Astronomers have discovered the most luminous galaxies ever seen in the Universe, with star systems so bright, they're at a loss for words to describe them.

According to the team from the University of Massachusetts Amherst, established galaxy descriptors such as 'ultra-luminous' and 'hyper-luminous' don't do these uber-mega-shiny (not a real term) galaxies justice, so they've had to invent a new category of their own to convey the actual level of brightness these galaxies can hit.

"We've taken to calling them 'outrageously luminous' among ourselves, because there is no scientific term to apply," said one of the researchers, Kevin Harrington.

The astronomers made the find using Mexico's Large Millimeter Telescope and data recorded by the European Space Agency's Planck and Herschel space observatories. They estimate that the newly observed galaxies are about 10 billion years old, and are likely to have formed about 4 billion years after the Big Bang.

When categorising luminous sources, astronomers use a unit of measurement called solar luminosity. An infrared galaxy is called 'ultra-luminous' when it has a rating of about 1 trillion solar luminosities. Beyond that, 'hyper luminous' describes objects of about 10 trillion solar luminosities.

But what if the galaxy you're looking at is in the measure of 100 trillion solar luminosities like the researchers' new find? "[W]e don't even have a name [for that]," said lead researcher, Min Yun.

"The galaxies we found were not predicted by theory to exist; they're too big and too bright, so no one really looked for them before," he added.

But now that we're aware of them, it will help scientists to learn more about the conditions of the early Universe.

"Knowing that they really do exist and how much they have grown in the first 4 billion years since the Big Bang helps us estimate how much material was there for them to work with," said Yun. "Their existence teaches us about the process of collecting matter and of galaxy formation. They suggest that this process is more complex than many people thought."

According to the scientists, the galaxies detected are not actually as large as they appear, as a phenomenon called gravitational lensing magnifies their light, making them seem about 10 times brighter from Earth than they really are.

But what makes the objects so luminous to begin with is their extremely high rate of star formation. While the Milky Way produces a few solar masses of stars each year, the researchers say these newly detected galaxies look as if they're forming a new star every hour or so.

"We still don't know how many tens to hundreds of solar masses of gas can be converted into stars so efficiently in these objects," said Harrington, "and studying these objects might help us to find out."

The findings are reported in the Monthly Notices of the Royal Astronomical Society.