The Milky Way Galaxy

Our Little Corner of the Cosmos

Milky Way Galaxy
An artist's concept of what our galaxy looks like from outside. Note the bar across the center and the two main arms, plus smaller ones. NASA/JPL-Caltech/ESO/R. Hurt

When we stare up into the heavens on a clear night, away from light pollution and other distractions, we can see a milky bar of light that spans across the sky. This is how our home galaxy, the Milky Way, got its name, and it's how it looks from the inside.

The Milky Way is estimated to span between 100,000 and 120,000 light years from edge to edge, and contains between 200 - 400 billion stars.

Galaxy Type

Studying our own galaxy is difficult, since we can't get outside of it and look back.

We have to use clever tricks to study it. For instance, we look at all parts of the galaxy, and we do so in all available radiation bands. The radio and infrared bands, for instance, allow us to peer through regions of the galaxy that are filled with gas and dust and see stars that lay on the other side. X-ray emissions tell us about where the active regions are and visible light shows us where the stars and nebulae exist.

We then use various techniques to measure the distances to various objects, and plot all of this information together to get an idea of where stars and gas clouds are located and what "structure" is present in the galaxy.

Initially, when this was done the results pointed to a solution that the Milky Way was a spiral galaxy. However, upon further review with additional data and more sensitive instruments, scientists now believe that we actually reside in a subclass of spiral galaxies known as barred spiral galaxies.

These galaxies are effectively the same as normal spiral galaxies except for the fact that they have at least one "bar" passing through the bulge of the galaxy off which the arms extend.

There are some, however, that claim that while the complex barred structure favored by many is possible, that it would make the Milky Way quite different from other barred spiral galaxies that we see and that it might be possible that we instead live in an irregular galaxy.

This is less likely, but not outside the realm of possibility.

Our Location in the Milky Way

Our solar system is located about two-thirds of the way out from the center of the galaxy, between two of the spiral arms.

This is actually a great place to be. Being in the central bulge would not be preferential as the star density is much higher and there is a significantly higher rate of supernovae, than in the outer regions of the galaxy. These facts make the bulge less "safe" for long term viability of life on planets.

Being in one of the spiral arms is not all that great either, for much the same reasons. The gas and star density is much higher there, increasing the chances of collisions with our solar system.

Age of the Milky Way

There are various methods that we use to estimate the age of our Galaxy. Scientists have used star dating methods to date old stars and found some as old as 12.6 billion years (those in the globular cluster M4). This sets a lower bound for the age.

Using cooling times of old white dwarfs gives a similar estimate of 12.7 billion years. The problem is that these techniques to date objects in our galaxy that would not have necessarily been around at the time of galaxy formation.

White dwarfs, for instance, are stellar remnants created after a massive star dies. So that estimate does not take into about the life time of the progenitor star or the time it took for form said object.

But recently, a method was used to estimate the age of red dwarfs. These stars live long lives and are created in large quantities. So it follows that some would have been created in the early days of the galaxy and would still be around today. One has recently been discovered in the galactic halo to be about 13.2 billion years old. This is only about half a billion years after the Big Bang.

At the moment this is our best estimate of our galaxy's age. Of course there are inherent errors in these measurements as the methodologies, while backed up with serious science, are not completely bullet proof.

But given the other evidence available this seems a reasonable value.

Place in the Universe

It was long thought that the Milky Way was located at the center of the Universe. Initially this was likely due to hubris. But, later, it seemed that every direction we looked everything was moving away from us and we could see the same distance in every direction. This led to the notion that we must be in the center.

However, this logic is faulty because we don't understand the geometry of the Universe, and we don't even understand the nature of the boundary of the Universe.

So the short of it is that we don't have a reliable way to tell where we are in the Universe. We may be near the center - though this is not likely given the age of the Milky Way relative to the age of the Universe - or we may be nearly anywhere else. Though we are fairly certain that we are not near an edge, whatever that even means, we're not really sure.

The Local Group

While, in general, everything in the universe is receding away from us. (This was first noticed by Edwin Hubble and is the foundation of Hubble's Law), there are a group of objects that are close enough to us that we gravitationally interact with them and form a group.

The Local Group, as it is known, consists of 54 galaxies. Most of the galaxies are dwarf galaxies, with the two larges galaxies being the Milky Way and the nearby Andromeda.

The Milky Way and Andromeda are on a collision course and are expected to merge into a single galaxy a few billion years form now, likely forming a large elliptical galaxy.

Edited by Carolyn Collins Petersen.