Have a look at this beautiful image captured by the James
Webb Space Telescope. Immediately, you’ll notice two types of objects.
First, you’ll see the nearby stars within our own Milky Way galaxy, which look like points of light
surrounded by diffraction spikes. But further in the background, you’ll see entirely new galaxies,
each containing billions of stars, smeared out into elliptical or spiralling blurs.
What you probably won’t see are distinct, individual objects within those other galaxies,
as they are far too distant and blend in with all of the other sources of light.
But there is one exception to that rule. One type of object that is so bright,
you can make it out from billions of light years away;
so bright that it stands out from all of the surrounding stars in its galaxy. A quasar.
But what if I told you that a whole bunch of tiny quasars were hiding in plain sight—right
within our own Milky Way? How can that be, when they are some of the brightest objects
in the universe? In this video, we’ll get to see how that’s possible—and how a team of scientists
used images of just one nearby microquasar to gain a deeper perspective into the inner workings of
quasars all over the universe. I’m Alex McColgan, and you’re watching Astrum. Join me today as we
unravel the story of these astronomical lighthouses, one cosmic photon at a time.
Astronomers have been pointing their telescopes up at the night sky for centuries. Yet,
it was only in the 1950s that they set out on a quest to map out what
the universe around us looks like in radio waves, rather than visible light. Think
of this as the astronomer’s version of putting on night-vision goggles.
At first, they didn’t see anything out of the ordinary: many of the same galaxies that emitted
visible light also emitted radio waves, so they could be seen with or without the goggles. But
then, among the stars and galaxies that were known to populate the night sky, astronomers
saw something new: something that shone very brightly in radio waves, but had never before
been seen in visible light. And there wasn’t just one of these objects—by 1960, observations
quickly grew, until hundreds of mysterious radio sources had been recorded across our sky.
At the time, astronomers didn’t have a clue what the origin of these radio waves could
be. For all they knew, the waves could come from a giant intergalactic network of alien
radios. But what they did learn from follow-up observations using more precise telescopes like
the Hale Telescope is that you could see these objects in the visible part of the spectrum—they
were just extremely small and faint. They were far too small to be galaxies—they appeared as points,
not smeared-out blobs in the sky—but they also couldn’t be regular stars,
because regular stars don’t typically emit much energy at all in radio waves. For this reason,
these mysterious objects came to be known as quasi-stellar radio sources, which quite
literally translates to “something like a star—but not a star—that emits a lot of radio waves.” That
name eventually got shortened to something that was a bit more palatable: quasars.
Even with the catchy name, it took astronomers and physicists nearly thirty
years to really pin down what these quasars were. And—unfortunately—it’s probably not alien radios,
although the real answer is still extremely fascinating. It turns out
that the explanation centres around black holes—and not just ordinary black holes,
but supermassive black holes that can be millions or even billions of times the mass of the Sun.
Such supermassive black holes are frequently found at the centres of large galaxies, and although
black holes can’t emit any light themselves, they can make quite the spectacle out of the
accretion disks orbiting around them. If you want all the details, you can queue up our older video
on quasars here, but for now, let me just give you a quick summary. Matter orbiting around a
black hole can spiral inward as it loses energy due to internal friction. Much of that energy
ultimately gets converted into light that streams outward from the black hole, but in some cases,
the accretion disk also spits out relativistic jets of ionized matter, collimated by powerful
magnetic fields along with the spin of the black hole and its surroundings. The particles in
these jets can propagate through the universe from galaxy to galaxy as cosmic rays, sending out radio
signals as they’re pushed around by intergalactic magnetic fields. Overall, quasars emit so much
energy, that a single quasar in a distant galaxy will outshine the other hundreds of billions of
stars in that galaxy combined. That’s why when quasars were first observed, they appeared as
small as a star but far brighter than any star outside of the Milky Way could have ever been.
Now, the Milky Way has its own supermassive black hole right in its centre, too, called Sagittarius
A* (pronounced “A-star”). But for better or for worse, the accretion disk around our black hole
is too thin and faint to call it anything close to a quasar—and until something dramatic happens,
like the collision with the Andromeda Galaxy that’s scheduled for a few billion years in the
future—Sagittarius A* will likely remain calm and mellow. On one hand, we’re kind of lucky,
because a quasar that close to Earth could emit enough high-energy radiation to be
potentially fatal to terrestrial life, which makes you wonder about the kinds
of life that could arise in galaxies that do contain quasars. But on the other hand,
we’re missing out on a second source of light in the sky illuminating our planet almost as
brightly as the Sun—and it would have been so cool to get to study a quasar from up close!
So you can imagine my surprise when I learned that we might actually have the best of both
worlds. Quasars, alive and active in the Milky Way, and yet not so powerful as to
wipe out our entire biosphere. How could this be possible? If the one supermassive
black hole at the centre of our galaxy isn’t a quasar, then where else could they be?
It took a couple of decades, but scientists eventually discovered that quasars can
form around ordinary black holes too—not just supermassive ones. These ordinary black holes,
just a few times more massive than the Sun, typically form when giant stars collapse,
so there are loads of them spread throughout practically any galaxy.
You probably wouldn’t expect one of these black holes to have its own accretion disk,
but once in a while, a nearby orbiting star might get sucked into its gravitational pull
and act as a source for a smaller, less deadly version of a quasar—a microquasar.
Even though the jets of a microquasar are much weaker, they can be easier to study
because of their proximity to Earth, allowing us to learn more details about the properties
and dynamics of both microquasars and their larger, full-scale counterparts.
The very first microquasar was discovered in 1979, and it was given the obvious name SS 433,
as it was originally documented as the 433rd entry in a catalogue of stars compiled by astronomers
Nicholas Sanduleak and Bruce Stephenson two years prior. To date, only a handful of microquasars
have ever been found, and SS 433 in particular has been a subject of widespread wonder and
research. Despite its name, SS 433 is more than just some ordinary star in a catalogue. It is a
type-A supergiant orbiting around a stellar-mass black hole in a binary system. As the black hole
accretes material from the supergiant star, it produces a pair of jets perpendicular to
the line of sight from Earth—meaning that the jets themselves will never actually hit us or reach our
detectors. But as the jets propagate outward, they emit light in all sorts of directions,
allowing us to image them at various wavelengths and learn more about how they are produced.
One of the first things we saw was that there are actually two types of jets emanating from SS 433:
The inner jets extend out just a couple of light years from the black hole before fading away,
while the outer jets appear 75 light years out and span an additional three hundred light years
or so. We still don’t know how exactly these outer jets form, or why they terminate after
300 light years, and we only have a rough idea of what they’re even made of—but a recent study
from the HESS collaboration may give us some new clues to answer these questions.
One such clue is the presence and distribution of high-energy photons
called gamma rays being emitted by SS 433. Ordinarily, stars or accretion disks would
not be expected to produce any gamma rays because their temperatures are far too low, but the outer
jets coming out of SS 433 seem to emit them in spades. What’s even more surprising is that,
even though the outer jets are extremely hot, the spectrum of light that they’re emitting isn’t
consistent with an ordinary thermal distribution of photons. So what’s generating all of these
gamma rays? The current theory is that the outer jets contain high-energy electrons that can bump
into lower-energy photons passing through the system and they transfer their energies to the
photons during those collisions. This process goes by the name of inverse Compton scattering.
The scientists at HESS realized that measuring the energy in these emitted gamma rays can give
us a lot of information about the energy of the electrons in the jets that produced them
in the first place—and what they found was in full support of the inverse Compton scattering theory.
The highest-energy gamma rays, carrying over 10 teraelectronvolts of energy per photon (or about
10 trillion times more energy than a photon of visible light), were primarily coming from
the innermost region of the outer jets. Gamma rays with lower energies, on the other hand,
came from correspondingly farther distances down the jets of SS 433. This distribution
is a clear indication that electrons farther down the jets have less energy to transfer to
photons than electrons right at the beginning—or in other words, as the electrons stream along the
outer jets, they tend to lose energy over time, presumably due to their collisions with photons.
But how did the electrons acquire such large energies to begin with? Models suggest that
their initial launch velocity from the black hole was just about one-quarter of the speed of light,
but by the time they reach the outer jets, these high-energy electrons appear to be
moving at speeds even greater than 99% the speed of light, multiplying their initial energies by a
factor of one billion. The HESS collaboration believes that this wild acceleration occurs
due to a shock wave at the base of the outer jets caused by complicated configurations of
magnetic fields. Each time an electron interacts with the shock front, it gains a boost of speed
as if it crossed a boost pad in Mario Kart. And due to the diffusion of electrons back and forth
within the jet, some electrons can cross over this boost pad over and over and over again,
like an infinite speed glitch. This process is called diffusive shock acceleration,
and it may be responsible for amping up electrons to energies in the hundreds of teraelectronvolts,
just enough to be able to produce the high-energy gamma rays observed by HESS.
But shedding light on the production of photons within the outer jets of SS 433 is
just the beginning. Microquasars scattered throughout the Milky Way can hold treasure
troves of information about the inner workings of even the most massive quasars in the centres
of distant galaxies. And now that we know to look for them, microquasars are less like
hidden gems and more like beacons in the sky, posing for our astronomers to capture their
beauty in an entire spectrum of photographs, and revealing ever more shocking surprises.
So what surprised you most from today’s video? Was it the possibility of shock
waves in deep space near black holes? Or was it the fact that the Milky Way is home
to dozens of little quasars of its own? Let us know your thoughts in the comments down below.
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