Howdy Star Gazers and welcome to Star Trails.
I'm Drew and I'll be your guide to the night
sky for the week of August 17th to the 23rd.
This is a week designed for patient eyes and
early alarms. We have slim crescents at dawn,
a lovely planetary trio before sunrise, and the
darkest weekend of the month for deep sky hunting.
Then in the second half of the show, we'll widen
the frame and talk about the Milky Way's hidden
companions. Dozens of ghostly little galaxies
that have been circling us all along, almost
invisibly. and how they're reshaping our sense
of home in the universe. Let's begin with the
moon. Right now it's a delicate waning crescent
hugging the pre -dawn eastern horizon. Each morning
it slims further and slides closer to the sun's
glare. By Friday it's a little more than a silver
thread in brightening twilight. The new moon
arrives early Saturday morning. This isn't just
any new moon. It's what astronomers and calendar
keepers call a seasonal black moon, the rare
case when one astronomical season contains four
new moons instead of the usual three, and this
is the third of the four. It's mostly a calendar
quirk, but practically speaking, it means Friday
and Saturday nights bring the darkest skies of
the month. If you've been waiting for the Milky
Way to leap out of the sky, or you want to take
a crack at the Andromeda galaxy with the naked
eye from a truly dark sight, this is your moment.
The pre -dawn sky is where the week's best show
plays out. On Tuesday, August 19th, Mercury reaches
its greatest western elongation for this apparition.
About 30 to 40 minutes before sunrise, scan very
low in the east northeast. Mercury will be bright
for a small planet, but still close to the horizon.
A pair of binoculars will punch through the haze
and help you spot it. Now look higher up that
same stretch of sky and you'll find Venus and
Jupiter still sharing the stage after last week's
conjunction. Venus blazes lower, Jupiter shines
above it. On Tuesday and Wednesday a paper -thin
crescent moon joins them in the morning twilight
turning the scene into a gorgeous quartet. Mercury
near the horizon, Venus brighter above it, Jupiter
higher still, and the moon as a thin accent.
Evening planets are quieter but worth a look.
Mars clings to the low western horizon right
after sunset. It's faint, small, and sets quickly,
so you'll need an unobstructed view and a bit
of luck with atmospheric clarity. By late evening,
Saturn is the reward, rising in the east -southeast
and climbing higher night by night as it heads
towards opposition on September 21st. Even a
small telescope will show its rings and, with
patient viewing, the tiny star -like point of
Titan nearby. If you haven't seen Saturn this
year, mark it down, late August through September
is prime time. The Perseid meteor shower peaked
last week, but it doesn't switch off like a faucet.
Perseids linger through late August, and a few
bright stragglers are very possible in the predawn
hours, especially now that moonlight will not
be washing out the sky. If you're up early for
the planetary lineup anyway, give yourself 15
or 20 minutes just to scan the sky. Face generally
northeast, but don't fixate on the radiant. Meteors
can streak across any part of the sky, and you'll
spot more by letting your peripheral vision do
the work. If you want targets for the dark evenings
later this week, let me recommend three summer
gems that don't get a lot of hype. The first
is Delphinus, the dolphin, a little diamond and
tail asterism just east of Altair that resembles
a leaping dolphin. The second is Scutum, the
shield. Small and unassuming, but home to M11,
the wild duck cluster. Through binoculars or
a small scope, it looks like a burst of stardust
against the Milky Way, dense, rich, and full
of texture. The third is Volpecula, the fox,
which hides the famous coat hanger asterism.
Six stars and a bar, with four forming the hook.
Sweep slowly between Altair and Vega, and once
you see the code hanger, you will never unsee
it. Under this weekend's moonless sky, all three
reward a patient scan. In recent episodes, we've
journeyed out to the darkness at the edge of
our solar system to explore objects in the Kuiper
Belt and beyond. But what will we find when we
venture out to the edge of our galaxy? That's
coming up after the break. Stay with us. Welcome
back. Today we're talking about the Milky Way's
hidden companions. When you stand under a truly
dark sky this weekend and the Milky Way glows
like a river of light, it is tempting to think
we know this city of stars well. We've mapped
its spiral arms, traced its dusty lanes, and
even weighed the black hole at its center. But
there's a surprise waiting just beyond the obvious.
Our galaxy is surrounded by dozens, maybe hundreds,
of ghostly little galaxies. quietly orbiting
the Milky Way like dim satellites. Some are so
faint that until recent years we didn't know
they even existed. Astronomers call them dwarf
galaxies and the faintest among them are known
as ultra -faint dwarfs. They are small collections
of stars and dark matter bound together by gravity.
A typical ultra -faint dwarf may contain only
a few thousand stars spread over hundreds of
light years. By comparison, the Milky Way holds
hundreds of billions of stars. If we think of
the Milky Way as a bright metropolis, these dwarves
are outlying towns, sparsely populated, dimly
lit, and far from the bustle of the core. Why
did we miss them for so long? The answer is partly
about surface brightness. It's not just how much
light an object emits, but how spread out that
light is in the sky. Many of these dwarves are
so diffuse that their stars melt into the background.
They don't pop the way a bright cluster or a
nebula does. The breakthrough came with enormous
digital sky surveys and sophisticated search
techniques. The Sloan Digital Sky Survey was
a pioneer here, and the Dark Energy Survey dramatically
expanded the census, revealing a host of ultra
-faint candidates by looking for tiny over -densities.
These are little knots where there are just a
few more stars than you would expect by chance.
Follow -up spectroscopy then distinguishes true
galaxies from star clusters by measuring how
the stars move. Galaxies show the signature of
dark matter, clusters do not. And the floodgates
are still opening. The Vera C. Rubin Observatory,
coming fully online soon, will map the southern
sky over and over for a decade, essentially making
a time -lapse movie of the universe. It's expected
to add dozens, maybe hundreds, more of these
ghostly satellites to the map. These little galaxies
matter in big ways. First, they are dark matter
laboratories. Dwarves are often dominated by
dark matter, so by measuring the motions of their
stars, astronomers can probe how dark matter
clusters on small scales. Second, they are crucial
clues to galactic evolution. Massive galaxies
like the Milky Way likely grew by merging with
smaller ones, and many dwarves are survivors
or new arrivals in that ongoing story. Some are
actively being torn apart, leaving long streams
of stars wrapped around our galaxy like faint
ribbons. Third, they speak directly to the missing
satellites problem. Computer simulations predict
that a Milky Way -sized galaxy should have hundreds
of tiny companions. We have found a few dozens
so far. Either most of the rest are simply too
faint to notice until recently, and we're just
now finding them, or our models need fine tuning.
Either way, the science gets sharper. A few examples
make this concrete. The Sagittarius dwarf Spheroidal
Galaxy is so close to us that our gravity is
shredding it. Its stars have been pulled into
great arcs, tidal streams that circle the sky
and are visible in deep survey data. The Canis
Major dwarf galaxy, discovered in 2003, may be
the closest galaxy to our own, hiding behind
the thick dust and star fields of the Milky Way's
disk, only 25 ,000 light -years from us. And
Segway 1, an ultra -faint dwarf discovered in
Sloan data, might be the dimmest known galaxy,
with only a few hundred visible stars. On an
image, it looks like nothing more than a slightly
suspicious smudge, yet its star's motions betray
a deep well of dark matter. These aren't showpieces
at the eyepiece, but they're gold mines for understanding
how galaxies, including ours, came to be. If
you live or travel in the southern hemisphere,
you can see two of the Milky Way's brighter companions
with your unaided eyes. The large and small Magellanic
Clouds, which look like detached pieces of the
Milky Way itself floating off the side of the
sky. From northern latitudes, you can make a
powerful connection by turning binoculars or
a small scope toward the Andromeda Galaxy, M31.
It's not a satellite of the Milky Way. We and
Andromeda are the two big cities in our local
group. But thinking of us as Andromeda's companion
gives a helpful sense of scale and relationship.
There is also a delightful binocular tie -in
you can do from anywhere in the northern hemisphere.
Look toward the rich star fields of Sagittarius
and find the globular cluster M54. This cluster
is actually embedded in the Sagittarius dwarf
galaxy. When you gaze at M54, you're peering
into another galaxy's star system. You can also
explore these faint satellites from the couch.
The Sloan Digital Sky Survey's online navigate
tools let you type in targets like Segway 1 or
Canis Major Dwarf, and zoom into the exact patches
of sky where they hide. The Dark Energy Survey
has public image galleries that show these ultra
-faint dwarves against deep star fields, and
they're striking once you know what you're looking
at. If you want a three -dimensional feel for
the neighborhood, free software like GaiaSky
can let you fly around the model of the Milky
Way and see where the known satellites live in
our halo. I'll drop links to all of these in
the show notes. There is a poetic link between
this week's seasonal black moon and these hidden
galaxies. The black moon hides in the sun's glare,
invisible not because it's absent, but because
of where and when we're looking. The dwarf galaxies
hide in the Milky Way's glare, invisible to our
eyes until we gather enough light and apply enough
patience. In both cases, the sky is reminding
us that the obvious story is rarely the whole
story. A darker sky reveals more. A deeper look
reveals the rest. Our galaxy is not an island
alone in the dark. It's part of a quiet archipelago
of companions, faint and fragile, sharing our
long orbit through the universe. If the stars
spoke to you this week or if a question's been
on your mind, I'd love to hear it. Visit our
website starrtrails .show where you can contact
me and explore past episodes. Be sure to follow
us on Blue Sky and YouTube. Links are in the
show notes. Until we meet again beneath the stars,
clear skies everyone.