How Many Galaxies Fill the Universe?

When you gaze up at the night sky, especially in the summer, you’ll notice a faint strip of stars stretching across the center of the sky. Our Sun is merely one of approximately 200 billion stars within the Milky Way, our home galaxy, which is just one among the many galaxies in the universe. So, the question remains: how many galaxies populate the universe?

This article will explore how galaxies were discovered, the different types that exist, their composition, internal structure, formation and evolution, distribution across the cosmos, and the extraordinary energy emissions from active galaxies.

What Defines a Galaxy?

A galaxy is a vast collection of stars, gas (primarily hydrogen), dust, and dark matter, all orbiting a shared center and bound by gravitational forces — you can think of them as "island universes."

Galaxies come in various shapes and sizes, each with its unique characteristics. They are extremely ancient, having formed early in the universe’s history. However, the exact processes behind their formation and evolution remain elusive.

By using powerful telescopes, astronomers peer deep into the universe and discover countless galaxies. These galaxies are separated by vast distances and are moving apart as the universe continues to expand.

In addition, galaxies are grouped into massive clusters and other structures, which may hold significant clues about the broader structure, formation, and ultimate fate of the universe.

Active Galaxies

Certain galaxies, known as active galaxies, radiate vast amounts of energy, often in the form of intense radiation. These galaxies might feature unusual structures like supermassive black holes at their core. Active galaxies are a key focus in astronomical studies.

The Luminosity-Distance Connection

Astronomers, whether professionals or enthusiasts, can gauge a star’s brightness (the amount of light it emits) using tools like a photometer or charge-coupled device attached to a telescope. By knowing the star’s brightness and its distance from Earth, they can compute its luminosity — the total energy it radiates (luminosity = brightness x 12.57­ x (distance)­).

Alternatively, if a star’s luminosity is known, its distance can be determined.

How Many Galaxies Exist in the Universe?

The universe might contain up to two trillion galaxies.

In the early 2000s, astronomers estimated there were around 200 billion galaxies in the universe. Yet, a 2016 study of Hubble Space Telescope data by the University of Nottingham revealed that the observable universe holds at least ten times that number [source: NASA].

In 2022, the James Webb Space Telescope captured "the deepest and clearest image of the distant universe ever taken" [source: Webb Space Telescope], significantly enhancing our ability to examine galaxies near and far.

Types of Galaxies

Galaxies exhibit a wide range of sizes and shapes. Some may contain as few as 10 million stars, while others boast up to 10 trillion stars (the Milky Way holds around 200 billion stars). In 1936, Edwin Hubble introduced the Hubble Sequence to classify galaxy shapes.

Elliptical Galaxy

These galaxies feature a subtle, rounded appearance, but lack gas and dust, and show no noticeable bright stars or spiral arms. They also do not possess galactic disks, which will be discussed below.

Elliptical galaxies are classified from E0 (circular) to E7 (most elliptical). It's estimated that about 60 percent of the galaxies in the universe are elliptical.

Galaxies vary greatly in size — while most are relatively small (about 1 percent the diameter of the Milky Way), some can be up to five times the Milky Way's diameter.

Spiral Galaxy

The Milky Way is considered one of the larger spiral galaxies. These galaxies are luminous and distinctly disk-shaped, containing hot gas, dust, and bright stars along their spiral arms. Because of their brightness, spiral galaxies make up the majority of visible galaxies, but they likely account for only about 20 percent of all galaxies in the universe.

Spiral galaxies are categorized into the following types:

Irregular Galaxy

These galaxies are small and faint, characterized by large clouds of gas and dust, but lack distinct spiral arms or bright centers. They contain a mix of older and younger stars and are typically small, ranging from 1 percent to 25 percent of the Milky Way's diameter.

Parts of a Galaxy

Spiral galaxies have the most intricate structures. Below is an illustration of how the Milky Way would look from an external perspective.

Galactic Disk

The Milky Way's more than 200 billion stars are primarily located within this region. The disk itself is divided into the following sections:

Globular Clusters

A few hundred globular clusters are dispersed above and below the galactic disk. The stars within these clusters are significantly older than those in the disk.

Halo

A halo is a vast, faint region that envelops the entire galaxy. It consists of hot gas and possibly dark matter.

Gravity

All the components of a galaxy orbit around the nucleus, and their movement is governed by gravity. Since gravity is dependent on mass, it might seem that most of a galaxy’s mass would be concentrated in the galactic disk or near its center.

However, by analyzing the rotation curves of the Milky Way and other galaxies, astronomers have determined that the majority of the galaxy's mass is actually found in the outer regions, such as the halo, where there is little light emitted by stars or gas clouds.

History of Galaxies

Let’s explore the history of how galaxies have been understood in the field of astronomy.

Early Observations

The ancient Greeks introduced the term "galaxies kuklos," meaning "milky circle," to describe the Milky Way. At the time, it appeared as a faint band of light, and its true composition was unknown.

When Galileo first observed the Milky Way through a telescope, he discovered that it was made up of countless stars.

For centuries, it has been known that our solar system resides within the Milky Way, as the Milky Way encircles us. We observe it in various parts of the sky throughout the year, with it being especially bright during the summer months when we are facing the galaxy's center. However, for astronomers in the 18th century and earlier, it wasn't obvious that the Milky Way was a galaxy and not just a collection of stars.

18th-century Findings

In the late 18th century, astronomers William and Caroline Herschel mapped the distances to stars in multiple directions. Their findings revealed that the Milky Way was a disk-like cloud of stars, with the Sun positioned near its center.

In 1781, Charles Messier cataloged various nebulae (faint patches of light) across the sky and identified several as spiral nebulae.

20th-century Discoveries

In the early 20th century, astronomer Harlow Shapely measured the distribution and positions of globular star clusters. He concluded that the Milky Way’s center is 28,000 light-years away from Earth, located near the constellations Sagittarius and Scorpio, and that the center is a bulge rather than a flat region.

Shapely later proposed that the spiral nebulae discovered by Messier were actually "island universes," or galaxies (keeping the Greek term). However, another astronomer, Heber Curtis, argued that these nebulae were simply part of the Milky Way.

The debate continued for years, as astronomers required larger, more advanced telescopes to resolve finer details.

21st-century Innovations

In 1924, Edwin Hubble resolved the debate. Using a massive 100-inch diameter telescope—much larger than the ones available to Shapely and Curtis—at Mount Wilson in California, he discovered that the spiral nebulae contained structures and stars known as Cepheid variables, similar to those in the Milky Way. (These stars exhibit periodic changes in brightness, with the luminosity directly linked to the length of their brightness cycle.)

Hubble utilized the light curves of Cepheid variables to calculate their distances from Earth, and he discovered that they were located much farther than the known limits of the Milky Way. As a result, these spiral nebulae were confirmed to be separate galaxies beyond our own.

While there remain numerous unanswered questions about how galaxies form, we will discuss some of the most well-accepted theories on the next page.

Light-years Away

Galaxies are widely spaced apart. The Andromeda galaxy, also known as M31 (Messier object #31), is the closest galaxy to us, situated 2.2 million light years away. Astronomers often express intergalactic distances in terms of megaparsecs:

The most distant observable galaxies are about 3,000 Mpc away, which is roughly 10 billion light years.

Galaxy Formation

We don't fully understand how galaxies formed and evolved into the diverse shapes we observe today. However, we have several theories about their origins and development over time.

Now, let's explore the era of galaxy formation.

Observations made by Edwin Hubble, and the Hubble Law derived from them, led to the conclusion that the universe is expanding. This expansion rate helps us estimate the age of the universe.

Since some galaxies are billions of light years away, we can infer that they began to form soon after the Big Bang (the further we look into space, the further back in time we peer).

While most galaxies formed early on, data from NASA's Galaxy Explorer (GALEX) telescope suggest that some galaxies have formed more recently — "recently" referring to the past few billion years, compared to the formation of early galaxies over 10 billion years ago.

Most theories about the early universe rest on two key assumptions:

Protogalactic Clouds

Based on these assumptions, astronomers propose that the denser regions slowed down the expansion just enough to allow gas to collect into small protogalactic clouds. Within these clouds, gravity caused the gas and dust to condense, leading to the formation of stars.

These stars burned out rapidly, eventually becoming globular clusters, but gravity continued to pull the clouds together. As the clouds collapsed further, they began to form rotating disks.

The rotating disks, drawn together by gravity, attracted more gas and dust, resulting in the creation of galactic disks. Inside these disks, new stars were born. What remained on the outer edges of the original cloud were globular clusters and the halo, consisting of gas, dust, and dark matter.

Two key factors from this process may explain the differences between elliptical and spiral galaxies:

When Galaxies Collide

Galaxies don't exist in isolation. While the distances between galaxies may appear vast, the sizes of galaxies themselves are also immense.

Relative to the vastness of stars, galaxies are quite close to each other. This proximity allows them to interact and, importantly, collide. However, during these collisions, the stars inside galaxies don't crash into each other due to the vast empty spaces between them.

Despite this, galaxy collisions can warp their shapes. Simulations indicate that when spiral galaxies collide, they often transform into elliptical galaxies (so, spiral galaxies likely haven't undergone many such collisions). Researchers suggest that up to half of all galaxies have experienced some type of collision in their history.

Gravitational forces at play during galaxy collisions can lead to several outcomes:

Do galaxies simply drift through space, or is there an unseen force that governs their movements? And what happens when they collide?

Galaxy Distribution

Galaxies are not scattered randomly across the cosmos; they tend to form groups called galactic clusters. The galaxies within these clusters are gravitationally bound and have a mutual influence on each other.

Astronomers Margaret Geller and Emilio E. Falco found that when they mapped the positions of galaxies and galactic clusters, it was evident that these clusters and superclusters are not randomly spread out across the universe.

In fact, they form clusters that are grouped in long filaments, separated by vast empty spaces, giving the universe a structure reminiscent of a cobweb.

The Intergalactic Medium

The intergalactic medium — the space between galaxies and galaxy clusters — is not entirely devoid of matter. While its precise composition remains unclear, it likely contains a sparse amount of gas.

The majority of the intergalactic medium is cold, at about 2 degrees Kelvin. However, X-ray observations have revealed that certain regions are much hotter, reaching millions of degrees Kelvin, and are abundant in metals.

A major area of ongoing research in astronomy is focused on understanding the intergalactic medium. This could provide crucial insights into the origins of the universe, as well as how galaxies come into being and evolve over time.

Hubble's Law

Let’s explore one final characteristic regarding galaxies and their distribution. For his measurements of galactic distances, Edwin Hubble examined the light spectra emitted by galaxies.

In every case, he observed that the spectra were Doppler-shifted toward the red end, indicating that these galaxies were moving away from us.

Hubble observed that no matter where he looked, galaxies appeared to be moving away from us. The further a galaxy was, the faster it seemed to recede. In 1929, he published a graph illustrating this relationship, which became known as Hubble's Law.

Hubble's Law can be described mathematically as stating that the speed at which a galaxy recedes (V) is directly proportional to its distance from us (d). This relationship is expressed as the equation V = Hd, where H represents the Hubble constant, or the constant of proportionality.

The most recent estimate of H is 70 kilometers per second per megaparsec. Hubble's Law is a key piece of evidence supporting the idea that the universe is expanding — his research laid the groundwork for the big bang theory regarding the universe's origin.

The Doppler Effect

Similar to how the pitch of a fire truck siren lowers as the truck moves away, the movement of stars alters the wavelengths of light we observe from them. This effect is known as the Doppler Effect.

The Doppler Effect can be measured by analyzing the spectral lines of a star and comparing them to those from a standard lamp. The amount of shift in these lines reveals the star's speed relative to us.

Additionally, the direction of the Doppler shift indicates the direction of the star's movement. If the star's spectrum shifts towards the blue end, it's moving toward us; if it shifts towards the red end, it is moving away from us.

Active Galaxies

In a typical galaxy, the majority of light emitted comes from the stars, appearing as visible light and evenly spread across the galaxy.

However, in certain galaxies, their nuclei emit extremely bright light. When observed in X-ray, ultraviolet, infrared, and radio wavelengths, these galaxies radiate vast amounts of energy, seemingly from their cores.

Active galaxies, which make up only a small fraction of all galaxies, fall into four distinct categories. However, the type we observe might be more dependent on our viewing angle than on actual structural variations:

Black Holes

To understand active galaxies, scientists must explain how these galaxies can release immense amounts of energy from such small areas in their centers. The most widely accepted theory is that a massive or supermassive black hole resides at the heart of each active galaxy.

Surrounding the black hole is an accretion disk of rapidly rotating gas, encircled by a torus — a donut-shaped ring of gas and dust. As the material from the accretion disk approaches the event horizon, it heats up to millions of degrees Kelvin and is expelled outward in powerful jets.

Seyfert Galaxies

First identified by Carl Seyfert in 1943, Seyfert galaxies (which account for 2 percent of all spiral galaxies) display broad spectral lines indicating the presence of hot, low-density ionized gas at their cores. The brightness of these galaxies' nuclei fluctuates over weeks, suggesting that the central objects are relatively compact, approximately the size of a solar system.

Through Doppler shifts, astronomers have observed that the velocities within the centers of Seyfert galaxies are roughly 30 times faster than those in regular galaxies.

Radio Galaxies

Radio galaxies, which make up only 0.01 percent of all galaxies, are elliptical in shape. Their centers emit powerful jets of high-velocity gas (close to the speed of light) that extend above and below the galaxy. These jets interact with magnetic fields, producing detectable radio emissions.

Quasars

Quasars, also referred to as quasi-stellar objects, were first identified in the early 1960s. Over 13,000 quasars have been discovered, though some estimates suggest there may be as many as 100,000. These highly energetic objects are located billions of light years away from the Milky Way.

The extraordinary brightness of quasars can change over a period of just one day, indicating that the energy is being emitted from an exceptionally small region. Thousands of quasars have been detected, and it is believed that they emanate from the centers of distant galaxies.

Blazars

Blazars are a category of active galaxies, with approximately 1,000 known to exist. From our vantage point, we are directly observing the jet of energy being expelled from the galaxy's nucleus. Much like quasars, the brightness of blazars can fluctuate rapidly, often within the span of a single day.

Starburst Galaxies

While most galaxies form stars at a slow rate of about one per year, starburst galaxies are prolific, producing over 100 new stars annually. With such a rapid pace, these galaxies deplete their gas and dust reserves in roughly 100 million years, which is brief when compared to the billions of years that most galaxies have existed.

The intense light emitted by starburst galaxies comes from a concentrated region of newly created stars and explosive supernovae. This has led astronomers to believe that starburst galaxies may represent a transient phase in galactic evolution, possibly occurring before a galaxy becomes an active galaxy.