Summer Nights Under The Milky Way’s Black Hole

The Milky Way is a spiral galaxy with a prominent dense bar of stars across its core. The sun and planets are located with a spiral arm some 25,000 light years from the center. Illustration: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)
The Milky Way is a spiral galaxy with a prominent dense bar of stars across its core. The sun and planets are located within a stub called the Orion Spur some 25,000 light years from the center. The arrows are sight lines. In summertime, we face toward the galactic center (Sagittarius region) at nightfall facing south. In winter, we face out the other side of the galaxy in the direction of the constellation Taurus. Illustration: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

Hidden behind a scree of interstellar dust 25,000 light years away in the constellation Sagittarius lies the center of the Milky Way galaxy. Billions of stars concentrate there surrounded by a a whorl of spiral arms. For every star we see in the nighttime sky, someone living near the galactic center would see one million. If we could sweep away the fine dust between us and the center, we’d see the galaxy’s core glow like an ember in the southern sky bright enough to casts shadows.

Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black
Invisible in ordinary light, telescopes like the orbiting Chandra X-Ray Observatory can see through the galactic dust using X-ray vision to photograph Sagittarius A*, site of the our galaxy’s supermassive black hole as seen in this July 2014 image. The glow comes from X-rays given off as the material swirls down the hole. Occasionally, a larger object strays too close to Sgr A*, gets pulled in and shredded as it disappears down the hole, causing a bright flare of light. Credit: NASA

At the center of it all lurks a supermassive black hole about the size of our solar system 4.5 million times more massive than the sun. It goes by Sagittarius A* (the asterisk is pronounced “star”) and it snacks on asteroids, hapless stars that pass nearby, dust and gas. Other black holes make up part its diet, too. As smaller holes across the galaxy encounter and devour other stars, the interaction causes them to gradually settle toward the galaxy’s center, where they’re eventually swallowed by the big dude at a rate of about one every million years. And so grows the hole.

 The black hole companion is shedding its outer envelope in a powerful wind, and some of this gas is captured by the black hole’s powerful gravity. + Click for high resolution (1.2 Mb) JPG Credit: Aurore Simonnet/Sonoma State University/NASA
Illustration of a black hole gobbling up gas from a nearby star. As the material crosses over the hole’s point of no return called the event horizon, it’s torn apart and heated, causing a sudden brightening around the hole. Credit: Aurore Simonnet/Sonoma State University/NASA

Although we can’t see see the black hole directly, astronomers have been watching stars whipping around it for decades. If nothing else, supermassive black holes have super-duper gravity, lots of “pull” in the neighborhood as it were. Astronomers can determine how massive an object is by measuring how much it tugs on something such as an orbiting star. That’s how we know that something incredibly massive but otherwise dark lies at the galactic center.

Black hole locator
Although you can’t see it directly, the center of the galaxy and Sagittarius A* sits just off the spout of the Teapot asterism in constellation Sagittarius the Archer. Bright Mars and Saturn will help guide you there. Map: Bob King, source: Stellarium

Most galaxies have supermassive black holes in their cores. Ours is relatively quiet, but some have a constant case of the munchies and flare brilliantly as material gets physically torn apart and heated to tens of millions of degrees as it whorls inexorably toward the singularity at the “bottom” of black hole. A singularity is little more than a point of zero size and infinite density. Almost too bizarre to contemplate without your brain going into overdrive, no one’s sure what happens at the singularity or whether our current physics is even capable of offering an explanation.

All massive objects create dimples in the fabric of spacetime (3D space woven together with time as the 4th dimension). The black hole takes warping to an extreme, creating a singularity of massive density and zero size at its core. Credit: NASA
All massive objects create dimples in the fabric of spacetime (3D space woven together with time as the 4th dimension). The black hole takes warping to an extreme, creating a singularity of infinite density and zero size at its core. Credit: NASA

Recently, a European team of astronomers used the new GRAVITY instrument at European southern Observatory’s Very Large Telescope to obtain detailed observations of the surroundings of the galaxy’s central black hole. They combined the light from all four of the 323-inch (8.2-meter) Unit Telescopes for the first time to probe the extremely strong gravitational fields close the central black hole. Together, the combined light of the four can provide the same resolution and precision as a single telescope around 426 feet (130 meters) in diameter!

This artist’s impression shows stars orbiting the supermassive black hole at the centre of the Milky Way. In 2018 one of these stars, S2, will pass very close to the black hole and this event will be the best opportunity to study the effects of very strong gravity and test the predictions of Einstein’s general relativity in the near future. Credit: ESO / L. Calcada
This artist’s impression shows stars orbiting the supermassive black hole at the centre of the Milky Way. In 2018 one of these stars, S2, will pass very close to the black hole and this event will be the best opportunity to study the effects of very strong gravity and test the predictions of Einstein’s general relativity in the near future. Credit: ESO / L. Calcada

Although the position and mass of the black hole have been known since 2002, by making precision measurements of the motions of stars orbiting it, GRAVITY will allow astronomers to probe the gravitational field around the black hole in unprecedented detail, providing a unique test of Einstein’s general theory of relativity. The GRAVITY team has used the instrument to observe a star known as S2 as it orbits the hole with a period of only 16 years.

The team will soon be able to obtain ultra-precise positions of the orbiting star, equivalent to measuring the position of an object on the Moon with half-inch accuracy. That will enable them to determine whether the motion around the black hole follows the predictions of Einstein’s general relativity — or not. Seems like nobody will ever leave Einstein alone. Yet his theory has capably handled nearly everything thrown at it these many decades.

In 2018 the S2 star will be at its closest to the black hole, just 17 light-hours away from it and travelling at some 18.6 million miles an hour (30 million kph), or 2.5% of the speed of light, an opportunity that won’t be repeated for another 16 years.

A warm summer night under the arch of the bright Milky Way offers a singular opportunity to reflect on everything fireflies to cosmic dust to singularities.