46 ASTRONOMY • APRIL 2021
Hα filters for amateur instru-
ments, as well as dedicated Hα
telescopes.
That’s fortunate, since this
wavelength is where the
action is. Even when the Sun
appears barren through
white-light filters, Hα filters
give us a ringside view of the
Sun’s magnificent chromo-
sphere, the layer directly
above the photosphere. Like
sunspots, Hα features come
and go depending on solar
activity.
The most spectacular Hα
features are prominences,
f lamelike protrusions that
gracefully reach out from the
solar limb. Even the smallest
of these prominences would
tower over our entire planet.
Some extend from the edge ofthe solar limb like a grove of
trees along a distant horizon.
Others hook and loop away
from the disk, forming giant
arcs. When observing the
Sun, keep tabs on these fea-
tures throughout the day and
you will see that their struc-
tures can dramatically evolve.
If a prominence appears
along the trailing edge of the
Sun, solar rotation can carry
it onto and across the disk. As
a prominence moves across
the face of the Sun, it appears
dark thanks to its contrast
against the brighter chromo-
sphere. This creates what
looks like a shadowy solar
serpent known as a filament.
Also keep an eye out for
bright patches across the pho-
tosphere called plages, whichoccur where a magnetic dis-
turbance has broken through
the photosphere. Plages often
coincide with sunspot groups,
although they may also
appear alone.
The most dramatic and
rarest Hα events, however,
are solar f lares. These sud-
den, violent eruptions occur
in and around sunspot
groups, resembling rivers of
white-hot lava f lowing
between the spots. Flares
usually last between five and
10 minutes, although some
may last for several hours.
Despite their brief existence,
individual solar f lares release
enough energy to power the
United States for roughly
100,000 years. They also
release a f lood of chargedparticles, and if the aim is
just right, these particles will
be drawn to Earth’s two mag-
netic poles. As they collide
with our upper atmosphere,
the air molecules glow,
creating the beautiful aurora
known as the northern and
southern lights.
Finally, while all these Hα
solar features come and go,
spicules are always visible.
These jets of plasma jut
upward from the chromo-
sphere all around the edge of
the solar disk. At lower magni-
fications, they’re reminiscent
of peach fuzz. But increasing
to 150x power or more will
resolve individual spicules,
which allows you to watch
them change before your eyes.
If viewing all these amaz-
ing solar features interests
you, make sure to pay extra
attention to your Hα filter’s
bandwidth — the range of
light frequencies that can pass
through the filter largely
unimpeded. A bandwidth of 3
to 4 angstroms (Å) will reveal
prominences, but you will
need a filter with a bandwidth
no more than 1 Å to spot
features like filaments.
Two terrific sources for the
latest news on solar activity
are https://spaceweather.com
and http://halpha.nso.edu.
Both show real-time images
of the Sun, the former in
white light and the latter in
Hα. Be sure to bookmark
them.
A stellar lineup of varied
solar sights awaits us every
sunny day. So, make sure to
take some time and explore
our wondrous host star; you’ll
quickly become a Sun-
worshipper. Just don’t forget
your sunblock.Phil Harrington is a longtime
contributor to Astronomy and
the author of many books. He
has also observed the Sun for
decades.Massive amounts of solar gas and plasma are blasted high above the surface when the Sun’s complex web of magnetic
field lines violently interact, creating arced prominences like those seen here. JEFF PADELL
Sunspot 2645 is seen here in Hɑ on April 1, 2017, through
the Kitt Peak National Observatory Visitor Center
Telescope. Earth is superimposed on the image for scale.
RON COTTREL
Sunspot AR2781 popped up during Solar Cycle #25. In this
Hɑ shot from November 4, 2020, the sunspot is swinging
around the Sun’s eastern limb. JOHN CHUMACK