Why are we interested in star clusters?

This blog is all about star clusters, which are pretty stunning and amazing astronomical objects, but some of you might be wondering why astronomers are so interested in them. So in this post I thought I'd give my Top 5 Reasons to Study Star Clusters! I hope you enjoy it!

The Orion Nebula Cluster, seen in X-rays and
optical light. By estimating the masses of the
young stars formed here we can study the
distribution of stellar masses that form
(Credit: Chandra X-ray Observatory)

1. Star formation. Perhaps the most important reason to study star clusters is because we believe the majority of stars form in groups or clusters and so by studying young star clusters we hope to learn something about how stars form. For example, by measuring the masses of all the stars in a cluster we can study the mass distribution of recently-formed stars (known as the initial mass function), which is one of the most important products of the star formation process.

Hubble Space Telescope image of an evaporating
protoplanetary disk (known as a proplyd) in the
Orion Nebula. The disk (dark silhouette in the
centre) is being eroded by radiation from a
nearby bright O-type star leading to the tail of
material stripped off(Credit: NASA/HST)

2. The impact of environment on star formation. Stars form in many different environments, from small groups of only a handful of stars, up to dense clusters with millions of stars and many thousands of luminous and massive OB stars. In dense clusters young stars are very close to each other, which can lead to close encounters that might disrupt binary systems or planetary systems. The bright OB stars that are present in the most massive clusters can also erode the disks around stars in which planets form, potentially hindering the creation of a full solar system like our own. Understanding how the environment that stars form in affects their final properties is therefore very important!

3. Stellar evolution. Just like humans, stars change as they age in many different ways, from subtle changes in their luminosity and slowing their rotation, to dramatic changes as they switch their source of nuclear fuel. To study these changes we need to know how old the stars are, but unfortunately its very difficult to measure how old individual stars are (we can't ask stars how old they are like we do with humans!). If you have a group of stars, however, you can often work out how old the group is by studying which stars have come to the ends of their lives and which haven't. This allows you to estimate the age of the cluster, and therefore all the stars in it. Once you know their ages you can study how the stars have evolved over time, an area of research known as stellar evolution.

The life cycle of a star like our Sun (upper row) and a more massive OB-type star (lower row).
Both types of star form in star forming regions and star clusters, but evolve through different
phases. By studying this process in star clusters with known ages, astronomers can
calculate how long this evolution takes (Image credit: SciOly.org)

4. Star clusters can be used to study star formation in distant galaxies. Its easy to study star formation in our own galaxy, the Milky Way, because we can observe stars forming deep within molecular clouds and the young stars that have recently formed. But in distant galaxies these things are too small and too faint to observe, so our understanding of star formation in other galaxies, whether it is different in any way, and how much star formation has been occurring, is limited. Star clusters however are bright and we can easily observe them in distant galaxies. Its also relatively simple to get a good estimate of their mass, the type of stars in them, and how old they are. This is really useful for astronomers because it means we can study not just the current star formation in these galaxies, but also star formation that occurred in the past. Astronomers call this the star formation history of a galaxy and its useful for understanding how galaxies evolve over cosmic time.

The Antennae Galaxies, two interacting galaxies with a rich and vibrant star formation history.
Astronomers have been able to study its star formation history by observing the many
star clusters (bright blue dots surrounded by red clouds) (Credit: Hubble Space Telescope)

The Jewel box Cluster (NGC 4755), one of many
clusters that are important for measuring distances
in astronomy! (Credit: APOD)

5. Star clusters are an important step on the cosmic distance ladder. In actual fact, they're two steps! The first step is a 'local' step on the cosmic distance ladder, and it comes from a process known as the moving cluster method, which is essentially a perspective effect whereby if you know the direction that all the stars in a cluster are moving you can estimate how far away it is. The second step is a much more 'distant' step on the ladder, which uses the luminosity of globular clusters to estimate the distance of the galaxy that they're in. The method stems from the assumption that the brightness globular cluster in a galaxy usually has the same luminosity as the brightest globular cluster in another galaxy. Based on this, if you can measure how bright the globular clusters in a galaxy are, you can estimate how far away the galaxy is.

There are many other reasons to study star clusters, but these are some of the most important and wide-ranging, spanning the formation of planetary systems to the size of the Universe!

So next time you hear about star clusters or new research into our understanding of these amazing objects, think about all the different scientific topics that might be influenced by those new results!

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.

Globular Clusters: the grandaddies of all clusters

This week I heard a fascinating talk about globular clusters, so I wanted to take this opportunity to tell you about globular clusters, and some of the science being done to study and better understand these amazing astronomical objects.

Globular clusters are a type of star cluster, but they're a very special type of star cluster, partly because they're very massive, but also because they're very old, almost as old as the Universe itself. For this reason they provide a glimpse of how stars, and star clusters, formed a long time ago.

The Omega Centauri globular cluster, the largest in
our galaxy (Credit: ESO)

Globular clusters are huge and contain millions of stars, considerably more than other types of star cluster. Although there are no fixed boundaries for the size of a globular cluster, they typically contain between 100 thousand and a few million stars, though there are some even larger than this.

The largest globular cluster in our galaxy is Omega Centauri, which is thought to contain approximately 10 million stars. Though it is quite distant it is relatively bright and can be seen with a pair of binoculars, as can many of the other globular clusters in our galaxy.

In addition to being massive, globular clusters are also very old. The ages of globular clusters can be determined by studying how the stars in the cluster have evolved based on our understanding of stellar evolution (the study of how stars evolve). The globular clusters in our galaxy seem to be about 11-13 Gyr old (1 Gyr = 1000 million years). This is just younger than the approximate age of the Universe, 13.7 Gyr, suggesting globular clusters were some of the first objects to form in the Universe.

The M80 'Scorpio' globular cluster (Credit: Baran Observatory)

It is thanks to their old age that globular clusters are so round and spherical. Globular clusters are held together by the mutual gravitational attraction of all the stars in the cluster, and over time this gravitational attraction has smoothed out any small structural differences in the cluster and created an almost perfect ball of stars. In fact globular clusters are really useful for studying how gravity affects clusters of stars, simply because they've been around for so long and have been relatively undisturbed by the galaxy.

The reason globular clusters haven't been disturbed by the galaxy they reside within is because of where within the galaxy globular clusters are found. The oldest stars in our galaxy are found not in the disk of the Milky Way galaxy, but in its halo, which is a spherical and diffuse cloud of stars within which the disk of our galaxy sits. Globular clusters are as old as the oldest stars in our galaxy and are found distributed across the Galactic halo in relative isolation compared to the spiral-shaped disk of the galaxy.

The structure of our galaxy, showing the globular clusters in the Galactic halo (Credit: University of Oregon)

Our galaxy contains approximately 150 known globular clusters, with experts suggesting that there may be 10 to 20 still to be discovered. Because of their brightness we can identify globular clusters in other galaxies and some large galaxies have many more globular clusters than the Milky Way, for example the Andromeda Galaxy may have as many as 500 globular clusters!

One of the great mysteries of globular clusters is how they formed, whether they formed in a single burst of star formation or whether they are made up of multiple generations of stars. This also raises the question of whether globular clusters are significantly different from some of the smallest type of galaxy we know of, for example the dwarf spheroidal galaxies. All of these unanswered questions are very important for understanding how stars and galaxies form, which makes globular clusters an important area of astronomical research.