ASTRONOMERS are close to plugging one of the most glaring holes in cosmology:
exactly how far is it from the Milky Way to the next galaxy? Their answer will
also help to pin down the age of the Universe.
At the moment astronomers have no way of judging distances accurately. As a
result, estimates of the distance to the edge of the visible Universe could be
out by up to 20 per cent. This has a knock-on effect because that figure is used
to calculate the Hubble constant, which in turn tells us the age of the Universe
and how fast it’s expanding.
So Kris Stanek at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts, is working on calibrating the “standard candle” system
for gauging intergalactic distances. “Standard candles” are stars with an
intrinsic brightness—wattage, if you like—that we can deduce just
from their appearance. Once we know this, their apparent brightness in the night
sky tells us how far away they must be. This makes them handy reference points
in space.
Advertisement
Some of the best standard candles are those in a class of stars called
Cepheids, which pulsate with clockwork regularity. Their pulse rate is directly
related to their intrinsic brightness, so two Cepheids that flicker at the same
rate must be equally bright. Compare the apparent brightnesses of the two and
you can tell how far away they are relative to one another.
But this rule doesn’t give absolute distances because we don’t know the exact
distance to any Cepheid—which would tell us its intrinsic brightness and
allow us to calibrate the scale. Stanek is trying to put that right by finding
stars that are close to known Cepheids and whose exact distance from Earth we
can calculate.
Stanek’s chosen stars are eclipsing binary systems—two stars orbiting
each other so that they periodically eclipse each other. From the duration of
these eclipses Stanek can work out how big the stars are, and from this he can
deduce their intrinsic brightness, and therefore how far away they are.
Stanek is currently measuring the distance to a binary system in the Milky
Way’s nearest neighbour, the Andromeda galaxy, he told last week’s MIT
Astrophysics Colloquium. Best estimates put Andromeda at 2 million light years
away, but with an error of about 15 per cent. Stanek says he can trim that error
to 5 per cent or less. And since Andromeda is also chock-a-block with Cepheids,
Stanek says his work should halve the error on Hubble’s constant.
Meanwhile, Ed Guinan at Villanova University in Pennsylvania is using the
same method to calculate the distance to a binary star system in the Milky Way,
in which both stars happen to be Cepheids. Researchers at Chile’s Optical
Gravitational Lensing Experiment discovered the system in 1999.
Michael Bolte of the Lick Observatory in Santa Cruz, California, who has
worked on measuring intergalactic distances, says the technique is promising but
warns that dust between Earth and the stars could affect its accuracy. “You want
not just one eclipsing binary, you want a dozen,” he says, “so I wouldn’t go out
and change my best estimate of the Hubble constant just yet.”