Latest / Star Trails: From Backyard Astronomy to Cosmic Wonder / The Black Moon & the Milky Way’s Hidden Companions

Transcript

The Black Moon & the Milky Way’s Hidden Companions · Star Trails: From Backyard Astronomy to Cosmic Wonder

• 0:07
  Howdy Star Gazers and welcome to Star Trails.
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  I'm Drew and I'll be your guide to the night
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  sky for the week of August 17th to the 23rd.
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  This is a week designed for patient eyes and
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  early alarms. We have slim crescents at dawn,
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  a lovely planetary trio before sunrise, and the
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  darkest weekend of the month for deep sky hunting.
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  Then in the second half of the show, we'll widen
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  the frame and talk about the Milky Way's hidden
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  companions. Dozens of ghostly little galaxies
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  that have been circling us all along, almost
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  invisibly. and how they're reshaping our sense
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  of home in the universe. Let's begin with the
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  moon. Right now it's a delicate waning crescent
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  hugging the pre -dawn eastern horizon. Each morning
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  it slims further and slides closer to the sun's
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  glare. By Friday it's a little more than a silver
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  thread in brightening twilight. The new moon
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  arrives early Saturday morning. This isn't just
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  any new moon. It's what astronomers and calendar
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  keepers call a seasonal black moon, the rare
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  case when one astronomical season contains four
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  new moons instead of the usual three, and this
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  is the third of the four. It's mostly a calendar
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  quirk, but practically speaking, it means Friday
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  and Saturday nights bring the darkest skies of
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  the month. If you've been waiting for the Milky
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  Way to leap out of the sky, or you want to take
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  a crack at the Andromeda galaxy with the naked
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  eye from a truly dark sight, this is your moment.
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  The pre -dawn sky is where the week's best show
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  plays out. On Tuesday, August 19th, Mercury reaches
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  its greatest western elongation for this apparition.
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  About 30 to 40 minutes before sunrise, scan very
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  low in the east northeast. Mercury will be bright
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  for a small planet, but still close to the horizon.
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  A pair of binoculars will punch through the haze
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  and help you spot it. Now look higher up that
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  same stretch of sky and you'll find Venus and
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  Jupiter still sharing the stage after last week's
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  conjunction. Venus blazes lower, Jupiter shines
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  above it. On Tuesday and Wednesday a paper -thin
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  crescent moon joins them in the morning twilight
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  turning the scene into a gorgeous quartet. Mercury
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  near the horizon, Venus brighter above it, Jupiter
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  higher still, and the moon as a thin accent.
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  Evening planets are quieter but worth a look.
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  Mars clings to the low western horizon right
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  after sunset. It's faint, small, and sets quickly,
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  so you'll need an unobstructed view and a bit
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  of luck with atmospheric clarity. By late evening,
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  Saturn is the reward, rising in the east -southeast
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  and climbing higher night by night as it heads
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  towards opposition on September 21st. Even a
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  small telescope will show its rings and, with
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  patient viewing, the tiny star -like point of
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  Titan nearby. If you haven't seen Saturn this
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  year, mark it down, late August through September
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  is prime time. The Perseid meteor shower peaked
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  last week, but it doesn't switch off like a faucet.
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  Perseids linger through late August, and a few
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  bright stragglers are very possible in the predawn
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  hours, especially now that moonlight will not
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  be washing out the sky. If you're up early for
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  the planetary lineup anyway, give yourself 15
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  or 20 minutes just to scan the sky. Face generally
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  northeast, but don't fixate on the radiant. Meteors
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  can streak across any part of the sky, and you'll
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  spot more by letting your peripheral vision do
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  the work. If you want targets for the dark evenings
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  later this week, let me recommend three summer
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  gems that don't get a lot of hype. The first
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  is Delphinus, the dolphin, a little diamond and
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  tail asterism just east of Altair that resembles
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  a leaping dolphin. The second is Scutum, the
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  shield. Small and unassuming, but home to M11,
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  the wild duck cluster. Through binoculars or
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  a small scope, it looks like a burst of stardust
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  against the Milky Way, dense, rich, and full
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  of texture. The third is Volpecula, the fox,
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  which hides the famous coat hanger asterism.
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  Six stars and a bar, with four forming the hook.
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  Sweep slowly between Altair and Vega, and once
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  you see the code hanger, you will never unsee
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  it. Under this weekend's moonless sky, all three
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  reward a patient scan. In recent episodes, we've
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  journeyed out to the darkness at the edge of
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  our solar system to explore objects in the Kuiper
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  Belt and beyond. But what will we find when we
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  venture out to the edge of our galaxy? That's
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  coming up after the break. Stay with us. Welcome
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  back. Today we're talking about the Milky Way's
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  hidden companions. When you stand under a truly
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  dark sky this weekend and the Milky Way glows
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  like a river of light, it is tempting to think
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  we know this city of stars well. We've mapped
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  its spiral arms, traced its dusty lanes, and
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  even weighed the black hole at its center. But
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  there's a surprise waiting just beyond the obvious.
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  Our galaxy is surrounded by dozens, maybe hundreds,
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  of ghostly little galaxies. quietly orbiting
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  the Milky Way like dim satellites. Some are so
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  faint that until recent years we didn't know
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  they even existed. Astronomers call them dwarf
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  galaxies and the faintest among them are known
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  as ultra -faint dwarfs. They are small collections
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  of stars and dark matter bound together by gravity.
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  A typical ultra -faint dwarf may contain only
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  a few thousand stars spread over hundreds of
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  light years. By comparison, the Milky Way holds
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  hundreds of billions of stars. If we think of
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  the Milky Way as a bright metropolis, these dwarves
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  are outlying towns, sparsely populated, dimly
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  lit, and far from the bustle of the core. Why
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  did we miss them for so long? The answer is partly
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  about surface brightness. It's not just how much
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  light an object emits, but how spread out that
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  light is in the sky. Many of these dwarves are
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  so diffuse that their stars melt into the background.
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  They don't pop the way a bright cluster or a
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  nebula does. The breakthrough came with enormous
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  digital sky surveys and sophisticated search
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  techniques. The Sloan Digital Sky Survey was
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  a pioneer here, and the Dark Energy Survey dramatically
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  expanded the census, revealing a host of ultra
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  -faint candidates by looking for tiny over -densities.
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  These are little knots where there are just a
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  few more stars than you would expect by chance.
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  Follow -up spectroscopy then distinguishes true
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  galaxies from star clusters by measuring how
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  the stars move. Galaxies show the signature of
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  dark matter, clusters do not. And the floodgates
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  are still opening. The Vera C. Rubin Observatory,
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  coming fully online soon, will map the southern
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  sky over and over for a decade, essentially making
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  a time -lapse movie of the universe. It's expected
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  to add dozens, maybe hundreds, more of these
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  ghostly satellites to the map. These little galaxies
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  matter in big ways. First, they are dark matter
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  laboratories. Dwarves are often dominated by
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  dark matter, so by measuring the motions of their
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  stars, astronomers can probe how dark matter
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  clusters on small scales. Second, they are crucial
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  clues to galactic evolution. Massive galaxies
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  like the Milky Way likely grew by merging with
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  smaller ones, and many dwarves are survivors
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  or new arrivals in that ongoing story. Some are
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  actively being torn apart, leaving long streams
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  of stars wrapped around our galaxy like faint
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  ribbons. Third, they speak directly to the missing
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  satellites problem. Computer simulations predict
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  that a Milky Way -sized galaxy should have hundreds
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  of tiny companions. We have found a few dozens
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  so far. Either most of the rest are simply too
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  faint to notice until recently, and we're just
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  now finding them, or our models need fine tuning.
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  Either way, the science gets sharper. A few examples
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  make this concrete. The Sagittarius dwarf Spheroidal
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  Galaxy is so close to us that our gravity is
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  shredding it. Its stars have been pulled into
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  great arcs, tidal streams that circle the sky
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  and are visible in deep survey data. The Canis
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  Major dwarf galaxy, discovered in 2003, may be
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  the closest galaxy to our own, hiding behind
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  the thick dust and star fields of the Milky Way's
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  disk, only 25 ,000 light -years from us. And
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  Segway 1, an ultra -faint dwarf discovered in
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  Sloan data, might be the dimmest known galaxy,
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  with only a few hundred visible stars. On an
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  image, it looks like nothing more than a slightly
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  suspicious smudge, yet its star's motions betray
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  a deep well of dark matter. These aren't showpieces
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  at the eyepiece, but they're gold mines for understanding
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  how galaxies, including ours, came to be. If
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  you live or travel in the southern hemisphere,
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  you can see two of the Milky Way's brighter companions
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  with your unaided eyes. The large and small Magellanic
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  Clouds, which look like detached pieces of the
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  Milky Way itself floating off the side of the
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  sky. From northern latitudes, you can make a
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  powerful connection by turning binoculars or
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  a small scope toward the Andromeda Galaxy, M31.
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  It's not a satellite of the Milky Way. We and
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  Andromeda are the two big cities in our local
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  group. But thinking of us as Andromeda's companion
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  gives a helpful sense of scale and relationship.
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  There is also a delightful binocular tie -in
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  you can do from anywhere in the northern hemisphere.
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  Look toward the rich star fields of Sagittarius
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  and find the globular cluster M54. This cluster
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  is actually embedded in the Sagittarius dwarf
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  galaxy. When you gaze at M54, you're peering
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  into another galaxy's star system. You can also
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  explore these faint satellites from the couch.
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  The Sloan Digital Sky Survey's online navigate
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  tools let you type in targets like Segway 1 or
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  Canis Major Dwarf, and zoom into the exact patches
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  of sky where they hide. The Dark Energy Survey
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  has public image galleries that show these ultra
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  -faint dwarves against deep star fields, and
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  they're striking once you know what you're looking
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  at. If you want a three -dimensional feel for
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  the neighborhood, free software like GaiaSky
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  can let you fly around the model of the Milky
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  Way and see where the known satellites live in
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  our halo. I'll drop links to all of these in
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  the show notes. There is a poetic link between
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  this week's seasonal black moon and these hidden
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  galaxies. The black moon hides in the sun's glare,
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  invisible not because it's absent, but because
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  of where and when we're looking. The dwarf galaxies
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  hide in the Milky Way's glare, invisible to our
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  eyes until we gather enough light and apply enough
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  patience. In both cases, the sky is reminding
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  us that the obvious story is rarely the whole
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  story. A darker sky reveals more. A deeper look
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  reveals the rest. Our galaxy is not an island
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  alone in the dark. It's part of a quiet archipelago
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  of companions, faint and fragile, sharing our
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  long orbit through the universe. If the stars
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  spoke to you this week or if a question's been
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  on your mind, I'd love to hear it. Visit our
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  website starrtrails .show where you can contact
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  me and explore past episodes. Be sure to follow
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  us on Blue Sky and YouTube. Links are in the
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  show notes. Until we meet again beneath the stars,
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  clear skies everyone.