What’s an intergalactic star?

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Intergalactic stars, also known as star outcasts or wanderers, are stars that don’t belong to a galaxy. They may have been ejected from a galaxy due to a merging process or a black hole. The Hubble telescope observed intergalactic stars in the Virgo cluster, and their estimated number could exceed a trillion. However, a view of the night sky from a planet orbiting an intergalactic star wouldn’t be very exciting. The existence of intergalactic stars was confirmed by comparing shots from the Hubble Deep Field with an image taken of a dark section of the Virgo cluster.

An intergalactic star is a star that does not belong to and is not in a galaxy. These are also known as star outcasts and another unofficial name is wanderer. The intergalactic stars probably formed in a galaxy, but an event of some kind could have ejected the stars, leaving them alone. The concept of an intergalactic star was hypothetical until 1997, when the Hubble telescope observed several in a region of the universe known as the Virgo cluster, a group of galaxies that, from Earth, appear to be in the constellation Virgo.

Apparently star outcasts aren’t that rare. The estimated number of intergalactic stars in the Virgo cluster alone could exceed a trillion. Despite the large amount of these stars, astronomers think a view of the night sky from a planet orbiting an intergalactic star wouldn’t be very exciting. The star isn’t in a galaxy, so while there might be some distant galaxies that would be visible, the inhabitants wouldn’t have the crowded, starry skies that humans can see from Earth. The effect would be even worse if the planet had no moon.

How the stars became intergalactic isn’t exactly known, but there could be a couple of possible ways. One is that the stars were part of colliding galaxies that ejected the stars in the process of merging. Another hypothetical process is a multi-star system that gets too close to a black hole, with one of the stars in the system crossing the event horizon and falling into the black hole, and the others are somehow repelled, eventually combining to form an intergalactic star.

The stars observed by the Hubble telescope were red giants. The first hint that intergalactic stars might actually exist came when astronomers found planetary nebulae outside the galaxies in the Virgo cluster. A planetary nebula forms as part of the process that occurs when a star is near the end of its lifespan, and if planetary nebulae were outside galaxies, this implied that there had previously been stars outside of those galaxies. The astronomers then compared shots from the Hubble Deep Field (HDF), an image of deep space galaxies, with an image taken of a relatively dark section of the Virgo cluster. If intergalactic stars existed there, astronomers thought they would find additional but faint points of light, and they did, confirming the existence of intergalactic stars.




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