Astronomers Measure the Universe

Triangulum Galaxy (M33)(Photo: NASA/JPL-Caltech/GALEX)
Triangulum Galaxy (M33)(Photo: NASA/JPL-Caltech/GALEX)

A person asking how far Cleveland is from Columbus would expect a straightforward answer. But what if that person asks how far the Triangulum Galaxy is? Figuring out the distances to stars and galaxies is not a simple matter. Astronomers throughout history have been trying measure the universe more accurately, as they develop new techniques and technology.

Astronomers have just calculated a new distance to the Triangulum Galaxy, also called M33. They found it’s 15% farther away than previously thought. OSU Astronomy Professor Kris Stanek is one of the astronomers involved in the project.

“Distance to something is a basic number. If we want to know anything about an object in the Universe, the very first thing we need to know is how far away they are,” he says.

When measuring astronomical distances, astronomers use a series of measurements they call the distance ladder. It’s easiest to measure distances to close objects like neighboring planets. Then, based on those measurements, astronomers calculate distances to farther objects like stars. Finding distances to even farther objects like other galaxies often involves more complicated steps – or rungs of the distance ladder. The problem is, inaccuracies accumulate with more rungs.

What sets this new measurement apart from previous ones is that astronomers have directly calculated the distance to M33 without intermediate steps. These new findings could be a more accurate, initial rung of the distance ladder.

Stanek says, “we do a lot of very basic measurements – they’re hard but they’re basic. We measure how big the stars are, we measure how hot they are, we measure how much light they emit. But that allows us to be pretty accurate and precise.”

One way astronomers calculate distances to the stars is by comparing how bright they appear and how bright they actually are. A candle at the end of a football field would look a lot dimmer than if it were just two feet away. By measuring how much dimmer the distant candle looks, you can figure out how far away that candle is.

For M33, scientists looked at a pair of stars that orbit around each other. As the stars orbit, one of them moves in front of the other, changing the amount of light the telescope receives. From the light, astronomers can then determine how big and how hot the stars are. It’s like knowing how big the candle is, and therefore how bright it would be if it were two feet away, instead of 100 yards away. Knowing how bright the stars actually are, and comparing with how bright they appear, astronomers can calculate a distance.

It took scientists 10 years to do this measurement of M33. Stanek says it was a tedious and time-consuming project, requiring some of the world’s largest telescopes and continually improving technology.

Accurate distance measurements are important because they tell astronomers how big the Hubble constant is. The Hubble constant is a number that astronomers use to determine the size and the age of the universe.

The fact that M33 is 15% farther away than previously thought could mean the universe is 15% larger and 15% older than previously thought. But Stanek won’t jump to conclusions.

“It could actually mean that the universe is 15% bigger, but I would not bet my money on this right now,” he says.

Currently, astronomers believe the universe to be about 14 billion years old. The recent findings, which have been accepted for publication in the Astrophysical Journal, show that M33 is about 18 million trillion miles away.

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