Sol Lucet Omnibus - ESO

The CHARA Array took
a picture of Altair, even
though it’s as difficult
as reading a newspaper
at a distance of 160
kilometres.
This complex process can actually
be summed up quite neatly:
stars are machines that convert
gravitational potential energy
into light using nuclear fusion to
power a near-balance between gas
pressure and the force of gravity.
If you think of a star that is static
in space — not moving, and more
importantly, not spinning — the
force of gravity and the pressure
of gas are the same everywhere
across the star’s surface, so it tends
to be round.
Getting in a spin
Static stars are however not very
likely in a Universe as dynamic
as ours. The Sun, in fact, rotates
roughly once a month, which is a
60
positively leisurely rate. However,
some stars rotate with substantially
more alacrity.
Let’s consider the case of a star
that is spinning very fast. The
two competing, but previously
balanced effects of gravity versus
gas pressure are now starting to
get out of whack. The reason is that
the outward-pushing gas pressure
is getting an assist from a similarly
outward-pushing centrifugal force.
Just try pulling a hand mixer out
of a bowl of cake batter while the
mixer is still spinning and you’ll
have a dramatic (and messy)
demonstration of centrifugal force.
However, this centrifugal force
that is competing for gravity’s
centripetal attention, along with
the gas pressure, is greatest at the
equatorial regions of the star, not
present at all at the poles of the
star, and scaling along the way
in the in-between latitudes. As a
child, you may have felt this sort
of effect, and its scaling, while
riding a merry-go-round or carousel
— at the edge, furthest from the
rotational axis, you felt the most
pull to the outside, while at the
centre, you may still have gotten
dizzy, but felt no pull towards the
outside.
The latitude dependence of
the centrifugal force versus the
non-latitude dependence of the
other two factors, gravity and gas
pressure, is what makes the star
deform into an oval or “oblate”
shape.
Taking pictures of
rapidly rotating stars
In 2001, after many failed attempts
by astronomers worldwide, my
team and I were successful in
measuring the effect of rotation
on a rapidly rotating star. We
looked at the star Altair, a nearby
hot, rapidly spinning star, with the
Palomar Testbed Interferometer, an
ultra-high resolution technology
prototype telescope designed to
test techniques for finding planets
about other stars. We found that
a simple size measurement of the
star was greater in one direction
than another, and deduced that
Altair’s rapid rotation was finally
revealing itself directly.
More recently, a newer array of
telescopes operated by Georgia
State University’s Centre for High
Angular Resolution Astronomy
(CHARA) has managed to image
Altair fully. By combining the
light from six small telescopes
spread over the top of Mount
Wilson in southern California, the
CHARA Array effectively creates a
telescope that has the resolution of
a 330-metre telescope — roughly
130 times more resolution than
the Hubble Space Telescope.
Investigators from the University
of Michigan were able to utilise
a special new instrument that
combined the light from those
individual telescopes and take a
picture of Altair.
Pretty pictures are nice, but
extracting interesting astrophysics
is even more compelling. One
intriguing aspect of imaging these
sorts of stars is that they appear
brighter at the poles than at their
equators. Indeed, the star Altair
shows a temperature difference
of nearly 2000 degrees between
its hot pole and cool equators;
this effect tells astronomers a
great deal about how the star is
put together on the inside, in the
presence of such extreme rotation.
This detection is a powerful
demonstration of the ability of a
simple image to contain a great
deal of meaningful astrophysics.
It has taken many decades of
telescope advancements to get to
this point where stellar surfaces
can be directly imaged!
There is a promising future for this
particular kind of work. Currently
there are more than five dozen
such targets that will be observed
with the CHARA Array and the
European Southern Observatory’s
own Very Large Telescope
Interferometer. Seeing the stars in
this new way will tell us even more
about how these markers in the
night time sky are put together, and
how they live and ultimately die.
61
An image of Altair (right)
in comparison to a model
(left) that incorporates
rapid rotation and bright
poles.
A graphical illustration
from xkcd.com of the
confusion that often
occurs between the
fictitious centrifugal force
and the centripetal force.