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| The Galactic Center in Radio from MeerKAT Ian Heywood & Juan Carlos Munoz-Mateos |
This is a radio image of the Milky Way's center. It was produced by the MeerKAT array of 64 radio dishes in South Africa. The image spans four times the angular size of the Moon (2 degrees).
The galactic center is in the direction of the constellation Sagittarius. Sagittarius A* is the bright spot in the center of the image. It is the super-massive black hole at the center of our galaxy. Yes, I know, it's an oxymoron to say that a bright spot is a black hole, but black holes attract stuff and big black holes attract a lot of stuff, so much stuff that it gets crowded trying to get in, and when it gets crowded, it gets hot. So what we are actually seeing is the super heated crowd of stuff getting sucking into the pit of their doom.
The key in the lower right corner shows that the colors are based on the spectral index, ranging from -1.8 to 1.0. The opening line of Wikipedia's page about Spectral index is:
In astronomy, the spectral index of a source is a measure of the dependence of radiative flux density (that is, radiative flux per unit of frequency) on frequency.
It goes on at some length babbling about bizarre mathematical functions, but I'm not going to get into that.
My rough approximation is that they measure the power at a frequency and then compare the power level (on some arbitrary scale) to the frequency. So a low frequency signal, say 1 Hz, with a 'power level' of 1 will have a spectral index of 1. A high frequency signal at 1GHz with a power level of one billion would also have a spectral index of 1. This is only a rough guess on my part because it doesn't explain how you could get a negative spectral index.
NASA's page has an annotated version. Mouse over the image and labels of the various features show up.
Via Borepatch
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