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.

Star formation, filaments, and the initial distribution of young stars

I've talked before about molecular clouds, the birthplaces of stars, but only in very general terms. Today I want to go a little deeper to look at how stars are distributed within molecular clouds when they form, and how this can lead to the formation of star clusters.

Molecular clouds aren't just big fluffy clouds of gas that uniformly form stars, they have considerable structure. This is clear if you look at molecular clouds using an infrared telescope, particularly one that's tuned to the far-infrared, such as the Herschel Space Observatory (the further into the infrared you go, the cooler the material you can study because cooler things radiate light at longer wavelengths, i.e. the far infrared).

A far-infrared image of the Taurus molecular cloud, showing the filamentary structure of the gas
(Credit: Herschel Space Observatory)

This image from the Herschel Space Observatory shows the incredibly filamentary structure of the gas in the Taurus molecular cloud. This structure is thought to arise due to a combination of shock compression (due to collisions between material) and self-gravity (meaning the filaments can form gravitationally-stable structures by themselves). These filaments can be seen on all spatial scales, from the large filament spanning the entire image, through to small filaments emanating from the larger ones.

Many of these filaments are dense, containing many times the mass of our Sun in molecular gas. This high density means that they can become gravitationally unstable, which can lead them to collapse and potentially form stars. Simulations of filaments suggest that a single filament may actually fragment into multiple stars distributed along it's length.

The NGC 1333 molecular cloud. Contours
show the distribution of dense gas, while
 the red dots show the very young stars that
are still forming (Credit: Gutermuth et al. 2008)

One of the predictions of this model of filamentary star formation is that the densest gas in star forming regions should be distributed in filaments and that the youngest stars - those that are actually still forming - should be found within these filaments.

The image to the right shows that the distribution of very young stars (red dots) in the star forming molecular cloud NGC 1333 are almost entirely projected against (and therefore likely to be within) the filamentary dense gas shown by the contours.

This provides strong evidence that stars form directly out of the dense, filamentary gas in molecular clouds.

Not only do the young stars trace the spatial distribution of the dense gas, but they also appear to have very similar motions. Numerous studies have measured the radial velocities (the speed of an object along the line of sight - i.e. towards or away from us) of stars such as these and found that they are almost identical to the velocities of the gas (e.g., Walsh et al. 2004, Hacar et al. 2016).

Figure showing the Orion A molecular cloud. The
background greyscale image show the dense gas
while the red dots show very young stars
(Credit: Megeath et al. 2012)

This picture where star formation occurs in dense gas is seen in all the star forming regions we've studied, and not just on small scales, but also on the much larger scales of giant molecular clouds such as the Orion A cloud, shown on the left.

This is important for understanding star formation because it not only tells us where stars will form, but also provides clues as to how stars form (i.e. the process requires very dense gas) and what must occur to set up the necessary conditions (i.e. the gas in molecular clouds must be compressed in dense filamentary structures that become self-gravitating).

It is also important for understanding star clusters because it tells us not just the initial spatial distribution of groups of stars that may go on to form clusters, but also their kinematics, i.e. how those stars are moving.

The next question to ask is how do these elongated and substructured groups of stars evolve into the classic centrally-concentrated images of star clusters we see so often? But that's a question for another day!

The nearest site of star formation, the Taurus Molecular Cloud

Following my recent post about how stars form I wanted to take some time to talk about a few of the most prominent star forming regions that astronomers have studied and what they have taught us about the star formation process. The first of these regions I want to talk about is the Taurus Molecular Cloud.

The Taurus Molecular Cloud is, unsurprisingly, in the constellation of Taurus. It is the nearest star forming region to our Sun and so it is the region that astronomers have been able to study in the most detail. The proximity of the Taurus Molecular Cloud means that it spans a relatively large area on the sky, almost 10 degrees across, but the lack of a bright emission nebula means that all that can be seen even with a good backyard telescope is the obscuration of the background stars due to the gas and dust in the cloud.

The image below shows where the Taurus Molecular Cloud can be found in the night sky, approximately halfway between Elnath (the second brightest star in the constellation of Taurus, to the upper left) and the Pleiades open star cluster.

Location of the Taurus Molecular Cloud within the constellation of Taurus, and near the Pleiades star cluster. North is up in this image, East to the left (Image adapted from one by David Malin)

The Taurus Molecular Cloud was discovered in 1852 by J.R. Hind, which he noted as a faint nebulous object on the sky. Astronomers were soon able to take a spectrum of the light from the nebula and were able to confirm that it was indeed a giant cloud of gas. However, its importance as a site of nearby star formation wasn't immediately recognised.

The archetypal young star, T Tauri, visible in the centre
of this image and surrounded by a small dusty cloud

(Credit: University of Alaska)

At the beginning of the 20th Century many astronomers became interested in a number of variable stars identified in the vicinity of the Taurus Molecular Cloud, the most prominent of which was named T Tauri after the constellation in which it was found.

Searches for other variable stars in the vicinity of dark nebulae produced many candidates in the 1940s and 1950s. These stars were often found in the vicinity of young OB stars (massive stars with particularly short lives, hence they must be young), leading many astronomers to believe that they were particularly young stars themselves. This discovery led to the realisation that the dark nebulae that these stars were found near was likely where these stars had formed. This was how the modern theory of star formation began!

The star T Tauri is now firmly recognised as the prototypical young star, and its name has been given to the class of young stars that share its properties, T-Tauri stars. It is thought to be less than a few million years old, already formed but still accreting material and growing in mass. The star itself is actually in a binary system with a fainter star, and is thought to be surrounded by a disk of material that is in the process of accreting onto the star, which partly explains the variability of the star that initially brought it attention.

An infrared view of the Taurus Molecular Cloud (Credit: FCRAO)

With the invention of infrared detectors in the second half of the 20th century astronomers were able to start peering into these dark nebulous clouds to study the star formation process within them, and as the nearest such cloud Taurus was a major target for early infrared astronomy.

The infrared image shown here penetrates the dusty molecular cloud and allows astronomers to see the giant gas clouds that are in the process of collapsing into stars. This image shows the cloud in immense detail, with filamentary tendrils of gas and dense cores where stars will one day form.

Because the Taurus Molecular Cloud is not large enough to be forming any really massive stars the region is spared the destructive powers that these stars can inflict on their surroundings. This means that there is still considerable molecular material in the cloud, including many molecular ices, despite the fact that many stars have already formed. This means that star formation is still ongoing and may be able to continue for a while, all thanks to the tranquil nature of the stars formed here.

The rise of infrared astronomy has also led to an increase in the number of young stars discovered in the cloud. Prior to this only the bright and optically visible young stars, such as T Tauri itself, had been identified. But infrared observations allowed astronomers to peer into the dark and obscuring clouds and identify many more young stars, and several hundred are now known.

The distribution of young stars (red stars and triangles) in the Taurus Molecular Cloud, show against a map of the molecular hydrogen in the cloud. Yellow diamonds, blue squares and green circles show young stars with known outflows. (Credit: Narayanan et al. 2012) 

This large sample of young stars has been vital in helping astronomers learn about star formation. For example, the distribution of these stars, as can be seen in the image above, coincides strongly with the distribution of the dense molecular gas, suggesting that stars form in regions of particularly dense gas. Furthermore with so many young stars all roughly of a similar age, astronomers have been able to produce models for how young stars of a given age would appear as they finished forming, and were then able to compare these models with the stars discovered in Taurus.

All in all the Taurus Molecular Cloud has been vital for how astronomers have learnt about the star formation process. It has provided a rich, nearby laboratory to study the dark nebulous clouds in which stars form and also to observe the final stages of the star formation process itself. Next time you look up and see the constellation of Taurus, see if you can spot the dark clouds of the Taurus Molecular Cloud and think about how important this region has been for astronomy!