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Solar Eclipses, Speeding Comets & the Enigma of Cosmic Event Horizons · Space Nuts: Astronomy Insights & Cosmic Discoveries

0:00
Andrew Dunkley: Hi there. Thanks for joining us on a Q and A
0:02
edition of Space Nuts. My name is Andrew
0:04
Dunkley. Thanks for your company. on today's
0:07
episode, questions from the audience. Rusty
0:10
is asking about the similarities between the
0:12
sun and the moon. And there are several, and,
0:14
most are, coincidental, as it turns out.
0:17
Trevor wants to talk about speeding through
0:19
space. Slap a couple of pay plates on and
0:22
you're on your way. Austin, has asked,
0:25
us about Hoag's object. This is really
0:28
interesting. And Dan wants us to explain
0:30
the cosmic event horizon. We'll do all of
0:33
that shortly on this edition of
0:36
space nuts. 15 seconds. Guidance is
0:39
internal. 10, 9.
0:42
Ignition sequence start.
0:44
Jonti Horner: space nuts.
0:44
Andrew Dunkley: 5, 4, 3, 2. 1, 2, 3, 4,
0:47
5, 5, 4, 3, 2, 1.
0:50
Jonti Horner: Space nuts.
0:51
Andrew Dunkley: Astronauts report it.
0:52
Jonti Horner: Neil's good.
0:53
Andrew Dunkley: And to help us along with all of those
0:55
questions is Jonti Horner, professor of
0:58
astrophysics at the University of Southern Qu
1:00
Queensland. Hi, Jonti.
1:02
Jonti Horner: Good afternoon. How are you going?
1:03
Andrew Dunkley: I'm well. Good to see you again. Nice T
1:06
shirt, by the way. Is that a comet that's
1:09
on a dinosaur? Yep.
1:11
Jonti Horner: Fighting off the oncoming comment. These T
1:13
shirts are great, actually. I use these for,
1:15
when I'm doing outreach talks. And they're
1:17
particularly good with kids. And I didn't
1:19
realize when I got them, I just thought,
1:21
these are fabulous. So these kind of printed
1:23
T shirts with dinosaurs and rocks from space
1:26
falling down and they're kind of funny and
1:28
they're kind of cute. I didn't realize when I
1:30
got them. I was giving all these talks and
1:31
someone came up to me and said, oh, is that
1:32
from Happy Little Dinosaurs? What's Happy
1:35
Little Dinosaurs? Turns out that there is a
1:37
card game. that's one of the many, many
1:40
kind of board game card games that are out
1:43
there. Fabulous. If I've got loads of games
1:45
on the shelves behind me. Board gaming is
1:47
great. game out there, which is basically
1:49
about the end of the times for the dinosaurs.
1:51
And they're not happy at all in actuality,
1:54
but it's apparently really good fun and it's
1:56
kid friendly. And apparently all these T
1:58
shirts I've got are free advertising for
2:00
game.
2:00
Andrew Dunkley: Oh, there you go.
2:02
Jonti Horner: Yeah, there you go. I'm a walking
2:04
billboard and didn't even realize it.
2:06
Andrew Dunkley: Yes, indeed. that's not a fat joke either.
2:08
All right, let's answer, some questions. Now.
2:11
I'm going to have to put an apology forward.
2:13
Straight up. I've, been to my optometrist
2:16
this week. I do have a Little bit of an eye
2:18
issue, so everything's super blurry. And
2:20
they're all text questions, as it turns out.
2:23
But, we'll, we'll do our best. Rusty from
2:25
Donnybrook is first up. I'm wondering just
2:28
how many coincidences there are between the
2:30
sun and the Moon. First, we have the almost
2:32
exact apparent size coincidence, which has
2:34
enabled us to,
2:37
learn an amazing amount about the sun during
2:39
solar eclipses. Yes, Fred and I have talked
2:41
about that many times. Second, is the
2:44
rotation rate, which is the same for the sun
2:46
and the Moon at about the solar latitude
2:49
where most sunspots are seen.
2:52
What will this coincidence enable as we
2:55
begin to inhabit the Moon permanently? Will
2:57
we be able to avoid most solar radiation
3:00
storms by moving to the night side when large
3:02
groups of sunspots, are visible? After all,
3:05
we don't want to be stuck dug in at, the
3:08
poles forever. could we do this with
3:11
spacecraft in large orbits around the Moon?
3:13
Does the coincidence mean that large deposits
3:16
of helium 3 may be found at certain lunar
3:19
longitudes? are there any more Sun, Moon
3:21
coincidence that, coincidences that you know
3:24
of? Thanks for the show. moving on up,
3:27
guys. you'll be number one soon. Well, we'll
3:30
see about that. But we're happy where we are.
3:32
Rusty, from Donnybrook. that was more than
3:35
one question, basically, but, all centered
3:37
around these, these
3:39
coincidences that exist between the sun and
3:41
the Moon.
3:42
Jonti Horner: Absolutely. And the coincidences are really
3:45
astonishing. And it's just our very good
3:48
fortune to live at exactly the right time in
3:50
the Earth's history where things line up like
3:51
this. Because when the Moon formed air, the
3:53
Earth was spinning much, much quicker, the
3:55
Moon formed much closer to us. And at that
3:58
point when the Moon moved in front of the
3:59
sun, it would block the sun out totally by a
4:02
long, long way. And, you'd have a fairly
4:04
underwhelming eclipse of the sun, to be
4:06
honest. And, as time has gone on, the Moon
4:08
has moved further and further away from the
4:09
Earth. The further it moves away, the slower
4:11
the Earth, spins, the longer the Moon takes
4:13
to orbit the, Earth and the slower it
4:15
recedes. And we're now in this very
4:18
privileged window where most
4:21
of the time the Moon and the sun are
4:24
so similar in size that if they line up
4:26
perfectly, the sun will be blocked out by the
4:29
disk of the Moon. And we can see the Sun's
4:31
our atmosphere. And we get the wonders of a
4:32
total solar eclipse. Now. Doesn't happen
4:35
all the Time, because when the Moon is near
4:37
apogee, when it's furthest from the Earth,
4:40
it is small enough that you instead get the
4:42
ring of fire type eclipse. You get an annular
4:45
eclipse, where you get an annulus or a ring
4:48
of the Sun's disk around the Moon. And
4:51
that is presaging what is to come in millions
4:53
of years into the future. So as time goes on
4:55
and the Moon keeps edging away from us, and
4:58
as the sun continues to get very, very, very
5:00
slightly bigger as well, because these things
5:03
also happen, what that means is that as time
5:05
goes on, we will get fewer and fewer total
5:07
eclipses and more and more annular eclipses.
5:10
And eventually there will come a day in
5:12
millions of years in the future where we no
5:15
longer get total eclipses at all. And we will
5:17
see the final ever total eclipse from the
5:18
Earth, which will probably be sad, but
5:21
it's so far away in the future that we don't
5:23
really have to worry about it right now. So
5:26
that's coincidence number one. And it's
5:27
effectively that the Moon is about, I think
5:29
it's 1 400th of the distance to the sudden
5:32
1 400th of the size of the Sun. So,
5:35
you know, similar triangles that we learned
5:37
at school and thought we'd never use mean
5:40
that they're about the same angle in the sky
5:41
and they block each other out. That's all
5:43
great. The second one, which
5:46
Rusty, has pointed out, I'd actually never
5:49
really given any thought to, to be honest.
5:50
But it is true that the length of the
5:53
lunar month, the length of the orbit of
5:56
the Moon around the, Earth, is about the same
5:58
as the period of rotation of the sun with an
6:01
asterisk. And, the rotation period of the
6:04
Moon when compared to the
6:06
background stars is the same
6:09
as its orbital period around the Earth
6:11
compared to the background stars. It's
6:13
tidally locked. It's trapped in this orbit.
6:15
So it always keeps effectively one face
6:17
towards the Earth, one face away from the
6:19
Earth. And, that's a natural result of the
6:21
tidal interaction between the two objects.
6:23
And it's tied to these effects that are
6:25
causing the Moon to gradually drift away.
6:29
Now, the sun is a fluid object. It
6:32
doesn't have the same rotation period at all,
6:34
locations. It rotates quicker at the equator
6:36
and slower at the poles. And that's thought
6:38
to be part of the reason we get the sunspot
6:40
cycles. What happens with that, is that the
6:43
equator rotates around once every 24
6:46
Earth days. The poles rotate about once every
6:48
32, 34 Earth days and locations
6:51
between have different rotation periods. So
6:54
there is a latitude on the Sun's disk that
6:56
rotates with a period of exactly one lunar
6:58
month. And so what Rusty's talking about here
7:01
is the idea that when sunspots are at that
7:03
latitude, if you've got one dominant group of
7:05
sunspots, then one side of the Moon will be
7:08
getting hammered when those sunspots are
7:10
creating activity. And the other side of the
7:12
Moon will see the sun when the sunspots are
7:14
on the other side and they won't see them.
7:16
The reason that
7:19
that doesn't mean you get one particular side
7:21
of the moon getting all the solar radiation
7:23
on the other side being protected, it is the
7:25
fact that sunspots form at different
7:27
latitudes on the sun and so go around at
7:30
slightly different speeds. Early in a solar
7:33
cycle, they're at higher latitude. Later in
7:35
the solar cycle, they're at lower latitudes,
7:36
so the rotation period changes a bit. Also,
7:39
you get multiple groups of sunspots at a
7:42
given time. And, the activity from a given
7:44
sunspot group can hit the Earth and influence
7:47
the Earth when it's at different locations on
7:49
the sun, depending exactly what the magnetic
7:51
field between the two is in the solar window
7:53
doing. So. I've seen cases in the past where
7:56
we've had a flare and they've been aurora.
7:58
And you look at it, and the sunspot that had
8:00
the flare wasn't dead center of the Sun's
8:02
disk. Your intuitive thing is that when the
8:04
sunspot is back in the middle of the Sun's
8:06
disk, anything it ejects will come directly
8:08
towards the Earth. But in actuality, we've
8:11
had a lot of aurora when the sunspot is
8:13
offset because things follow a curved path,
8:15
all this kind of stuff. So in that sense,
8:18
I don't think there is any particular
8:21
longitude on the moon that would be favored
8:23
over the others because, on longer
8:25
timescales, everything would totally smooth
8:27
out, totally average out. On a
8:30
given sunspot group on a given event,
8:33
you'll obviously have one half of the moon
8:35
exposed and the other half protected by
8:37
looking the other way. Effectively, I think
8:40
in terms of people wandering around on the
8:42
Moon and our, spacecraft on the Moon, it
8:44
would be very inefficient to have to move
8:46
halfway around the Moon to get into shelter.
8:48
Yeah, I think it makes more sense to drill
8:50
down and hide a few meters below the surface
8:53
where the rocks shield you. Yeah. And so
8:56
while I love the idea of us having the
8:59
ability to duck around the corner, you'd
9:01
actually have to move quite a long way. And
9:03
that would be inefficient and time, time
9:05
intensive and all the rest of it. When what
9:08
you can do is just say, well, we're just
9:09
going to nip indoors and watch movies for a
9:11
couple of days. You know, we'll be under the
9:13
shielding of all the rock and the rubble
9:14
above us. That makes more sense.
9:17
And, in terms of other
9:21
coincidences, the one that I think of that
9:24
isn't really a coincidence at all is if you
9:26
look at the bulk composition of
9:29
the solid material in the solar system
9:32
is to first order, the same as the bulk
9:34
composition of the sun minus the gases.
9:37
And that's just because everything formed
9:39
from the same stuff. What's interesting is
9:42
that the Moon is depleted in the heavier
9:45
elements and enriched in the lighter
9:47
elements. Compared to that.
9:49
And in the flip side, the Earth is actually a
9:51
bit enriched in the heavier elements and
9:53
depleted in the lighter elements. They formed
9:56
at the same place in the solar system at the
9:58
same time. So you'd expect them to form from
10:00
the same stuff. And so the difference in
10:02
their compositions actually telling us about
10:04
the story of their formation and the fact
10:06
that the Earth formed as a single object and
10:08
then was smashed and torn asunder and formed
10:11
the Earth and Moon. And the Moon was formed
10:12
from the lightest stuff that had floated to
10:14
the top, so primarily from the crust and the
10:16
mantle. And therefore you get this different
10:18
in composition where the overall
10:21
bulk composition will be the same as the bulk
10:23
composition of those materials in the, sun.
10:25
But the differences are what tell us the
10:26
story. So it's not really a coincidence, but
10:29
it is an interesting link between a lot of
10:31
them because we all formed at the same place
10:33
at the same time from the same stuff. But the
10:36
formation then shaped us.
10:37
Andrew Dunkley: M Fascinating. There you go, Rusty. Hopefully
10:40
that covered all your bases. Thanks for the
10:43
question.
10:46
0G and I feel fine. Space nuts.
10:48
And our next question comes from Trevor in
10:51
Port Macquarie. with Comet 3I
10:54
Atlas heading through our solar system at a
10:56
record speed of around 60 kilometers per
10:59
second, it's got me thinking about how fast
11:02
an object like a comet or indeed a
11:04
spacecraft could reach. I'm assuming
11:07
Comet 3i Atlas has acquired some of this
11:09
speed with maybe a slingshot around
11:12
another star or two in the past.
11:15
And as it passes, the sun,
11:17
it will probably pick up additional speed.
11:20
Is there any limit to how fast a
11:23
comet like this could reach if it continues
11:26
to receive gravitational assistance from
11:28
stars in the future? I know with a spacecraft
11:31
we use gravitational assistance by going
11:33
around, say, Venus and back to earth to pick
11:35
up speed. But what would happen if
11:38
we did, say, 50 times,
11:42
before heading off into the direction we want
11:45
to go? So 50 passes, he's saying, ah, could
11:47
he reach speeds well in excess of, what deep
11:50
space probes currently travel at? Or is
11:53
there a level of diminishing return to this
11:56
approach? Looking forward to your answer.
11:57
Jonti Horner: Trevor.
11:58
Andrew Dunkley: Love this question.
12:00
Jonti Horner: Yes, this is really, really good fun. So,
12:02
it's a bit of both, to be honest. The long
12:05
and short of it is that if you could set up
12:08
the perfect chain of
12:10
slingshot assists with
12:13
ever more massive objects moving at ever
12:15
greater speeds around, there is no real limit
12:18
up until the point you get very close to the
12:19
speed of light, because you can't get faster
12:21
than the speed of light. But the
12:24
challenge inherent in that is that you're
12:26
transferring a bit of the kinetic energy from
12:28
one object to another. You're sealing
12:30
momentum through a gravitational slingshot.
12:33
So if you fly by Jupiter and Jupiter gives
12:34
you a kick to speed you up, Jupiter actually
12:36
slows down a little bit. Because Jupiter is
12:39
much, much, much, much, much, much, much more
12:41
massive than you are. The change in its speed
12:43
is much, much, much smaller. Momentum is
12:45
speed times mass, effectively. But
12:49
you're taking a bit of that kinetic energy.
12:51
And because of that, there's no real limit to
12:53
how much you can boost. But any given
12:55
encounter will only give you so much.
12:58
The closer you get to the mass, the more of a
13:01
kick you can get. If the orientation is
13:02
right, the faster the mass is moving compared
13:05
to you, the more of a kick you can get as
13:07
well. So you can imagine setting up a
13:09
situation where you have a few flybys and you
13:11
gradually get more and more boosts. And
13:13
that's how people have taken the cheapskate
13:15
route to the outer solar system. You get
13:17
these missions that have a flyby of the Earth
13:19
and a flyby of Venus and a flyby of the Earth
13:21
and a flyby of Venus and a flyby of the Earth
13:23
and a flower flyby of Mars and so on. And
13:25
they get gradual, little incremental kicks to
13:27
get out to Jupiter. It's also
13:30
how we saw the Voyager spacecraft get a kick
13:33
from Jupiter to Saturn, then Voyager 2 got a
13:34
kick from Saturn to Uranus, and Uranus kicked
13:37
it onto Neptune, and you got to speed up
13:39
every time. The problem becomes
13:42
you eventually hit the escape velocity for
13:44
the solar system, and Jupiter is really good
13:45
at doing that. Once you're at the escape
13:47
velocity, you're not coming back for a second
13:48
pass by Jupiter. So what are you going to get
13:50
your next slingshot by? Well, if everything's
13:53
lined up just right, you can do like the
13:55
Voyager spacecraft did and do the grand tour
13:57
where you get one to the next to the next.
13:58
Yeah, but that's got a bit of diminishing
14:00
returns because they're only so massive. So
14:02
you can only get so much out of a given kick.
14:05
You could possibly in theory aim out of our
14:07
solar system to fly past Alpha
14:10
Centauri and get a kick from that, if you
14:12
oriented right and try and aim your
14:14
spacecraft to get a kick from Alpha Centauri
14:16
to kick you onto, I don't know, Sirius and
14:18
get a kick there and bounce around, and
14:20
bounce around and gradually accumulate speed.
14:22
But eventually you'd again reach the escape
14:24
velocity of the galaxy and eventually you'd
14:26
be on a one way trip out of our galaxy. So it
14:29
would be very hard to set you up to get to a
14:31
speed that is ludicrous speed, you know, a
14:34
speed that is approaching the speed of light.
14:36
You'd probably need to come incredibly close
14:38
to a black hole to do that. And you need to
14:40
engineer things such that you didn't get
14:42
destroyed by the radiation and everything
14:44
from the accretion disk around it and you had
14:46
your angle right so that you gained enough
14:48
momentum to get much quicker. So you could
14:50
probably play games like that. It's certainly
14:53
really useful in the solar system. It's very
14:55
effective at getting things up to high speed
14:57
without requiring huge amounts of fuel. You
15:00
can supplement it by timing the burn of your
15:03
rockets and using your limited propellant to
15:05
get the maximum kick possible. And I've read
15:07
about, special science fiction style
15:09
maneuvers where you whiz around and
15:12
you do your burn at the very closest approach
15:15
to an object to get the maximum change in
15:16
velocity and the maximum energy change for
15:19
that given packet of fuel. I
15:21
think it's called the Oberth manoeuvre or
15:23
something like that. And it's really
15:25
interesting because it, at, ah, first thought
15:27
it defeats common sense.
15:30
Do you think if you're speeding up by 1 meter
15:32
per second, doing it anywhere in the orbit
15:34
would have the same effect, but in fact
15:36
speeding up by 1 meter per second when you're
15:38
traveling really quickly creates a bigger
15:40
change in your final speed once gravity and
15:43
everything works out, than doing it when
15:44
you're moving really slowly? There's all
15:46
sorts of odd things like that going on. So
15:48
there are games you can play with it and you
15:50
could set up a theoretical path that could
15:53
take you millions of years to complete, where
15:54
you Chain path objects. You have a
15:57
little bit of control in your spacecraft to
15:59
refine your orbit, refine your path,
16:02
so that you aim perfectly to the next target,
16:05
and effectively get a slingshot from each.
16:08
But the practical limits are that unless you
16:10
could get very close to a black hole, you're
16:12
going to be limited to some degree. And it
16:14
becomes, like you said, diminishing returns.
16:17
Not because the physics of it prevent you
16:20
from getting any arbitrary speed you want,
16:23
but because the faster you're moving, the
16:25
less targets there are to hit and the longer
16:27
you've got to wait to get there.
16:29
Andrew Dunkley: Do you think the day will come, though, where
16:32
we develop technology that stops us
16:34
needing gravity assist,
16:37
maybe ion engines or scramjets
16:40
or whatever the things they're developing at
16:42
the moment?
16:44
Jonti Horner: We're always developing new things. And, as
16:46
I've said before, you know, the one
16:48
prediction you can make is that all
16:49
predictions are wrong. Science fiction is
16:52
amazing at coming up with things that have no
16:54
grounding in current physics, but that
16:57
suggest possible routes to do things that
16:59
break the current laws of physics. And to
17:01
some degree, everybody writes them off as
17:03
purely fiction because they can't work in our
17:05
current understanding of physics. But I
17:07
remember that a long time ago, a lot of
17:09
people were arguing that it was utterly
17:11
impossible and physically impossible to have
17:13
heavier than air travel. And our
17:15
understanding of, physics and the cosmos is
17:17
limited by the quality of observations that
17:20
we have. And our theories are
17:22
exceptionally good at explaining things to
17:24
the level that our observations can currently
17:26
be carried out and a bit more. But it's
17:29
entirely feasible that as we get better at
17:31
making observations, we reach the limits of
17:33
where our current models, are accurate.
17:36
And we need new and better physics, and new
17:38
things will come out of that. We'll also get
17:40
more efficient at, using the tools we know do
17:42
work. And things like the ion engines which
17:45
they have tested with spacecraft are
17:47
interesting. Our common conventional kind of
17:49
chemical rockets apply a very large amount of
17:52
thrust for a very short period of time and
17:54
burn fuel very intensively. And so you get a
17:57
high acceleration for a short time. Ion
18:00
engines are almost exactly the opposite in
18:01
that they give you a very small acceleration,
18:03
but that can be applied continuously for very
18:05
long periods of time. And so they're much
18:08
more efficient. And you can imagine stuff
18:10
like that being scaled up. But the science
18:13
fiction that
18:16
leads to people moving at significant
18:18
fractions of the speed of light nearly
18:19
universally uses technologies that are beyond
18:22
what our physics currently allows. And, it's
18:25
where you get the blurring of hard sci Fi and
18:27
soft sci fi of the degree to which you have
18:30
things grounded in current science and things
18:32
grounded in fantasy almost in
18:34
science that is so advanced that it's magic
18:36
to us because we can't explain how it works
18:38
or know that it will. And so trying to say
18:41
whether we'll ever get to a stage where
18:44
our technology allows us to do these things
18:48
quickly, to get to Mars in a week, to get to
18:50
Pluto in a month or a week,
18:54
you can imagine it is possible if you do the
18:56
maths. If you accelerate at 1g, the
18:59
acceleration due to gravity and you can
19:00
maintain the acceleration for a while, you
19:03
get to very high speeds very quickly. So if
19:05
you accelerate at 10 meters per second per
19:08
second, after 100 seconds you're going at
19:10
1 kilometer a second, after 200 seconds,
19:13
you're going at 2 kilometers a second. So you
19:15
gain speed very, very, very quickly. But
19:18
you've got to use fuel very efficiently to do
19:20
that. And a sample of a lot of hard
19:23
science fiction that's kind of near future
19:24
sci fi relies on the idea
19:27
of some form of drive system that allows
19:30
you to maintain that level of acceleration
19:32
for periods of hours or weeks or months.
19:35
And if you could do that, the travel time to
19:38
get to places like Saturn becomes
19:40
manageable. On a holiday you could do what
19:43
Fred's doing, but instead of going to
19:44
Scandinavia, you could go and visit Titan.
19:47
If you've got that level of technology and
19:49
you do it in a certain degree of comfort
19:51
because what you do is your Spacecraft's
19:52
accelerating at 1G, so you feel an
19:54
acceleration of 1G and it's like you're on
19:56
Earth. You don't need artificial gravity. You
19:59
just have that short period halfway through
20:00
the flight where you're weightless while the
20:02
spacecraft turns dark, turns around and
20:04
starts accelerating at 1G in the other
20:05
direction to slow you down for approach.
20:08
And that's a step level mode of the more
20:10
realistic sci fi. But it still
20:13
requires technology that we don't yet have
20:15
and we may never develop or we might. And you
20:17
know, I'd love it if we did, but we'll just
20:19
have to see.
20:20
Andrew Dunkley: Time will tell. but you know, Flash Gordon
20:23
was the first to put rockets,
20:26
you know, that could reland themselves. And
20:28
now, now we can do that. so yeah, there's
20:31
all sorts of weird and wonderful stuff in sci
20:33
fi that has become reality. Thanks,
20:37
so much for the question, Trevor. This is
20:39
Space Nuts with Andrew Dunkley and John de
20:41
Ora.
20:44
0G and I feel fine. Space
20:46
Nuts. Our next Question comes from
20:49
Austin. He said, I've, just been watching a
20:52
long YouTube Music presentation on Hoag's
20:54
object. A lonesome galaxy with
20:57
lots of features that don't seem to fit with
20:59
present knowledge. Do objects like that
21:02
require a complete rethink of the standard
21:05
model or are, there ready
21:07
explanations of its features now?
21:10
Hoag's object is
21:12
astonishing, to say the very least. It's a
21:15
very unusual ring galaxy.
21:18
I know you've been doing some research on
21:20
this, so we can sort this one out for Austin.
21:23
Jonti Horner: I have. It's beautiful. The photos are
21:25
amazing. I do encourage people to look at
21:27
this. It is one of a
21:30
number of galaxies that we've seen that have
21:32
this kind of ring like structure. And the
21:34
typical explanation is that you've had some
21:37
kind of violent encounter between two
21:39
galaxies where one has pushed through the
21:42
other really quickly and set up a shockwave.
21:45
And that shockwave has propagated outwards.
21:47
And, the shockwave causes gas and dust pile
21:49
up and drive the bursts of star formation.
21:52
And as we know, the bulk of the
21:55
most luminous stars in a galaxy are the
21:57
hottest, brightest, shortest lived ones.
21:59
Andrew Dunkley: Yes.
22:00
Jonti Horner: And so it's natural that you'd get a ring
22:03
where the shock wave has recently passed,
22:05
where you've had a lot of massive stars
22:07
formed that have not yet died. And you'd have
22:09
a few reddish stars dotted through there,
22:11
which are the stars that are about to die.
22:13
And that the middle of the galaxy would look
22:14
fairly yellowish and dull because there's no
22:16
young stars there because all the gas and
22:18
dust have been swept out. And, we do see a
22:20
lot of these kind of galaxies, but I've never
22:23
seen any of them that is as
22:25
symmetrical and beautiful as Hoag's object.
22:28
Hoag's object looks like if you
22:31
got somebody to make a perfect fried egg
22:34
and then they got two of those weird little
22:36
poached egg rings and they put one around the
22:38
yolk, and then they put a much wider one
22:41
around near the outside of the white.
22:43
Everything between those two rings was lifted
22:45
out and thrown away. And then the rings were
22:46
taken away and you were left with a yolk in
22:48
the middle and then an empty gap and then a
22:50
ring of white around the outside. Although in
22:52
this case a ring of blue. It is beautifully
22:55
symmetric. There's a little bit of a hint of
22:57
rotation there. It almost looks like a
22:59
turning salt to its blade when you look at
23:00
it.
23:01
Andrew Dunkley: It does.
23:01
Jonti Horner: which is leftover of the original rotation
23:04
of the galaxy itself.
23:07
The level of symmetry and stuff is
23:09
really Unusual. Now, what it
23:12
suggests is that, and it should be said
23:15
nobody's absolutely certain how this object
23:17
formed, but there are ready explanations
23:20
that people have put forward. One which seems
23:23
to have been shot down is that this is an
23:25
extreme example of what's described as a
23:28
bar, instability. So you get these barbed
23:31
spiral galaxies where you get a big, long,
23:32
straight central bar and then beautiful
23:34
curved spiral arms coming off the end of the
23:36
bar. You can almost imagine the curved arms
23:39
joining up and then the bar disappearing. For
23:41
some reason, you'd be left with something
23:42
that almost looks like this. That seems to
23:45
be very disfavored for this object
23:48
because of the shape of the central blob. The
23:51
central blob is fairly circular rather than
23:53
an elongated. And bad spirals tend to have an
23:56
elongated central blob. So that seems to rule
23:59
that out. Although there is still debate,
24:01
the explanation that would make most sense
24:04
is that, this was the result of a collision
24:06
between two galaxies at high speed, where
24:08
you've almost got one galaxy punching through
24:11
very near to the center of Hurd's object at,
24:14
high speed, triggering this shockwave and
24:17
then running off and vanishing. Now, given
24:19
the scale of that ring, the suggestion will
24:22
be that the collision happened about 3
24:24
billion years ago. Billion with a B, not
24:26
million with an M. The problem with this is
24:29
that nobody can see any object. That would be
24:31
the bullet seems m. That would
24:34
have disappeared. However, 3 billion
24:37
years is a lot of time for things to happen.
24:39
So I think the suggestion that most people
24:40
have is that, the bullet,
24:43
has been lost in 3 billion years. There's
24:46
also the fact that this is so, perfectly
24:48
symmetrical, so perfectly face on,
24:51
that you could possibly wonder whether the
24:52
bullet is actually hidden behind that central
24:54
blob. In other words, the bullet has moved
24:57
perfectly along our line of sight and we
24:58
don't see it because the galaxy's in the way.
25:00
Yeah, that would make sense to me. And that
25:03
sounds like it's vanishingly unlikely. But to
25:05
have this thing be so symmetrical means it
25:07
must be almost perfectly face on, which means
25:10
that the impact that created the shockwave
25:11
must have been quite close to our line of
25:13
sight. And so I can see some logical
25:16
consistency there. But this
25:19
is, and I know I say this all the time, but
25:21
this is a really good example of that
25:23
interplay between theory and observation,
25:26
where whenever we see something new, we
25:27
struggle to explain it. We get a better
25:29
understanding of how the universe works and
25:31
what all the models should say and what they
25:33
should tell us, and that improves our
25:35
understanding of other objects, and then we
25:36
find something else that pushes the
25:38
boundaries of that knowledge. It is clear
25:40
that we do not have a defined, definitive
25:43
answer for this yet. And that's true for many
25:45
of these objects in space. I think the
25:48
argument of it being formed through a
25:50
collision is
25:53
fairly compelling, even though, you know,
25:56
there are a lot of reasons why a typical
25:58
formation will be unlikely. I'm just looking
26:00
here at, the summary of this on Wikipedia, to
26:03
be honest, and that's got links to a few
26:04
papers that have discussed this. now I'm
26:07
sure some of the listeners are probably
26:09
recalling in shock at me looking on
26:10
Wikipedia.
26:11
Andrew Dunkley: No, no, we actually do have it quite a lot
26:14
because there's m. Some very valid stuff in
26:17
there. You just got to work your way through
26:18
the garbage.
26:20
Jonti Horner: Stress this a lot to my students that, you've
26:22
been brought up through high school that
26:23
Wikipedia is unreliable because it's an
26:26
alterable resource and people can change it.
26:28
And I've known friends who are teachers who
26:30
set their class some really obscure
26:33
topic to research and then deliberately go in
26:35
and edit the Wikipedia page to say something
26:37
wrong and then change it back after their
26:39
class have done the work to demonstrate.
26:41
Andrew Dunkley: You're kidding.
26:42
Jonti Horner: Cruel, but entertaining.
26:45
what I stress to my undergrad students is
26:47
that for astronomy, Wikipedia is a good first
26:50
look quite often. And that's because
26:53
we have people around the world who
26:56
are into astronomy, have a lot of knowledge,
26:58
and tend to be very obsessive. And I mean
27:01
that in a really positive light and have
27:03
their favorite objects. And if something is
27:05
wrong, they're not backward in coming forward
27:07
at fixing it. Added to which, a lot of the
27:10
astronomical sites are not controversial, and
27:12
they're not the kind of places that
27:14
youngsters who want to be a bit rebellious
27:16
will go to to put something funny in. They're
27:18
not going to be your prime targets for
27:20
malfeasance, let's say. And, so what
27:23
that combined means is that a lot of
27:25
Wikipedia articles out there are, a
27:27
relatively good first stab for astronomy
27:29
topics. Now, they're not often spot on. You
27:32
know, I found things that have not included
27:35
my own research when they've been talking
27:37
about a subject. And that makes me sad. and
27:39
sometimes therefore give a different opinion
27:41
to what I'd have. And so it's always a case
27:43
of use it with caution, go to the primary
27:46
resources. But Wikipedia is really good
27:49
at pointing you to some of the primary
27:51
resources to get a good feel for it. And so I
27:53
actually find using Wikipedia is often for
27:56
astronomy things fairly Reliable and
27:58
fairly accurate. Because when an error creeps
28:00
in, it is fixed very, very quickly.
28:02
Andrew Dunkley: Yes, that's absolutely true. I actually have
28:05
found it very helpful in the past. when I was
28:07
writing, the book about my grandfather in
28:10
World War I, researching the minute by
28:12
minute processes of the particular battles
28:15
that he was in,
28:19
I found a lot of good data on Wikipedia.
28:21
So, yeah, don't write it off unless you're
28:23
doing it.
28:24
Jonti Horner: It wouldn't surprise me. Yeah, the kind of
28:27
community that would look into that kind of
28:29
thing probably has a lot in common with
28:30
astronomy and that they'd be very detail
28:32
oriented, are very precise and
28:35
very knowledgeable about their particular
28:36
topic. But again, I'd have thought that it's
28:39
unlikely that someone who wanted, to do
28:41
something funny would go to the report of a
28:43
particular world, one battle and edit it.
28:45
They'd probably go to Taylor Swift's
28:47
Wikipedia page. I suspect the editors for
28:49
Taylor Swift's Wikipedia page have been
28:50
working very hard. I'm sure they are.
28:54
Andrew Dunkley: Yeah.
28:55
Jonti Horner: So, yeah, it does not surprise me, is a
28:57
fabulous resource.
28:59
Andrew Dunkley: Yes, indeed. yeah, I think most of the people
29:01
who want to stir things up go to, Facebook,
29:03
Instagram or TikTok. That's, that's generally
29:06
where it all ends up. thank you, Austin.
29:08
Great question. And I, would encourage people
29:10
to have a look at Hoag's object online
29:12
because it is quite a spectacular
29:15
galaxy. Well worth a look. Our final
29:18
question today, Jonti, comes from Dan in
29:20
California. he said, a
29:23
new term, I heard the other day was the
29:25
cosmic event horizon. could
29:28
you talk about this a bit? Thanks, Dan. Yes,
29:31
we, we. This is another one that's a bit of a
29:34
mystery. I mean, it's a thing,
29:38
it's well known. It's. It's not something
29:40
that someone's sort of suggested may exist.
29:43
the question is, you know, where, where
29:46
is the, the limit of
29:49
this object? it's not really an object. It's
29:52
a status, I suppose.
29:53
Jonti Horner: Yes, yes, it's an interesting one.
29:56
And more generally,
29:59
an event horizon
30:02
is a line that marks the
30:04
boundary between things we can observe and
30:06
things we cannot observe in the simplest
30:09
possible terms. So the event horizon of a
30:11
black hole. Anything outside the event
30:13
horizon we can see happening. Anything
30:17
interior to the event horizon we can't see
30:19
because light can't escape. So the event
30:21
horizon in that sense marks a boundary
30:23
between what is observable and what is not.
30:26
And that has led people to this concept of
30:28
the cosmic event horizon or cosmological
30:30
Event horizon. And when you look into it,
30:33
there's actually two different definitions,
30:35
two different types. One is
30:39
effectively the maximum distance,
30:42
at which a source can be and have
30:44
emitted light in the past that we can see
30:46
today. and that distance is
30:49
smaller than the real extent of the universe.
30:52
It's kind of set by the point of the
30:54
cosmological microwave background, in
30:56
actuality, because before that, the universe
30:58
was foggy and the light couldn't escape.
31:00
Now the universe is very roughly 14 billion
31:03
years old. I know the numbers are more
31:05
accurately than that, but we'll call it 14
31:07
billion billion years. So you're. And
31:10
certainly my naive expectation would be that
31:12
the most distant thing we can see is 14
31:15
billion light years away. And that
31:17
is true for a given version of true.
31:21
What it actually is, the case is that, the
31:23
things we see that are 14 billion
31:26
light years away, we're seeing,
31:29
and I've got to word this very carefully,
31:32
we're seeing where they were 14
31:35
billion years ago, and the light
31:37
has traveled from that point for 14 billion
31:40
years to reach us today. So we see them
31:43
14 billion years ago at the distance
31:46
of 14 billion light years, but they are
31:48
heavily redshifted. They're moving away from
31:51
us. So the objects that emitted that light 14
31:53
billion light years 14 billion years ago
31:57
at, a distance of 14 billion light years,
31:59
would now be 47 billion light years
32:02
away. And light they emit now would never
32:05
reach us because of the expansion of the
32:06
universe.
32:07
Andrew Dunkley: Yep.
32:08
Jonti Horner: So what that means is that one sense of the
32:10
cosmic event horizon is
32:13
that we can see objects that can be as
32:16
distant from us today as 47 billion
32:19
light years in any direction. But
32:21
we see them as they were 14 billion years
32:24
in the past, when they were only 14 billion
32:27
light years away from us, and they've moved
32:30
away since. So that is the event horizon in
32:33
terms of events that we can see today that
32:35
happened in the past. The other
32:38
version, which makes my head hurt slightly
32:41
more, to be honest, is the concept in
32:43
cosmology that there is an event
32:46
horizon which is the
32:49
most distant objects that if they
32:51
emitted a photon of light today, could ever
32:54
be seen from the Earth in the future.
32:57
Now they're moving away as well. Now, this
33:00
seems to be well defined, and there's maths
33:02
around it, and people have discussed it, that
33:04
because of the event horizon. Sorry, because
33:06
of the expansion of the universe, you can't
33:09
have an object that's arbitrarily far away
33:12
emit a photon of light and expect that it
33:14
would ever reach us, because the expansion of
33:16
the universe is such that before that photon
33:18
of light reaches us, we will be moving away
33:21
from it at a speed faster than the speed of
33:23
light. And so it can never catch us up. It's
33:25
a bit like the hare and the tortoise. And,
33:28
you know, at some point the hair is going so
33:31
quickly that if you throw ping pong balls
33:33
after it, they won't catch it up because it's
33:34
going quicker than the speed of a ping pong
33:36
ball. Yeah. Hugely mixed metaphor, but you
33:38
can kind of see what I mean there. Now, this
33:41
is fairly well defined as a concept. It's
33:43
this idea that objects far
33:46
away from us emit light towards us, but
33:48
they're receding and they're receding from us
33:50
at an ever increasing speed. And, we're
33:52
receding from them at an ever increasing
33:54
speed because the expansion of the universe
33:57
doesn't matter where you are, it's all
33:58
expanding. Yeah. And so there must be a
34:00
horizon at some point, at some distance from
34:03
us, where an object at that distance today
34:06
would emit a photon of light and that light
34:08
would never reach us because of the
34:11
expansion. I cannot find
34:14
a definitive number for that size
34:16
anywhere. I've looked around.
34:19
There's a lot of mathematical equations that
34:21
people use to quantify it. But the reason
34:23
that we don't have a definitive number for
34:24
that is that there are many models that look
34:27
at the expansion of the universe into the
34:29
future that all have very slightly different
34:32
expansion rates going into the future. And
34:34
different expansion rates would move this
34:36
event horizon to different distances.
34:39
If the universe is expanding quicker, then
34:42
that event horizon will get nearer to us
34:44
because you'd have to be closer to us to
34:46
overcome this barrier of the expansion. If
34:48
the expansion is a bit slower in the future,
34:51
the event horizon will be further away. If
34:53
you go again, big up to the Wikipedia page.
34:56
If you go to the Wikipedia page on Event
34:58
Horizon and go down to the article
35:01
heading Cosmic Event Horizon, there is a
35:04
figure on the right that shows the reachable
35:05
universe as a function of time and distance
35:08
in the context of the expanding universe that
35:11
has lots of different things on it. And it
35:14
seems to suggest that if light were emitted
35:16
from the Milky Way galaxy now, right now, if
35:19
we shone a laser up into the sky, the most
35:22
distant object from us that could
35:25
ever reach is at 26
35:28
billion light years from the edge of the
35:31
Big Bang. that's not the same
35:34
as saying 25 billion light years from us.
35:36
This is where it all gets really, really,
35:39
really confusing. They've got these light,
35:41
light curves and things like this light ray
35:44
emitted at 13 gig years from now
35:47
would reach further out. So that figure
35:49
suggests that, event
35:52
horizon is something like 13 or 14 billion
35:54
light years from us right now. If we
35:57
emitted that light now, something further
35:59
away from that than that could never see us.
36:02
Yeah, but it's all up in the air. I can't,
36:04
like I said, find an exact answer. And I
36:06
think the reason I can't find an exact
36:08
calculated distance is if you even
36:11
vary the expansion rate of the universe by a
36:13
very small amount, you change the location of
36:16
that event horizon by a very large distance.
36:18
So it's just hugely uncertain. So at the
36:20
minute it remains a kind of theoretical
36:22
conceit, a philosophical concept, but one
36:25
that is important in us actualizing,
36:28
in terms of. In us conceptualizing, I
36:30
guess, the idea that no matter how far in
36:33
the future or the past you go, there will
36:36
never have been a time when the entire
36:37
universe is visible from here. So it's a bit
36:40
like walking around on the surface of the,
36:42
Earth. No matter where you are on the, Earth,
36:43
you cannot see the whole of our planet unless
36:45
you look at photos. Because from your
36:47
location, no matter how high above sea level
36:50
you are, there is always some of the Earth
36:52
heat you cannot see even when you're in
36:55
space. It's absolutely.
36:57
Andrew Dunkley: There's always another side to it. That's.
36:59
Jonti Horner: So when we get. It makes people's head hurt
37:02
hugely, the idea that the universe can be
37:05
infinite and finite at the same time, that we
37:07
only see a small fraction of the universe
37:09
that's out there. But to me, it does
37:12
make sense when you think about it in the
37:13
context of the Earth. I look out of my window
37:15
here and I can see the beautiful bunny
37:16
mountains in the distance. They're about 60
37:19
kilometers away, something like that.
37:21
That's a long way away. I can see a large
37:23
part of the Earth's surface, but that's a
37:25
trivially small amount of the Earth. I'm
37:27
aware at this instant of a pool of, Earth
37:30
around me. And in all honesty, the rest of
37:32
the Earth could have disappeared and I
37:33
wouldn't know now. I mean, obviously we're
37:35
still on our connection, so that hasn't
37:37
happened. But we can visualize in
37:40
that sense that the whole of something can be
37:42
bigger than the fraction that we see. The
37:45
whole of the Earth is bigger than the
37:46
fraction of the Earth that you can see from
37:47
any one location. The universe is a bit like
37:50
that. And this is another way of discussing
37:52
that, where it's discussing the maximum
37:54
extent that you could see in the past, the
37:55
maximum extent that you could see in the
37:57
future. Yeah, so hopefully that makes a
37:59
bit of sense, and gives you some direction.
38:02
If you want to read more about it, I guess.
38:04
Andrew Dunkley: Yes, yes, it's, it's when you do
38:07
research on it, it yeah, you come up with all
38:09
sorts of theories about the universe. Is it
38:12
infinite, is it finite? But you know, beyond
38:15
our capability to see because of its
38:18
expanding rate and you know, the position of
38:20
objects emitting light that could not reach
38:22
us because of this, that, it just goes on.
38:25
Jonti Horner: And on and on.
38:26
Andrew Dunkley: It's, it is really interesting and it, it
38:28
does get your, your mind swimming. but the
38:31
bottom line is that the objects
38:34
behind, beyond the cosmic event horizon,
38:38
they just, there's just not enough time for
38:40
that light to ever reach Earth. that's the
38:42
bottom line, isn't it? So hopefully that
38:45
explains it for you. Dan, thanks for the
38:47
question. It's been great. A couple of real
38:50
thought provoking questions today. Much
38:52
appreciated. And if you do have a question
38:54
for us, please send it through via our
38:57
website. we've got a new batch of audio
38:59
questions, two thirds of which come from one
39:02
person. But that's okay.
39:04
we'll get through those, but we, we do need
39:06
more. if you've ever considered sending a
39:09
question, just never got around to it, jump
39:10
on our website, Space Nuts SpaceNutspodcast
39:13
uh.com spacenuts IO
39:16
and click on the AMA link at the top and send
39:18
us text or audio questions. That away
39:21
and we look forward to hearing from you.
39:23
Don't forget to tell us who you are and where
39:25
you are from. And thanks to all our
39:28
patrons too. I don't thank you enough. these
39:30
are the people who enjoy the program and
39:32
pitch in with a couple of dollars here or
39:35
there to keep us afloat. you are amazing
39:38
people. We never asked for that. But we
39:40
certainly appreciate your support and if
39:41
you'd like to become a patron, you can find
39:44
out more on our website. Not mandatory,
39:46
but certainly appreciate it. Jonti,
39:49
you're appreciated too. Thank you very much.
39:52
Jonti Horner: That's absolute pleasure. Thank you for
39:53
having me. And yeah, the more questions the
39:55
better.
39:55
Andrew Dunkley: Yes, they're good fun. It's a great segment.
39:57
I'm glad it's developed into that. It used to
39:59
just be something that we tacked onto the end
40:01
of one episode, but we've made it its own
40:03
show. It's become bigger than Ben Hur,
40:05
really. and I'm wearing a wristwatch in the
40:08
scene as well. Some people will understand
40:10
that. and, yeah, thanks, Jonti. We'll catch
40:12
you soon. Jonti Horner, professor of
40:14
Astrophysics at the University of Southern
40:16
Queensland. Also, thanks to Huw in the
40:17
studio, who couldn't be with us, he's taken a
40:19
holiday beyond the cosmic event horizon.
40:22
So, we can't see his light, but I'm sure
40:24
it'll return and he can tell us how he got
40:26
there when he gets back in about 47 billion
40:29
years. And from me, Andrew Dunkley, thanks
40:31
for your company. Catch you on the next
40:32
episode of Space Nuts. Bye. Bye.
40:36
Jonti Horner: You'll be listening to the Space Nuts
40:38
podcast, available
40:41
at Apple Podcasts, Spotify,
40:43
iHeartRadio, or your favorite podcast
40:46
player. You can also stream on
40:48
demand@bytes.com M.
40:50
Andrew Dunkley: This has been another quality podcast
40:52
production from bytes.com.