Latest / Star Trails: From Backyard Astronomy to Cosmic Wonder / Where the Milky Way Begins… and Ends

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Where the Milky Way Begins… and Ends · Star Trails: From Backyard Astronomy to Cosmic Wonder

• 0:07
  Howdy Star Gazers and welcome to this episode
• 0:10
  of Star Trails. My name is Drew, and I'll be
• 0:13
  your guide to the night sky for the week of May
• 0:16
  3rd to the 9th. This month we're launching into
• 0:19
  a new topic, galaxies. These are among the grandest
• 0:24
  structures in the universe, and perhaps the most
• 0:27
  complex. We'll begin by examining the one we
• 0:31
  live in, the Milky Way. Later in the show, we'll
• 0:34
  take a look at this week's night sky, and I'll
• 0:37
  share a quick observation report from a recent
• 0:40
  rocket launch I saw. Whether you're tuning in
• 0:43
  from the backyard or the balcony, I'm glad you're
• 0:45
  here. So grab a comfortable spot under the night
• 0:48
  sky and let's get started. Tonight we're going
• 0:54
  to try to do something that sounds simple at
• 0:57
  first, and then becomes increasingly difficult
• 1:00
  the longer we think about it. We're going to
• 1:02
  try to find the edge of the Milky Way. And as
• 1:06
  straightforward as that sounds, it leads us into
• 1:09
  one of the more unsettling realizations in astronomy.
• 1:13
  Because the deeper we go, the harder it becomes
• 1:16
  to answer a question that feels like it should
• 1:19
  have a clean, satisfying solution. Where does
• 1:22
  a galaxy actually end? If you step outside on
• 1:27
  a clear dark night, far from city lights, you
• 1:30
  can see the Milky Way stretching across the sky
• 1:33
  as a faint luminous band. A kind of celestial
• 1:37
  river made up of countless unresolved stars,
• 1:41
  soft and diffuse. For most of human history,
• 1:45
  we looked at that glowing band as a feature of
• 1:48
  the sky rather than a place within it. something
• 1:52
  distant and decorative rather than something
• 1:55
  we were embedded inside. But what we now understand
• 1:59
  is more disorienting because that pale streak
• 2:02
  of light is not above us at all. It surrounds
• 2:06
  us, envelops us, and defines our position within
• 2:10
  a vast rotating system of stars, gas, and dust.
• 2:15
  The Milky Way is what astronomers call a barred
• 2:18
  spiral galaxy. A structure that, if viewed from
• 2:21
  far enough away, would resemble a flattened disc
• 2:25
  with a bright central bar of stars stretching
• 2:28
  across its core. From whose ends long, sweeping
• 2:32
  spiral arms extend outward in graceful arcs,
• 2:36
  not as solid structures, but as regions where
• 2:39
  stars, gas, and dust are more concentrated. Those
• 2:43
  spiral arms exist because of something called
• 2:46
  density wave theory, which describes them not
• 2:49
  as fixed collections of stars, but as moving
• 2:52
  waves of higher density. Something like cosmic
• 2:56
  traffic patterns through which stars and gas
• 2:59
  pass over time. The bright blue stars we associate
• 3:03
  with spiral arms are really just temporary highlights,
• 3:07
  forming and fading as material moves through
• 3:10
  these regions. Also, the Milky Way is still making
• 3:14
  stars, forming roughly 1 -2 solar masses worth
• 3:18
  of new stars every year, mostly within these
• 3:22
  spiral arms. Our solar system lies in a minor
• 3:26
  spur known as the Orion Arm, about 26 ,000 light
• 3:31
  years from the galactic center, orbiting at roughly
• 3:34
  500 ,000 miles per year. One trip around the
• 3:39
  Milky Way takes about 225 million years. To put
• 3:43
  that into perspective, the dinosaurs lived and
• 3:46
  died just in the last galactic year. At this
• 3:50
  point, it's tempting to assume something that
• 3:53
  feels almost obvious. If the Milky Way is a flattened
• 3:58
  disk formed by the rotation of a massive cloud
• 4:01
  of gas over billions of years, and if our solar
• 4:05
  system is also a flattened disk, formed by the
• 4:08
  rotation of a much smaller cloud of gas and dust,
• 4:12
  then surely these two discs must line up. You
• 4:16
  might expect the planets to orbit along the same
• 4:18
  path traced by the Milky Way in our sky, as if
• 4:22
  the solar system were simply a smaller version
• 4:24
  of the galaxy itself. But that's not what we
• 4:28
  see. Instead, the path of the Sun, Moon, and
• 4:32
  planets, the line we call the ecliptic cuts across
• 4:36
  the Milky Way at a steep angle, tilted by roughly
• 4:40
  60 degrees relative to the galactic plane, and
• 4:44
  the reason for that turns out to be surprisingly
• 4:46
  simple. The Milky Way's flat shape is the result
• 4:50
  of its overall angular momentum inherited from
• 4:53
  the enormous cloud of material that formed the
• 4:56
  galaxy billions of years ago, causing everything
• 4:59
  within it to settle into a rotating disk that
• 5:02
  shares a common orientation. But the solar system
• 5:06
  formed much later, from a relatively small, localized
• 5:10
  pocket of gas and dust within that much larger
• 5:13
  galaxy. And that local cloud did not have to
• 5:17
  align with the galaxy as a whole. It had its
• 5:20
  own motion, its own rotation, and its own history,
• 5:24
  shaped perhaps by nearby stellar explosions,
• 5:28
  turbulent gas flows, or gravitational interactions.
• 5:32
  collapsing into a spinning disk with an orientation
• 5:35
  that was essentially independent of the Milky
• 5:38
  Way's structure. So what we're left with is a
• 5:41
  kind of cosmic misalignment. The Milky Way defines
• 5:45
  one plane, and that's the structure of the galaxy
• 5:48
  itself. The solar system defines another, the
• 5:51
  orbital plane of the planets. And those two planes
• 5:55
  intersect near Sagittarius and Gemini. So even
• 6:00
  though we're embedded within the Milky Way, we're
• 6:02
  not aligned with it. And at the center of all
• 6:05
  this, hidden behind dense clouds of gas and dust,
• 6:09
  lies something even more mysterious. A supermassive
• 6:13
  black hole known as Sagittarius A -star, containing
• 6:17
  about 4 million times the mass of our sun. It's
• 6:22
  worth noting that this is not unusual. Most large
• 6:25
  galaxies appear to host similar black holes at
• 6:28
  their centers. And while these objects can have
• 6:31
  dramatic effects on their immediate surroundings,
• 6:34
  heating gas, launching energetic outflows, and
• 6:38
  in some cases powering intensely bright galactic
• 6:41
  cores, they are not what holds the galaxy together.
• 6:45
  The overall structure of the Milky Way is governed
• 6:48
  by the combined gravity of its stars and, even
• 6:52
  more significantly, by an enormous halo of unseen
• 6:56
  matter extending far beyond what we can see.
• 7:00
  Of course, we've never actually seen the Milky
• 7:02
  Way from the outside. Every image you've ever
• 7:06
  seen of our galaxy, those beautiful spiral illustrations
• 7:09
  with glowing arms and a bright central bar, are
• 7:13
  reconstructions. built from decades of observation,
• 7:17
  mathematics, and careful inference. Because we're
• 7:21
  embedded inside the disk itself, our view is
• 7:24
  obscured by enormous clouds of interstellar dust
• 7:27
  that block visible light, especially toward the
• 7:30
  galactic center. So, to map our own galaxy, astronomers
• 7:35
  had to learn to see it in other ways. Using radio
• 7:38
  telescopes, they tracked clouds of neutral hydrogen
• 7:42
  gas emitting at a wavelength of 21 centimeters,
• 7:46
  a natural radio signal produced by hydrogen atoms
• 7:49
  throughout the galaxy. By measuring subtle shifts
• 7:53
  in that signal, a phenomenon known as the Doppler
• 7:56
  effect, astronomers could determine which gas
• 7:59
  clouds were moving toward us, which were moving
• 8:01
  away, and how fast. Over time, that allowed us
• 8:05
  to build a three -dimensional map of the Milky
• 8:08
  Way, while never actually leaving it. With that
• 8:11
  picture in mind, a rotating disk, spiral arms
• 8:15
  as density waves, a hidden central black hole,
• 8:19
  we can begin our journey outward. Imagine rising
• 8:23
  slowly above the plane of the galaxy, watching
• 8:26
  as the dense band of stars begins to thin. the
• 8:29
  structure softening as you leave behind the bright
• 8:32
  cohesion of the galactic disk and enter a more
• 8:36
  diffuse region known as the stellar halo, populated
• 8:39
  by older stars and ancient globular clusters,
• 8:43
  some of which, like Omega Centauri, may in fact
• 8:47
  be the remnants of smaller galaxies that were
• 8:49
  pulled apart and absorbed long ago. And here,
• 8:53
  as the stars grow fewer and the sky begins to
• 8:56
  open up, something unexpected happens. We expect
• 9:00
  the light of the galaxy to fade cleanly into
• 9:03
  darkness. But it doesn't quite do that. Because
• 9:07
  out here, in regions that appear almost empty,
• 9:11
  there exists something so faint that it escaped
• 9:14
  detection for most of human history. A diffuse
• 9:17
  network of dust clouds known as Integrated Flux
• 9:21
  Nebula. These clouds don't emit their own light,
• 9:25
  nor are they illuminated by a single nearby source,
• 9:29
  but instead glow faintly by reflecting the combined
• 9:33
  light of the entire Milky Way. The integrated
• 9:37
  output of billions of stars scattered across
• 9:40
  space and gently revealed through long exposure
• 9:43
  astrophotography. In recent years, photographers
• 9:47
  have begun capturing these structures in remarkable
• 9:50
  detail, particularly in regions near the North
• 9:53
  Celestial Pole around Polaris, where what once
• 9:57
  appeared to be an empty space now reveals subtle,
• 10:01
  smoky textures, evidence that even far from the
• 10:04
  bright disc the galaxy's presence is still being
• 10:07
  felt. And then we reach another layer entirely.
• 10:12
  Because beyond the stars, beyond the halo, beyond
• 10:16
  even this faint reflected glow, there exists
• 10:19
  something much larger that we cannot see at all.
• 10:24
  Surrounding everything we've described is a vast
• 10:27
  halo of dark matter, whose presence is inferred
• 10:31
  through observations like the galaxy rotation
• 10:34
  curve, which reveal that stars orbit the galaxy
• 10:38
  at speeds that can't be explained by visible
• 10:40
  matter alone. This invisible structure may extend
• 10:44
  out to nearly a million light -years far beyond
• 10:48
  the visible boundaries of the Milky Way, complicating
• 10:51
  any attempt to define where the galaxy truly
• 10:54
  ends. Astronomers sometimes use a concept called
• 10:58
  the Virial Radius to approximate a boundary,
• 11:02
  marking the region where the galaxy's gravity
• 11:04
  dominates over the expansion of the universe.
• 11:08
  But even this is more a useful definition than
• 11:11
  a physical edge. Eventually, we leave the galaxy
• 11:15
  behind. We move out into a universe that, on
• 11:19
  the largest scales, is organized into a vast
• 11:23
  network known as the Cosmic Web, where galaxies
• 11:27
  gather along filaments that stretch across space
• 11:30
  surrounding enormous regions known as Cosmic
• 11:33
  Voids. expanses that can span hundreds of millions
• 11:37
  of light years and contain very little at all.
• 11:41
  From within one of these voids, the universe
• 11:44
  would not appear rich or crowded, but distant
• 11:47
  and quiet, with only a few faint galaxies scattered
• 11:51
  across an otherwise overwhelming darkness. And
• 11:55
  now for one final shift in perspective. Imagine
• 11:59
  returning to Earth, standing beneath the night
• 12:02
  sky and removing one thing. the Milky Way itself.
• 12:07
  If we could somehow erase it, there wouldn't
• 12:09
  be much of anything left in our night sky, because
• 12:12
  everything we see in the night sky, save for
• 12:15
  some very distant and bright objects, are part
• 12:18
  of our home galaxy. All the stars we see in our
• 12:22
  night sky are in the Milky Way. If we could somehow
• 12:26
  lift our solar system out of the Milky Way and
• 12:29
  plop it down by itself into empty space, We'd
• 12:32
  see our sun and moon and our planets and whatever
• 12:36
  other objects are passing by, comets, asteroids
• 12:39
  and so on. Maybe even a wandering star that's
• 12:43
  been ejected from another system. We'd be able
• 12:46
  to trace the faintest hints of neighboring galaxies
• 12:49
  within the local group, like Andromeda, one of
• 12:52
  the most distant objects we can see with the
• 12:54
  naked eye. But mostly, the sky would be very
• 12:58
  empty. So, where does a galaxy begin, and where
• 13:02
  does it end? The answer is that it doesn't, at
• 13:06
  least not in the way we expect. A galaxy is not
• 13:10
  a sharply bounded object, but a gradual transition,
• 13:14
  a region where matter becomes more concentrated,
• 13:17
  then less so, then less still, until it blends
• 13:21
  into the larger structure of the universe. It's
• 13:25
  not unlike our own solar system in that regard.
• 13:28
  We once thought our own system ended somewhere
• 13:31
  around Pluto's orbit, but we now know there's
• 13:34
  a vast region of smaller objects, the Kuiper
• 13:37
  Belt, farther out, tracing the outer disk of
• 13:40
  the solar system. Farther still, we're enveloped
• 13:44
  by the bubble of the Oort cloud, which may extend
• 13:48
  halfway to the nearest stars. In space, large
• 13:52
  areas don't just end abruptly, they gradually
• 13:56
  fade away. Our galaxy is a system shaped by motion
• 14:01
  and gravity, containing spiral waves of stars,
• 14:06
  a hidden central black hole, ancient stellar
• 14:09
  remnants, invisible halos of dark matter, and
• 14:13
  even the faint echo of its own light reflected
• 14:17
  in distant dust. Next week we'll take this one
• 14:21
  step further because galaxies aren't still. They
• 14:25
  move, they interact, and sometimes they collide.
• 14:29
  Our home, the Milky Way, is already on a path
• 14:32
  toward becoming something entirely new. After
• 14:47
  a quick break, we'll be back with a brief observation
• 14:49
  report and a look at this week's night sky. Stay
• 14:53
  with us. Welcome back. I have another rocket
• 15:09
  launch observation to report, and this time I
• 15:12
  didn't bother driving around to find a good observing
• 15:15
  location. I simply stepped out on the rear balcony
• 15:18
  of my house. Around 8 .50 p .m. on the evening
• 15:22
  of April 27, an Atlas V rocket lifted off carrying
• 15:27
  Amazon's latest batch of LEO satellites, part
• 15:31
  of the company's growing low Earth orbit broadband
• 15:34
  constellation, formerly known as Project Kuiper.
• 15:38
  Launching from the Cape Canaveral Space Force
• 15:41
  Station, the rocket followed a standard ascent
• 15:44
  profile, placing its payload into orbit roughly
• 15:47
  280 miles above Earth, where the satellites will
• 15:51
  gradually spread out and integrate into the growing
• 15:54
  constellation. Following the live broadcast about
• 15:58
  two minutes after launch and looking east, I
• 16:00
  first spotted the rocket just above the tree
• 16:03
  line as a bright reddish point of light with
• 16:06
  a distinct conical plume trailing behind it.
• 16:10
  The rocket was traveling northeast, steadily
• 16:12
  climbing in elevation as it moved leftward across
• 16:15
  my field of view. Over the course of a minute
• 16:18
  and a half, the object maintained a smooth, continuous
• 16:21
  trajectory, gradually dimming before finally
• 16:25
  winking out. Through binoculars, the trailing
• 16:28
  plume became more pronounced, giving the object
• 16:31
  a comet -like appearance. I talked about how
• 16:34
  to spot rocket launches back in episode 101,
• 16:38
  so be sure to check that out if you missed it,
• 16:40
  particularly if you live near the east or west
• 16:42
  coast of the U .S., where launches may be more
• 16:45
  common than you think. As we step into the first
• 16:55
  full week of May, the night sky carries the lingering
• 16:58
  glow of the recent full moon. and that brightness
• 17:02
  will shape much of what we can see in the days
• 17:04
  ahead. The full flower moon peaked on May the
• 17:08
  1st, and now as we move into this week, the moon
• 17:12
  begins to wane, rising later each night and gradually
• 17:16
  surrendering the evening sky back to the stars.
• 17:20
  Early in the week you'll find a bright waning
• 17:22
  gibbous moon dominating the late evening and
• 17:25
  overnight hours. washing out fainter objects
• 17:28
  but offering excellent opportunities to explore
• 17:31
  lunar features, craters, maria, and mountains
• 17:35
  along the terminator. By the end of the week
• 17:38
  on May 9th, the moon reaches its last quarter
• 17:41
  phase, rising around midnight and leaving the
• 17:44
  early evening skies noticeably darker. This week
• 17:48
  also brings one of the quieter but more elegant
• 17:51
  meteor showers of the year. the Eta Aquarids,
• 17:55
  debris left behind by none other than Halley's
• 17:58
  Comet. The shower peaks in the early morning
• 18:01
  hours around May the 5th with activity stretching
• 18:03
  a few days on either side. These meteors are
• 18:07
  fast, streaking into Earth's atmosphere at high
• 18:09
  velocity, often leaving persistent glowing trails.
• 18:14
  But there's a catch. The waning gibbous moon
• 18:17
  will still be fairly bright during peak mornings,
• 18:20
  So the best strategy is to head out before dawn,
• 18:23
  place the moon behind a tree or building, and
• 18:26
  let your eyes adjust to the darker portions of
• 18:29
  the sky. Even under less than perfect conditions,
• 18:32
  you might catch a handful of these quick streaks
• 18:35
  just before sunrise. In the early evening sky,
• 18:40
  Jupiter continues to hold court, shining prominently
• 18:43
  high overhead and slowly descending toward the
• 18:46
  horizon as the week progresses. It's one of those
• 18:49
  reliable anchor points, easy to spot even in
• 18:53
  twilight, and a great target for binoculars or
• 18:56
  a small telescope where you can begin to pick
• 18:58
  out its Galilean moons. Meanwhile, if you're
• 19:02
  willing to wake up early, the pre -dawn sky is
• 19:05
  beginning to wake up as well. Saturn is rising
• 19:08
  earlier each morning, becoming easier to spot
• 19:11
  low in the eastern sky before sunrise, hinting
• 19:15
  at its return to prominence in the coming months.
• 19:18
  This is one of those transitional periods where
• 19:20
  the evening sky is slowly losing its brightest
• 19:23
  planets, and the morning sky is quietly preparing
• 19:27
  to take over. As the week progresses and the
• 19:30
  moon begins to rise later, the sky opens up again,
• 19:34
  revealing some of the deeper, more distant structures
• 19:36
  that define this time of year. And, in keeping
• 19:40
  with our theme tonight, this is Galaxy Season.
• 19:44
  If you're looking for something bright and relatively
• 19:47
  easy to track down, start with the Sombrero Galaxy,
• 19:51
  Messier 104, low in the constellation Virgo.
• 19:56
  Even under moderate moonlight, its bright central
• 19:59
  core can punch through the glow, appearing in
• 20:01
  a small telescope as a compact, luminous oval.
• 20:05
  With a bit more aperture or in darker skies,
• 20:08
  you may begin to notice its defining feature.
• 20:11
  a dark lane of dust slicing cleanly through the
• 20:14
  galaxy, giving it that iconic hat -like appearance.
• 20:19
  It's one of those objects that feel structured,
• 20:22
  symmetrical, and strangely precise. Now, if you
• 20:25
  want something that connects directly to what
• 20:27
  we talked about earlier in the show, something
• 20:30
  that helps you visualize the Milky Way from the
• 20:33
  outside, turn your attention to the faint constellation
• 20:36
  Coma Berenices and seek out the Needle Galaxy.
• 20:43
  NGC 4565. This is a spiral galaxy seen perfectly
• 20:47
  edge -on. In other words, you're looking at a
• 20:50
  galaxy much like our own, but from the outside
• 20:53
  stretched into a thin, delicate line of light,
• 20:56
  with a dark band cutting through its center.
• 20:59
  And when you see it, even faintly, something
• 21:02
  clicks. That narrow sliver of light is what the
• 21:06
  Milky Way would look like to a distant observer.
• 21:09
  And then there's our wow object for the week.
• 21:12
  High in the evening sky in the constellation
• 21:15
  Canes Venetici lies the Whirlpool galaxy, Messier
• 21:20
  51. This one is a little more challenging, especially
• 21:24
  earlier in the week with the moon still bright.
• 21:27
  But as the skies darken toward the weekend, it
• 21:29
  becomes a rewarding target. Through a small telescope,
• 21:33
  it may appear as a faint, misty patch with a
• 21:36
  brighter core. But under steady skies and with
• 21:39
  a bit of patience you may begin to notice something
• 21:42
  remarkable, a second glow nearby, its companion
• 21:46
  galaxy. These two galaxies are locked in a slow
• 21:50
  gravitational interaction, their mutual pull
• 21:53
  distorting their shapes and enhancing the spiral
• 21:56
  structure we see. I have one more bit of housekeeping
• 22:06
  before I sign off this week. I once again apologize
• 22:11
  for abandoning our book selection. Next week
• 22:15
  I hope to wrap up my commentary on the final
• 22:18
  chapters in Nightwatch. We're right near the
• 22:21
  end with chapters 10, 11, and 12. That's going
• 22:29
  to do it for this week. If you found this episode
• 22:32
  interesting, please share it with a friend who
• 22:34
  might enjoy it. The easiest way to do that is
• 22:37
  by sending folks to our website, StarTrails .Show.
• 22:42
  And if you'd like to support the show, use the
• 22:44
  link on the site to buy me a coffee. That really
• 22:47
  helps. Be sure to follow Star Trails on Blue
• 22:50
  Sky and YouTube. Links are in the show notes.
• 22:54
  Until we meet again beneath the stars... Clear
• 22:57
  skies everyone.