“Praise you the Lord:
for it is good to sing praises unto our God…
He determines the number of the stars;
he calls them all by their names.
Great is our Lord, and of great power:
His understanding is infinite.” -Psalm 147:1-5
A light-year is a unit of length equivalent to about 6 trillion miles (or 10 trillion kilometres). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.
The light-year is mostly used to measure distances to stars and other distances on a galactic scale. Note that the light-year is a measure of distance rather than, as is sometimes misunderstood, a measure of time.
Imagine the distance of 4 BILLION light years.
That’s the length of a recently-discovered largest structure in our Universe.
Agence France-Presse reports that on Jan. 11, 2013, astronomers said they had observed the largest structure yet seen in the cosmos, a cluster of galaxies from the early Universe that spans an astonishing four billion light years.
The sprawling structure is known as a large quasar group (LQG), in which quasars — the nuclei of ancient galaxies, powered by supermassive black holes — clump together.
From Wikipedia, here’s an artist’s rendering of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun.
Photo credit: ESO/M. Kornmesser
The discovery in the deep Universe was made by a team led by Roger Clowes at the Jeremiah Horrocks Institute at Britain’s University of Central Lancashire.
It would take a spaceship traveling at the speed of light four thousand million years to get from one end of the cluster to the other.
To give a sense of scale, our galaxy (the Milky Way) is separated from its nearest neighbor, the Andromeda galaxy, by two and a half million light years.
Clowes said in a press statement issued by Britain’s Royal Astronomical Society (RAS): “While it is difficult to fathom the scale of this LQG, we can say quite definitely it is the largest structure ever seen in the entire Universe. This is hugely exciting, not least because it runs counter to our current understanding of the scale of the Universe.”
The paper appears in a RAS journal, Monthly Notices of the Royal Astronomical Society.