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When Galaxies Collide · 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
the 10th to the 16th. This week we continue our
0:20
journey through the world of galaxies, not by
0:23
asking what galaxies are, but by asking what
0:26
galaxies actually do. We'll explore how galaxies
0:30
collide, eat smaller neighbors, fling stars into
0:34
the darkness between galaxies, and even exchange
0:38
material across intergalactic space. Along the
0:42
way we'll revisit the evolving story of the Milky
0:45
Way and the Andromeda Galaxy, and discover that
0:49
what many of us thought was our galaxy's inevitable
0:52
fate may not be written in stone after all. Later
0:56
in the show, we'll wrap up our long -delayed
0:58
Star Trails book club discussion of Nightwatch,
1:02
and we'll step outside together for this week's
1:05
Night Sky Report, where a waning moon gives way
1:08
to darker skies, spring galaxies overhead, and
1:12
a pre -dawn meeting between the moon, Mars, and
1:15
Saturn. Whether you're tuning in from the backyard
1:18
or the balcony, I'm glad you're here. So grab
1:21
a comfortable spot under the night sky, and let's
1:24
get started. Before we kick off this week's discussion,
1:28
I wanted to back up and revisit something from
1:30
last week's episode. A listener mentioned to
1:34
me that he was surprised to learn the Milky Way
1:36
is a barred spiral galaxy, as opposed to just
1:40
a normal spiral. Many of us may have had the
1:44
same thought, because I distinctly remember seeing
1:47
artists' illustrations of it in books through
1:50
the years. The Milky Way as a graceful pinwheel
1:53
of stars and dust, none featuring a central bar.
1:58
This isn't a false memory because the bar at
2:01
the center of our Milky Way is a fairly recent
2:04
discovery. Astronomers suspected the Milky Way
2:08
might be barred as early as the 1960s, based
2:11
on stellar motions and radio observations of
2:15
gas near the galactic center. But because we
2:18
live inside the galaxy and because the galactic
2:21
core is buried behind thick dust clouds, it's
2:25
incredibly difficult to map our own structure
2:28
directly. The real breakthrough came with infrared
2:31
observations, especially from the Spitzer Space
2:35
Telescope. Infrared light can penetrate dust
2:39
far better than visible light, allowing astronomers
2:42
to see through the crowded core. By the mid -2000s,
2:46
Spitzer's Glimpse Survey helped confirm that
2:50
the Milky Way contains a large central stellar
2:53
bar. making the barred spiral classification
2:56
much more secure. In my chat with this listener,
3:00
we both wondered how something like a straight
3:03
bar shape could be formed in space. It turns
3:06
out cosmology is weirder than I thought. Bars
3:10
do seem to arise from gravity and rotation, but
3:13
in a novel way. Over time, if a galactic disk
3:17
becomes unstable, Stars can begin settling into
3:21
elongated orbits near the center. Instead of
3:24
orbiting in a neat circular path, many stars
3:27
begin moving in coordinated, stretched paths.
3:31
If enough stars start moving in these long, narrow
3:34
orbits, a bar pattern eventually emerges. And
3:38
it's not just decorative. These bars act like
3:41
gravitational conveyor belts, channeling gas
3:45
inward toward the galactic center. which can
3:48
trigger star formation, feed the central black
3:51
hole, and further shape spiral arms. The bar
3:55
becomes one of the engines that helps a galaxy
3:58
evolve. Spiral and barred spirals are two of
4:02
the main galaxy types. Both feature a flat rotating
4:05
disk, spiral arms, and ongoing star formation.
4:10
Barred spirals are different in that the spirals
4:13
extend from the bar rather than the central disk.
4:16
And we think about two -thirds of the spiral
4:19
galaxies in the nearby universe have bars, making
4:22
them more common than plain spirals. These grand
4:26
design spirals are quite different from the other
4:29
galaxy types we see. Elliptical galaxies resemble
4:34
eggs in space. They tend to have older stars,
4:37
very little gas and dust, and very little new
4:41
star formation. The two satellite galaxies we
4:44
see in images of Andromeda are good examples
4:47
of these. Then we have irregular galaxies which
4:51
defy any firm categorization. They can be distorted
4:55
by gravity, rich in star formation, and are chaotic
4:59
or asymmetric in appearance. Our satellite galaxy,
5:04
the Large Magellanic Cloud, is a good nearby
5:07
example. With that, let's turn to this week's
5:11
main topic, because these galaxies aren't out
5:14
there just looking pretty. On the contrary, they
5:17
can be quite violent. As we closed the last episode,
5:25
I left you with a teaser. One day, perhaps four,
5:28
five, or even ten billion years from now, our
5:31
own Milky Way may have a close encounter, or
5:35
even a full collision, with our giant neighboring
5:38
galaxy, Andromeda. Galaxies aren't isolated objects.
5:43
They move. They evolve. They interact. They steal
5:47
from one another. They fling stars into deep
5:50
space. They exchange matter across unimaginable
5:53
distances. And sometimes in the glacial language
5:57
of gravity, they consume one another entirely.
6:01
When we look at those breathtaking images released
6:04
by NASA, the European Space Agency, or the Hubble
6:07
Space Telescope, galaxies often appear peaceful.
6:11
Elegant pinwheels of starlight, suspended in
6:14
darkness, frozen in time, as though they've always
6:18
existed in exactly the shape we see them now.
6:21
But that image is misleading. Galaxies are dynamic
6:25
ecosystems, vast gravitational cities, made of
6:30
hundreds of billions of stars, clouds of gas,
6:34
dust, rich and heavy elements, dark matter halos
6:37
we can't directly see, and invisible magnetic
6:41
structures threading through it all. And like
6:44
cities, or perhaps like civilizations, galaxies
6:47
have histories. They grow, they change, they
6:51
carry scars. And one of the strangest things
6:54
astronomers have discovered is that many of the
6:56
largest galaxies in the universe appear to have
6:59
grown not just by forming stars, but by consuming
7:03
smaller galaxies around them. Astronomers call
7:07
this galactic cannibalism, and as dramatic as
7:11
that sounds, it's not that rare. Our own galaxy
7:14
is doing it right now. Orbiting the Milky Way
7:18
is a small companion called the Sagittarius Dwarf
7:22
Spheroidal Galaxy, and it wasn't even discovered
7:25
until 1994. Think about that for a moment. We've
7:30
studied the sky for thousands of years, we've
7:32
charted constellations, built observatories,
7:36
landed on the moon, and sent probes beyond the
7:39
outer planets. And yet, an entire neighboring
7:42
galaxy remained hidden from us until the 1990s.
7:46
That's because Sagittarius lies almost directly
7:50
behind the crowded and dusty central regions
7:53
of our own galaxy. hidden among the star clouds
7:56
of the Milky Way's core, essentially camouflaged
8:00
by our own structure. The Sagittarius dwarf is
8:04
a small, faint, ancient kind of galaxy known
8:07
as a dwarf spheroidal. It's only about 10 ,000
8:11
light -years across, tiny compared to the Milky
8:14
Way's 100 ,000 light -year span. And it contains
8:18
mostly older stars with very little fresh gas,
8:22
or active star formation left. If the Milky Way
8:26
is a great ocean liner crossing a dark cosmic
8:29
sea, Sagittarius is more like a small raft that
8:33
wandered too close and got sucked into the ship's
8:36
propeller. And over billions of years, the Milky
8:40
Way's gravity has been pulling it apart, slowly
8:43
stretching it, star by star, until those stars
8:46
begin to drift free of their home. Today, astronomers
8:50
can actually trace those lost stars. They form
8:54
what's known as the Sagittarius Stream. Enormous
8:58
rivers of stars wrapping around the Milky Way
9:01
like ghostly ribbons. Some of them looping around
9:05
our galaxy multiple times. These stars once belonged
9:09
to Sagittarius. Now they've become part of our
9:13
galaxy's halo, their original home slowly being
9:17
dismantled around them. And the story gets even
9:20
stranger. Data from the Gaia mission suggests
9:24
that Sagittarius may have plunged through the
9:27
disk of the Milky Way multiple times in the distant
9:30
past. And those encounters may have sent ripples
9:33
through our galaxy itself. Waves of stars moving
9:37
up and down through the galactic disk, almost
9:41
like dropping a stone into a pond. So this little
9:45
galaxy isn't just being destroyed by the Milky
9:48
Way. It may have left scars on our galaxy in
9:52
the process. And if you'd like to see what a
9:55
larger galactic collision looks like, astronomers
9:58
don't have to rely on simulations or theory.
10:02
We can actually see it happening. About 70 million
10:06
light years away, in the direction of the constellation
10:09
Corvus, lies one of the most spectacular galactic
10:13
train wrecks in the observable universe, the
10:16
Antenna galaxies. These are two spiral galaxies,
10:21
once separate, now locked in a gravitational
10:24
encounter that has been unfolding for hundreds
10:26
of millions of years. And if you've ever seen
10:30
an image of them, the name makes sense. Gravity
10:34
has stretched their spiral arms outward into
10:36
enormous tidal tails. Vast streams of stars,
10:41
gas and dust extending hundreds of thousands
10:44
of light years into intergalactic space. Distances
10:48
so vast the human mind almost refuses to process
10:51
them. And yet what's happening inside those galaxies
10:55
may be even more beautiful than the destruction
10:58
we can see from the outside. It's hard to believe
11:01
but when galaxies collide stars almost never
11:05
hit each other directly. The space between stars
11:09
is simply too immense. Instead, gravity reshapes
11:13
everything else. Clouds of hydrogen gas collide,
11:18
dust compresses, regions collapse under their
11:21
own weight, and suddenly in the middle of all
11:24
that chaos, new stars begin to form. The antenna
11:28
galaxies are, in many ways, a giant stellar nursery
11:32
where entire clusters, containing millions of
11:35
newborn stars, are being ignited in a beautiful
11:38
paradox, creation born directly from destruction.
11:44
But stars aren't the only things galaxies exchange.
11:48
When galaxies interact, entire clouds of gas
11:51
can be pulled loose. Dust, rich in carbon, oxygen,
11:55
silicone, and iron, the elements forged in ancient
11:59
stars, can be flung far beyond the visible boundaries
12:03
of a galaxy. and woven invisibly through it all
12:06
are magnetic fields. Galaxies possess magnetic
12:11
fields stretching across thousands and sometimes
12:14
tens of thousands of light years. These invisible
12:18
structures help guide charged particles, influence
12:22
the movement of gas, and may even help determine
12:25
where future generations of stars are born. A
12:29
few weeks ago here on Star Trails, we had the
12:31
privilege of speaking with astrophysicist Enrique
12:35
Lopez Rodriguez, whose work is helping astronomers
12:39
map these invisible magnetic structures. And
12:42
it turns out those magnetic fields may extend
12:45
even beyond the galaxies themselves. Right here
12:49
in our cosmic neighborhood, the large and small
12:51
Magellanic clouds appear to be connected by something
12:54
called the Magellanic Bridge. A stream of gas,
12:58
dust, young stars, and possibly a shared magnetic
13:02
field stretching roughly 75 ,000 light years
13:06
through intergalactic space. And farther out,
13:09
in violent mergers like the system known as ARP
13:13
-220, astronomers, including Dr. Rodriguez himself,
13:18
have found evidence of what some researchers
13:20
have described as a magnetic superhighway. invisible
13:24
structures guiding gas, plasma, dust, and charged
13:28
particles through the wreckage of colliding galaxies.
13:32
Since these structures are called superhighways,
13:35
I was curious to understand exactly how fast
13:38
material could be racing along them. If you recall
13:41
Maxwell's equations, magnetism does propagate
13:45
at the speed of light. But in general, matter
13:48
following a magnetic field gets nowhere close
13:51
to the speed of light. For example, in ARP220,
13:56
some of the outflowing material was measured
13:58
at around 1 million miles per hour. That sounds
14:02
fast, and it is, but it's only about 0 .17 %
14:08
the speed of light. That's nowhere near relativistic.
14:12
However, things get more interesting at the quantum
14:15
level. Individual particles, especially cosmic
14:19
rays like electrons or protons, can move much
14:22
faster, sometimes very close to the speed of
14:25
light, and they spiral around magnetic field
14:29
lines and corkscrew -like paths. And that leads
14:33
to one of my favorite thoughts in this entire
14:36
series. We often say, as Carl Sagan famously
14:40
reminded us, that we're made of star stuff. But
14:44
on the largest scales, some of that star stuff
14:48
may not even be native to its own galaxy. The
14:52
iron in a future planet, the carbon in some distant
14:55
atmosphere, the silicone in an alien mountain
14:58
range, may have begun its journey in the death
15:01
of a star a long time ago in a galaxy far, far
15:05
away. My apologies to George Lucas. If gas and
15:11
dust can be bounced out of a galaxy, it only
15:13
makes sense that stars can suffer the same fate.
15:17
Some stars are flung outward by gravity itself,
15:21
accelerated by galactic mergers, tidal interactions,
15:25
or close encounters with supermassive black holes.
15:29
Astronomers call some of these hypervelocity
15:32
stars. They're moving so fast they can escape
15:35
their galaxies entirely. Imagine a star born
15:38
beneath the glow of billions of neighboring suns,
15:42
spending millions or billions of years as part
15:45
of a galactic family. only to be thrown clear
15:48
of its home forever, exiled into the darkness
15:52
between galaxies. Which brings us back to the
15:55
question we teased last week. What about us?
15:59
What about the Milky Way and Andromeda? Even
16:03
if you think you've heard this story before,
16:05
listen on, because the science is changing. For
16:09
decades, astronomers believed the ending was
16:11
already written. In roughly four to five billion
16:15
years, the Milky Way and Andromeda would collide,
16:19
merge, and eventually become one giant galaxy,
16:23
sometimes nicknamed Milkomeda. It was one of
16:26
those cosmic facts people repeated with absolute
16:29
confidence. But science kept observing, and newer
16:33
measurements from Hubble and Gaia have changed
16:36
the story. That collision may no longer be inevitable.
16:41
Right now, some of the best models suggest something
16:44
closer to a 50 -50 chance over the next 10 billion
16:47
years. And I love that, because it reminds us,
16:51
even on the scale of galaxies, the future may
16:54
still be unwritten. Will Andromeda collide with
16:58
us? Will it pass close enough to tear at our
17:01
spiral arms before drifting away? Or will gravity
17:05
eventually pull both galaxies into one final,
17:08
slow -motion embrace? The truth is, we're still
17:12
learning. But if that collision does happen,
17:15
the night sky of some unimaginably distant future
17:19
Earth, or whatever remains of it, would be extraordinary.
17:23
Andromeda would slowly grow larger over millions
17:26
of years, eventually spanning huge portions of
17:29
the sky. Spiral arms would stretch. Stars would
17:34
be flung into intergalactic space. Clouds of
17:37
gas would collapse into new generations of suns
17:41
and at the hearts of both galaxies Two supermassive
17:45
black holes would begin an even slower dance
17:48
Spiraling toward one another over hundreds of
17:51
millions of years until eventually they too may
17:55
merge After a quick break, we'll be back with
18:16
this week's Sky and we'll wrap up our book club
18:19
selection on Night Watch. Stay with us. Welcome
18:34
back. As we move into this week, the moon is
18:38
finally beginning to get out of our way, and
18:41
for deep sky observers, that's always welcome
18:43
news. We begin the week just after last quarter
18:47
moon, so each night the moon will rise later
18:50
and appear thinner, slipping into a graceful
18:53
waning crescent as the week unfolds. By the evening
18:57
of May the 16th, we arrive at New Moon, giving
19:00
us some of the darkest skies of the month. If
19:03
you've been waiting for a serious observing session,
19:05
this is your window. Planet -wise, the western
19:09
sky after sunset remains home to a nice pairing.
19:13
Venus is still dominating the early evening,
19:16
shining like an unmistakable beacon in the twilight.
19:19
Its brightness literally stopped me in my tracks
19:22
last night. Nearby, Jupiter is also hanging in
19:26
the western sky, lower now than it was earlier
19:28
in the season, but still bright enough to command
19:31
attention. These two planets are slowly drawing
19:34
closer together as the month progresses, making
19:37
for a lovely naked eye scene if you catch them
19:40
shortly after sunset. Farther along in the week,
19:43
before dawn, both Mars and Saturn begin to creep
19:47
back into view for early risers, sitting low
19:50
in the eastern sky as morning twilight approaches.
19:54
Now, let's talk deep sky, because with darker
19:57
skies returning, this is a wonderful week to
20:00
revisit some of Spring's lesser -talked -about
20:02
treasures. First, point your scope toward the
20:05
constellation of Coma Berenices and seek out
20:08
the Black Eye Galaxy, Messier 64. Through a modest
20:14
backyard telescope, it appears as a soft oval
20:18
glow. But with larger aperture and darker skies,
20:21
you may begin to notice the feature that gives
20:23
it its name. A dark dust lane crossing one side
20:27
of the galaxy's core, almost like a cosmic bruise.
20:32
This galaxy lies roughly 17 million light years
20:35
away and is one of Spring's most underrated targets.
20:40
Next, if you're looking for something a little
20:42
more challenging, turn toward the constellation
20:44
Canes Venetici and find the Whirlpool galaxy.
20:48
We've touched on galactic structure recently,
20:52
but if this week's episode is about what galaxies
20:54
do, M51 feels especially appropriate. This famous
21:00
interacting galaxy pair is one of the clearest
21:03
examples of a spiral galaxy being gravitationally
21:06
influenced by a companion. Under dark skies,
21:10
even smaller telescopes may reveal its bright
21:13
core and companion galaxy. while larger scopes
21:17
can tease out hints of the spiral arms. For something
21:20
a little different, try the Owl Cluster in the
21:23
constellation Cassiopeia, also nicknamed the
21:27
ET or Dragonfly Cluster. This isn't a galaxy,
21:32
but a nice open cluster that can be fun for beginners.
21:35
Two brighter stars form what looks like glowing
21:38
eyes, with fainter stars creating wings or outstretched
21:42
arms, depending on your imagination. And, if
21:46
you're out very late this week, or up before
21:49
dawn under truly dark skies, something else begins
21:52
to return. The bright core of the Milky Way itself.
21:57
By mid -May, especially near New Moon, the summer
22:01
Milky Way starts climbing in the southeastern
22:03
sky during the pre -dawn hours. This is one of
22:07
the first real hints that summer observing season
22:09
is on the way. And if you're far from city lights,
22:13
you may begin to notice those rich star clouds,
22:16
dark dust lanes, and the crowded regions near
22:19
the constellations Sagittarius and Scorpius beginning
22:23
to reappear. As for special events this week,
22:27
the Eta Aquarid meteor shower, born from the
22:30
debris of Haley's Comet, peaked last week, but
22:33
don't give up just yet. A few stragglers may
22:36
still be visible before dawn during the early
22:39
part of the week. especially now that the moon
22:41
is becoming less intrusive. These meteors tend
22:45
to be swift and graceful, entering Earth's atmosphere
22:48
at around 65 kilometers per second. And one final
22:53
celestial treat this week comes before dawn on
22:56
the morning of May the 13th. If you're willing
22:59
to set the alarm a little early and find a clear
23:02
eastern horizon, you'll be rewarded with a beautiful
23:04
little planetary gathering. A thin crescent moon,
23:09
just a delicate sliver of light, will appear
23:11
beside Saturn and Mars in the morning twilight.
23:15
All three objects should fit comfortably in binoculars,
23:19
making for a lovely little cosmic tableau. Saturn
23:23
will shine with a steady golden light, Mars with
23:26
its softer reddish glow, and between them the
23:29
fading crescent moon, quietly escorting both
23:33
planets into the dawn. As we close out our journey
23:43
through Nightwatch, the final chapters of the
23:45
book feel like an invitation to stop studying
23:48
the sky and begin building a personal relationship
23:51
with it. In chapter 10, Comets, Meteors, and
23:55
Auroras, author Terence Dickinson shifts our
23:58
attention away from the fixed architecture of
24:01
the night sky and towards something more fleeting.
24:05
A meteor flashing overhead and vanishing in an
24:07
instant. A comet slowly brightening over the
24:10
course of weeks, sometimes becoming a binocular
24:13
object, and occasionally, if we're lucky, a naked
24:17
eye spectacle. And then there are auroras, perhaps
24:21
the most emotional of all, where the sky itself
24:24
seems to come alive. One of the things Dickinson
24:27
does beautifully here, and he's always been good
24:30
at this throughout Nightwatch, is reminding readers
24:33
that not every astronomical experience requires
24:36
gear. Sometimes your own eyes are enough. In
24:41
a hobby that can occasionally become obsessed
24:44
with equipment, I find that refreshing. Now,
24:48
I need to make a small confession here. I own
24:51
the fourth edition of Nightwatch, not the newer
24:54
fifth edition, which means I actually missed
24:57
out on some of the expanded astrophotography
25:00
material in Chapter 11, particularly the newer
25:03
contributions from astrophotographer Alan Dyer.
25:07
And honestly, that's a little painful because
25:10
from everything I've seen, the astrophotography
25:12
updates in the new edition look excellent. But
25:16
even so, the photography sections in the edition
25:19
I do own still reveal something important about
25:22
the philosophy of this book. Dickinson and Dyer
25:26
don't treat astrophotography as some elite branch
25:29
of astronomy reserved for people with observatory
25:32
domes and $5 ,000 mounts. Instead, they present
25:37
it as a natural extension of observing itself.
25:42
You begin by looking up. Then maybe you put a
25:45
camera on a tripod. Then perhaps you try a star
25:49
tracker. Then one day, before you know it, you're
25:52
collecting photons from a galaxy 20 million light
25:55
years away. And then we arrive at chapter 12,
25:59
Wonders of the Southern Heavens. After spending
26:03
so much time helping northern observers learn
26:05
their sky, Dickinson essentially says, now imagine
26:09
an entirely different one. Here we leave behind
26:13
familiar northern anchors like Ursa Major and
26:16
Cassiopeia, and instead travel far below the
26:20
equator to encounter sites many of us in North
26:23
America may never see firsthand. The Crux, the
26:27
Large Magellanic Cloud, the Small Magellanic
26:30
Cloud, Entire star fields and celestial landmarks
26:34
that, to many of us, feel almost mythical. There
26:38
are entire celestial cultures and perspectives
26:41
waiting beyond our own horizon. And before I
26:45
wrap up this little book club, I do have one
26:48
personal criticism, though perhaps that's too
26:51
strong of a word. I've said throughout this series
26:55
how much I adore the photography, the illustrations,
26:58
the charts, and the overall design of Nightwatch.
27:02
It's a beautiful book. But there's a practical
27:05
part of me that wishes I could read it electronically.
27:09
I'd love to have a Kindle version. I'd love to
27:12
pull it up on my laptop while traveling, or read
27:15
it in bed without needing to spread the book
27:17
across a table under a bright reading lamp. Of
27:21
course, the irony is that one of the reasons
27:23
an electronic version probably doesn't translate
27:26
easily is precisely because the physical book
27:29
is so visually rich. The star charts, the photographs,
27:34
the illustrations are part of the experience.
27:37
You spread it out, you flip back and forth, you
27:40
compare charts, and you lose yourself in the
27:43
imagery. I don't have any ideas for future book
27:46
club selections, but if this is something anyone
27:49
would like me to continue, shoot me a message
27:52
over at the show website and we can discuss it.
27:56
I've always wanted to read Wrinkles in Time by
27:59
George Smoot. It's a book about cosmology by
28:03
one of the physicists who won a Nobel Prize studying
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the cosmic microwave background. If that sounds
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intriguing to you, let me know. If tonight's
28:16
episode sparked your curiosity or maybe gave
28:19
you something new to think about the next time
28:21
you look up, I'd be honored if you shared Star
28:24
Trails with someone who might enjoy the journey.
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You can always find the latest episodes, show
28:30
notes, and extras at StarTrails .Show. And if
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you'd like to help support the show, there's
28:37
also a little Buy Me a Coffee link there on the
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site. It genuinely helps keep these stories coming.
28:45
Be sure to follow Star Trails on Blue Sky and
28:48
YouTube. Links are in the show notes. Until we
28:51
meet again beneath the stars, clear skies everyone.