On a clear, dark summer or winter night, a band of cloudy light can be seen crossing the sky.
This light is known as the Milky Way.
In reality, the Milky Way has nothing to do with dairy. Rather, it is the term for the light from hundreds of millions of stars that are so far away that we cannot see them as individual points of light. Instead, we see their combined brightness as a bright, diffuse band of light.
While the ancient Greeks may have speculated that this was so, Galileo discovered that observing the brightness of the Milky Way was due to starlight using his “optical tube.” Although Galileo was not the inventor of the telescope, he is the first we know of who used a similar device to study the stars.
In the photo below, a magnified portion of the Milky Way reveals the countless stars it contains.
See if you can see the Summer Triangle in the enlarged portion (hint: Deneb is on the left, Altair on the bottom right with Vega above and left of center in the enlarged image).
In a galaxy far, far away
The Milky Way is the name of both the band of cloudy light that we see and the name of our galaxy.
It is difficult to understand that this band of light is actually a galaxy, especially since we reside within the Milky Way and orbit its center.
Since no one has yet to travel beyond the Milky Way, we have to conceptualize our galaxy and our internal position from what we see of other galaxies.
We also use what we can measure and understand from the many types of technologies, observations, and theories that have been developed over the past several hundred years.
The plate example
To better understand the Milky Way, we must imagine that we are looking at it from the outside and then travel above it to see its structure.
To do this, let’s imagine two plates stacked in front of each other (photo below left). This represents the approximate shape of our Milky Way as seen from the outside.
It is somewhat similar in shape to the photograph on the right taken by the European Southern Observatory. From this perspective, the Milky Way is shaped like a flattened disk with a central bulge.
Now, let’s imagine that we are going to travel above the top of our galaxy.
As we do so, we’ll see the flattened disk extend outward like in our plate (bottom left) and in this NASA view of our Milky Way galaxy (bottom right).
Finally, seen from above we see the full shape of the disk and all its splendors.
This is evident in the NASA concept (below right). On the left, we can perhaps only imagine the plates used to hold a Milky Way chocolate bar.
All about galaxies
Our galaxy is one of an estimated two billion galaxies that may exist in the part of the universe that we can see.
A galaxy is an oasis of hundreds of billions of stars, gas and dust in the middle of a relatively arid universe.
There is very little matter between galaxies. But inside, a huge amount of material produces stars, planets and life, at least as we know it here on Earth.
Galaxies come in many different shapes and sizes. Our type of galaxy is known as a “barred spiral.” This is because the arms have a spiral structure, but a large central bar spans much of the middle.
Our Milky Way has a supermassive black hole at its center. This affects stars and other materials in the core.
The Milky Way is about 100,000 light years in diameter and about 20,000 light years thick. The sun and all objects in our solar system are located approximately halfway from the center (or about 25,000 light years).
Like most galaxies, the Milky Way rotates slowly and we follow it. It takes about 240 million years to make one revolution. Since our Sun is about 4.6 billion years old, we have circled the center of our galaxy more than 19 times.
Why the Milky Way looks different depending on the season
Understanding our place in the Milky Way as seen in NASA’s concept image (above) makes it easier to understand why the summer Milky Way appears so much brighter than the winter one.
As the Earth revolves around the Sun, we see different stars in the night sky at different times of the year. During the summer, we look toward the central part of our Milky Way (the center is located to the right of our August constellation: Sagittarius).
However, during winter, we have moved halfway around the sun and, as a result, are facing the opposite direction to that of summer. This means that we look outward from our galaxy and see much less of the structure of the Milky Way.
Although much fainter, the dark skies of most of our parks will offer the viewer a good view of the winter Milky Way. One of the best times to see the Milky Way in winter is during the early hours of dawn, just before sunrise, during the fall months.