How to make a star cluster

In my last two posts I've been discussing how stars are distributed at the time they're born. This is an important question because many of our theories for how stars form suggest that their environment can play a crucial role in how they accrete material. Our current observations suggest that stars form directly out of the dense gas that is found in filamentary structures across molecular clouds. But if stars form with an elongated and filamentary distribution, how do spherical star clusters form?

The answer to this question has eluded astronomers for many decades, and though there is still considerable debate in the community, a picture is emerging whereby star clusters arise when filaments of dense gas merge.

Star clusters forming in the Rosette Molecular Cloud
(Credit: Schneider et al. 2012)

The figure to the right shows an image of filaments and star clusters in a star forming region known as the Rosette Molecular Cloud (so-called because its very close to the famous Rosette Nebula). The background image shows the distribution of dense gas in the cloud, with the density of the gas ranging from low-density (black) to high-density (green and red).

On top of this are marked (in white) the positions of the filaments that make up the molecular cloud, and on top of that (the turquoise stars) are the positions of known star clusters.

If you inspect the image closely you'll see that the majority of the star clusters (which were known about well in advance of this study) sit at the intersections between the filaments. In fact out of the 14 star clusters in this molecular cloud, 13 of them are found at these intersections. This is unlikely to be a coincidence, so it appears that the formation of star clusters is closely linked to overlapping or merging filaments.

Over the last decade astronomers have seen various strands of evidence pointing towards this picture (a good summary of the early evidence can be found here). However it wasn't until the launch of the far-infrared Herschel Space Observatory in 2009 that the filamentary structure of molecular clouds became so apparent, and soon after that the relation between clusters and filaments began to emerge.

So if stars clusters are found where filaments overlap, this suggests that the collision between the filaments might create the necessary conditions for a star cluster to form. The question this then poses is whether the filament collision occurs before, after, or even during the star formation process.

If the filament collision occurs before star formation then the collision is effectively bringing together large volumes of dense gas into a small space. This would allow star formation to proceed very rapidly in a very dense cluster of gas, leading to the formation of stars in a highly clustered distribution. This has sometimes been referred to as clustered star formation or in-situ cluster formation.

Alternatively, the filament collision might occur after star formation has begun, in which case the filament collision would be bringing together stars that have already formed, depositing them in a highly clustered distribution. This is usually referred to as conveyor-belt cluster formation.

Which of these two scenarios is right has big implications for how stars form and how the environment affects the star formation process. There are strands of evidence in favour of both scenarios, though neither has been conclusively shown to be true yet. Of course its possible that both scenarios might occur, perhaps in different environments, in which case it would be interesting to understand which process occurs more often, and whether the clusters that form from the two processes differ in some way. Hopefully that's a question we can answer soon!

What sort of environment do stars form in?

Last week we talked about the initial spatial distribution of young stars and how their distribution follows that of the dense gas in molecular clouds. But we also know that stars form in groups with a wide variety of sizes and densities, which astronomers think is really important for determining the type and sizes of the star clusters that form.

Distribution of young stars in the Perseus Molecular Cloud
(red, green and blue dots) projected against the gas
distribution (Credit: Evans et al. 2009)

The image on the right shows the distribution of young stars across the Perseus Molecular Cloud. These young stars were all detected by the Spitzer Space Telescope, an infrared telescope that was particularly effective in detecting young stars due to the copious amounts of infrared light they emit.

The molecular cloud is very elongated, as the image clearly shows, but even within that elongated structure the young stars are not evenly distributed, they're clumped into groups. Many of these groups represent the well-studied embedded star clusters typically found in molecular clouds, such as IC 348 and NGC 1333.

In addition to these dense and compact clusters there are also smaller groups, such as the clumps of young stars labelled B1 and B5, as well as numerous young stars that appear relatively isolated.

It appears that while young stars do like to form in groups, there are almost as many young stars that form alone - so is there a typical group size and density that stars form in? And if so, what is it?

One way that astronomers have attempted to tackle this problem is to study the distribution of densities that stars are forming at. To do this astronomers have measured the density of stars surrounding each young star. The distribution of densities is usually referred to as the surface density distribution of young stars.

The figure below shows such a distribution compiled from Spitzer Space Telescope observations of numerous nearby star forming regions. Mid-infrared observations from the Spitzer Space Telescope were chosen for this because it allows astronomers to peer deep within molecular clouds and hopefully identify all the young stars that are present. Hopefully this means no stars were missed!

The surface density distribution of young stars (both Class I and Class II young stars) identified from
Spitzer Space Telescope observations (Credit: Bressert et al. 2010)

The figure shows the fraction of stars born at various densities, from low densities on the left (surface densities of 1 star per square parsec) to high densities on the right (hundreds to thousands of stars per square parsec). The former represent stars that have formed in relative isolation, while the latter represent stars that have formed in dense groups or clusters.

Most notable in this figure is the fact that there is a smooth distribution from low to high densities, which suggests that stars don't just form at low and high densities (in isolation and in clusters), but at a wide range of densities, with groups and clusters existing over a variety of densities.

This is important for our understanding of star formation because it tells us about the conditions under which stars form, as well as the sort of environment where planetary systems form. A planet forming in a dense cluster faces very different conditions compared to one born around a relatively isolated star. In a dense cluster there could be multiple interactions or collisions between stars and planets, as well as a very powerful radiation field due to the close proximity of so many other stars, which could damage a forming planet's atmosphere.

Hopefully as we start to learn more about the various types of planetary system that exist, and especially once we start studying the atmospheres of these planets, we can hopefully address the question of what impact the birth environment has on a forming planetary system.