Laniakea

If you read the news, and I’m sure you do, you doubtless have read about Laniakea. But in case you missed that particular news item, let me briefly recap. Laniakea is a Hawaiian word that is roughly translated as “spacious heaven,” and it is the new name for the local supercluster.

Now you may ask, what is the local supercluster? Or for that matter, what is a supercluster, local or otherwise?

My explanation starts with the sun. Our sun is a member of the Milky Way Galaxy, along with about 100 to 400 billion other suns. (We call them stars, of course, and our sun is a star.) The sun travels through space, but it doesn’t travel alone. We have all the stars in the Milky Way Galaxy traveling with us.

Just as stars form groups called galaxies, galaxies form groups, too. Our Milky Way is a member of a group of galaxies we call the Local Group, which includes all the galaxies within 5 million light years of the Milky Way. In addition to the Milky Way and its satellite galaxies (yes, there are small galaxies that orbit the Milky Way), the Local Group includes the Andromeda Galaxy and its satellite galaxies, the Triangulum Galaxy, and dozens of other galaxies. The Local Group contains more than 54 galaxies. The exact number of galaxies is not certain, as from time to time a new one is found.

The Local Group is part of a larger collection of galaxies called the Virgo Cluster, which contains at least 1300 galaxies and possibly as many as 2000 galaxies. As large as it is, the Virgo Cluster is only one galaxy cluster in a much larger group called the Virgo Supercluster. The Virgo Supercluster is, or was, our local supercluster and was thought to be 110 million light years in diameter.

Now a team of astronomers have published a new way of defining galaxies and they tell us that the Milky Way Galaxy is a part of a new supercluster called the Laniakea Supercluster. They tell us that Laniakea is about 520 million light years in diameter. (Furthermore, Laniakea seems to be gravitationally bound to an even larger assembly of galaxies called the Shapley Concentration, but that’s another story.)

These are awfully big distances and therefore difficult to grasp. To put all this vast scale into a more understandable perspective, I want to shrink the Milky Way Galaxy down to the size of our sun. To do the calculations I am going to use scientific notation. Scientific notation is a way of expressing numbers by using powers of 10. For instance, 100 is written 1.0e+2, 1000 is written 1.0e+3, 5,000,000 is written 5.0e+6. Fractions have a negative exponent: 0.01 is written 1.0e-2, 0.001 is written 1.0e-3, 0.000005 is written 5.0e-6. And so on.

The Milky Way’s diameter is estimated to be 100,000 to 120,000 LY (light years). Let us choose to use the average which is 110,000 LY or 1.1e+5 LY. Using the metric system, the Milky Way’s diameter in meters is 1.1e+5 LY x 9.46e+15 meters/LY = 1.04e+21 meters. (If we weren’t using scientific notation we would have to write down the distance using this number: 1,040,000,000,000,000,000,000 and you can see how awkward that becomes.)

Our sun has a diameter of 1,391,684 kilometers (1.392e+9 meters). Therefore the ratio of the sun's diameter to the Milky Way's diameter is (1.392e+9)/(1.04e+21) = 1.338e-12. Just by looking at that number we know that the sun’s diameter is about a millionth of a millionth (a trillionth) of the diameter of its parent galaxy. If Laniakea were shrunk so that the Milky Way Galaxy was the size of our sun, the sun would be shrunk by the same factor, so we can calculate its new diameter:

Sun’s new diameter = 1.392e+9 meters x 1.338e-12 = 1.86e-3 meters = 1.86 millimeters

A US dime is 1.35 millimeters thick. If the Milky Way Galaxy was the size of our sun, our sun would then have a diameter slightly more than the thickness of a dime. We can perform a similar calculation on our home planet, which has an average diameter of 12,742 km – slightly more at the equator, slightly less at the poles.

Earth’s new diameter =  1.2742e+7 x 1.338e-12 = 1.705e-5 meters = 17 microns

The diameter of a strand of human hair can range from from 17 to 181 microns. So Earth’s new diameter would be the same as the finest (meaning thinnest) human hair.

I’ve already stated that the Laniakea Supercluster has a diameter of about 520 million light years. If it were shrunk so that our Milky Way Galaxy was the size of our sun, how big would Laniakea then be? First, let’s convert its length from light years to meters.

Laniakea’s diameter = 5.2e+8 LY x 9.46e+15 meters/LY = 4.9192e+24 meters

Laniakea’s new diameter =4.9192e+24 x 1.338e-12 = 6.5818896e+12 meters

How big is this? How can we visualize it? The planet Saturn is 9.537 times as far from the sun as is planet Earth, which amounts to 1.427e+12 meters. The diameter of its orbit around the sun would be twice that, or 2.854e+12 meters. The diameter of the newly shrunken Laniakea Supercluster would be more than twice the current (non-shrunken) diameter of Saturn’s orbit around the sun.

So here’s the summation:

If we shrink space so that Laniakea has a diameter twice as large as Saturn’s orbit about the sun, then our Milky Way galaxy will have the diameter of the sun, the sun’s diameter will be about the thickness of a dime, and the Earth’s diameter will equal the thinnest human hair. And if you are a human who is 6 feet (1.83 meters) tall, how tall would you be in our shrunken supercluster? You’d be 2.45e-12 meters tall. What else is that small? Not much. In fact, it would take a thousand of you, lined up head to toe, to equal the length of the smallest virus humans have studied. Our local supercluster is a pretty big thing, compared to us humans, compared to our home planet, compared to our home star, and even compared to our home galaxy. And the part of the Universe we can observe holds at least a million superclusters.