Sky & Telescope - December 2018 - 26

Hexagon and the stars and clusters
within and around it, I feel like a
kid again every time I see it, even
after all these years.
The Winter Hexagon wraps six
ﬁrst-magnitude stars together like
a giant present in the sky. AldebaNo data
No data
Coronagraph
ran and Capella (Alpha Tauri and
mask
Alpha Aurigae, respectively) are
Star
already glinting in the dusk after
×
Scattered
sunset. On long winter evenings,
light
Ring
it's fun to pull back the curtains
center
Ring
from time to time and watch as the
inner edge
rest of the Hexagon ﬁlls the sky as
the night goes on. Keep an eye open
Ring
and watch Orion rise with them,
outer edge
his shield leading the way as he
holds off Taurus for another night.
Next we anticipate the appearp DUSTY DISK OF DEBRIS In 2005, the Hubble Space Telescope directly imaged the disk around
Fomalhaut. The coronagraph mask blocks the otherwise blinding light from the star - but some still
ance of Rigel, also known as
manages to seep through, seen here as scattered light. The center of the ring is offset from the star,
Beta (β) Orionis, and Pollux
indicating that an unseen body tugs at the disk. The inset shows observations from 2004 and 2006,
(Beta
Geminorum), followed by
combined into a single image. The dot is the planet, Fomalhaut b, which has moved in the time separatProcyon (Alpha Canis Minoris).
ing the two observations. Subsequent observations have confirmed the motion and that the planet is
indeed in orbit around the star.
Alpha Canis Majoris, famously
known as Sirius, is the last of the
Hexagon's
stars
to
rise.
It's
unmistakably the brightest star in
To ﬁ nd Fomalhaut, look at Altair in the last of the glowthe night, more than twice as bright as the second-brightest,
ing twilight, then turn to the south - toward your left.
the southern star Canopus. Sirius easily outshines everything
Alpha Piscis Austrini will be the next bright star you come
else in the nighttime sky except the Moon, Venus, and Jupiter.
to. Without any other even moderately bright stars nearby
The Sirius we see is actually a pair: a white star about twice
to act as signposts, it can be tough to spot, so be patient.
When you see it, there's no doubt. It's the only bright star in the size of the Sun (Sirius A) and a dim companion (Sirius B),
sometimes called "The Pup," in reference to Sirius's moniker,
what's an otherwise humble and quiet part of the sky. It'll
be at about the same altitude as Altair and will set at around the Dog Star. Sirius B is a white dwarf that has about the
same mass as the Sun squeezed into a sphere approximately
the same time.
the size of the Earth.
That one point you see in the sky is actually three stars
Every year, I like to grab a friend or my kids at this moment
- Fomalhaut A and its two companions, Fomalhaut B and
and step outside to have a look just before the horns blow.
Fomalhaut C. The three stars are all part of the same system,
even if they're separated by several degrees. Fomalhaut A is
a fairly young star, around 200 million years old, and like
Vega approximately 25 light-years away. Its diameter is about
twice that of the Sun. When you look at it, bear in mind that
you're also looking at an exoplanet embedded in a broad disk
of dust, ice, and rock in a long orbit around its parent star. In
2008, the Hubble Space Telescope discovered Dagon (Fomalhaut b), the ﬁrst exoplanet detected by direct imaging in visible wavelengths, rather than by other indirect methods, such
as the transit method. It's incredible to think that we're able
to look across that distance and actually see a world orbiting
another star (with Hubble, of course!).

The Winter Hexagon
Now that we have the ﬁrst four stars on our tour in the bag,
let's turn toward the east. For my money, there's no sight in
the skies more incredible with the naked eye than the enormous Winter Hexagon. Between the stars that make up the
26

DECE MBE R 2 018 * SK Y & TELESCOPE

p TWINS IN SPACE Castor and Pollux are the heads of the twins in
the Gemini constellation. Pollux, one of our first-magnitude stars, is a
red giant and has a gas giant planet orbiting it. Castor, not far behind at
magnitude 1.94, is a bluish-white main sequence star and is part of a
multiple-star system.

FO M A LH AU T: N ASA / ESA / P. K A L AS / J. G R A H A M / M. CL A MPIN;
CASTOR A ND POLLUX: ROG ELIO BER N A L A NDREO

Bright Stars

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018