Thursday, 10 December 2015

What are OB associations and why should you care?

Over the last year I've talked a lot about different types of star cluster, but I've also mentioned another type of stellar group known as an OB association, and you may be wondering what are these OB associations? In this post I'm going to talk about OB associations, what they are, and why they're so important for understanding star clusters.

OB associations are very similar to young star clusters in that they are a group of young stars. They're not as compact as star clusters though, and the stars in them are spread out over a larger area of space than for star clusters.

OB associations were first identified just over a century ago by Arthur Eddington who noted the presence of groups of stars distributed across large areas of the sky but sharing a common motion. The actual term OB association was first penned by Victor Ambartsumian in 1947, who noted that these associations appeared to consist mostly of bright and blue stars known as O- and B-type stars. The image below shows the distribution of these stars near the Sun, showing how they appear to fall into a number of rough groups.

The distribution of nearby O-type (filled circles) and B-type
(empty circles) massive stars from Hipparcos. The boxes show the
positions of known OB associations (Credit: Preibisch & Mamajek 2008)
Ambartsumian noted that the low density of the OB associations meant that they weren't gravitationally bound (meaning that they weren't held together by their own gravity - in the same way that dense star clusters are). This means that they must be in the process of expanding and dispersing, and also implies that they must be relatively young structures if we are able to observe them before they have dispersed.

This revelation came at the same time as other scientists were beginning to understand how long stars of different types lived for. This revelation came from understanding the nuclear fusion that gives stars their energy, and then comparing the source of energy that each star has with the rate at which it is radiating that energy away. The O- and B-type stars are amongst the most luminous of all stars (as well as being the most massive), meaning that they are radiating away their energy the fastest and therefore have the shortest lives of all stars. This discovery confirmed the youthfulness of the OB associations that Ambartsumian had hypothesised.

If OB associations are not gravitationally bound then this means that they're probably in the process of expanding and dispersing into the Galactic field. And if OB associations are expanding then this means they were probably smaller and more compact in the past. This has led some astronomers to suggest that OB associations are the expanded remnants of compact star clusters that have been disrupted by some process.

The most common explanation for this disruption is that young star clusters are thought to be held together by the giant gas clouds that they formed in, and when star formation finishes this gas is blown away by powerful winds that come from massive stars. Without this gas the star cluster doesn't have enough mass to hold themselves together by gravity, and so the cluster begins to expand and disintegrate. The schematic below illustrates this sequence of events.

The sequence of events leading to the disruption of a star cluster: (1) The star is born embedded within a cloud, (2) the winds from the young stars disperse the cloud, and (3) without the gravitational potential of the cloud holding the cluster together the star cluster disperses and is briefly visible as an OB association. (Credit: Nick Wright)

There are other suggestions for how star clusters might be disrupted, such as tidal heating of the cluster, or possibly that some of the young star clusters that appear so ubiquitous may not actually be gravitationally bound in the first place and would therefore naturally expand and disperse.

This all means that OB associations are quite important objects to study, because by studying them we can effectively observe the process of star clusters being destroyed. Unfortunately they're not easy objects to study because the stars in an OB association are often spread over a large area of the sky and they can sometimes be difficult to distinguish from the older stars that make up the Milky Way Galaxy. For this reason our knowledge of OB associations, and our census of those that exist in our galaxy is rather slim.

The Scorpius Centaurus association
(Credit: Akira Fujii)
The most well studied OB associations are those nearest to us (this is often the case in astronomy). Associations such as the Scorpius-Centaurus association (see image on the left), the Perseus OB association, and the Lacerta I association were all discovered thanks to the bright and blue O- and B-type stars in them.

Other, slightly more distant OB associations include a number of slightly larger and more populous OB associations in the constellation of Cygnus (some of which I've discussed before here and here), as well as the Orion I association that surround the bright clusters in that constellation. Sometimes OB associations include a number of smaller clumps or clusters of stars within them, for example the double clusters h and chi Persei are part of the larger Perseus OB associations discovered in 1943.

There are a few OB associations that you can see without the aid of a telescope, but not many unfortunately because they are often very diffuse. The Alpha Persei cluster in the constellation Perseus is part of the larger Perseus OB association and is easy to observe. And while you may not be able to see most OB associations there are a number of young clusters you can observe that may one day become OB associations!

Wednesday, 4 November 2015

The distance to the Moon

I've recently been watching the wonderful HBO series From the Earth to the Moon, a dramatisation of NASA's Apollo missions to the Moon, which I'd highly recommend. As I've been watching it I've been reminded of some of the amazing contributions to science that these missions made. Its certainly true that their original motivation wasn't scientific, but that doesn't mean they didn't achieve anything scientifically.

One of their most important achievements is helping to precisely measure the distance to the Moon. Knowing the distance to the Moon and how this changes is important for understanding the orbit of the Moon, which influences tides on Earth, and whether the Moon is spiralling towards or away from the Earth.

Calculations from the ancient Greek astronomer
Aristarchus used to estimate the distance to the Moon
(Credit: Wikipedia)
Prior to the Apollo missions there were various methods to measure the distance to the Moon, including using radar and simple trigonometry. The ancient Greeks were the first to try to measure the distance to the Moon using trigonometry.

The astronomers Aristarchus and Hipparchus both succeeded in using this method, with the latter measuring a distance of about 410,000 km, an estimate which is only off by about 25,000 km, or 7% of the total distance.

To improve the distance measurements that were available at the time, the Apollo programme decided to take special reflectors to the Moon and leave them on the lunar surface. These specially-designed reflectors allow the distance between the Earth and the Moon to be measured by aiming lasers on Earth at the positions of these reflectors and then timing the amount of time it takes for the laser to be reflected back to Earth. Since we know how fast light moves through the vacuum of space we can use the time this journey takes to calculate the distance travelled.

The lunar ranging equipment, as left by Apollo 11 (Credit: NASA)

Once the reflectors were installed by Neil Armstrong and Buzz Aldrin of Apollo 11, a number of telescopes around the world were able to use them to measure the distance to the Moon. Additional reflectors were also left on the lunar surface by the Apollo 14 and 15 missions, with the latter using a particularly large reflector array that was three times the size of the other two reflectors. The majority of distances measurements to the Moon since then have used the Apollo 15 reflector due to its size.

Thanks to the precise measurements that these reflectors have allowed we have learnt a considerable amount about how the distance between the Earth and the Moon is changing. The distance to the Moon, which is approximately 385,000 km, is now known with an accuracy of better than one part in 10 billion. The exact distance changes throughout the Moon's orbit around the Earth, as well as due to a number of smaller effects.

One of those small effects is that the Moon is very slowly moving away from the Earth, at a rate of about 3.8 cm per year. While this is only a tiny fraction of the total distance between the Earth and the Moon it is surprisingly high! Don't worry though, at that rate it would take millions of years for there to be any perceptible change in the Moon's appearance from Earth!
Artist's rendering of the lunar core (Credit: NASA)

Variations in lunar rotation and orbit, measured thanks to these reflectors, have also provided evidence that the Moon probably has a liquid core. This core is thought to be about 20% of the Moon's radius. Seismographic measurements since then have refined this picture, suggesting that the Moon may have a solid inner core surrounded by a fluid and partially-fluid outer core.

Perhaps the most important use of knowing the precise distance to the Moon is the impact of the Moon on the rotation of the Earth, due to tidal gravitational forces. We now know, for example, that the Moon has a small effect on the length of the Earth day, which is changing very slowly due to this effect.

Nick Wirght (2015), The distance to the Moon (Blogger).

Wednesday, 28 October 2015

I'm a Scientist, Get me out of here!

In Spring 2014 I took part in a wonderful outreach event called I’m a Scientist Get Me Out of Here! This initiative brings scientists and school children together using a combination of internet chat events between scientists, school children and their teachers, and question boards that scientists post answers to. The objective is to give school children a chance to meet real scientists and ask them the sort of questions they want to ask them. I found it great fun and I got to chat to a lot of students and heard what they thought about many of the big science questions of the day.

After a fortnight of regular chat events, and trying to answer as many of the student's questions as possible, I learnt that the students had voted me their favourite of all the scientists taking part that month. I was touched to hear that I had connected with so many students and I hope they had all learnt something from me as well! The reward for winning the event was a small cash prize to spend on my outreach work, which was a wonderful gift from the organisers.

I wanted to find a way to put the prize money towards a project that provided school children with exposure to real scientific research in a fun and simple way. My original plan was to produce a series of posters that shared cutting-edge scientific ideas in astronomy with school children through simple but attractive imagery. In the end I did spend a portion of the money on this project, but I also wanted to do something more proactive that would make a longer-term impact on school children across the country.

Two of the science posters made to try to explain complex scientific ideas using attractive astronomy images.
Copies of both of these posters are still available, so please message me if you'd like one sent to your school.

The British Science Association runs an excellent scheme called CREST Awards, whereby 11-19 year old school students complete projects that help build their scientific skills. The work for these is typically done in school STEM clubs, which also provide an extra opportunity for students to engage with science. You can read more about them or find out how to get involved here.

Suitable scientific projects for CREST Awards are not easy to prepare, particularly astronomy projects and this was evidenced from the shortage of such projects currently available for schools from the CREST website. I wanted to remedy this by designing a number of astronomy-related CREST projects that used real astronomy data, yet were easy for school teachers to use, and were also really fun for students.

Most astronomy data is actually free and readily available for anyone to use, including high-quality images that can be used to answer exciting and cutting-edge questions in science. It is my hope that by helping school children access this data and providing projects that allow them to study and use this data in a real scientific experiment we could enthuse the children to pursue scientific subjects in school and possibly take up a career in science. So I started designing a few projects that used real and freely available data.

As it happened at that time a local school in Hertfordshire made a request to our local STEMNET coordinator for someone to help them set up a new STEM club. I thought this would be a perfect opportunity not only to test my CREST projects, but also to work with a school going through the process of setting up a STEM club, and in doing so see the process through the teacher’s eyes.

Me with part of an inflatable Solar System I bought with the prize money
to help teach students about the planets.

Working with the school and the teachers I was able to refine and fashion the projects, making them easy to use for the teachers, while also being new and exciting for the school children. I used the prize money to visit the school regularly, give talks to the students, and advise the teachers on how to run the CREST Award projects I had designed. By being directly involved in the STEM club itself I was also able to see how the students reacted to the projects, what worked for them and what they enjoyed the most. It was a really enjoyable experience and taught me a lot about how teachers interact with students and what scientists can most helpfully provide teachers with to help them.

These projects will soon be available through the CREST website, with information for how they can be run, where the data can be obtained, and how the results can be interpreted by students and teachers. This is all thanks to "I'm a Scientist, Get me out of here!", so thank you to all involved!

Monday, 19 October 2015

Now blogging from Keele University

The last month has been rather hectic (hence the lack of posts!), having moved both house and university. I've left my position at the University of Hertfordshire and moved to Keele University in Staffordshire to take up an STFC Ernest Rutherford Fellowship. I'm very excited to be working at Keele University and honoured to have been awarded such a high profile fellowship.

Keele Hall, part of Keele University in Staffordshire
Keele University is home to a good-sized astrophysics department, with many prominent scientists working in a wide variety of areas. As well as experts in young stars and star clusters (whose interested overlap with mine), there are also researchers studying massive stars and planetary systems. Hopefully there will be lots of opportunities for collaboration while I'm here!

Now that I'm settled into my new house and my new department I hope to be able to return to active blogging - stay tuned for more posts!

Monday, 14 September 2015

Hatfield Cafe Scientifique starts tonight!

I'm very excited because today we're launching the first University of Hertfordshire Cafe Scientifique here in Hatfield! Cafe Scientifique provides an opportunity for people to gather and discuss science while having a coffee, a pint or a glass of wine. Since it was formed in 1998 almost 50 separate Cafe Scientifiques have been set up in towns and cities across the UK, and today we start the first such gathering at the Red Lion pub in Hatfield, Hertfordshire.

Cafe Scientifique in action - discussing science!
(Credit: Cafe Scientifique)
We chose to start this venture because we (a mixed group of scientists from the University of Hertfordshire) wanted to find a way to discuss our scientific ideas and research with the general public in a venue different from the lecture halls and school rooms in which we often communicate such ideas. Furthermore since there wasn't yet such an initiative in either Hatfield or the neighbouring town of St Albans, we thought it was about time there was one!

So tonight we kick off a monthly series of talks and discussions about various scientific topics. Tonight, Dr Keith Davis will be talking about Art vs Science and how our perceptions of the two cultures have changed in the 50 years since C.P. Snow's seminar lecture. The event will start at about 7pm and is completely free (though I'm sure the pub would appreciate it if you had a drink or a bite to eat while you're there). If you're in the area feel free to come along, I hope to see you there!

Thursday, 27 August 2015

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!

Monday, 24 August 2015

Why is there a giant ring of young stars surrounding us?

A few weeks ago I talked about one of the nearest (and most important) regions of star formation, the Taurus Molecular Cloud. This region however isn't alone in being a relatively nearby region of star formation, in fact it forms part of a group of such regions that circles us in the night sky and which is known as the Gould Belt.

The Gould Belt is a ring of nearby star forming regions and young star clusters that surrounds our Sun. It was first observed by John Herschel (son of the famous William Herschel who discovered infrared radiation), who noted it as a band of bright stars that circled the sky, tilted away from the plane of the Milky Way, which also circles the night sky.

The bright stars of the Scorpius-Centaurus OB association,
part of the Gould Belt of young stars and star forming regions
(Credit: Akira Fujii)
The belt was named after an American astronomer called Benjamin Gould who performed the first detailed study of the structure in the 1870s. Gould was a pioneering and prominent astronomer, the first American to earn a doctorate in astronomy, and would go on to found the Astronomical Journal, one of the most prominent astrophysical journals in the world.

In the early 20th century astronomers were able to use spectroscopy to measure the speeds of the stars in the belt and found that they were moving with similar motions. This meant that all the stars in the Gould Belt were part of a single coherent structure. The distinctive pattern of the stellar motions has also revealed that the belt is both expanding and rotating. The rotation of the belt is thought to be caused by the fact that it sits embedded within the Milky Way galaxy, which is itself rotating as well.

The system appears to be broadly flat and pancake-shaped, approximately 2000-3000 light years across, but only about 400-500 light years thick. There is also evidence that the belt isn't perfectly circular and is more oval-shaped, a distortion that is also thought to be due to the rotation of the Milky Way as it stretches and twists the belt.

Illustration of the Gould Belt across the night sky relative to the distribution of molecular clouds in our galaxy. The Gould Belt is shown in red and the Galactic Plane is shown in blue. (Credit: Nick Wright / Thomas Dame)

The discovery of molecular clouds of hydrogen gas in the second half of the 20th century revealed that the Gould Belt was made up of many such clouds. When it was later realised that such clouds were where stars form it was quickly recognised that the Gould Belt represented a major site of star formation. It is now known that these star-forming molecular clouds make up most of the mass of the Gould Belt. This includes many well known structures such as the Taurus Molecular Cloud, the Orion Nebula, and the Rho Ophiuchi cloud complex.

While there are still many stars forming in the Gould Belt there is also a considerable history of star formation dating back almost 60 million years. This has lead to a huge collection of young star clusters (such as the Pleiades and Alpha Persei clusters), many OB associations (the famous Scorpius-Centuaurus association amongst others), and a number of luminous supergiant stars (such as Antares, the bright red star in the constellation of Scorpius). Supergiants are massive stars that are coming to the ends of their short lives and these objects were most likely born in one of the star forming regions in the Gould Belt. In fact the Gould Belt includes the majority of massive stars in the solar neighbourhood.

The young star clusters and OB associations that make up the Gould Belt,
shown in 3D relative to the plane of the Milky Way.
(Credit: New Scientist)

Since the discovery of the Gould Belt astronomers have been trying to understand how such a large and coherent structure formed within our galaxy. It was first thought that it formed when a massive star exploded as a supernova. The shock wave from the supernova would have swept up huge clouds of gas, compressing them and triggering the formation of new stars within them. However, if this were the case the ring should be aligned with the Galactic Plane and not pointing out of the plane (as the above image shows), so this theory appears to be ruled out.

A recent suggestion is that the Gould Belt was produced when a massive cloud of gas collided with our galaxy, in the same way that dwarf galaxies are known to collide with our Milky Way galaxy. This collision would have lead to a giant ring of expanding gas in the Milky Way that would be inclined at the same angle to the Milky Way of the collision itself. The shock wave from this would lead to the triggering of star formation and the creation of young star clusters, just as in the structure we see today.

This theory represents the best explanation astronomers have for the Gould Belt at this present time. Recently weight was added to this theory when astronomers found evidence for similar structures to the Gould Belt in other galaxies, suggesting that this phenomena may not be that rare.

The Gould Belt is just one of many structures in our galaxy that we can observe, from local star-forming clouds up to massive spiral arms. Understanding how these structures relate to the continual process of star birth and death and the evolution of our galaxy is one of the major tasks for astronomers today. Next time you look up at the night sky and see the bright stars and star clusters that are part of the Gould Belt think of how these objects are a part of the continual evolution of our galaxy!

Wednesday, 12 August 2015

Tracing the spiral arms of the Milky Way

Our galaxy, the Milky Way, is thought to be a huge spiral galaxy like many such galaxies we see across the Universe. One of the tasks that modern-day astronomers are trying to achieve is to map the size and structure of our galaxy so we can better understand how it formed and how it will evolve in the future.

The spiral galaxy Messier 100 - similar to our own Milky Way? (Credit: ESO)

One of the key tasks in such work is to map the spiral arms of our galaxy. This is important because spiral arms are thought to be where the majority of dense gas is found in galaxies, and therefore where the majority of star formation takes place. Spiral arms aren't fixed objects though, the stars in our galaxy actually move in and out of the spiral arms as they orbit within our galaxy. Spiral arms are actually thought to be density waves that rotate around our galaxy, independently of the stars in our galaxy, just like waves in the ocean move independently of the water in them.

Spiral arm model of the Milky Way with four arms.
The Sun is located towards the top of this image.
(Credit: Georgelin & Georgelin 1976)
Identifying spiral arms is easy when you're outside of a galaxy and looking at it face on, but its much harder when you're embedded within the galaxy and all you can see is the plane of our galaxy. We can't directly see the spiral arms of our galaxy, but we can trace their presence by looking for signposts that identify them. Signposts such as giant molecular clouds, star forming regions, and bright young stars are all indicators of where spiral arms are found.

The Milky Way was first identified as a spiral galaxy thanks to the work of William Morgan from Yerkes Observatory who showed that the distribution of bright and hot OB stars, which are known to be very young objects, appear to be distributed in spiral arms. Morgan identified three spiral arms, which he labelled the Perseus, Orion and Sagittarius arms.

Later studies that attempted to discern the spiral structure of the Milky Way used the radio emission from hydrogen gas to trace its structure, but it can be tricky to determine the distance to such gas, making it hard to reveal the 3-dimensional structure.

A major breakthrough came in the 1970s when scientists combined radio measurements of hydrogen gas with optical measurements of the distances to the young stars associated with the gas. This work lead to a model made up of four spiral arms called the Norma, Scutum-Centaurus, Sagittarius and Perseus arms. While many researchers debated the distances to the various star forming regions used for this model (and therefore the exact structure and number of spiral arms the model predicted), this picture was for over 30 years the standard model of the spiral structure of the Milky Way.

The model changed again in 2008 thanks to data from NASA's infrared Spitzer Space Telescope, which allowed astronomers to count the number of stars all the way across our galaxy. The number of stars they counted suggested that there weren't four spiral arms, but only two, with a number of smaller spiral arms lying in between them.

Artist's conception of our new view of the Milky Way's structure thanks to results from the Spitzer Space Telescope.
The Sun's position is marked towards the bottom of this image.
(Credit: NASA)
This new model suggests that the Perseus and Scutum-Centaurus arms are the two major arms, while the Norma and Sagittarius arms are actually relatively minor arms. The two major arms connect up with the inner Galactic Bar, which dominates the central part of our Milky Way and may also play a role in the origin of the spiral arms.

Recently a flurry of results have taken this work even further with suggestions of a new and distant spiral arm that wraps completely around one side of the galactic centre, while other researchers have started using the distribution of star clusters to trace the structure of the Milky Way. Further improvements in the model of our galaxy's structure have come thanks to improved distance estimates for many of the stars and clusters in our galaxy, allowing the exact size and extent of the galaxy to be better determined.

Upcoming missions such as the Gaia observatory that will determine the distances to a billion stars across our galaxy will dramatically improve our understanding of our galaxy's size and shape. The motions that the Gaia spacecraft will measure will allow astronomers to study the orbits of these stars as well, improving our understanding of our galaxy from a purely structural model to a more advanced dynamical model.

Wednesday, 29 July 2015

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
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!

Thursday, 23 July 2015

How do stars form?

Stars, like our Sun, are forming all the time across out Galaxy and in other distant galaxies. Understanding how stars form and what causes different types of star to form is one of the most important areas of research in astrophysics. Today I'd like to discuss the star formation process, what we know about it, and what we are still trying to understand.

Stars form out of dense clouds of gas (mostly made of hydrogen and helium) known as molecular clouds, so-called because many of the atoms in them have cooled and formed molecules. These molecular clouds are huge and are mostly found in the spiral arms of galaxies such as our own. The clouds are very cold, with temperatures of only 10 to 20 Kelvin (about -253 Celsius) and made of molecular gases such as H2 and CO.

The Whirlpool galaxy imaged in visible light (left) showing young stars and star-forming regions delineating the spiral
arms and a radio image (right) showing emission from the CO molecule tracing the molecular clouds in which stars form (Credit: NASA / PAWS)

These molecular clouds are thought to be held in balance between the inward force of gravity (which tries to make them collapse) and the outward pressures of magnetic fields and the motions of the molecules in the cloud (which are trying to make the cloud expand and disperse).

Eventually though something has to give and some part of the molecular cloud will begin to collapse. As it does so it will also cool as the molecules in the cloud release energy through as process known as radiative cooling, which helps the cloud collapse further. If the molecules weren't able to cool down while the molecular cloud contracted then the increase in density would cause them to heat up and the molecular cloud would expand and disperse, so this cooling is critical for star formation.

The dark cloud Barnard 68 (Credit: Marco Lombardi)
As this happens the molecular cloud will begin to fragment into smaller and smaller clumps of gas, each becoming denser and denser as they contract in towards their centres. In fact the density can reach so high that no light can penetrate to the centres of these clumps, making them so dark that they even block the light from background stars. We call these objects dark clouds, because they appear as dark patches on the night sky!

Once these dark clouds are dense enough that they can block out starlight then they cool even faster because they are no longer being heated by the light from nearby stars. Once these clouds have cooled even further then they can even block infrared radiation and become so cool as to not even emit infrared radiation. Only the coldest objects in the Universe are so cold as to not emit infrared radiation!

Once the centre of the clump has collapsed considerably a dense, gravitationally stable core forms in the centre, known as a protostar, which begins to heat up as it continues to contract. The protostar continues to grow in size by accreting more material from the surrounding molecular cloud, its core getting denser and hotter as it does so, and after a while the protostar begins to radiate energy into the surrounding molecular cloud.

A forming protostar surrounded by a disk of material accreting onto it
(Credit: ESO)
At this point the protostar is massive enough that it attracts considerably more material from the surrounding molecular cloud, which falls towards the star. Due to the conservation of angular momentum this material spirals in towards the star and forms a disk of material that orbits the star, slowly accreting onto the star in bright bursts that illuminate the surrounding cloud. With each burst of accretion the star becomes hotter and more massive.

Eventually the core of the protostar becomes so dense and hot that the temperature is high enough for nuclear fusion to take place. At first the star can only burn deuterium, but as it gets hotter it will eventually burn hydrogen just like our own Sun. The star is now beginning to shine quite brightly and the radiation from the star prevents further material accreting onto the star and may even begin to disperse the remaining material in the disk that still surrounds the star.

Once the star has started fusing hydrogen into helium we say that it has fully formed. Hydrogen fusion is the process by which the vast majority of stars create their energy, and the star can usually maintain this for billions of years before it runs out of hydrogen in its core.

This is the rough process by which we think stars form, and there is a lot of evidence to support this picture, including observations of forming stars and computer simulations that try to model the entire process. There are however a number of outstanding questions that scientists are still trying to answer, such as: How are stars clustered when they form (for example in clusters and OB associations) and what causes this? What causes stars to form with different masses? And what brings the star formation process within a molecular cloud to a halt? These are questions that astronomers such as myself are actively trying to answer!

Tuesday, 14 July 2015

Big data from new telescopes

I read an article recently on the Guardian's science website called Big universe, big data, astronomical opportunityThe article discussed the rise of large datasets in science, particularly in astronomy, and the need for astronomers to adapt to the challenges faced by this by acquiring new skills for analysing such data.
The Large Synoptic Survey Telescope
(Credit: LSST)

The article made many good points about the rise of big data. We are, after all, on the verge of a number of large projects that will produce more data than we've ever seen before. These include the Gaia satellite (which I've talked about before), new sub-mm and radio telescopes such as ALMA and the Square Kilometre Array, and the Large Synoptic Survey Telescope. The latter is particularly noteworthy because once it enters operation in 2022 it will produce 30TB of data each night, more than most telescopes produce in a year!

Astronomers will need to learn not just to manage these datasets, but to effectively sort through and process them. New algorithms will need to be designed, utilising artificial intelligence and advanced machine learning methodologies, as well as new approaches to visualise and understand the results. Astronomers will have to embrace this because this is the way science is going, and the telescopes that will produce this data are already being built, or in some cases are coming online as we speak.

Despite making these good points the article then appears to suggest that these new telescopes and satellites, designed to answer the most far-reaching questions humanity has ever asked, aren't what we need. "Have we leaned all that we can from the data that we have?", the author asks, suggesting that we should analyse all the data we have before collecting more. The article even suggests that the data we have could "hold answers to some of the fundamental questions of the Universe we are seeking".

I strongly disagree with the author on this point and find this attitude rather short-sighted. The data that we have may provide answers to some questions, but certainly not the big questions we want to answer, and that's where we should be investing our efforts. We won't find answers in our existing data to questions such as how the Sun and the Earth formed, how the Universe began, or whether there is life out there, it's just not possible.

Astronomical research is the continuous pursuit of answers to the greatest questions we can ask. We usually can't answer these questions in one go so instead we answer them step-by-step, performing experiment after experiment, refining the question as we go. For each experiment we design the equipment needed to answer the question, and then when that question has been answered we take the research further. If the same equipment or telescope can be used to answer the next question then we use it, but often the next question involves looking further, deeper and at different wavelengths, requiring new telescopes and equipment.

Just because a lot of data already exists doesn't mean that the answers to our questions can be found there. The great explorers of the past didn't discover new continents by searching around at home, they built great ships and went in search of discovery. Astronomers shouldn't be searching around in old data, but building great telescopes and searching the skies for their discoveries!

Wednesday, 17 June 2015

X-rays from stars

Tomorrow I'm going to a small conference at the University of Warwick to talk about X-ray emission from stars and our Sun. It may surprise many of you to learn that our Sun is a relatively bright source of X-rays, and it certainly surprised most astronomers when this was discovered in the 1940s!

Why is this such a surprise? Well, the surface temperature of the Sun is a relatively modest 6000 C, which may seem like a high temperature to you and I, but its certainly not hot enough to emit X-rays, which require temperatures of millions of degrees. And yet when astronomers launched the first rockets above our atmosphere in the 1940s they found that the Sun was a bright X-ray source!

X-ray image of our Sun taken from the Solar Dynamics Observatory
(Credit: NASA/SDO)

As it turns out these X-rays don't actually come from the surface of the Sun but from a region above the surface called the corona. You can see this in the X-ray image above, which shows the corona reaching above the surface of the Sun. The corona is a hot, low density plasma of gas, which reaches temperatures of millions of degrees, making it very bright in X-rays but almost completely invisible in the optical part of the spectrum.

So why is the corona so hot if the surface of the Sun is much cooler? Well this is actually an open question that many astronomers are still trying to answer, and its a question I've tried to answer by studying other stars. Our current best guess is that the corona is heated by the release of magnetic energy generated deep within the Sun in a type of dynamo. Unfortunately we can't see within the Sun to work out how and why this is happening, which is why this is still an unanswered question. Hopefully tomorrow's meeting will shed some light on this topic!

Friday, 29 May 2015

Tycho Brahe's gin

Last year I attended a conference in Copenhagen and while I was there I wanted to visit Tycho Brahe's observatory. For those who don't know, Tycho Brahe is considered to be one of the greatest astronomical observers in history, particularly in the pre-telescope era in which he lived, and he made many notable contributions to astronomy, including reporting on the 1572 supernova that now bears his name.

Hven gin, made on the island that was once home to
Tycho Brahe's observatory.
In 1576 he constructed an observatory called Uraniborg, which was apparently very impressive. It was built on the island of Hven in the Baltic Sea between Sweden and Denmark, and I was interested in visiting the observatory to see what remained. Unfortunately, as I found out,  the observatory doesn't exist any more, which is a great shame. Luckily for me though I was at least able to bring some part of Hven home with me when I found out that the island is now home to a gin distillery, and since my wife enjoys gin I was able to find a bottle in a Copenhagen liquor store and bring her a bottle home. I'm told it's very good, Tycho would have been proud!

Friday, 1 May 2015

Which parties will improve science in the UK at this year's general election?

While this year's general election in the UK may not be focussed on science and engineering, these are areas that are very important to the economy of our country. Despite this, science and research have suffered over the last decade, particularly under the last government. As a fraction of GDP the UK's research spending is currently the lowest amongst the G8 and the world's major economies, and in 2012 research spending dropped below 0.5% of GDP for the first time ever.

Science funding as a percentage of GDP (Credit: The Guardian)

The UK is clearly dropping behind these other economies in terms of research spending and productivity, which will seriously affect our future productivity and economy. Since this is an important issue it can be useful to know where the major parties stand on science education and funding, and what they plan to do about these issues if they were to get into power.

To answer this question I've trawled through the various parties' manifestos, as well as their letters to the Campaign for Science and Engineering, to try to find out what the different parties plan to do about science after the election. This meant a lot of sorting through vague statements of support for science and education to find actual plans and promises that might mean something over the next 5 years. Here's what I found.

The two main parties, the Conservatives and Labour, aren't really making any strong promises. The Conservatives are promising to continue the current science budget for another year, though since that is a fixed budget it would likely equate to a drop in actual spending power due to inflation. Labour are promising to ensure that all young people study Maths until age 18, which would probably benefit scientific literacy levels, but only as a by-product.

The leaders of the four major UK parties, but who is best to
improve science in the UK? (Credit: BBC
The Liberal Democrats go a little further, partly by promising to continue the ring-fencing of the science budget, but also by ensuring that by 2020 both research capital and revenue spending increase in line with inflation. This is an improvement over the Conservative's fixed science budget, though it doesn't go as far as other parties are promising to go, as we'll see later. The Liberal Democrats also have a more promising track record on these issues, by attempting to get more scientifically-literate MPs into parliament and also by introducing a 5p plastic bag charge into supermarkets.

The UK Independence Party are promising to abolish tuition fees, though only for students of STEM subjects who stay and work in the UK for 5 years following their graduation. Offsetting this strong move though UKIP are also promising to leave the European Union, which they claim will reduce the regulations which hamper science and technology (I can't speak for any of these regulations myself), but this will also lose UK science of up to 1 billion Euros of funding that we currently receive from the EU every year. Furthermore UKIP are promising to repeal the Climate Change act of 2008, which includes carbon budgets and targets for this half century, and this, I think, is a dangerous move given the way the world's climate is going at the moment.

The Green Party
(Credit: Wikipedia)
The Green party have, perhaps unsurprisingly, made some of the most impressive promises, including doubling public spending on research over the next ten years to reach 1% of GDP, the sort of levels already close to being reached by major research powerhouses across the world such as the USA and South Korea. In addition to this they want to end undergraduate tuition fees and reintroduce student grants, both initiatives that should encourage more people to go to University, study the sciences, and therefore improve the scientific literacy of our workforce.

And finally, while I myself can't vote for either the Scottish National Party (SNP) or Plaid Cymru, since I do not live in either Scotland or Wales, it is worth noting that both parties promise to establish free education for all, including abolishing University tuition fees.

To the majority of people in the UK, there's a stark choice between the commitments the major political parties have made to science. The two largest parties, Labour and the Conservatives, clearly think that science is important, but not important enough to make clear and ambitious promises for how to improve science and engineering in the UK. The Liberal Democrats are offering a little more, as do the SNP and Plaid Cymru (if you are able to vote for them), but none of the parties go as far as the Green Party in making commitments to science for the next parliament.

Hopefully this has been useful for deciding who to vote for in the next election. Whoever you vote for, and whoever gets into power, I hope that science and engineering do well under the next government, for the benefit of our own country and everyone in it.

Friday, 24 April 2015

Happy 25th Birthday Hubble Space Telescope!

On this day 25 years ago the Space Shuttle Discovery was launched from the Kennedy Space Centre on Cape Canaveral in Florida. After launch the Space Shuttle rose to an altitude of 380 miles and began to orbit the Earth. In the days that followed the crew of the Space Shuttle opened the shuttle's cargo bay doors and deployed its valuable payload, the Hubble Space Telescope.
The launch of Space Shuttle Discovery, with the
Hubble Space Telescope on board (Credit: Wikipedia)

That was 25 years ago, and ever since then the Hubble Space Telescope has acted as the world's premier astronomical observatory. In that time Hubble has made an immeasurable contribution to our understanding of the cosmos, from measuring the speed of the expansion of the Universe, finding the first evidence for dark energy, and discovering planetary systems forming in the Orion Nebula.

The Hubble Space Telescope is not the only astronomical observatory in space, in fact there are dozens of them. It wasn't the first such observatory and it certainly won't be the last, but it is probably the most important, not just for the scientific discoveries it has made, but also for how it has brought those discoveries, as well as thousands of beautiful images of the cosmos, to the public. In this article I want to share some of those images, and the science behind them, with you all.

The Pillars of Creation in the Eagle Nebula, imaged by the
Hubble Space Telescope in 1995
(Credit: Hubble Space Telescope)
Perhaps one of the most famous images taken by the Hubble Space Telescope, and one of the first to lodge itself firmly in so many people's hearts, is the magnificent image of forming stars in the Eagle Nebula. The image, which was quickly dubbed the Pillars of Creation because of the stars being created within the nebula, showed for the first time the amazing detail in star forming regions such as this. The pillars themselves, sometimes referred to as elephant trunks, are giant clouds of gas and dust that are being slowly eroded by a cluster of massive stars just above this image. Those stars are sculpting and eroding this cloud of gas and dust, and potentially, as was later shown, halting the star formation process within them. The Hubble Space Telescope revisited this image as part of the 25th Anniversary celebrations this year, producing a new, larger and higher-resolution image of this amazing nebula.
The merging Antennae Galaxies, imaged by the
Hubble Space Telescope in 2006
(Credit: Hubble Space Telescope)

The Hubble Space Telescope didn't just spend its time imaging star forming regions like this, it also produced a huge number of very detailed images of distant galaxies. One of my favourite images of these galaxies is that of the Antennae Galaxies that has been imaged by Hubble multiple times, most recently in 2006. The Antennae are actually two galaxies that are in the process of merging as they interact, and this interaction has quite radically torn these galaxies apart, as the image shows. This apparent destruction has, rather paradoxically, led to a very brief but intense period of star formation that astronomers refer to as a starburst. The Hubble Space Telescope images are so detailed that they have allowed astronomers to study the star formation in these distant galaxies and even resolve individual star clusters within them. Getting such a detailed view of this important phase of galaxy evolution has been really useful for astronomers to understand how galaxies merge.

The Hubble Deep Field, imaged by the
Hubble Space Telescope in 1995
(Credit: Hubble Space Telescope)
Perhaps one of the most unique images taken by the Hubble Space Telescope is that of the Hubble Deep Field, which was imaged in 1995 from 6 days of exposure of an apparently empty patch of sky. This tiny area, one 24-millionth of the entire sky, was chosen because it was almost completely devoid of any stars and galaxies. The questions astronomers were effectively asking by taking this image was, what will we find in the darkest and emptiest areas of space?

The answer was that this apparently empty area of space was actually full of galaxies! Almost all of the 3000 objects in this image are distant galaxies, billions of miles away. Some of the galaxies are so distant that it has taken almost the entire age of the Universe for their light to reach us, allowing us to see what they looked like when the Universe was very young. Images like this have been vital for helping astronomers understand both the large-scale structure of our Universe as well as how galaxies have changed over the lifetime of the Universe.

Finally I want to end with a new image taken by the Hubble Space Telescope very recently. This image was released to the public yesterday to celebrate the 25th Anniversary of the launch of the Hubble Space Telescope, and you can see this amazing image below.

The Hubble Space Telescope's 25th Anniversary special image release showing the massive star cluster
Westerlund 2 and surrounding nebulosity (Credit: Hubble Space Telescope)

The image shows the massive star cluster Westerlund 2, one of the most massive clusters of young stars in our Galaxy (and one which I have studied in the past and talked about before on this blog). This image is so large and detailed that not only can you make out many hundreds of young and massive stars in this cluster, but you can also see the beautiful nebula that surrounds the cluster and make out young stars forming within it! This is an amazingly detailed image, which I encourage you all to have a look at in more detail here.

You can see more images like these on the Hubble Space Telescope's gallery webpage, or follow the various events celebrating this anniversary on the Hubble Space Telescope's 25 Years webpage. Over the next few months I'll talk more about some of the amazing discoveries from the Hubble Space Telescope, the history of how this great observatory came to be, and the exciting telescope being built to replace Hubble in the next few years.