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Sci-Tech

A messy galaxy that defies description

A new Hubble photo shows a gloriously messy galaxy that is difficult to confine to a single category.

ESA/Hubble & NASA

It's not really a particularly well-known galaxy, but NGC 4861 sure is interesting. It's located 30 million light-years away from Earth, in the small, northern hemisphere constellation of Canes Venatici (the Hunting Dogs), a location in which a number of interesting deep-space objects can be found.

NGC 4861 is small, too, a dwarf galaxy that astronomers are still trying to classify. It has some spiral galaxy physical properties, such as its size, mass and rotational velocity. But it's also, as you can see in this image released by the European Space Agency, a bit like a comet, with a long, trailing tail of gas and stars.

This is more consistent with an irregular dwarf galaxy.

It's also interesting because it's what is known as a starburst galaxy, a galaxy with a very high rate of star formation. And not just any stars; NGC 4861 has a high number of a rare type of star called Wolf-Rayet stars, stars that are extremely massive when young, at least 20 times the mass of the sun. They are also extremely hot and extremely bright, with surface temperatures ranging between 30,000 and 200,000 Kelvin (the Sun's surface temperature is around 5,778 Kelvin).

This furious light and heat comes at a price. They also lose mass at an accelerated rate, with the intense radiation pressure driving powerful solar winds. They don't live long in comparison to other stars, and are thought to explode dramatically in a stunning supernova when their time is up.

This makes NGC 4861 what is known as a Wolf-Rayet starburst galaxy. According to the ESA, these galaxies are great to study, because they are so small that matter moves around in them quite easily compared to other galaxies; there is less mass, which means less gravity holding everything together.

This means the stellar winds produced by the Wolf-Rayet stars, in birth, life and spectacular supernova, would in theory join what is known as a galactic wind, which blows charged particles out of galaxies into the interstellar medium. NGC 4861 should be a perfect candidate for studying this wind, except for one problem: recent studies haven't found it.

This doesn't mean it isn't there, though. Galactic winds are hard to observe, because the density of material is very low, which means their optical emission lines and x-ray emission lines are both difficult to detect.

If you'd like a wallpaper-sized version of this image, you can grab it here.

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