http://www.skyandtelescope.com.au 19KEEP
There determine
the dis laxy:VARIABLE STAR- a particular and
star – is related to
a Cepheid’s perio
that to its appare
Similar period-luminosityrelations r
types of variable stars, most notab.
The relationships need to be calibr g
distances to Milky Way variables by means of the
parallax method.
ECLIPSING BINARIES By spectroscopically measuring
the orbital speeds of the two components of an
eclipsing binary star (a close binary in which the stars
mutually eclipse each other), and combining this
information with the observed light curve, it is possible
to geometrically calculate the physical dimensions
of the two stars. Comparing this with their observed
temperature and apparent brightness yields a distance.
Eclipsing binaries were used in 2013 to determine the
distance to the Large Magellanic Cloud to a precision
of 2%.
RED GIANT STARS At the end of their lives, Sun-like
stars turn into bloated red giants, slowly increasing
in size and luminosity. Just before they experience
the so-called helium flash, they all produce the same
amount of energy. By observing a large number of red
giants in another galaxy, it is possible to derive the
apparent brightness of this so-called tip of the red
giant branch (TRGB). Comparing that to the known
absolute brightness yields the distance. The TRGB
method has been calibrated by observing red giants
in the Large Magellanic Cloud, which is at a precisely
known distance.MEGAMASERS Excited by X-rays from a supermassive
black hole, water molecules in orbiting gas clouds can
be stimulated to emit maser light (just like laser, but
at microwave frequencies). These maser regions can
be seen orbiting the black hole as a disk through very
long baseline interferometry (VLBI). Combining the
apparent size of the megamaser disk with the actual
size that would match the clouds’ velocities, derived
from spectroscopic observations, yields a precise
distance, to within some 3% in the case of M106.
Unfortunately, water megamasers are not extremely
common, but the technique has proved to be useful in
calibrating other “standard candles.”STEP3:Astronomersthendeducethepropertiestheprimordial
plasmahadtohavein ordertoproducethisspectrum, including the
speed of sound in the plasma and the density of different types of
matter. From those, researchers work out what the CMB patterns’
true physical size must be. Since they know the apparent size, the
comparison provides the distance, which depends on the history of
cosmic expansion.
EarthDistanceH 0 = 67.4 km/s/MpcClarity will come
In the closing session of the symposium, Schmidt stoically
observed that “clearly, we have not solved things today”. The
hope is that new and better observations will shed light on
the Hubble constant controversy. Astronomers look forward
to more precise parallax data from the European astrometric
satellite Gaia, to detailed supernova observations by the
James Webb Space Telescope, to high-precision measurements
of the CMB by the future Simons Observatory in northern
Chile, and to planned surveys of the large-scale structure of
the universe at various look-back times in cosmic history.
By far the best, however, would be a direct measurement
of the expansion of the universe. “It’s hard, but worth it,”
says Rachel Webster (University of Melbourne). The idea
would be to precisely measure the redshift (and, thus, the
recession velocity) of a distant quasar, and then to repeat
the measurement ten years later, to determine how much it
has increased. The expected change in redshift would be on
the order of one part in a billion, but with future facilities
such as the European Extremely Large Telescope (ELT) or the
Square Kilometre Array (SKA), this is “potentially doable,”
according to Webster.
Colless, like most of his colleagues, remains optimistic.
“The nice thing about this field is that many outstanding
questions will be answered in due time, unlike the situation
with cosmic inflation or string theory,” he says. “Five years
from now, we’ll have a much clearer view.”
GOVERT SCHILLING writes about astronomy and space
science from his hometown of Amersfoort, The Netherlands.
Since he was born, the universe has expanded by about
0.00000044%.