Confirmed: The Milky Way's monstrous black hole

A young star and a belt of gasses give the game away.

(Triff/ESO/Big Think)
  • Scientists have provided the first confirmation that what's at the center of the Milky Way is a supermassive black hole.
  • The discovery caught the interaction of gasses and a small star spinning around the mysterious object.
  • This is thought to be compelling proof of the black hole's central role in a galaxy.

At the center of the Milky Way, about 25,000 light years away, is a faint source of radio noise. It's huge, estimated to weigh the equivalent of the 4.14 million suns. Astronomers have long suspected it's a supermassive black hole, and they've named it "Sagittarius A*." This week, the European Southern Observatory (ESO) announced that an international collaboration led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) has collected the most definitive proof that this is exactly what Sagittarius A* is.

While astronomers can't directly observe a black hole — light doesn't escape it — they might, however, be able to see some of what goes on around one. Genzel and other scientists across the globe collected information regarding a small star called "S2" and the belt of gas, or accretion disc, that spin around Sagittarius A*. It's in the interaction between the two that the new discovery lies, and it was made possible by a breakthrough in imaging.

The imaging breakthrough

Photo credit: MPE/GRAVITY team

The ESO has a four-telescope array, the Very Large Telescope (VLT) of the Paranal Observatory, rising 2635 meters above sea level in Chile's Atacama Desert. The amazing device that ultimately allowed the team to confirm Sagittarius A*'s identity leverages the Paranal telescopes. It's called "GRAVITY," and it combines all four in a single interferometer that has the resolution of a single mirror resolution of a single mirror 130 meters in diameter. "All of the sudden, we can see 1,000 times fainter than before," said Genzel when GRAVITY went into use.

S2 and the redshift

Image source: ESO/MPE/S. Gillessen

Every 16 years, a young blue star dubbed "S2" or S-02"completes an elongated orbit that brings it perilously close to Sagittarius A*, about 11 billion miles.

Many scientists feel that black holes — of which Einstein himself was unconvinced — are predicted by general relativity. (They were only finally confirmed a couple of years back when two black holes collided.) Einstein's theory, though, also predicts that if S2 is indeed orbiting a black hole, the speed of the light waves bouncing off it when it draws Sagittarius A*near should slow down, shifting the light it reflects to a more reddish hue.

In July of this year, Genzel's team announced that they had observed via GRAVITY the center of the Milky Way, and had seen the predicted redshift, allowing them to pinpoint S2's closest approach to Sagittarius A*. New York Times reports that as the results were being read off at the Munich announcement, the room broke out into applause.

Accretion disc flares

Also spinning around Sagittarius A* is an accretion disc that travels at nearly 30 percent the speed of light, zooming 150 million miles around the object every 45 minutes. According to relativity, whenever S2 — or any hot object — reaches its innermost, or stable, orbit, bits of it should cross the event horizon and be instantly vaporized as they fall into the black hole, sparking brief infrared flares.

Thanks to GRAVITY, the MPE scientists have been able to see that this actually happens at S2's closest fly-by. "GRAVITY's tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail," another MPR scientist, Oliver Pfuhl, tells ESO. "It's mind-boggling to actually witness material orbiting a massive black hole at 30 percent of the speed of light."

The predicted flares were spotted, actually, as the MPE team was observing S2 in the research that led to July's announcement, though it took until now to prepare supporting materials for publication. "We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*," Pfuhl recalls. "During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!"

Now we know what lies at the center of the Milky Way

Image source: ESO

Genzel refers to the discovery of the flares as a "resounding confirmation of the massive black hole paradigm." Astronomers believe that black holes likely lie at the core of other galaxies as well, so this announcement has far-reaching implications. "This always was one of our dream projects but we did not dare to hope that it would become possible so soon," he concludes.

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It looks like a busy hurricane season ahead. Probably.

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Surprising Science
  • Before the hurricane season even started in 2020, Arthur and Bertha had already blown through, and Cristobal may be brewing right now.
  • Weather forecasters see signs of a rough season ahead, with just a couple of reasons why maybe not.
  • Where's an El Niño when you need one?

Welcome to Hurricane Season 2020. 2020, of course, scoffs at this calendric event much as it has everything else that's normal — meteorologists have already used up the year's A and B storm names before we even got here. And while early storms don't necessarily mean a bruising season ahead, forecasters expect an active season this year. Maybe storms will blow away the murder hornets and 13-year locusts we had planned.

NOAA expects a busy season

According to NOAA's Climate Prediction Center, an agency of the National Weather Service, there's a 60 percent chance that we're embarking upon a season with more storms than normal. There does, however, remain a 30 percent it'll be normal. Better than usual? Unlikely: Just a 10 percent chance.

Where a normal hurricane season has an average of 12 named storms, 6 of which become hurricanes and 3 of which are major hurricanes, the Climate Prediction Center reckons we're on track for 13 to 29 storms, 6 to 10 of which will become hurricanes, and 3 to 6 of these will be category 3, 4, or 5, packing winds of 111 mph or higher.

What has forecasters concerned are two factors in particular.

This year's El Niño ("Little Boy") looks to be more of a La Niña ("Little Girl"). The two conditions are part of what's called the El Niño-Southern Oscillation (ENSO) cycle, which describes temperature fluctuations between the ocean and atmosphere in the east-central Equatorial Pacific. With an El Niño, waters in the Pacific are unusually warm, whereas a La Niña means unusually cool waters. NOAA says that an El Niño can suppress hurricane formation in the Atlantic, and this year that mitigating effect is unlikely to be present.

Second, current conditions in the Atlantic and Caribbean suggest a fertile hurricane environment:

  • The ocean there is warmer than usual.
  • There's reduced vertical wind shear.
  • Atlantic tropical trade winds are weak.
  • There have been strong West African monsoons this year.

Here's NOAA's video laying out their forecast:

But wait.

ArsTechnica spoke to hurricane scientist Phil Klotzbach, who agrees generally with NOAA, saying, "All in all, signs are certainly pointing towards an active season." Still, he notes a couple of signals that contradict that worrying outlook.

First off, Klotzbach notes that the surest sign of a rough hurricane season is when its earliest storms form in the deep tropics south of 25°N and east of the Lesser Antilles. "When you get storm formations here prior to June 1, it's typically a harbinger of an extremely active season." Fortunately, this year's hurricanes Arthur and Bertha, as well as the maybe-imminent Cristobal, formed outside this region. So there's that.

Second, Klotzbach notes that the correlation between early storm activity and a season's number of storms and intensities, is actually slightly negative. So while statistical connections aren't strongly predictive, there's at least some reason to think these early storms may augur an easy season ahead.

Image source: NOAA

Batten down the hatches early

If 2020's taught us anything, it's how to juggle multiple crises at once, and layering an active hurricane season on top of SARS-CoV-2 — not to mention everything else — poses a special challenge. Warns Treasury Secretary Wilbur Ross, "As Americans focus their attention on a safe and healthy reopening of our country, it remains critically important that we also remember to make the necessary preparations for the upcoming hurricane season." If, as many medical experts expect, we're forced back into quarantine by additional coronavirus waves, the oceanic waves slamming against our shores will best be met by storm preparations put in place in a less last-minute fashion than usual.

Ross adds, "Just as in years past, NOAA experts will stay ahead of developing hurricanes and tropical storms and provide the forecasts and warnings we depend on to stay safe."

Let's hope this, at least, can be counted on in this crazy year.

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