Science Articles

The Science of the Supernova

Sun, 21st Nov 2010

What the death of a star can tell us about astrophysics...

Jeannie Moulton

Much like being blinded when someone turns on the lights to wake you up before dawnSWIFT GRB, on June 21, 2010, the Swift X-ray Telescope was blinded by the largest Gamma Ray Burst ever recorded.

Gamma Ray Bursts (GRBs) are believed to occur when a massive star, meaning one much larger than our own Sun, suddenly collapses, explodes and spawns a black hole. But what is a gamma ray, why should a star suddenly collapse, and why would it explode?

What is a Gamma Ray?

A gamma ray is a form of electromagnetic radiation or light, although our eyes cannot see it. In the same way that our Sun radiates visible light; other bodies in space also emit radiation, some of it visible to us, some of it, like gamma rays, invisible. As the illustration of the electromagnetic spectrum shown to the left illustrates, the more energy (heat) involved in the process causing the radiation, the higher the frequency of radiation. Conversely, the higher the frequency of radiation, the more energy it carries.

To understand this, Electromagnetic spectrumthink about our Sun, which radiates a lot of visible light, so that we can see and so that plants can grow, but it also emits light that we cannot see, such as infrared (IR) and ultraviolet (UV) wavelengths. UV has a higher frequency and more energy than visible light, and this is why UV light can cause problems like skin cancer, because it has sufficient energy to damage DNA within skin cells, leading to cancer-causing mutations. The illustration also shows that X-rays have even more energy than UV, meaning that they can readily penetrate soft tissues. This is why doctors can use them to image bones. Gamma rays are even more energetic still. Luckily, Earth's atmosphere absorbs X-rays and gamma rays arriving from space, keeping us safe. But while this is a bonus on the one hand, it creates a problem for astronomers and astrophysicists because physicists and astronomers need to put their detectors into orbit around Earth - above the atmosphere - to see them.

Signals recorded by these orbiting detectors, of which SWIFT is one, are fed into computers, which build up an image for us to see. And by looking at many different forms of radiation, we can obtain a much more complete view of our universe than visible light alone can reveal. This is because objects and events in space might not generate very much visible light, or might be shrouded in a dark blanket of dust and gas, but other higher or lower energy forms of radiation can nonetheless make it through.

Why does a Star Collapse?

A star can only exist when the opposing forces caused by gravity and fusion are balanced. What keeps a star together in the first place is gravity. Gravity is an inward force. Just like you are pulled toward the center of Earth by our gravity, all of the particles in a star are pulled inward by its gravity, holding it together. For very large stars, this inward force is very strong and must be counteracted for stars to exist. The force that counteracts gravity is caused by fusion. Fusion is the process in a star that creates heat. For example, our Sun fuses hydrogen into helium, but you can think of it as the Sun burning hydrogen as a fuel. When something burns, it creates heat and causes the surrounding material to expand. The heat created by burning fuel inside the star thus creates an outward pressure. The outward pressure created by fusion counteracts the inward pressure of gravity, and the star is balanced.

This balancing act canít go on forever, because the fuel supply will some day run out. When the fusion stops, there is no more outward pressure and an imbalance of force results. Gravity takes over, the star succumbs and begins to collapse inward.

Star in gravity balance

Collapsing Star

Gravity causes stars to collapse. This force is counteracted by expansion generated through the heat of the fusion process.

As the star exhausts its fuel supply it cools and gravity becomes the dominant force, collapsing the star.

Why does a Star Explode?

As the star collapses inward under the force of gravity the particles are pushed so close together that they fuse with each other, creating new elements such as iron. Reactions also occur on a sub-atomic level. These reactions create a build-up of pressure and small, high-energy particles called neutrinos. As the volume of the star continues to shrink, the energy from the reactions continues to build up, and eventually becomes so large that it releases itself in the form of a magnificent supernova, violently exploding and sending the new elements and neutrinos away from the star. All that remains after the supernova is a core, whose gravitational inward force was strong enough to survive the blast. For a massive star, this remaining core is a black hole.

Star shrinks under gravity

Supernova explosion

As the star collapses, particles are pushed closer together, forcing them to fuse. This creates new elements, such as iron.

As the star continues to shrink, the energy from the reactions continues to build and the outward pressure eventually becomes so large that it releases itself in the form of a supernova explosion, ejecting the stars contents into the cosmos. What remains is a core, whose gravitational inward force was strong enough to survive the blast. For a massive star, this remaining core is a black hole.

Back to the Swift Telescope and GRBs

What was observed on June 21, 2010 may have been the culmination of these events, except in proportions scientists did not think were possible. It is still unclear exactly what causes GRBs, but the Swift Telescope will help scientists gather the data that they need.

GRBs are often accompanied by a tail-off of lower frequency radiation including X-rays. This is called an after-glow. The Swift Telescope carries equipment to measure gamma radiation, X-radiation and UV/optical radiation. The gamma rays are detected and used to find the crude direction of the source of the burst. The equipment is then pointed in this direction in order to get the best possible image using X-rays from the after-glow. Because the telescope can be aligned to receive x-rays during the after-glow, the image obtained from the x-rays can accurately pin-point the direction of the GRB.

Travelling as it did for five billion years across space, the bright burst disrupted data recording equipment, but luckily, the scientists at Penn State University were able to recover the data lost during the disruption (it certainly is some valuable data!). As a comparison, the burst was 140 times larger than the brightest continuous gamma ray source in the sky. This rare event will surely provide much needed data to better understand the origin of GRBs and the universe.

References:

heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html

www.swift.psu.edu/xrt/

Test it yourself

To observe a similar effect, try the following kitchen experiment. We wonít be creating a supernova, but youíll get to see a process similar to that of a star collapsing when it runs out of fuel.

What youíll need: a drinking glass, chewing gum, wooden match, plate and some water.

Chew the gum, and then stick it in the center of the plate. Fill the plate with 3-5 mm of water, and light the match and stick the match in the gum so it stands upright. Cover the match and gum with the glass. What happens when the match burns out?

When the match burns out, the water is sucked up into the glass. Why? Itís because there is a pressure imbalance between the outside of the glass and the inside of the glass, which must be neutralized.

When the match is burning, it creates heat, the air inside the glass expands and creates and outward pressure. If you look closely, you may see bubbles coming out from under the glass when the flame is lit. When the oxygen in the glass runs out, the flame burns out and there is no more outward pressure from the heat to compensate for the inward pressure due to air pressure differences. The net pressure switches inward and as a result, the water gets sucked up into the glass.

Now imagine that you have a much larger set up with a very large, hot flame. Presumably, you could create such a pressure imbalance that when the water was sucked up into the glass, it would rush in with a force large enough to cause the glass to explode Ė a kitchen supernova! Donít try this one, please.

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