Unfolding the Milky Way

One of our favorite activities at the family cottage is star gazing. We do it at home as well, but even though we are located in a small town on the outskirts of the city, the ever present lighting tends to block out the fainter objects in the sky. The cottage is located in the wilderness, and artificial lights are rare. The luminous flood from all this artificial lighting is called “light pollution”. We are finding ways to reduce it in urban areas, but we will save that for another posting.

The most impressive thing that we can see at the cottage that we find hard to see at home is the Milky Way. In the summer, the Milky Way dominates the upper eastern part of the night sky. Most readers will be familiar with it, but in today’s urban society, a lot of people aren’t sure what to look for, or whether they have seen it or not.

The Milky Way is a hazy band of light that runs vertically across the sky. What we are looking at is our own galaxy from the inside. We are out near the edge and when we look at the Milky Way, we are looking toward the centre. Every star in the sky is part of the same galaxy. But, for star gazers, the Milky Way refers to the dense band of light itself.

Humans discovered that in modern times. Ancient peoples had their own stories about what the Milky Way was. The Australian Aborigines called it a river in the Sky World. The Cherokee believed that a celestial dog had stolen some corn meal and spilled a trail behind him. The Egyptians said that the Milky Way was a pool of cow’s milk. In Greco-Roman times, natural philosophers speculated that the Milky Way was a cluster of stars that were so close together, we couldn’t make them out individually. As usual, they were ahead of their time.

When Galileo first turned his telescope on the night sky in 1615, the milky haze resolved into a plethora of individual stars, no different from the others. Immanuel Kant proposed in 1755 that mutual gravity must hold these stars together. He also said that they must be shaped like a rotating disk, much like our solar system. William Herschel drew a partial map of the Milky Way in 1785.

As we have discussed in earlier posts, in 1920, Edwin Hubble proved that certain nebulae in the night sky are galaxies outside our own. What’s more, the further we look, the more galaxies we see. Today, we estimate that the universe has hundreds of billions galaxies spread evenly throughout space.

Now that we know that the universe is not one galaxy, but billions of them, new questions arise. Why isn’t there one big galaxy? How does each individual galaxy form? And what holds the stars in the galaxies in place?

Researchers at the Institute for Astrophysics, Canary Islands (AIC) in Spain provided some new answers to these questions a couple of weeks ago. They published their study in the journal Nature Astronomy.

The earliest stars in our galaxy formed about 13 billion years ago. We know that they were clustered into small dwarf galaxies, which collided to form the larger galaxies we see in the sky today. Using the Gaia Space Telescope, astronomers at the AIC measured the position, brightness and distance of more than a million stars, all within 6,500 light years from earth.

Looking at the outer halo of our galaxy, they noticed two categories of stars. Some were bluish and some were more reddish. Others had shown that the blue stars came from a dwarf galaxy, which they had named Gaia-Enceladus. After thorough analysis, the IAC team found that the red stars contained more metals. From this, the IAC team has pieced together how our galaxy formed.

Thirteen billion years ago, two groups of stars formed. One was Gaia-Enceladus, and the other was the first version of the Milky Way. The red stars were in the Milky Way and contained more metals. The Milky Way was also four times larger than Gaia-Enceladus.

About ten billion years ago, the two galaxies collided. This threw the stars into chaotic motion, causing the outer halo. This was followed by violent star formation until about 6 billion years ago. At that point, the Milky Way Galaxy settled into the thin disk that we are used to seeing in artist’s renderings today.

Little by little, the story of our universe is revealing itself to us. The underlying theme that we see in this story is that the universe is somehow self-organizing. It seems as if galaxies are imbued with creativity. As we saw above, after the collision of Gaia-Enceldus and the Milky Way, there was a burst of creativity, resulting in about 4 billion years of new star creation.

Today’s post tells two stories on which we can reflect with awe. One is the way in which the human mind has been able to go from thinking that the milky way was a pool of cow’s milk to realizing that the Milky Way is one of hundreds of billions of starry galaxies. The other is the way that the elements that came forth from the Big Bang creatively organized themselves to form those galaxies.

There is always more to learn if we dare to know.

Learn More:
Instituto do Astrofisica de Canarias (IAC)
Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

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