Content co-written by:
Heather Sickels
March 17th, 2017

NGC 4845
Image Credit: NASA

Looking up into our galaxy, the Milky Way surrounds us–encircling the Earth we peer out from within. We have the view of an insider embedded within one of its spiraling arms. Our solar system lies approximately 25,000 light years away from the Galactic Center (GC) where a supermassive black hole resides. It is understood that many galaxies have black holes at their centers. Keep in mind that our star the Sun is just 8 light seconds away from Earth. The Milky Way is a vast place and it is one of billions of galaxies in the universe.

A galaxy is defined as a gravitationally bound system of stars and matter. Due to the vastness of space and the huge scale on which it operates, scientists still have many unanswered questions about distant galaxies. There are, however, many things that science has so far uncovered about the universe and our own small place within it.

Facts about Galaxies

  • There may be as many as 400 Billion stars within the Milky Way.
  • Galaxies are classified into three main types: Sprial, Elliptical and Irregular.
  • Andromeda and The Milky Way are sister galaxies. It is predicted that they will collide with one another in approximately 4 Billion Years.
  • At the center of many of these space phenomena are supermassive black holes.
  • The Milky Way is 100,000 light years “wide”. Some galaxies are well over 1 million light years across.

How Do Scientists Know About Galaxies?

Before understanding what a galaxy is, it’s helpful to know how scientists know they’re out there in the first place. No human has ever visited another galaxy, and it would be impossible to do so with our current levels of technology. Fortunately, stars are very bright, and this means that they can be seen across vast distances.

When you look up at the sky, you can see thousands of twinkling stars. In rural areas where there is little light pollution, these stars are much more visible. Nearby star clusters like the Andromeda Galaxy and the Large and Small Magellanic Clouds can be viewed without a telescope from certain places on earth.

Astronomers use high-powered telescopes located in areas with clear sky for star viewing. This allows them to see other galaxies clearly despite the great distance. Space-based telescopes, such as the Hubble Telescope, can take high-resolution photographs of distant galaxies without the interference of Earth’s atmosphere. Everything we know about galaxies has come from this kind of observation and mathematical models.

What is a Galaxy?

If you imagine that the universe is a country, each galaxy could be considered an individual state, with solar systems making up the cities and individual planets serving as neighborhoods within each city. Just as individual states have their own unique shapes, sizes and other properties, galaxies have their own unique features.

A galaxy is a cluster of solar systems, gas and dust. These clusters of star systems are held together by gravity, and they can take the shape of a spiral, ellipsis or irregular blob. Our own galaxy, the Milky Way, is spiral-shaped and rotates slowly around a black hole at its center. The exact number of star systems in our the Milky Way is unknown, but scientists estimate that there may be anywhere from 100 to 400 billion stars. If each of these stars hosted a solar system like the one we live in, there could be as many as 3.2 trillion planets in the Milky Way!

How Are Galaxies Formed?

It’s estimated that galaxies began to take form within a billion years of the Big Bang. There are two prevalent theories about their origins. One suggests that the freshly-formed universe was filled with “clumps” of matter that would draw together through gravitational pull and eventually form star systems. Another idea is that enormous clouds of gas and dust may have broken apart and condensed into stars.

It’s possible that both theories are correct, or that the truth lies somewhere in-between. What scientists do know, however, is that galaxies are constantly being formed and changing shape. Because they are held together by gravity and are in a state of constant motion, they sometimes collide with one another. These collisions and mergers can dramatically alter the shape and size of a galaxy.

Types of Galaxies

There are dwarf sized galaxies all the way up in size to what are called giant galaxies. They are organized into different types and are usually defined by their morphology. There are so many shapes and oddities that categorizing is somewhat messy.

Spiral, such as the Milky Way, Andromeda and Pinwheel galaxies, have spirally arms and a bulging center dominated by older stars. The Milky Way is more specifically defined as a barred spiral galaxy, describing the bar shaped center with the arms radiating from the ends of the bar.

Elliptical lack the structure of spiral galaxies and have an ellipsoidal shape. It is theorized that these types of galaxies are the result of two or more galaxies colliding in the past. It is predicted that our Milky Way and the Andromeda galaxy are on a collision course. There is often so much void space between the stars in colliding galaxies that there is no actual collision of the stars, but rather a gravitational effect that disrupts the structure of the original galaxies.

Irregular are a catch all description used to categorize the many oddly shaped and anomalous galaxies that are still being discovered and categorized today. The closest galaxy is the Canis Major Dwarf galaxy which is an example of an irregular galaxy. It is being cannibalized or gravitationally torn apart by the Milky Way galaxy.

Hubble Classification of Galaxies

Class Attributes
Elliptical Galaxies
E0 – E7 Elliptical Galaxies
Spiral Galaxies
S0 Bright Bulge, Disk but no arms
Sa Tightly wound Arms, Bright Bulge in Center
Sb Less tightly wound Arms, Fainter Bulge
Sc Loosest Arms, Faintest Bulge
Barred Spiral Galaxies
SBa Tightly wound Arms, Bright Bulge and Bar in Center
SBb Less tightly wound Arms, Fainter Bulge and Bar
SBc Loosest Arms, Faintest Bulge and Bar
Irregular Galaxies
Irr I No defined central bulge, bar, or shape
Irr II Same as Irr I but smoother in appearance

The Andromeda Galaxy

Andromeda Galaxy is the closest major galaxy and the largest in our galactic neighborhood known as the Local Group, which includes 51 identified galactic members including ours. Andromeda is our big spiral galactic sister and is so closely related that it will dance into, colliding with our Milky Way is an estimated 4.5 billion years. Andromeda is currently a distant 2.5 million light years away. Even on a clear moonless night, from our viewpoint it looks like a hazy star or nebulous smear. This ‘smear’ contains 400 billion stars and was first described in 964 AD. It was not properly identified as a separate galaxy, an object outside of our own galaxy, until 1923. Think of how much humanity have discovered and realized about the universe in the last 100 years alone.

Almost all of the stars seen by the naked eye from Earth are within our own galaxy and within a small local section of our local galactic arm. Many of the stars within the Milky Way are too far to be seen without a telescope. When looking at Andromeda you are viewing an entirely different galaxy!

GN-z11, the oldest known galaxy

The oldest and therefore most distant galaxy identified as of March 2016, lies at 32 billion light years away. It is known as GN-z11 and is estimated to have formed 400 million years after the Big Bang, relatively quickly on an astronomical timescale. Looking the furthest in space means looking the furthest back in time.

Current and Future Galaxy research

There are estimated to be one hundred billion galaxies in the observed universe. Telescopes are being designed to look deeper in space and further back in time. Many scientists estimate that 90% of galaxies have yet to be seen by our current technology. It is also said that there is such an abundance of galaxies throughout our universe that every patch of sky contains part of a galaxy. This is true even despite the immense voids of space and time between them.

Galaxies are enormous, and astronomers are learning more about them each day. Although there are still many unanswered questions about their origins and the star systems they contain, more answers are being uncovered through the use of superior telescopes and other technologies. Some day we may be able to see distant galaxies first-hand; until then, the universe will continue to be filled with mysteries waiting to be explored.