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This whirling image features a bright\u00a0spiral\u00a0galaxy\u00a0known as MCG-01-24-014, which is located about 275 million light-years from Earth. In addition to being a well-defined spiral galaxy, MCG-01-24-014 has an extremely energetic core, known as an\u00a0active galactic nucleus\u00a0(AGN), so it is referred to as an active galaxy. Even more specifically, it is categorised as a Type-2 Seyfert galaxy. Seyfert galaxies host one of the most common subclasses of AGN, alongside\u00a0quasars. Whilst the precise categorisation of AGNs is nuanced, Seyfert galaxies tend to be relatively nearby ones where the host galaxy remains plainly detectable alongside its central AGN, while quasars are invariably very distant AGNs whose incredible luminosities outshine their host galaxies.<\/p>\n
There are further subclasses of both Seyfert galaxies and quasars. In the case of Seyfert galaxies, the predominant subcategories are Type-1 and Type-2. These are differentiated from one another by their\u00a0spectra\u00a0\u2014 the pattern that results when light is split into its constituent wavelengths \u2014 where the spectral lines that Type-2 Seyfert galaxies emit are particularly associated with specific so-called \u2018forbidden\u2019 emission. To understand why emitted light from a galaxy could be considered forbidden, it helps to understand why spectra exist in the first place. Spectra look the way they do because certain atoms and molecules will absorb and emit light very reliably at very specific wavelengths. The reason for this is quantum physics: electrons (the tiny particles that orbit the nuclei of atoms and molecules) can only exist at very specific energies, and therefore electrons can only lose or gain very specific amounts of energy. These very specific amounts of energy correspond to certain light wavelengths being absorbed or emitted.<\/p>\n
Forbidden emission lines, therefore, are spectral emission lines that should not exist according to certain rules of quantum physics. But quantum physics is complex, and some of the rules used to predict it use assumptions that suit laboratory conditions here on Earth. Under those rules, this emission is \u2018forbidden\u2019 \u2014 so improbable that it\u2019s disregarded. But in space, in the midst of an incredibly energetic galactic core, those assumptions don\u2019t hold anymore, and the \u2018forbidden\u2019 light gets a chance to shine out towards us.<\/p>\n
[Image Description:<\/i>\u00a0A spiral galaxy. It appears to be almost circular and seen face-on, with two prominent spiral arms winding out from a glowing core. It is centred in the frame as if a portrait. Most of the background is black, with only tiny, distant galaxies, but there are two large bright stars in the foreground, one blue and one red, directly above the galaxy.]<\/p>\n<\/p><\/div>\n<\/p><\/div>\n