Galaxies contain gas, dust, dark matter, and stars. So many stars. And gravity binds all of it together. Supermassive black holes sit at many galactic centers, while others are gigantic stellar nurseries. Galaxies collide with one another, creating new stars and exoplanets while they provide the building blocks of our universe.
Although billions of galaxies exist in our universe, you can only see three (beyond the Milky Way) without a telescope. The Large and Small Magellanic Clouds are visible in the Southern Hemisphere, and the Andromeda Galaxy is in the Northern. And all are visible to your naked eye with dark skies. Interestingly, the Magellanic Clouds got their names from Ferdinand Magellan, who used them to navigate.
Let’s delve into facts about galaxies, these fascinating cosmic islands that capture our imaginations with their mystery and beauty.
Table of Contents
Types of Galaxies
Three major galaxy classifications exist, according to astronomers. They are spiral, elliptical, and irregular galaxies. Some are vast, while others are dwarf. Some contain 100 million stars, while others host a trillion.
Beyond these three major categories, scientists identify other galaxies with unusual shapes in various stages of development. And we’ll look at some facts about unusual galaxies too. But first, the main types.
Facts About Spiral Galaxies
You’ll find Earth in a spiral galaxy. We’re in the Milky Way, around 26,000 light years from the galactic center. Our solar system resides on the Orion-Cygnus Arm’s inner edge between the Perseus and Sagittarius Arms.
Here are some facts about spiral galaxies.
- They have yellowish center bulges with blue-white cosmic discs of gas, dust, and stars.
- They actively form stars.
- Most named galaxies in the universe are spirals.
- Normal spirals have arms that start from the central bulge.
- Barred spirals get their name from the bar of stars that pass through the central bulge.
- Their arms start at the bar’s ends rather than from the bulge.
Facts About Elliptical Galaxies
Spiral galaxies have swirling arms, but ellipticals are more rounded, like a circle stretching out. Some are completely round, while others have an elongated egg shape.
Cygnus A is likely the most famous elliptical galaxy. It first appeared in Carl Sagan’s science fiction novel “Contact” in 1985. About ten years later, Hollywood turned the book into a film of the same name starring Jodi Foster and Matthew McConaughey. An instant space movie hit!
- About a third of all named galaxies are ellipticals. More exist, maybe up to 60% of all galaxies, but they are fainter than spirals and harder to spot. So fewer are identified, confirmed, and named.
- They don’t have as much gas and dust as spirals.
- Ellipticals host older stars and no longer form new stars.
- Rare giant ellipticals can stretch across space for about 300,000 light-years.
- They likely form when smaller galaxies collide and merge.
- Dwarf ellipticals are more common and only a few thousand light-years across.
Facts About Irregular Galaxies
A catch-all name for a catch-all category, irregular galaxies are neither disc-like with arms nor elliptical. Scientists think they are older than their more easily classified counterparts. Irregulars appear abundantly in the early universe.
The Small and Large Magellanic Clouds (SMC and LMC) are the best-known examples of irregular galaxies. These Milky Way satellites float in space alongside our home galaxy. Both are stellar nurseries with vast gas clouds that slowly collapse, forming new stars. The SMC is about 200,000 light-years away, so scientists study star formation there.
Check out the astounding view of the Small Magellanic Cloud below. It’s a compilation so you can see Chandra’s X-rays in purple, Hubble’s visible light in blue, green, and red, and Spitzer’s infrared also in red.
Here are some irregular galaxy facts.
- There is no defined structure or shape.
- They form from collisions with other galaxies or from internal chaos and violence.
- Old and young stars live within them.
- They contain significant gas clouds and bright pockets of star births.
- They range in size due to the diverse objects in this catch-all category.
- Irregulars are usually more faint than spiral galaxies.
How Are Galaxies Classified?
The Hubble tuning fork diagram organizes galaxies by their shapes. American astronomer and scientist Edwin Hubble devised the system in 1926. Scientists now know that Hubble’s scheme is too simple to classify all galaxies, but the basic principles still apply.
There are two parts to the diagram: elliptical and spiral galaxies. Ellipticals appear on the fork’s handle, starting with “E0” for the most spherical and progressively stretching to “E7s” for the most elliptical galaxies. E7s look much longer than wide when viewed from Earth but would appear closer to perfect circles if we could view them head-on.
Spirals make up the fork’s tines and have letter designations to characterize the tightness or compactness of their arms. So “Sa” galaxies have tightly wound arms, whereas an “Sc” designation means loosely wound galaxies.
In addition, the bulges in the spirals grow more significantly when the arms wind tightly. So scientists think there is a close connection between elliptical galaxies and those with more prominent bulges (S0, Sa, and Sb.)
Although astronomers have identified more spirals than ellipticals, they still think ellipticals likely dominate the cosmos. Spiral galaxies are bright with new and more brilliant star nurseries, while ellipticals are dim, with older red stars that give off less light. But when scientists study specific sky regions, they see a concentration of more elliptical galaxies. And those counts are likely consistent throughout space.
What Is Dark Matter Inside a Galaxy?
Vera Rubin, an astronomer in the late 1970s, discovered dark matter. While studying the manner in which galaxies spin, she noticed the Andromeda Galaxy (a spiral) rotated oddly. In what she termed a direct violation of Kepler’s and Newton’s Laws, edge materials moved just as quickly as those at the center.
Rubin saw that the center held most of the mass concentration. But she determined a non-visible mass held the galaxy together. She called it dark matter and soon found it existed in every galaxy she examined.
Now fifty years later, scientists know that dark matter makes up about 84% of the universe’s material. But they don’t know what it is. Dark matter is invisible, but its presence changes how stars move within their galaxies. It also affects how matter clumped into groups during the early universe. And it impacts how galaxies tug on one another or collide.
The Bullet Cluster
The cluster of galaxies 1E 0657-556, or the Bullet Cluster, shows some of science’s best evidence for dark matter’s existence. Two large galaxy clusters collided, leaving the Bullet Cluster and causing one of the universe’s most energetic events since the Big Bang.
Gas clouds, stars, and dark matter had different behavior in the collision so that scientists could study them as individual parts. The Magellan and Hubble telescopes observed the stars in visible light and found them mostly unaffected. Instead of colliding, they passed right through.
Then the Chandra X-ray Observatory examined the hot gases in the colliding clusters. The gas clouds contained most of the normal matter. But scientists indirectly detected the dark matter through the gravitational lensing of background objects.
You can’t see dark matter since it doesn’t reflect, refract, or emit light. But during the collision, clumps of dark matter moved ahead of the normal matter (hot gas), which separated the two forms: dark and normal.
Light gets significantly distorted from gravitational lensing around objects with large masses. So if the hot gas were the clusters’ most massive component, you wouldn’t see the light distortion. But instead, observations showed that dark matter must exist, even though it is invisible. Dark matter also must have enough mass to create a robust gravitational effect.
Chandra X-rays show the hot gas as two pinkish clumps where the normal matter resides. The compilation image below shows a bullet-shaped hot gas clump on the right that passed through the larger cluster when they collided. Hubble and Magellan telescope images show the galaxies in white and orange.
Blue areas show most of the clusters’ mass, clearly separated from the pink normal matter. Scientists say the blue regions are direct evidence that most of the matter within the clusters is dark.
Facts about galaxies: Dark matter exists in the universe and affects how stars and galaxies move and interact, sometimes influencing the collisions of galaxies.
What Happens When Galaxies Collide?
Galaxy diameters are vast, but they are still relatively close to each other compared to the distance between stars. As a result, galaxies interact and collide, passing through one another.
An interesting fact about galaxies is that the stars don’t crash into other stars when galaxies collide. And that’s because they’re so far apart. But gravitational interactions during galactic collisions spur extraordinary happenings.
- They create new stellar nurseries.
- Supernovas form.
- Black holes form.
- The galaxy’s shape gets distorted, with computer modeling suggesting that spiral galaxies could eventually make ellipticals.
Even our Milky Way is fated to collide with Andromeda (M31), our neighboring spiral galaxy, in a few billion years. When that happens, it will likely fling the Sun into a new region of the galaxy. But neither the Earth nor the solar system faces destruction because of the collision.
Andromeda is currently around 2.5 million light-years away. But the mutual gravity pull between our galaxies and the surrounding dark matter has Andromeda falling toward our galaxy.
Hubble data computer simulations show that it’ll take two billion years after the collision for Andromeda and the Milky Way to merge under gravity’s pull completely. Scientists think they’ll reshape into an elliptical galaxy. In addition, our solar system will get tossed closer to the galactic center than its current position.
Check out the simulation of how Earth’s night sky might look with Andromeda closing in on the Milky Way.
Colliding And Merging Galaxy Examples
The Hubble Space Telescope has captured several instances of galaxies falling into one another and colliding in a celestial dance.
The Coma Berenices constellation is bout 300 million light-years away and contains colliding galaxies that scientists dubbed “The Mice” or NGC 4676. Each galaxy has long tails of gas and stars trailing them. Eventually, the two will merge into a single enormous galaxy.
In this image of the colliding pair, you can see a bright blue patch of youthful, hot blue stars. Their formation resulted in the gravitational interaction of the galaxies. You can also observe streams of material making its way between the pair.
When exploring facts about galaxies, it is stunning to think of them dancing and swirling through space, colliding as they go. But that’s exactly what happens. This next pair ended up in a candy cane-like shape, called the Antennae galaxies. Superstar clusters pop up in the stellar nursery regions of merging NGC 4038 and 4039.
Both are spiral galaxies, and they started interacting a few hundred million years ago, a short time in the universe. They contain tens of thousands of young stars. And in one of the most incredible images we’ve seen below, you can see the original distinct galaxies as orange blobs. They contain old stars and brown dust filaments.
But what is especially exciting are the bright blue star-forming regions. You can see the nurseries surrounded by hydrogen gas that glows pink. Astronomers can age date the clusters and realize that 90% of the superstar clusters won’t survive past 10 million years. Instead, they’ll disperse, and the individual stars will spread throughout the galaxy.
Researchers believe around a hundred of the most dense clusters will form regular globular clusters, like those within the Milky Way. The Antennae galaxy shows scientists what could happen when Andromeda falls into the Milky Way in a few billion years.
Here’s another example of two spiral galaxies that grazed each other as they passed in space. About 110 million light-years away, near the Canis Major constellation, the two galaxies below nearly collide as they pass. The image comes from Hubble’s Wide Field Planetary Camera 2 and reminds us of an owl’s wise eyes as he gazes back toward Earth.
Facts about galaxies: When two galaxies collide, the stars pass next to one another without crashing since they lie so far apart.
Facts About Galaxies: How They Form
Galaxies interact with star groups and other galaxies, which help form their shapes over billions of years. Scientists don’t know precisely how galaxies formed into the shapes we currently see. But they use supercomputers to look back through time to simulate what might have occurred.
Based on its expansion rate, scientists believe the universe is about 13.8 billion years old. Edwin Hubble was the first to propose that the universe is indeed expanding. As researchers look deep into space, it is like looking back in time, so they conclude that galaxies residing several billion light-years away formed shortly after the big bang.
Some evidence exists that other galaxies formed in the past few billion years, making them younger than those from the early universe. Even though Edwin Hubble understood that the universe continually expands and gave a preliminary diagram of its types, galaxy evolution is far more complex than he realized. Galaxies collide with other galaxies, and star births and deaths occur. And these changes alter how the galaxy forms and evolves.
Scientists think the early universe held dark matter, helium, and hydrogen. Some concentrations were dense, and they eventually spun in on themselves to collapse. The gas accumulated in the dark matter clumps, forming the first stars, clusters, and galaxies.
The Hubble telescope can’t see the first galaxies but can track how they developed. The spiral galaxies below come from the Great Observatories Origins Deep Survey (GOODS.) And since the image groups show their distance away, it also shows their ages. So you can see how galaxies change over billions of years.
Spiral Galaxy Examples
One of our favorite spiral galaxies is the Pinwheel Galaxy, M101, located 25 million light-years from Earth in the Ursa Major constellation. It has an apparent magnitude of 7.9, so you can see it through a small telescope during May.
The Pinwheel Galaxy is about 170,000 light-years wide, almost twice that of the Milky Way. Notice the brighter blue areas that represent the galaxy’s stellar nurseries. Scientists estimate a trillion stars populate the galaxy’s spiral arms in giant gas clouds of molecular hydrogen.
The Hubble Telescope took 51 exposures over ten years to get the compiled version of the galaxy you see below. Astronomers filled in missing galaxy portions with ground-based images, so even the best home astrophotographer won’t see this type of detail when capturing photographs of the Pinwheel.
When looking at facts about galaxies, you must include the dust-laced lanes of stars and gas that make up the most astonishing grand-design spiral galaxies. The graceful lines, or arms, are full of compressing and swirling hydrogen gas, collapsing in on itself to create new star clusters.
M51, the Whirlpool Galaxy, is one of these stunning beauties, appearing as a grand spiral staircase. A smaller galaxy glides near the outer tip of one of the Whirlpool’s arms. And astronomers think the interaction between the two triggers new star formation. NGC 5195 has tugged on the Whirlpool Galaxy’s arm for hundreds of millions of years, and the tidal forces create stellar nurseries.
You can see the Whirlpool Galaxy best during May with a small telescope.
Another example of spiral galaxies is the HCG 16 group. The image below captures four of the seven close galaxies.
- NGC 839
- NGC 838
- NGC 835
- NGC 833
Notice their wispy gas tails and glowing golden centers.
Elliptical Galaxy Examples
One of the universe’s most giant galaxies in an E0 elliptical. Messier 87 houses trillions of stars, about 15,000 globular clusters, and a supermassive black hole. When it comes to learning facts about galaxies, astronomers learn much from studying this one.
To appreciate the enormity of M87, consider that the Milky Way only has a few hundred billion stars with only approximately 150 globular star clusters. It gives you a more clear perspective on M87’s size.
Residing about 54 million light-years away in the Virgo constellation, M87 is observable in May with a telescope from dark sky areas.
Another large elliptical galaxy within the Vigo cluster is M59. It also centers around a supermassive black hole, and we mean supermassive! It’s about 270 times the Sun’s mass!
The galaxy has about 2,200 globular star clusters, still many more than the Milky Way, although much fewer than M87. It’s about 60 million light-years away and best seen in May.
You can see about half of M59 in the Hubble image compilation below. The brighter light points are globular clusters. Astronomers use these observations to understand better early-type galaxies, often ellipticals found near galaxy cluster centers.
Irregular Galaxy Examples
Many shapes and sizes present themselves in the catch-all category of irregular galaxies. Let’s look at the peculiar galaxy, NGC 3256, part of the Hydra-Centaurus supercluster complex. Peculiars aren’t their own classification of galaxies; they are two merging systems. The collision distorts the shapes and gives them a peculiar appearance.
In the case of NGC 3256, it is more aptly the relict of a clash of two galaxies that merged in the distant past. You can see the two extended tails streaming out from the galaxy, where a high density of star clusters resides. A double nucleus also shines from the center amidst tangled brown dust lines.
Interacting galaxies get their name because of their influence on one another. They may result in a collision, merger, or unique galaxy formation. The Hubble Space Telescope captured the wondrous image below where it looks like the left galaxy, NGC 2799, is pouring into the right galaxy’s center (NGC 2798.) It almost looks like a waterspout dripping stars.
Galactic mergers can take hundreds of millions to a billion years to complete and form a single irregular entity. And while you’d think the union is chaotically violent, stars drift past one another without colliding.
Let’s look at one more slightly disheveled irregular galaxy, NGC 7292 (below.) It is about 44 million light-years away in the Pegasus constellation. NGC 7292 has a stretched-out core that pulls into a distinct bar, as you’d see in a spiral galaxy. But instead of arms, it has a hazy shape.
The galaxy is also rather faint and barely distinguishable against the night sky, so it gained the classification of a low surface brightness galaxy. This type of galaxy contains more dark matter and gas than stars.
The Hubble Team inspected this galaxy to learn more about the aftermath of its Type II supernova, SN 1964H. The massive star collapsed and then catastrophically exploded in 1964. Astronomers hope to estimate the star’s initial mass before it went supernova. And they’re searching for any surviving stellar companions to study.
Wrap-Up: Facts About Galaxies
Three main classifications of galaxies exist: spiral, elliptical, and irregular. But astronomers acknowledge the many types of galaxies that fall between classifications or are just beyond a category’s boundaries.
By studying the different types of galaxies, scientists learn their history, how they form and will evolve. They identify the nature of galaxies to cluster, fall into one another, and even collide. But interestingly, even in the collision of two galaxies, stars are spaced so far apart that they glide past one another without harm.
Galaxies form the universe’s building blocks through the formation and lifecycles of their stars.