http://www.skyandtelescope.com.au 55Korean astronomers recorded at this
location in 1073.
If these explosions on the white
dwarf continue on schedule, R Aquarii
should next go nova in the late 2400s.
A system that’s telescopically faint
now will then shine at perhaps 2nd
magnitude — as obvious to the naked
eye as Beta Ceti appears now, over there
to the right of it. Swing your telescope
there and try to imagine the change.white dwarf ventures far enough from
the giant that the Very Large Array radio
telescope was able to resolve the two at
a separation of 0.055 arcsecond. Their
orbit is thought to be highly elliptical.
Another source of light also adds to
the spectrum: the faint nebula Cederblad
- Although it dominates the photo
at left, the nebula is extremely difficult
or impossible to detect visually. It’s
expanding and changing as we watch;
all or much of it was apparently thrown
off violently only about 250 years ago (as
seen from Earth).
There’s more. The narrow, vertical
S-shaped feature visible in the photo
is strikingly bright in X-rays. Blobs
of X-ray-hot material within it are
moving outward at a tremendous 600
and 850 kilometres per second. The S
results from a bipolar jet of hot material
flying from the poles of an accretion
disk surrounding the white dwarf. The
material that the dwarf is accreting
surely comes from the giant, which like
all Mira stars is shedding a thick wind.
For more than a century, visual
variable star observers have tracked
some very unusual behaviour for a Mira
giant. At roughly 44-year intervals its
annual fadings become less deep, the
system’s overall colour near minimum
turns bluer — and the giant’s maxima
turn fainter. The added blue light surely
comes from the white dwarf and/or its
accretion disk flaring up. But how could
that suppress the red giant’s own, much
greater luminosity at maximum?
One theory is that a large, semi-
transparent dark cloud surrounds the
white dwarf and its accretion disk as
they orbit the giant; the cloud eclipses
the giant for several years. If that’s the
case, the next episode of giant-dimming
should run from about 2018 to 2026.
Variable-star observers are paying
attention.
And why has the dwarf tended to
brighten at around these same times?
Perhaps because this is when, in its
elliptical orbit, it passes closest to the
giant and collects the heaviest dose of
outflowing wind. That’s indeed when
you would expect, statistically, the
cloud-eclipses to be most likely to occur.
Much more spectacular things
happen at longer intervals, as the nebula
attests. The fresh hydrogen building up
on the dwarf’s surface erupts at intervals
as a nova explosion. The nebular ring,
astronomers judge from its expansion
rate, was blasted off by a nova outburst
around 1773. A dimmer, outer nebula
too faint to show on the facing page
likely came from a nova explosion thatSIn the same sky area as
Neptune (now near Lambda
(h) Aquarii as shown on
page 52) you can look in on
a very different interesting
point of light. R Aquarii is
close to a little triangle of
5th-magnitude stars almost
halfway from Lambda
Aquarii to 2nd-magnitude
Beta Ceti, as seen at left.
The large chart on page 52
provides a wider context
for this scene. The little
black rectangle shows
the area covered by the
comparison-star chart for
R Aquarii above. There,
stars’ visual magnitudes
(courtesy AAVSO) are given
to the nearest tenth with the
decimal point omitted.t^1
t^2CETUSAQUARIUSSCULPTORR_`bfhq0 h 30 m 0 h 00 m 23 h 30 m 23 h 00 m–10°–20°Star magnitudes Fomalhaut –30°2 3 4 5 6 7t^1t^264955310310611311711080746762
8392 4650R23 h 40 m–14°23 h 50 m 23 h 48 m 23 h 46 m 23 h 44 m 23 h 42 m 23 h 38 m–15°–16°Star magnitudes7658
9
10
11