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In the 1970s, astronomers discovered a bunch of gamma rays
that stumped them for almost 20 years.
The light was spotted by NASA’s second Small Astronomy Satellite, or SAS-2,
which scanned the sky for gamma rays in 1972 and ‘73.
These rays have the most energy of any kind of light,
and they come from the most extreme stuff in the universe,
like neutron stars and matter around black holes.
So pretty quickly, astronomers figured out which objects made most of the light
the satellite detected…
except for one spot.
It’s elegantly called 2CG 195+04, but it’s nicknamed Geminga.
And 50 years later, it still hasn’t revealed all of its secrets.
The name “Geminga” is actually very good
it’s somewhere between pun and acronym:
It’s it is near the constellation Gemini, and it emits gamma rays.
So scientists squashed those words together to get “Geminga”,
which is similar to a phrase in the Milanese dialect of Italian
that means “it’s not there” or “it doesn’t exist”.
Which is… appropriate.
Because when astronomers tried to learn more about it,
they came up largely empty-handed.
They did find X-rays in the same area where the SAS-2 satellite detected the
gamma rays, but neither measurement could pin down an exact position.
So, scientists couldn’t be sure all that high-energy light
came from the same source.
And when they checked the region in other kinds of light
they couldn’t see anything.
There was no visible light and, weirdest of all, no radio waves.
That’s especially confusing considering that the most likely culprit
for this light was a pulsar.
Pulsars are a type of neutron star.
They’re super hot, dense remnants left behind after massive stars explode,
and they are the most common object in space that gives off gamma and X-rays.
But as pulsars rotate, they also send pulses of radio waves in our direction,
especially from around their magnetic poles.
And since the researchers didn’t detect any radio waves near Geminga…
what was this thing?
Astronomers throughout the 1970s and ‘80s proposed all sorts of ideas
for what might be going on.
But the first real breakthrough did not come until 1987, when a pair of scientists
found a tiny blip of a star in the same region as the gamma and X-rays.
It was 40 million times dimmer than anything you could see with the naked eye,
but it was the start of a decade-long bonanza.
By 2000, teams around the world had proved that the gamma rays, X-rays,
and that tiny star’s visible light all waxed and waned about four times a second.
With that many similarities, these kinds of light had to be from the same source,
and that source had to be a pulsar.
After all, that waxing and waning suggested the star was spinning,
and anything bigger than a pulsar could not spin that fast.
Also, anything less extreme wouldn’t make gamma rays.
But Geminga sure is an odd pulsar.
For one, it’s really old considering how many gamma rays it emits.
Neutron stars spin slower as they age, and spinning about four times a second
puts Geminga around 300,000 years old.
Other pulsars that emit similar levels of gamma rays are only
a couple thousand years old.
Still, that age could help explain why it’s so dim in visible light,
since neutron stars also cool off as they get older.
But this doesn’t explain the radio silence.
One popular idea here is that Geminga’s magnetic field is so intense that when
particles travel along its magnetic field lines and smash together,
these collisions produce a shower of subatomic particles.
And that shower absorbs radio waves near the star, almost like a cloaking device.
Astronomers have also calculated that for a lot of pulsars, the cone of gamma-ray
light they send out from their poles is naturally wider than the cone of radio waves.
So, maybe we’re just not at the right angle to see Geminga’s radio pulses.
Either way, this radio mystery continues to fascinate astronomers,
just like it has for 50 years.
And even though we have not cracked the case yet,
Geminga has been teaching us about the universe in other ways.
It even resolved what seemed like an unrelated mystery!
In the early 2000s, scientists were detecting way more high-energy particles
than they expected.
Specifically, electrons and their antimatter counterparts, called positrons.
It can be hard to find where these particles come from because they don’t travel in
straight lines: Magnetic fields in the galaxy twist their paths all over the place.
So, some physicists wondered if these particles
could be evidence of dark matter interacting with interstellar dust somehow.
Dark matter is that invisible kind of matter scientists are just beginning
to understand, and if it was influencing these high-energy particles,
we could learn a lot about it.
But it wasn’t dark matter.
It was Geminga!
Turns out, electrons and positrons are the same particles
that produce a lot of Geminga’s gamma rays, and this pulsar
has been tossing them into space for hundreds of thousands of years.
And since this object is pretty close by,
a lot of them go on a twisty journey and then end up right here on Earth.
And even though they don’t come straight from their source to us,
instruments have found that there’s a big cloud of positrons around Geminga
that’s about as wide as the Big Dipper we see in the sky.
So, Geminga continues to be a mystery: Even half a century later, astronomers
haven’t been able to answer one of the first questions they had about this object.
But along the way, this weird,
ancient pulsar is still teaching us more about the complex universe we live in.
Thanks for learning about cool space technology with SciShow Space.
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