The massive stars of Cygnus OB2

A couple of weeks ago I talked about an OB association known as Cygnus OB2, one of the largest groups of young stars in our galaxy, and an exciting location to study star formation on the grandest scales. This week I want to tell you about some research I recently carried out to better understand this region, research which has recently been published.

The Cygnus OB2 association, as seen though a combination of
X-ray (blue), optical (yellow) and infrared (red) light
(Credit: Chandra X-ray Observatory)

There has been a lot of work carried out recently by many other astronomers to understand some of the really massive stars in Cygnus OB2, which are all very interesting objects, many of which are unique and can tell us exciting things about how massive stars live their short and turbulent lives. Thanks to this work we're now in a position to put all this information together and use it to better understand the entire group of stars as a whole, and that's what I did!

I was able to gather information about 169 massive stars in Cygnus OB2, including some stars as massive as 100 times the mass of our Sun. For each of these stars I was able to determine their mass and age, by comparing their measured properties with the predictions from models of how massive stars evolve throughout their lives. One of the main advances in astronomy over the last few decades has been the development of models that describe not just how stars change throughout their lives, but how they appear during this time. These models, known as stellar evolution models, allow astronomers to estimate how old and how massive the stars that they see are.

The positions of massive stars (red, green and yellow dots) across the Cygnus region, shown against a black and white infrared image of the region. The white circle denotes the area covered by Cygnus OB2 and studied in my paper
(Credit: Nick Wright)

With this information we were able to determine the approximate ages of all the massive stars, allowing us to determine what's known as the star formation history of the region. The star formation history tells us when all the stars formed, and that's important to know if we want to understand how massive OB associations like this formed.

The simple view would be that all the stars formed at the same, or at least very similar, times. This is what's known as instantaneous star formation, or star-burst, because all the star formation occurs in a quick burst when the conditions in the molecular cloud become right for star formation.

The centre of the Cygnus OB2 association - or is it multiple associations?
(Credit: Nick Wright)

However that wasn't what we found. Instead we found that the ages of the stars were spread out over quite a long time period, almost 10 million years. That's a long period of time for star formation, because most astronomers think star formation occurs quickly, within only 1-2 million years or less. But here we're seeing that the star formation didn't happen all at once but was spread out, happening almost constantly for 10 million years.

What does this mean? Is the star formation that has occurred here any different from star formation taking place elsewhere? Probably not. What probably happened here is that the star formation didn't just take place over a long period of time, but probably also took place over a large area of space, almost like multiple small star formation events! These individual star formation events have since merged and combined so that we see them now as this large and homogeneous group of young stars.

It's a theory anyway. One of the great joys of science is discovering something you didn't expect to find, thinking of a new theory to describe what you saw, and then testing your theory. Science is not set in stone but is continually evolving with new theories being proposed, and existing theories being tested, and then refined or discarded. We call this the scientific method, and it underpins all of science.

You can read the full paper here if you're interested to learn more.