Star birth and the Sea Serpent

What do sea serpents have to do with star formation I hear you ask! Well normally the two may not be connected, but this sea serpent is no ordinary snake, it is the giant constellation of Hydra, otherwise known as the Sea Serpent! Within this constellation lies a very important young star known as TW Hydrae, which has the honour of being the closest T-Tauri star to the Sun.

T-Tauri stars, as I've discussed before, are young stars, named after the prototypical example T Tauri. They are stars which have almost completely formed but are still accreting some material from their surroundings, which can cause them to glow brightly. This accretion process is not fully understood and so astronomers try to study as many different accreting stars as possible so that they can better understand how this is happening and how the accretion is affecting the star.

Artists impression of the TW Hydrae star
and its protoplanetary disk (Credit: NASA)

TW Hydrae is particularly important because it is the closest T-Tauri star at a distance of only 176 light years, and therefore it is the easiest such object to study. It was first recognised as a young star by American astronomer (and later astronaut) Karl Gorden Henize, who noted extreme H-alpha emission, typical of strongly accreting objects, in 1976.

The isolation of the star, not near any other T-Tauri stars or known star forming regions, suggested to some astronomers that it might not be a truly young star. However thanks to further observations of the star a few years later it was shown to be a bona fide and fully accreting young T-Tauri star, despite its apparent isolation.

The star itself is similar to our Sun, a little less massive and therefore a little redder, but much younger. Current estimates put its age at about 5-10 million years old, considerably younger than the 4.6 billion year old Sun that we orbit.

Fortunately for us of course the Sun is not a young star, because young stars like TW Hydrae can be very volatile objects. Young stars are surrounded by a disk of gas and dust known as a protoplanetary disk. These disks are created as material falls towards the star and gathers in a disk around it, due to a process known as the conservation of angular momentum (which basically means that things that spiral in towards a star like to keep spiralling, or at least orbiting the star and this can sometimes stop the material from falling onto the star).

Material from this disk is occasionally accreted onto the star, channeled along magnetic field lines before pummelling onto the star's surface. When this material hits the stellar surface it is heated to incredibly high temperatures, approximately 3 million degrees, which causes the star to briefly shine brightly. You don't want to be near such stars when they're accreting!

The stars and motions of the TW Hydrae association (shown in red and labelled "TW Hya" and with new members shown in black) relative to other nearby OB associations (Credit: Murphy et al. 2015).

Recently astronomers have uncovered other low-mass stars in the vicinity of TW Hydrae, creating a small group of about 20-30 stars known as the TW Hydrae association. These stars are all very young and share a common motion through our Galaxy, suggesting that they probably formed together. This discovery will be useful for astronomers looking for other young stars to study, as well as for studying how stars form in groups such as these. And importantly, TW Hydrae isn't alone any more!

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!