10 AUGUST 2006
Short of space?
Then you'll appreciate some good news: The universe could be 15 percent bigger
than scientists figure. That means gobs of extra room for parking lots, traffic
jams and gambling palaces.
Take our word for it: The universe is big, and stars are far away. But figuring out exactly how big and how far is tricky. We know light falls off with the square of distance: A lighted car-wash sign looks four times dimmer from a distance of 2 kilometers than from 1 kilometer.
The car-wash principle: Brightness falls off with the square of distance. The apparent brightness of this Seattle sign suggests how near it is. Photo © David Tenenbaum
This also works backwards: If two car-wash signs make the same amount of light, and one looks four times brighter, it must be twice as close.
So far so good. But snafus arise when you try to estimate the mileage to car washes — or stars — in other galaxies. How do you know two stars are equally bright? And even if you solve that, how do you know the distance to either star? (The car-wash principle, after all, only tells relative, not absolute, distance).
The spiral galaxy M33 may be 15 percent further away than
previous estimates, fully 3.2 million light years distant from your garage. A
new method for finding celestial distances finds the universe to be 15 percent
larger, and 15 percent older, than conventional wisdom. Photo: NASA
How big is Big?
Now we hear about a new study that used a streamlined method for estimating the distance to stars in the galaxy M33. According to Kris Stanek, an associate professor of astronomy at Ohio State University, M33 is 3.2 million light years distant, 15 percent further than previous estimates (we're definitely going to have to recalculate your next bus ticket to M33). His study also indicates that the universe may be 15 percent older than the accepted estimate, about 14 billion years.
During a 10-year project, Stanek and colleagues, including Alceste Bonanos, now a postdoctoral fellow at the Carnegie Department of Terrestrial Magnetism, performed an elegant three-step yard-stick trick:
1. They found a pair of stars that orbit each other so one periodically blocks light from the other, which allowed them to calculate each star's diameter.
2. They analyzed the wavelength of the starlight to find the star's surface temperature, which showed how much light is coming off each square centimeter.
3. They compared total brightness (area x brightness per square centimeter) to apparent brightness. Using the car-wash principle (light falls off with the square of distance) they calculated the distance to the binary stars.
The new distance calculation was 15 percent larger than the previous estimate of M33, says Stanek. Since the new estimate had used fewer steps, it had less chance for error. Nonetheless, Stanek and company spent a year recalculating the distance using infrared rather than visible light, and found the same result.
The giant 10-meter Keck Telescope measured the movement of the orbiting stars in galaxy M33, to help judge their distance. Photo: NASA
Convincing Constant?
If you're not short of elbow room and don't observe the anniversary of the Big Bang, you may not care about the age of the universe, or even how fast it is swelling. But the distance of M33 plays a role in one of the most important measurements of all: the Hubble Constant.
In 1929, astronomer Edwin Hubble realized that the deeper he looked into space, the faster the stars were receding from Earth. Thinking "There ought to be a law," he cooked up the Hubble Constant, which relates distance from Earth to that moving-away speed. To find the Constant, you must measure how fast stars are receding (easy, using the Doppler effect) and how distant they are (difficult, as described). Previous estimates of M33's distance may have caused as much as a 20 percent error in the value for the Hubble Constant, Stanek adds.
We e-mailed astronomer Wendy Freedman, director of the Carnegie Observatories, who lead a recent effort to calculate the widely accepted value for the Hubble Constant. "I think this represents a solid piece of work on the distance scale," she wrote back. "Beyond this, I think that it will take more than one eclipsing binary in one galaxy to determine the Hubble constant accurately and quantify the uncertainties in this method. It is also worth noting that eclipsing binaries in the LMC [the Large Magellanic Cloud, a nearby galaxy] give a distance 15 percent CLOSER, so there is further work to be done to understand the systematics in the method. I think it has promise for the future."
By revealing the rate of universal expansion since the start, the Hubble constant
becomes a basis for calculating when the Big Bang lowered the green flag on
the universe. A second cosmological concern connected to the Constant is dark
energy — a bizarre something-or-other that seems to be causing the universe to expand faster with age, 
confounding conventional physics. A refined estimate of the Hubble Constant, Stanek says, would give a better handle on "whether dark energy is constant or if it changes over time."
The discovery that the universe is expanding faster has rattled physicists, so any tool that bears on expansion, like the new celestial yardstick, should be welcome.
"Ten years ago, we did not even know about dark energy," adds Stanek, even though scientists now believe it comprise three-quarters of the "stuff" in the universe. "But now we know how much dark energy there is better than we know the Hubble Constant, which has been around for almost 80 years."
— David Tenenbaum
Bibliography
• The First DIRECT Distance Determination to a Detached Eclipsing Binary in M33, A. Z. Bonanos et al,
Astrophysical Journal.
• Calculating the Hubble Constant.
Related Why Files
• A bar in the Milky Way.
• Defining new planets.
• Super space telescope.
