Sky & Telescope - April 2020 - 33

across your field of vision away from your nose, but it varies
from person to person. The only way to find yours is by noting where dim objects consistently appear brightest in your
field of view.
Step 3: Steady seeing is the hard part.
The contrast between the galaxy's fuzzy core and the
thin, sharply delineated jet makes it seem like this shouldn't
be such a tough observation. That's how this cookie crumbles, though, and the jet can be seen only when the air is
steady, unless you're using a really large scope. A trembling
atmosphere blurs the jet into the overall fuzziness of M87,
and unfortunately there are few places that have consistently steady seeing.
For many of us in the Northern Hemisphere M87 is never
high enough to get the most out of a steady night. Sometimes
it happens anyway, though, and depending on your observing
site it may take many nights before the atmosphere gets out
of the way enough for a successful observation.

Visual Observations
My experience trying to see the jet certainly followed that
pattern. Before even suspecting that I'd seen the jet, my
notebooks are filled with observations that boil down to
"tried to see M87's jet, no luck again." I first tried throughout the 1980s with my homemade 12.5-inch f/7.8 Dob, and
continued trying when I replaced the Dob with a 20-inch f/5
Obsession in 1991. I had one observation with the 20-inch
that gave brief but repeated suggestions of the jet, but I can't
say that I really saw it. Frustrating.
In 2007 I had my first taste of the jet with my 28-inch f/4
Dob, but I couldn't convince myself I'd really detected it.
While hints of the jet are enticing, they're also disappointing.
Three years later all that changed on a memorable night at
Kitt Peak.

SUPERLUMINAL MOTION: G REGG DINDER M A N / S&T

Capturing the Jet
You don't need a large telescope or digital detector to image
the jet. For this 90-minute exposure captured on Fuji Provia
400F slide film, amateur astronomer John Boudreau used a
12-inch Meade SCT scope at f/11 and an Olympus OM1n
camera. He took the photo on March 20, 2001, at Myles
Standish State Forest near Plymouth, MA.
Want to see M87's jet in a big scope?
Here are links for public viewing reservations at the three
largest publicly available telescopes in the U.S.:
*
82-inch Otto Struve scope at McDonald
Observatory: https://is.gd/mcdonald_visitors
*
100-inch Hooker telescope on Mt. Wilson:
https://is.gd/mtwilson100_visitors
*
60-inch telescope on Mt. Wilson:
https://is.gd/mtwilson60_visitors
*
Note: The 90-inch Bok telescope at Kitt Peak
doesn't accept reservations for public viewing.

How Apparent Superluminal
Motion Works
M87's jet has the perfect setup for the appearance of superluminal motion, but the concept itself can be a slippery one
to grasp at first. A little math and a simple diagram of the
geometry will help you understand what's going on. Bear in
mind that the diagram below isn't to scale, but it gives you a
general idea of the layout.
B Phot
o

at β
oving

m
Blob
A

n2

θ

Line of sight
C

Photon 1

D

Point A in the diagram is the stationary source of the jet
that's expelling matter - a blob, say, or a shock wave - at
a velocity that's a significant fraction of the speed of light,
which we'll call β ( = v / c). The blob travels at an angle θ
to our line of sight. The blob emits a photon at A, and then
some time later, when it has arrived at Point B, it emits another photon. During that time (the photons will take a long
time to arrive at the detectors at Point D, though!), the blob
appears to have covered the transverse distance BC.
Making simplifications, we have the following formula for
the blob's transverse velocity (its speed across our line of
sight):
v_transverse =

β sin θ
(1 - β cos θ)

This is where the fun part begins: Grab your favorite calculating device and start plugging in numbers.
Let's start with a blob traveling at 90% the speed of light
(so, β = 0.9) at an angle of 15° to our line of sight. Substituting for those numbers:
0.9 sin 15°
v_transverse =

= 1.8c
(1 - 0.9 cos 15°)

The blob appears to travel at nearly twice the speed of light!
What happens if we slow the blob down to 60% the speed
of light? Substituting 0.6 for β gives us:
v_transverse = 0.4c
And 25% the speed of light? We get,
v_transverse = 0.09c
Notice how the actual speed of the blob, v, has
to be close to the speed of light for the superluminal effect to occur?
sk yandtelescope.org * APRIL 2 02 0

33


https://www.is.gd/mcdonald_visitors https://www.is.gd/mtwilson100_visitors https://www.is.gd/mtwilson60_visitors http://www.skyandtelescope.org

Sky & Telescope - April 2020

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