Date: March 22, 2009
Title: Star Formation
Podcaster: Robert Simpson
Organization: Orbiting Frog http://www.orbitingfrog.com/blog/
Description: Star formation is a vital and busy area of astronomical research. In 2009 two amazing new instruments — SCUBA-2 and Herschel — will begin operation. Together they will answer some of the burning questions of star formation and reveal the coldest parts of our galaxy, where stars initially form.
Bio: Robert Simpson is a PhD student studying star formation at Cardiff University. He has been running the Orbiting Frog website since 2006. He organised the first .Astronomy conference on astronomy and the Internet in 2008. Robert is the producer of the Channelzine website, and participates in the Science or Fiction and Web Quiz podcasts.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Damian Handzy and is dedicated to “all the women and men who have perused careers in science because they have improved the lives of more people than any others.”
Transcript:
My name is Robert Simpson and I am studying for my PhD at Cardiff University in the UK. I study star formation – this is an important field of research in astrophysics and so I thought I would try to summarise it for you in this podcast.
Star formation, in a nutshell, is about gravitational collapse: the contraction of a region of gas under the influence of gravity. These regions collapse into prestellar cores that evolve into protostars and then eventually fully-fledged stars – a process I’ll overview in this podcast. In 2009 astronomers will begin seeing the fruits of two advanced technologies: SCUBA-2 and Herschel. Using these, we will be able to peer into the objects that go onto to form stars and understand how and why they form where, when they do. But before I get into that, let’s start at the beginning…
Star formation is found in many galaxies in what’s called giant molecular clouds: huge, cold regions of mostly hydrogen and helium gas. Molecular clouds are usually too diffuse to collapse under gravity. However, they are made up of smaller regions of higher or lower density. In the regions of higher density, star formation may occur.
Molecular clouds are usually found in the spiral arms of galaxies. In fact the reason we see spiral arms is because there are more stars there to be seen! Density waves travel around the galaxy and like a giant, interstellar traffic jam, the material in the galaxy bunches up. When it bunches up, it is more likely to create the higher-density regions where stars can form. Next time you’re on the road and hit you hit traffic, you might notice that traffic jams usually travel in spurts. For a time you will travel slowly then suddenly the density of traffic lowers and you speed up for a minute. Then you again hit an area of more cars and slow down. You are passing through density waves in the traffic jam, just like material in the galaxy passes through spiral arms. Who’d have though that astronomy had a connection to traffic jams?
Once a sufficiently dense region begins to contract, the process begins to run away with itself. As it contracts the density increases and makes the contraction more rapid. At this point, we call the object “prestellar”, meaning that it is going to become a star but is not quite there yet. Prestellar cores are the subject on my own studies at Cardiff University and my PhD supervisor coined the term.
As the region collapses, any slight spin that it has will be flattened out by conservation of angular momentum. As the cloud contracts, it will spin more rapidly, turning the cloud into the shape of a disk. You can try out this spin-up effect by sitting on an office-style chair and spinning around with your arms outstretched. Once you’re spinning, pull your arms into your chest and you will feel yourself spin even faster. The energy required to spin your outstretched arms is larger than required to spin your clutched arms – this energy can’t go anywhere so instead you spin faster, conserving your angular momentum.
In the middle of this collapsing cloud, the density is growing. As material falls onto the center it will heat up to very high temperatures. Eventually there is enough material in the center to form a protostar – an object that will soon go on to form a star. This protostar will emit powerful radiation and energetic winds. As matter continues to fall onto the central protostar, the energy in the system, and its powerful magnetic fields, produce huge outflows from the protostars North and South pole. We call these bipolar outflows, or jets.
This protostar now gets hotter and denser all the time. Eventually the temperature and pressure inside the protostar becomes so great that something very important happens. At around 15 million degrees the hydrogen nuclei – protons – are traveling so fast that they can overcome the electric repulsion they exert on each other. The positive charges of the protons repel each other – just as all all like charges repel. Imagine though that eventually they can move fast enough that this repulsion is not sufficient to stop them hitting each other. When this happens, they are able to react with each other. This is a nuclear reaction and when protons collide they can form a Helium nucleus. This is a very important reaction and it releases a huge amount of energy. This is called hydrogen burning. This is nuclear fusion and it is one thing that makes a star a ‘star’.
This reaction releases a lot of energy and heats the protostar further, increasing the pressure inside it. Once enough fusion is going on inside the protostar, it begins to release an incredible amount of energy in the form of photons. Some of these photons are in the form of light that we see. After a short time, there is enough force from these photons, to push back against the gravitational collapse that has been driving the whole process. Now we have a star!
The radiation bellowing out of the new star is strong enough to blow all the lighter elements, such as hydrogen and helium, out into the farther reaches of this new stellar system. The heavier material is left nearer the centre around the star and it may begin to coalesce into small rocky fragments in a disk around the star. These may go on to become rocky planets. Although we will not cover this in this podcast, it is enough to say that planet formation is still not fully understood. The next generation of instruments, including an array of telescopes being built in Chile – called ALMA – may allow us to see planets forming. ALMA will not be ready for some years still but is being built at the time of recording.
The process of star formation varies depending on the conditions of the initial cloud. If the prestellar core is dense enough, the star that forms could be massive – or it might not quite be dense enough and the star may not make it to the hydrogen burning phase. Then you may end up with a small not-quite-star called a brown dwarf. We see stars in many sizes, but we seem to see more of them at, or just slightly lower than, the mass of the Sun. These are relatively long-lived stars that burn hydrogen steadily for most of their lives.
Although we can see stars with our eyes, we cannot see protostars and prestellar cores very well, even with conventional telescopes. The large cloud that collapses all though the process I have just described obscures the central object from view. To see a young protostar, for example, you would need to look through the layers of dust and gas that surround it. This is not so easy to do.
Young protostars do emit radiation but it is absorbed by the surrounding cloud before it gets to us. It does have an effect on the cloud though – in that it heats it up. This causes the cloud to emit infrared radiation which we can see. If we use infrared telescopes, flying high above the Earth’s absorbing atmosphere, we can see many protostars and young objects. However to see the really young objects – the prestellar cores – we need to go even colder and further along the spectrum into what is called the submillimetre wavelength regime. We want to look at prestellar cores because they tell us what becomes a star and what doesn’t. If protostars are the baby version of stars then prestellar cores represent the embryonic star.
This year, 2009, will see a new instrument come into use by astronomers. It is called SCUBA-2 and is in fact the successor to a similar device that was decommissioned a few years ago. The James Clerk Maxwell Telescope in Hawaii will use the SCUBA-2 camera to take high-resolution images of objects such as prestellar cores. With fast scanning abilities it will allow researchers to make maps of the places where stars form and begin to determine the fine properties of one prestellar core, compared to another. It will allow us to understand the properties of prestellar objects – how massive they are, how they are distributed and how small and large they can be. SCUBA-2 does all this from the ground, taking advantage of the excellent weather conditions on Mauna Kea in Hawaii and the amazing technological advances made by a team working predominantly in Edinburgh but also in several other institutions around the world, including my own home, Cardiff University.
Cardiff is also involved in another 2009 milestone. In April, the European Space Agency’s Herschel Space Observatory will launch – or just Herschel to its friends. This enormous telescope has a 3.5 metre mirror – the largest ever put in space. It is cooled to just a few degrees above absolute zero and will allow not only the study of star forming regions but also of the coldest and most distant objects in the universe! The observatory has several cutting-edge instruments on board that will allow a a new phase of submillimetre astronomy to begin.
There are a lot of star formation astronomers very excited about 2009. We will hopefully have much to report by the end of the IYA2009 from both Herschel and SCUBA-2, as well as other instruments I have not mentioned in this podcast. I hope you’ll follow our progress via the ESA and Herschel websites and also on many blogs, including my own orbitingfrog.com. Herschel is also the topic of a future podcast from the 365 Days of Astronomy, so look out for that later in the year. I’m Robert Simpson and you can find me on Twitter @orbitingfrog. I hope you’ve enjoyed this podcast. Thanks for listening.
End of podcast:
365 Days of Astronomy
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