Sagittarius A*: Beyond the Horizon

Narrator (April Hobart, CXC): For a long time people believed that the Earth was flat and that if you sailed too far you'd fall over the edge! It seems funny they could have thought that, because now we're lucky enough to have pictures of our entire planet and we can see its shape (take a look at image 2). But it took some pretty impressive technology to get these pictures, which wasn't available to our ancient ancestors. Did you know you have to travel about 20,000 kilometres from Earth to be able to see the entire planet?

Now imagine how far into space you'd have to travel to fit all the 300 billion stars of the Milky Way (our Galaxy) into one shot! This is way beyond our abilities at the moment, but we can photograph small sections of the Galaxy. This picture from the Chandra X-ray Observatory shows the very centre of the Milky Way. This is the most chaotic and dangerous part of the Galaxy, and home to a supermassive black hole.

Anything that gets too close to a black hole is pulled into it with such a strong force that it has no chance of escape. The boundary that marks the point of no return is called the event horizon. Past this not even light will return: this monster will pull it in forever. The blue haze in this picture includes piping-hot gas floating perilously close to the event horizon of our Galaxy's supermassive black hole. But astronomers have found that just a tiny amount of this gas will be gobbled up by the black hole, and the rest will be "spat out" before it gets too close.

Return to Podcasts

HD 189733: NASA's Chandra Sees Eclipsing Planet in X-rays for First Time

Narrator (April Hobart, CXC): HD 189733b: An exoplanet in orbit around a star about 63 light years from Earth. It has been nearly two decades since the first exoplanets – that is, planets around stars other than our Sun – were discovered. Now for the first time, X-ray observations have detected an exoplanet passing in front of its parent star. The observations, made by NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton Observatory, took advantage of the alignment of a planet and its parent star in HD 189733. This alignment enabled the observatories to observe a dip in X-ray intensity as the planet moved in front of, or transited, the star. This technique is the one used so successfully at optical wavelengths by NASA's Kepler telescope. In earlier studies using optical light, astronomers discovered that the main star in the HD 189733 system had what is known as a "hot Jupiter" around it. This means the planet is about the size of Jupiter, but in very close orbit around its star. The planet – that has been named HD 189733b -- is over 30 times closer to its star than Earth is to the Sun, and goes around the star once every 2.2 days. The new X-ray data suggest that this planet has a larger atmosphere than previously thought. This, in turn, may imply that radiation from the parent star is evaporating the atmosphere of HD 189733b more quickly than expected. The results on HD 189733 demonstrate how we need information from many different telescopes that detect different types of light to get a fuller picture of these mysterious worlds that we are now able to explore.