The first image of Sagittarius A*, the supermassive black hole at the center of our galaxy.
Credit: © EHT Collaboration

Finally, the faceless monster lurking at the center of our galaxy has been photographed. Sounds like science fiction? It’s true! On Thursday, May 12, scientists with the Event Horizon Telescope (EHT) team published a photograph of Sagittarius A* (saj uh TAIR eeuhs AY star). Sagittarius A* is a supermassive (huge) black hole in the center of our galaxy, the Milky Way.

A black hole is a region of space whose gravitational force is so strong that nothing can escape from it, not even light. The event horizon is the “point of no return” for a black hole: Anything that crosses this horizon is sucked into the black hole forever. Supermassive black holes at least a million times more massive than the sun lurk at the center of many galaxies.

The EHT is a global network of ground-based telescopes that use a technique called radio interferometry to produce images of black hole event horizons. The collection of radio telescopes participating in the EHT project stretches from Hawaii to Europe and all the way south to Antarctica. Several dozen of the world’s leading observatories and universities contribute to the project. Supercomputers process the image data using special algorithms in a process called correlation.

Astronomers began using the EHT to make observations of Sagittarius A* in 2017. In 2019, the EHT released an image it had captured of the event horizon surrounding the supermassive black hole at the center of the Messier 87 galaxy (M87*). It was the first time an event horizon had been photographed.

If light can’t escape a black hole, how did EHT photograph M87* or Sagittarius A*? Technically, it didn’t: all black holes are invisible and cannot be directly photographed. But matter trapped in orbit near the event horizon is extremely energized and emits large amounts of light. The black hole lurks in the circular “shadow” within the halo of high-energy matter. Thus, the EHT can take a picture of a black hole much as we can take a picture of a doughnut hole.

Despite being closer to Earth, Sagittarius A* was still harder to image than M87*. Sagittarius A* is still 27,000 light-years from Earth. It’s also more than 1,000 times smaller and less massive than M87*. So, although the high-energy matter is orbiting both black holes at about the same speed, the matter completes an orbit around Sagittarius A* in a matter of minutes. Furthermore, Sagittarius A* is “quieter”: it emits far less energy than M87*. All these factors made it difficult for EHT to capture an image of it.

Now that EHT has imaged two black holes, astronomers can compare them. Both look remarkably similar, despite their differing sizes. Both confirm what was predicted by Einstein’s theory of relativity. But plenty of questions remain. For example, are the “blobs” in the picture actual elements, or are they artifact of the correlating process? The EHT is also considering the feasibility of creating a short video of Sagittarius A* by stringing together multiple consecutive images. And plenty more supermassive black holes are waiting for their photographic debut. A major observation campaign that concluded earlier this year featured even more telescopes. Expect more exciting results soon regarding these most extreme objects in the universe.

September 24, 2018

Earlier this year, in April, the European Space Agency (ESA) released the second trove of data gathered by the space probe Gaia (named after the ancient Greek goddess Gaia). Gaia’s precise measurements of the position, brightness, motion, and color of over 1.3 billion stars will transform astronomers’ understanding of our galaxy, the Milky Way.

In this artist’s impression, Gaia maps the stars of the Milky Way. Credit: ESA/ATG medialab/ESO/S. Brunier

The Milky Way contains the sun, Earth, and other objects in our solar system. It also includes hundreds of billions of stars besides the sun. Huge clouds of gas and dust lie throughout the galaxy, and they constantly form new stars. The Milky Way is so massive that about 10 smaller galaxies orbit it like satellites revolving around a planet.

Because our solar system is located within the Milky Way, vast clouds of dust and dense swaths of stars prevent astronomers from determining the galaxy’s exact structure. It is a bit like trying to see the outside of one’s home from the inside. Until now, astronomers had to guess what the Milky Way looks like by observing other galaxies with similar traits—the equivalent of looking at other houses from a window. Astronomers know the Milky Way is a spiral galaxy, with long arms extending out from a central core. But no one knows how many arms it has, what kinds of stars the arms contain, or whether the galaxy is a traditional spiral or a subtype called a barred spiral galaxy where the central core is elongated.

The space probe Gaia will tell us if the Milky Way is a traditional spiral galaxy, as in this illustration, or a barred spiral galaxy with an elongated central core. Credit: Artwork © Jon Lomberg and the National Air and Space Museum

Gaia is like a flying drone taking photos around the neighborhood. Launched in 2013 by the ESA, Gaia traveled to a point behind Earth and began measuring the positions of over a billion stars. Gaia measures stellar distances using a technique called parallax. It first images a star against a background of other stars. Halfway around the sun, it takes another picture of the same star. Because the two pictures are taken from far distant positions, the star moves slightly in relation to its background. This is the same optical effect that makes an object at arm’s length to appear to change positions when viewed from just one eye or the other. Because scientists know how far away the two star pictures were taken, and how much the star appeared to move, they can precisely calculate its distance from Earth.

Gaia’s portrait of the Milky Way also extends into the fourth dimension—time. Over the course of its five-year mission, Gaia will map the sky 29 times. From those time-lapsed maps, astronomers can determine the speed and direction in which the galaxy’s stars and gas clouds are moving through space. This information tells us the origins of Milky Way objects and can even warn of galactic collisions or violent stellar explosions. This time-based study of the galaxy is called galactic archaeology.

Gaia is gathering more than just information about stars. The second data release also had the locations of 14,000 known asteroids within our solar system, allowing astronomers to precisely map their orbits. Future data releases will map even more solar system objects.

ESA released the first Gaia dataset in 2016. For each release, scientists have raced to mine the data for new discoveries. Usually, data from such spacecraft are temporarily reserved for scientists directly associated with the mission. But mission managers have released Gaia’s data to everyone at the same time. This has led to another impressive statistic: over 1,000 peer-reviewed scientific papers that use Gaia’s data have already been written since the first data release. Many thousands more will be written before all Gaia’s insights are revealed.

September 23, 2016

The Milky Way Galaxy is our home, but we know surprisingly little about it. Last week, on September 14, the European Space Agency (ESA) released a wealth of data gathered by the space probe Gaia (named after the ancient Greek goddess Gaia). The probe pinned down the precise position and brightness of over a billion stars. As massive as this dataset seems, it represents just a taste of what is to come from Gaia, which promises to revolutionize the study of our galaxy in the next several years.

This artist’s impression shows Gaia mapping the stars of the Milky Way. Credit: ESA/ATG medialab/ESO/S. Brunier

The Milky Way contains the sun, Earth, and other objects in our solar system. It also includes hundreds of billions of stars besides the sun. Huge clouds of gas and dust lie throughout the galaxy, and they constantly form new stars. The Milky Way is so massive that about 10 smaller galaxies orbit it like satellites revolving around a planet.

Because our solar system is located within the Milky Way, vast clouds of dust and dense swaths of stars prevent astronomers from determining the galaxy’s exact structure. It’s a bit like trying to figure out what the outside of one’s home looks like while being trapped inside. Until now, astronomers had to guess what the Milky Way looks like by observing other galaxies with similar traits—the equivalent of looking at other houses from a window. Gaia, however, is like a drone taking photos around the neighborhood. Launched in 2013 by the ESA, Gaia traveled to a point behind Earth and began measuring the positions of over a billion stars as it orbited the sun.

Gaia measures stellar distances using a technique called parallax. It first images a star against a background of other stars. Halfway around the sun, it takes another picture of the same star. Because the two pictures are taken many millions of miles apart, the star’s position changes slightly in relation to its background. This is the same effect that causes an object at arm’s length to appear to change positions when viewed from one eye or the other at a time. Because scientists know how far away the two star pictures were taken, and how much the star appeared to move, they can calculate its distance from Earth.

Gaia’s first map shows a two-dimensional plot of star density in the Milky Way. As Gaia orbits the sun, it will gather information on more and more points of light. It will then be able to refine its data to better define star positions. Astronomers are confident that future data from Gaia will allow them to create extremely accurate, three-dimensional maps of the galaxy.

Furthermore, as Gaia’s mission continues, its portrait of the Milky Way will extend into four dimensions (including time). As it observes over the years, Gaia will be able to detect the speed and direction in which stars are moving through space. This information will allow astrophysicists to see what the galaxy looked like in the recent past and to predict what it will look like in the near future.

July 13, 2012

For nearly 100 years, scientists have known that the Andromeda Galaxy, our nearest galactic neighbor, was heading our way. But they didn’t know whether an encounter would be a glancing blow or a head-on collision or if Andromeda would miss our Milky Way altogether. Now, thanks to measurements made using the Hubble Space Telescope, scientists with the Space Telescope Science Institute in Baltimore have calculated that Andromeda, which is hurtling toward us at a speed of about 250,000 miles (400,000 kilometers) per hour, will plow directly into the Milky Way in about 4 billion years. The encounter will produce some dramatic changes, the scientists said. But the destruction of the sun and the solar system will not be one of them.

The Milky Way, our "home" galaxy, is a spiral galaxy. Astronomers believe that after neighboring galaxy Andromeda--also a spiral galaxy--smashes into the Milky Way, the two will form an elliptical galaxy. (Artwork © Jon Lomberg and the National Air and Space Museum)

As Andromeda gets closer, it will fill more and more of our night sky. Eventually, the two spiral galaxies will begin to merge. A blaze of new stars will appear in the sky as clouds of dust and gas are compressed by the gravitational forces tearing at the galaxies. Stars within galaxies are so far apart that the sun or planets will not collide with other space bodies. But scientists think the solar system will be flung into a different part of the Milky Way, probably even farther from the galactic core than it is today. Over the next 2 billion years, the two spiral galaxies will combine to form an elliptical galaxy that some scientists are calling “Milkomeda.”

Earth and the sun will probably not be around to witness the final product, however. Scientists have estimated that about 5 billion years from now, the sun will have used up its hydrogen fuel. Eventually, it will expand enormously, probably nearly to the current orbit of Mercury, and swallow Earth.

Additional World Book articles:

•  Galaxy
• The Formation of Galaxies and Other Structures  (a Special Report)

Jan. 13, 2012

Planets orbiting stars other than the sun may be the rule rather the exception, an international team of scientists has reported. In fact, the Milky Way alone–an average-sized galaxy among the billions of galaxies in the universe–probably contains at least 160 billion planets. And even that, the scientists said, is a conservative estimate.

The scientists reached that number after making a statistical analysis of planets found using gravitational lensing, one of the main methods for hunting for extrasolar planets. They calculated that each star in the Milky Way has an average of 1.6 planets. Then they multiplied this number by the 100 billion stars estimated to belong to the Milky Way. Gravitational lensing, however, is not useful for finding exoplanets less than five times larger than Earth. Nor is it especially useful for finding planets that orbit closer to their star than Venus or farther from their star than Saturn. So the total number of alien planets is certainly much higher.

Three rocky planets smaller than Earth--the first alien planets that small found outside the solar system--have been found orbiting a red dwarf about 120 light-years from Earth. The smallest of the three planets (foreground), which is about the size of Mars, actually orbits the farthest from the star. NASA/JPL-Caltech

Evidence for that conclusion was the discovery of the first exoplanets smaller than Earth outside the solar system. Astronomers studying data imaged by the Kepler space telescope found the three rocky planets about 120 light-years from Earth. One of the planets is about the size of Mars, about half the size of Earth. All three orbit so close to their star, a red dwarf, that their sizzling surface temperatures rule out the existence of life as we know it. The Kepler scientists said the galaxy might have many more small planets than it has large planets. Astronomer John Johnson, one of the authors of the study, said that small exoplanets were “kind of like cockroaches. If you see one, then there are dozens hiding.”

Additional World Book articles:

• Back in Time (Astronomy 1996)
• Back in Time (Astronomy 1999)
• COROT
• In Search of Other Worlds (A Special Report)
• Kepler, Johannes
• Transit