly the speed of light in a magnetic field enmeshed in the gaseous remnant. Radiation generated in this way is called synchrotron radiation and is associated with various types of violent cosmic phenomena besides supernova remnants, as, for example, radio galaxies.

Dust clouds

The Eagle Nebula. Stars are forming in this column of cold dust and gas, which is 9.5 light-years in length. (more)

NGC 4013, a spiral galaxy, which has a prominent dust lane like the Milky Way Galaxy, in an image taken by the Hubble Space Telescope. (more)

The dust clouds of the Galaxy are narrowly limited to the plane of the Milky Way, though very low-density dust can be detected even near the galactic poles. Dust clouds beyond 2,000 to 3,000 light-years from the Sun cannot be detected optically, because intervening clouds of dust and the general dust layer obscure more distant views. Based on the distribution of dust clouds in other galaxies, it can be concluded that they are often most conspicuous within the spiral arms, especially along the inner edge of well-defined ones. The best-observed dust clouds near the Sun have masses of several hundred solar masses and sizes ranging from a maximum of about 200 light-years to a fraction of a light-year. The smallest tend to be the densest, possibly partly because of evolution: as a dust complex contracts, it also becomes denser and more opaque. The very smallest dust clouds are the so-called Bok globules, named after the Dutch American astronomer Bart J. Bok; these objects are about one light-year across and have masses of 1–20 solar masses.

More complete information on the dust in the Galaxy comes from infrared observations. While optical instruments can detect the dust when it obscures more distant objects or when it is illuminated by very nearby stars, infrared telescopes are able to register the long-wavelength radiation that the cool dust clouds themselves emit. A complete survey of the sky at infrared wavelengths made during the early 1980s by an unmanned orbiting observatory, the Infrared Astronomical Satellite (IRAS), revealed a large number of dense dust clouds in the Milky Way. Twenty years later the Spitzer Space Telescope, with greater sensitivity, greater wavelength coverage, and better resolution, mapped many dust complexes in the Milky Way. In some it was possible to view massive star clusters still in the process of formation.

Thick clouds of dust in the Milky Way can be studied by still another means. Many such objects contain detectable amounts of molecules that emit radio radiation at wavelengths that allow them to be identified and analyzed. More than 50 different molecules, including carbon monoxide and formaldehyde, and radicals have been detected in dust clouds.

The general interstellar medium

Central regions of the Milky Way Galaxy. The image on the left is in visible light, and the image on the right is in infrared; the marked difference between the two images shows how infrared radiation can penetrate galactic dust. The infrared image is part of the Two Micron All Sky Survey (2MASS), a survey of the entire sky in infrared light. (more)

The stars in the Galaxy, especially along the Milky Way, reveal the presence of a general, all-pervasive interstellar medium by the way in which they gradually fade with distance. This occurs primarily because of interstellar dust, which obscures and reddens starlight. On the average, stars near the Sun are dimmed by a factor of two for every 3,000 light-years. Thus, a star that is 6,000 light-years away in the plane of the Galaxy will appear four times fainter than it would otherwise were it not for the interstellar dust.

Another way in which the effects of interstellar dust become apparent is through the polarization of background starlight. Dust is aligned in space to some extent, and this results in selective absorption such that there is a preferred plane of vibration for the light waves. The electric vectors tend to lie preferentially along the galactic plane, though there are areas where the distribution is more complicated. It is likely that the polarization arises because the dust grains are partially aligned by the galactic magnetic field. If the dust grains are paramagnetic so that they act somewhat like a magnet, then the general magnetic field, though very weak, can in time line up the grains with their short axes in the direction of the field. As a consequence, the directions of polarization for stars in different parts of the sky make it possible to plot the direction of the magnetic field in the Milky Way.

The dust is accompanied by gas, which is thinly dispersed among the stars, filling the space between them. This interstellar gas consists mostly of hydrogen in its neutral form. Radio telescopes can detect neutral hydrogen because it emits radiation at a wavelength of 21 cm. Such radio wavelength is long enough to penetrate interstellar dust and so can be detected from all parts of the Galaxy. Most of what astronomers have learned about the large-scale structure and motions of the Galaxy has been derived from the radio waves of interstellar neutral hydrogen. The distance to the gas detected is not easily determined. Statistical arguments must be used in many cases, but the velocities of the gas, when compared with the velocities found for stars and those anticipated on the basis of the dynamics of the Galaxy, provide useful clues as to the location of the different sources of hydrogen radio emission. Near the Sun the average density of interstellar gas is 10 −21 gm/cm 3 , which is the equivalent of about one hydrogen atom per cubic centimetre.

Even before they first detected the emission from neutral hydrogen in 1951, astronomers were aware of interstellar gas. Minor components of the gas, such as sodium and calcium, absorb light at specific wavelengths, and they thus cause the appearance of absorption lines in the spectra of the stars that lie beyond the gas. Since the lines originating from stars are usually different, it is possible to distinguish the lines of the interstellar gas and to measure both the density and velocity of the gas. Frequently it is even possible to observe the effects of several concentrations of interstellar gas between Earth and the background stars and thereby determine the kinematics of the gas in different parts of the Galaxy.

Companion galaxies

Hear about the prediction of the Milky Way colliding with the Andromeda galaxy, which might happen in about four billion years

An overview of the predicted collision of the Andromeda and Milky Way galaxies, expected to occur in some four billion years. (more)

Most of the globular cluster NGC 1850 consists of yellow stars; the bright white stars are members of a second, open cluster about 200 light-years beyond NGC 1850. This picture is a composite of images taken by the Hubble Space Telescope. (more)

The Magellanic Clouds were recognized early in the 20th century as companion objects to the Galaxy. When American astronomer Edwin Hubble established the extragalactic nature of what we now call galaxies, it became plain that the Clouds had to be separate systems, both of the irregular class and more than 100,000 light-years distant. (The current best values for their distances are 163,000 and 202,000 light-years for the Large and Small Clouds, respectively.) Additional close companions have been found, all of them small and inconspicuous objects of the dwarf elliptical class. The nearest of these is the Sagittarius dwarf, a galaxy that is falling into the Milky Way Galaxy, having been captured tidally by the Galaxy’s much stronger gravity. The core of this galaxy is about 90,000 light-years distant. Other close companions are the well-studied Carina, Draco, Fornax, Leo I, Leo II, Sextans, Sculptor, and Ursa Minor galaxies, as well as several very faint, less well-known objects. Distances for them range from approximately 200,000 to 800,000 light-years. The grouping of these galaxies around the Milky Way Galaxy is mimicked in the case of the Andromeda Galaxy, which is also accompanied by several dwarf companions.

Milky Way Galaxy: All You Need To Know

In this article, we gathered answers to the most popular questions about the Milky Way. Keep reading, and you’ll learn what it is, where is our place in the galaxy, and when is the best time to view the Milky Way.

Contents

• What is the Milky Way?
• The Size of the Milky Way
• Why is it called the Milky Way?
• What type of galaxy is the Milky Way?
• Where is the Earth in the Milky Way?
• What is at the center of the Milky Way?
• How do we know what the Milky way looks like?
• How to see the Milky Way?
• F.A.Q.
  • How many stars are in the Milky Way?
  • How many planets are in the Milky Way?
  • How many Solar Systems are in the Milky Way?
  • How many constellations are in the Milky Way?

  What is the Milky Way?

  The Milky Way galaxy is a huge collection of dust, gas, and stars, including our Sun. The Earth is located inside this galaxy, so it is often called “our home galaxy” or simply “our galaxy.”

  It might be hard to believe, but that starry band across the night sky that we can see from the Earth is actually a huge galaxy that extends billions of kilometers around our planet. How big is it? Let’s find out.

  The Size of the Milky Way

  The Milky Way is the second-largest galaxy in the Local Group of galaxies; the first place goes to Andromeda. The Milky Way is 105,700 light-years wide while the Andromeda Galaxy is 220,000 light-years in width. By the way, the Local Group — a group of multiple galaxies including the Milky Way — extends for roughly 10 million light-years around us in space.

  Why is it called the Milky Way?

  The name of our home galaxy, like the names of many other astronomy objects, came from the ancient Greek and Roman cultures. Both the Greeks and Romans saw the starry band as the river of milk. The Greeks believed that it was milk from the goddess Hera who spilled it across the sky, and the Romans myth said that the Milky Way was milk from their goddess Ops.

  Other cultures had their own myths and beliefs regarding the starry band of light in the night sky. People in eastern Asia called it the Silvery River of Heaven; the Finns and Estonians believed it was the Pathway of the Birds; in Southern Africa, it’s called the Backbone of Night.

  What type of galaxy is the Milky Way?

  There are four main types of galaxies: spiral, elliptical, peculiar, and irregular. The spiral-shaped Milky Way belongs to the first type; if you could see it from the top (or the bottom), it would look like a spinning pinwheel.

  To be more specific, the Milky Way is a barred spiral galaxy, which means it has a central bar-shaped straight structure composed of stars. This bar contains the galaxy’s nucleus in the center and has two spiral arms attached to its ends. If the Milky Way was a normal spiral galaxy, its arms would lead right to its center (or nucleus) like in the Andromeda Galaxy.

  In total, the Milky Way has four known arms — two major connected with the bar (Scutum-Centaurus and Perseus) and two minor (Norma and Sagittarius) located between them. Previously scientists thought that all of these arms were major, but with the help of infrared images from NASA’s Spitzer Space Telescope, they found otherwise.

  Where is the Earth in the Milky Way?

  Speaking about our location inside the Milky Way, we’re far away from its center, which is good news (unless you’ve always wanted to neighbor a huge black hole). Our Sun is located nearly 27,000 light-years from the Milky Way’s nucleus, or about halfway between its center and the edge.

  Our Solar System is placed between two main arms — Scutum-Centaurus and Perseus, within the small partial arm named the Orion Arm or Orion Spur. This arm is about 3,500 light-years wide and more than 20,000 light-years long. It got its name after the constellation Orion. Our location inside it is the reason why we see so many bright objects within the constellation Orion — we’re simply looking at our local spiral arm.

  What is at the center of the Milky Way?

  The center region of the Milky Way is called the Galactic Center, and it contains a supermassive black hole of about 4 million solar masses called Sagittarius A*. To see the black hole, you’ll need a special radio telescope.

  A casual observer can view the Galactic Center, which is very bright despite its enormous distance from the Earth (27,000 light-years). However, its brightness is easy to explain — there are around 10 million stars within one parsec of the Galactic Center.

  How do we know what the Milky way looks like?

  From our position inside the Milky Way, it’s quite hard to figure out its shape. We don’t have pictures of our galaxy from the side as we can’t actually leave it for now. However, we have several clues that helped to figure out what it looks like:

  1. Astronomers observe the other galaxies and compare them with the behavior of the one we live in. For example, when they measured the velocities of stars and gas in the Milky Way, they saw that an overall rotational motion differs from random motions. This is a characteristic of a spiral galaxy.
  2. As the Milky Way appears to us as the long stripe across the sky, it means its shape is more likely a disk we see edge-on. We also can find the bulge at the center, and from observing the other galaxies, we know that the spiral ones are disks with central bulges.
  3. The gas fraction, color, and dust content of our Milky Way are like in the other spiral galaxies.

  How to see the Milky Way?

  The good news is the Milky Way is visible all year round, no matter where you are on the Earth. However, as our planet rotates, the galaxy also moves across the sky, and so does its core — the Galactic Center — the brightest and most spectacular part. And sometimes, the core disappears from our view.

  Here are things you need to know to get the best of the Milky Way and the Galactic Center:

  • The Galactic Center is located in the constellation Sagittarius and like the constellation, it can be visible only from latitudes between +55º and -90º. If you live above +55º latitude, you won’t see the Galactic Center! You’ll catch only part of the core, and the best time is before and after summer.
  • From the Northern Hemisphere, the Galactic Center is visible from March to October.
  • From the Southern Hemisphere, the core is visible from February to October.
  • The Milky Way’s core isn’t visible for the rest of the months around the world because, during this time, it’s located too close to the Sun.
  • From the southern latitudes, the observation conditions are better as the peak of visibility there happens in winter when the nights are longer and darker.
  • At the beginning of its visibility season, the Galactic Center can be seen shortly before sunrise. Over time, it becomes visible for a longer period each night and reaches its peak in June-July. During these months, the core is visible all night long.
  • You need a truly dark place free of light pollution. These tools will help you to find such a place: NASA’s Blue Marble, International Dark Sky locations, Dark Site Finder. Or find the closest observatory — they’re always located in dark sites.
  • The skies should be cloudless and clear. You can use an astronomy app with a stargazing forecast that indicates observational conditions. For example, Sky Tonight — it’s free and works without an internet connection.
  • The Moon phase is vital. A new Moon is ideal, as it doesn’t interfere with observations.
  • If you plan to photograph the Milky Way and its core, use tools to visualize the galaxy’s position in the sky over time. Our advice is the Ephemeris app, which predicts Milky Way visibility, its core’s exact position, and more. Ephemeris also helps to quickly find and check the detailed information about the Sun, the Moon, and the Milky Way for any date, time, and place.

  F.A.Q.

  How many stars are in the Milky Way?

  It’s difficult to give an exact number, but there are at least 100 billion stars in the Milky Way. Scientists’ current estimate is between 100 to 400 billion stars.

  How many planets are in the Milky Way?

  Scientists consider that there are at least 100 billion planets in the Milky Way, and more than 10 billion of them are terrestrial.

  How many Solar Systems are in the Milky Way?

  Well, there is only one Solar System in our galaxy, as only ours is officially called so. But astronomers have found more than 3,200 other stars with planets orbiting them in the Milky Way.

  How many constellations are in the Milky Way?

  As seen from the Earth, the Milky Way occupies the sky area that includes 30 constellations. The brightest part of our galaxy, the Galactic Center, lies in the constellation Sagittarius.

  Hopefully, in this article, we answered all the major questions about the Milky Way. Don’t hesitate to ask us any questions on social media and share your Milky Way observation experience.

  We wish you clear skies and happy observations!