This illustration shows how the precision stellar distance
measurements from NASA's Hubble Space Telescope have been extended 10
times farther into our Milky Way galaxy than possible previously. This
greatly extends the volume of space accessible to refining the cosmic
yardstick needed for measuring the size of the universe. This most solid
type of measurement is based on trigonometric parallax, which is
commonly used by surveyors. Because the stars are vastly farther away
than a surveyor's sightline, Hubble must measure extremely small angles
on the sky.
Illustration Credit: NASA, ESA, and A. Feild (STScI). Science Credit: NASA, ESA, A. Riess (JHU/STScI), S. Casertano (STScI/JHU), J. Anderson and J. MacKenty (STScI), and A. Filippenko (University of California, Berkeley)
Even though NASA's Hubble Space Telescope is 24 years old,
astronomers are still coming up with imaginative, novel, and
groundbreaking new uses for it. The latest is an innovative technique
that improves Hubble's observing accuracy to the point where rock-solid
distance measurements can be made to Milky Way stars 10 times farther
away than ever accomplished before.
To do this, Hubble observations and subsequent analysis were
fine-tuned to make angular measurements (needed for estimating
distances) that are so fine that if your eyes had a similar capability
you could read a car's license plate located as far away as the Moon!
This new capability allows astronomers to use even more distant stars
as yardsticks to refine estimates. In addition, it is expected to yield
new insight into the nature of dark energy, a mysterious component of
space that is pushing the universe apart at an ever-faster rate.
As proof of concept for this new long-range precision, Hubble was
used to measure the distance to a bright star of a special class (called
Cepheid variables) that is located approximately 7,500 light-years away
in the northern constellation Auriga. The technique worked so well that
additional Hubble distance measurements to other far-flung Cepheids are
being measured.
Such measurements will be used to provide firmer footing for the
so-called cosmic "distance ladder." This ladder's "bottom rung" is built
on measurements to Cepheid variable stars that, because of their known
intrinsic brightnesses, have been used for more that a century to gauge
the size of the observable universe. They are the first step in
calibrating far-more-distant intergalactic milepost markers, such as
Type Ia supernovae.
The most reliable method for making astronomical distance
measurements is to use straightforward geometry where the
186-million-mile diameter of Earth's orbit is used to construct a
baseline of a triangle, much as a land surveyor would use. If a target
star is close enough, it will appear to zigzag on the sky during the
year as a reflection of Earth's orbit about the Sun. This technique is
called parallax.
Astronomical parallax works reliably for stars within a few hundred
light-years of Earth. For example, the position of the nearest star
system to our Sun, Alpha Centauri, varies due to parallax by only one
arc second on the sky during the year, which is equal to the apparent
width of a dime seen from two miles away.
But the farther away the star, the smaller the angle of its apparent
back-and-forth motion, until the offset is so small it can barely be
measured. Astronomers have pushed to make ever smaller angular
measurements to extend the parallax yardstick ever deeper into our
galaxy.
Noble Laureate Adam Riess of the Space Telescope Science Institute
(STScI) and the Johns Hopkins University in Baltimore, Md., in
collaboration with Stefano Casertano of STScI, developed an ingenious
technique to use Hubble to make measurements as fine as five-billionths
of a degree on the sky. (A degree is twice the angular width of the full
moon.)
Riess imagined that if Hubble could take numerous exposures of a
target star quickly, he could combine the data to measure extremely
small angles on the sky. But rather than taking multiple exposures,
Riess had the stars trail across Hubble's imaging detector to leave
linear streaks. Riess says that he got the idea for how to do the
observation while swimming laps in lanes that are long, linear swaths,
like the stellar images streaked across the detector. Infinitesimal
offsets in the streaks that could be caused by parallax were measured
through new image analysis techniques developed by Casertano and Riess.
To make a distance measurement, exposures of the target Cepheid star
were taken every six months, when Earth is on opposite sides of the Sun.
A very subtle shift in the Cepheid's position was measured to an
accuracy of 1/1000 the width of a single picture element (pixel) in
Hubble's Wide Field Camera 3 (which has 16.8 megapixels total). A third
exposure was taken 12 months after the first observation to allow for
the team to subtract the effects of the subtle space motion of stars,
with additional exposures used to remove other sources of error.
Riess shares the 2011 Nobel Prize in Physics with another team for
his leadership in the 1998 discovery that the expansion rate of the
universe is accelerating, a phenomenon widely attributed to a
mysterious, unexplained "dark energy" filling the universe. His goal is
to refine estimates for the universe's expansion rate to the point where
dark energy can be better characterized.
CONTACT
Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Adam Riess
Space Telescope Science Institute/Johns Hopkins University, Baltimore, Md.
410-516-4474
ariess@stsci.edu
Source: HubbleSite
