How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

The ESA video ESOcast 173: First Successful Test of Einstein's General Relativity Near Supermassive Black Hole includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a presumed supermassive black hole. This isn't visible light because it's obscured by dust, so it may be radio or long wave infrared, but I don't know.

In the middle, I can see something flashing at whatever wavelength this image has been produced from.


  1. How are these images obtained, and
  2. what process is it that is believed to be causing that flashing?

GIF made from video at around02:50:

Six annotated frames from GIF highlighting the flashing that I'm seeing.

Question: How are these images obtained?

Later in the video the narrator says they took the images using ESO's VLT.

03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO's Very Large Telescope to the limits.

(Source: ESO transscript)

Over the whole observation period multiple telescopes and imaging instruments were used. Early observations were accomplished using the NTT. Since around the year 2002 the VLT observed Sagittarius A* with the NACO and GRAVITY instruments too.

This graph puts the rotation period of Sagittarius A* in relation to the time and the observing instruments:


Question: What process is it that is believed to be causing that flashing?

ESO provides older footage over a minutes-timescale showing a flare of Sagittarius A* in May 2003: Flashes of light from disappearing matter

ESO Press Video eso0330 shows the detection of a powerful flare from the centre of the Milky Way galaxy. These and other adaptive optics (AO) images (with resolution 0.040 arcsec in the near-infrared H-band at wavelength 1.65 µm) of the central region of the Milky Way were obtained with the NACO imager on the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory on May 9, 2003. [… ] The position of the 15-year orbiting star S2 (cf. ESO Press Release eso0226) is marked by a cross and the astrometric location of the black hole is indicated by a circle.

The cause of the flickering is probably the same for the video sequence you showed.

A October 2018 ESO publication states:

ESO's GRAVITY instrument on the Very Large Telescope (VLT) Interferometer has been used by scientists from a consortium of European institutions, including ESO [1], to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, [… ]. The observed flares provide long-awaited confirmation that the object in the centre of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole's event horizon - making these the most detailed observations yet of material orbiting this close to a black hole.


ESO also has another video showing the therory with more detail: skip to about 1:00.

Flares associated with Sgr A* are also described in this answer.

Artist's illustration of heated gas orbiting the black hole:

(Screengrab from 1:25 of that video)

See also

  • (2019): Black hole at the center of our galaxy appears to be getting hungrier
  • Do et al. 2019 Unprecedented Near-infrared Brightness and Variability of Sgr A*
  • Chen et al. 2019 Consistency of the Infrared Variability of SGR A* over 22 yr

Figure 1 from Do et al. 2019

Figure 1. Top row: a series of K' images taken on 2019 May 13 centered on Sgr A* showing the large variations in brightness throughout the night. The first image from the left is the brightest measurement ever made of Sgr A* in the near-infrared. Also labeled are nearby stars S0-2 (K' = 14 mag) and S0-17 (K' = 16 mag) for comparison. Bottom panel: K' (black) and H-band light curves of Sgr A* from 2019 May 13. On this night, we alternated between H and K' observations. The H-band magnitudes are offset using H − K' = 2.45 mag. There appear to be no significant color changes during the large change in brightness. Red circles show the location of the four images in the panels above.

How did we discover the shape of our galaxy?

Since nothing man made (that I know of) has left the galaxy how did we figure out it was spiral and not something else?

Yes. Nothing man made has left our milky way galaxy. Few man made things have left our solar system. We figured out the shape due to a couple of different pieces of evidence. One being that we live in a 3 spacial dimension universe. And so, when gravity sucks in gasses to collect matter, they flatten out and then spiral. Minutephysics has a good video on this. Our solar system is another example of this and is why almost all galaxies and solar systems are so flat. Another piece of more observable proof is statistically it should be. Looking at other galaxies and noticing that a vast majority are spiral makes sense for ours to be as well. And finally, you can see it. With the naked eye. When it's dark enough out, you can look up and see a smooth line of our galaxy with rounded ends (because youre viewing a disk from the side). And additionally with that you can kinda make out the spiral.

I'm a physics student. But because im just a student I don't know every piece of evidence. There is more than likely more that i don't know about, but this is what I know.

There's an ingenious way to do this! Hydrogen emits radiation with a wavelength of about 21 centimeters, and this radiation is easy to detect. And where there's hydrogen, there are stars.

Unfortunately, this isn't enough if you just look straight at the center of the Milky Way, you'll see plenty of 21 cm radiation, but you won't know how far away it's coming from.

The trick is to use the Doppler shift. If the emitting atoms are moving away from us, the wavelength will be slightly larger than 21 cm. If they're moving towards us, slightly less. So instead of just getting radiation at 21 cm, you get a distribution of wavelengths around 21 cm.

We know how fast stars orbit the center given their orbit radius, from galaxy rotation curves. And if we know how fast a star is moving relative to us, we can figure out how much their 21 cm line is shifted.

As a result, when you point your telescope at some direction in the Milky Way, and see a bump near 21 cm, you can do some trigonometry and figure out that it came from stars, say, 1000 light years away in that direction. You can repeat this process for many different directions, eventually arriving at a full map of the spiral arms. It looks like this.

We base it on pictures we have of similar galaxies, as we know they occur in general classes, or groups.

We also, to some degree, can map the speeds and positions of the various components of the spiral arms and that is confirmation that we have the general shape correct, although from memory, we did recently discover some new galactic features that previously had been hidden by dust. As John says in his answer, that is a difficult feat to achieve.

As well as visual observations, we use radio waves, infrared and ultraviolet wavelengths to try to peer through obscuring dust, to establish the most accurate map. As Rob Jeffries points out in his comment below, mapping of the hydrogen line (at a wavelength of 21 cm), is a vital part of this process. For more on this, please see Hydrogen Mapping in the Milky Way, or Kevin's answer on this page.

This is NGC 6744, which according to Wikipedia, may closely resemble the Milky Way.

And, just to finish off and see how far we have come, here is a William Herschel map, from the mid 1700s. Hershel, just in case the map is unclear, was mapping looking edge on into our galaxy, as we normally see it.

Although the map is, understandably, crude by modern standards, personally speaking, I feel this should not take away from the time and effort put into making it, considering the cloudy English skies he worked under.

We determine the shape of the Milky Way simply by measuring the positions of objects within it. Well, I say "simply", but this is an extraordinarily hard thing to do for lots of reasons. As a result there is still lots that we don't know about the Milky Way. For example although we believe the Milky Way is a barred galaxy we don't know how big the bar is.

The good news is that the Gaia mission will eventually measure the positions and velocities of about 1% of all the stars in the Milky Way. That should give us a pretty good idea of its structure.

Before satellites or aircraft were invented, how did people make maps? You start at one point on the ground and carefully measure the angle to some landmark, say a mountain. Then you measure the distance. You make many many such measurements of angles and distances between various objects. Then you scale all the measurements down to something manageable and carefully draw all these on a piece of paper. Stand back and you can see what the land would look like from space.

The same thing is true in principle of the Milky Way. You can directly measure angles to the stars. When astronomers developed ways to measure distances, they could plot, this angle from north, this many light years, so relative to us that star is here. Plot many stars this way and you will see the shape of the galaxy.

Yes, there's more to it than that. For one thing there's the complexity that the Earth is rotating, then it's also revolving around the Sun, and the Sun itself is moving. You have to be able to subtract out all these motions.

And measuring distances to the stars is not trivial, but I suppose discussion of how astronomers do that is a different question.

Actually it is quite simple and this method, Integrated Tomographic Imaging, has produced what is a fairly accurate 'top down' image of the Milky Way. You start with a catalog of known stars their elevation, azimuth, actual estimated brilliance and estimated distance. Sort by brilliance.

You then can polar plot in a 3 axis grid system the stars location. assigning a brilliance, that has been estimated, from the catalog with one or two methods (depending on your graphics program like Rhino, or MicroStation that can handle the large XYZ values a galaxy would have) a. a light source whose brilliance is directly proportional to the data or b. a sphere whose radius is proportional to the brilliance or a function of both. If your catalog has a spectral data value you can then adjust the star point/sphere color to match.

Run the render command (Rhino can actually show you the photographic end product using the GL render viewport/all view ports option on the tool bar, allowing you to do a really fun spin and tip manipulation real-time with your mouse)

Then using any of the view you would have a perspective , top front or 'side' view of the galaxy. NASA has such a work product on their web site. There are a huge number of Milky Way top down drawings on the web that used this method. Google "Milky Way Maps" there are hundreds of them, my favorite is the 'subway' from oneminuiteastronomer. as you can see from my icon.

the same technique was used to create the fantastic image of the known universe using the data from the SDSS, Sloan Deep Sky Survey. very humbling.

The SDSS is for galactic structures in the known or visible universe. not our galaxy specifically. more along the lines of galactic superclusters.

There’s a chance the black hole at the center of our galaxy is actually a wormhole

The odds are slim, but a new analysis shows it’s possible.

Science fiction writers love wormholes because they make the impossible possible, linking otherwise unreachable places together. Enter one, and it’ll spit you back out in another locale—typically one that’s convenient for the plot. And no matter how unlikely these exotic black hole relatives are to exist in reality, they tend to fascinate physicists for exactly the same reason. Recently, some of those physicists took the time to ponder what such a cosmic shortcut might look like in real life, and even make a case that there could be one at the center of our galaxy.

The most surefire way to confirm a wormhole’s existence would be to directly prod a black hole and see if it’s hiding a bridge to elsewhere, but humanity may never have that opportunity. Even so, researchers could rule out some of the most obvious scenarios from Earth. If the monster black hole residing in the churning center of the Milky Way, for instance, is more door than dead end, astronomers could tease out the presence of something on the other side. Black hole researchers have tracked the orbits of stars, such as one called S2, circling this galactic drain for years. Should those stars be feeling the tug from distant doppelgängers beyond the black hole, they’d perform a very particular dance for anybody watching, according to a recent calculation.

“If astronomers just measure the orbit of S2 with higher precision so that we can narrow it down [and notice such a dance],” says Dejan Stojkovic, a theoretical physicist at the University at Buffalo who helped calculate the result, “that’s it. That’s huge.”

Wormholes represent one strange shape of space theoretically allowed under the auspices of Einstein’s theory of gravity, but only black holes have the oomph required to actually sculpt one. One way to check if a given black hole has managed to put a pleat in the fabric of space would be to pull an Interstellar and try to send a probe through, but we’d have to wait thousands of years for any spacecraft to reach the nearest candidates.

To make such a mission even more quixotic, most physicists agree that human-traversable, sci-fi-style bridges can’t exist. The only way to fight their natural tendency to collapse, according to Einstein’s equations, is to put in a type of repulsion that other laws of physics forbid on large scales—negative energy (physics students may remember that energy, unlike velocity or acceleration, always comes out positive). Stojkovic says he and his collaborators avoided such “hocus pocus” in their previous work, describing a wormhole that would work in our universe.

However, just because astronauts can’t travel through a large wormhole doesn’t mean that nothing can. Working within the framework of Einstein’s theory of gravity, in the previous work the group found a way to build a big, stable wormhole kept open by the force driving the expansion of the universe. The new work extends the old, calculating that while most particles and electric fields stopped short, the force of gravity can sail smoothly through. That means, theoretically, objects on our side could feel the tug of something especially massive on the other side. “We were kind of surprised,” Stojkovic says, “but what else would you expect? Gravity is the property of spacetime itself.”

The new research, recently published in the journal Physical Review D, goes on to ask whether astronomers could detect such subtle gravitational tugs on stars in the Milky Way.

The ideal target, Stojkovic and his colleagues propose, is Sagittarius (Sag) A—the black hole that allegedly sits at the heart of our galaxy. More specifically, they calculated the possible effects on S2, a star that orbits Sag A. If the black hole harbors a wormhole within it, similar stars would likely orbit on the other side, somewhere else in the universe, and S2 might feel the gravitational pull of a distant twin traveling through the cosmic connection between them.

Any resultant swerves S2 might make would be slight, but after more than 20 years of observation astronomers have clocked the star’s acceleration to four-decimal-place precision. With roughly 100 times more accuracy than that, Stojkovic estimates, astronomers would have the sensitivity to test his wormhole hypothesis—a benchmark he says current experiments should naturally reach in a couple more decades of data collection. If S2’s motion brings no surprises at that point, he says, then Sag A* must either be an everyday black hole, or a wormhole linking to a rather empty area of space.

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But while Stojkovic and his colleagues analyzed their large wormhole using Einstein’s equations, other theorists studying the (as yet theoretical) microscopic properties of space and gravity aren’t so sure these conclusions hold at the particle level. Daniel Jafferis, a physicist at Harvard University, says that since no one has proposed a way for large wormholes to form, any odd jigs by S2 would raise more questions than they would answer. “Someone would probably have had to have made the wormhole intentionally,” he says. And the only thing less likely than a real wormhole might be a real wormhole constructed by super advanced aliens.

Furthermore, he suggests that the realities of particle physics may clash with conclusions drawn purely from Einstein’s equations, and that without the “magic” of negative energy, non-traversable really means non-traversable, full stop. “Nothing can get through, including gravitons [the hypothetical gravity particle],” Jafferis says. “So it seems [the wormhole] cannot be seen or detected from the outside.”

Stojkovic, who says he was motivated to do the calculation purely from personal curiosity, fully acknowledges the astronomically long odds. Nevertheless, since astronomers are collecting the data anyway, he loses nothing by waiting with an open mind. “If one worm hole is found, then there is no reason to believe that there aren’t many others,” he says. “When we found the first candidate for a black hole, then suddenly we saw millions of them.”

Correction: This article has been updated to accurately delineate the calculations of the group’s two publications.

Charlie Woodis a journalist covering developments in the physical sciences both on and off the planet. In addition to Popular Science, his work has appeared in Quanta Magazine, Scientific American, The Christian Science Monitor, and other publications. Previously, he taught physics and English in Mozambique and Japan, and studied physics at Brown University. You can view his website here.

I believe this is a mistake by Rowling

The O.W.L. exams are taken in June:

. June had arrived, but to the fifth years this meant only one thing: Their O.W.L.s were upon them at last.
Harry Potter and the Order of the Phoenix - Chapter 31: O.W.L.s

When Harry goes to take his Astronomy exam, it is 11pm:

When they reached the top of the Astronomy Tower at eleven o’clock they found a perfect night for stargazing
Harry Potter and the Order of the Phoenix - Chapter 31: O.W.L.s

The entries they make on their charts are implied to be what they are currently observing:

Professors Marchbanks and Tofty strolled among them, watching as they entered the precise positions of the stars and planets they were observing.
Harry Potter and the Order of the Phoenix - Chapter 31: O.W.L.s

I believe this quote suggests Harry was previously looking at Orion, before making an adjustment to look at Venus:

As Harry completed the constellation Orion on his chart, however, the front doors of the castle opened directly below the parapet where he was standing, so that light spilled down the stone steps a little way across the lawn. Harry glanced down as he made a slight adjustment to the position of his telescope and saw five or six elongated shadows moving over the brightly lit grass before the doors swung shut and the lawn became a sea of darkness once more.
Harry put his eye back to his telescope and refocused it, now examining Venus.
Harry Potter and the Order of the Phoenix - Chapter 31: O.W.L.s

He 'completed the constellation Orion on his chart', makes 'a slight adjustment to the position of his telescope' before 'now examining Venus'. I think this string of events heavily implies he was actually looking at Orion.

Using a planisphere, I can confirm that the constellation Orion is not visible in the June sky in the Northern Hemisphere at 11pm. In fact, it doesn't appear in the sky at all in the hours that it would be dark in Scotland in June, from around 10pm to 5am. From quotes, his exam seems to take around 2 hours, but this is beside the point - his exam could have lasted the entire night and he still wouldn't have been able to see it. There are no quotes saying magic is used for observations during the exam, so I think it's safe to rule that out. It is simply a mistake on the author's part.

I can't say this with 100% confidence, but I've a suspicion that he couldn't see Venus then either given he's observing it after 11pm, and a June 1996 astronomy almanac makes no mention of it.

4. Fine-tune how much data is used on a 5G connection

If you're happy with 5G performance, here's a network-related setting you should check out. Go to Settings > Cellular > Cellular Data Options > Data Mode where you'll find three different options: Allow more data on 5G, Standard and Low Data Mode.

Even though there are brief descriptions below the three different settings, they don't paint a complete picture for the first option. According to an Apple support document, allowing more data on 5G will give you high-quality video and FaceTime calls, and it also means that your phone can download software updates, stream high-definition Apple TV and Apple Music content, and allows third-party developers to also improve their respective apps.

The default setting on this page will depend on your carrier and your data plan, so it's a good idea to check your iPhone 12 and make sure it's set to your preference.

Astronomy 1a: Introduction

Ever wondered how the Earth developed and exists in the vastness of space? How do the scientific laws of motion and gravity play a role in its existence? Discover answers to these questions and explore the origin of the universe, the Milky Way, and other galaxies and stars, including the concepts of modern astronomy and the methods used by astronomers to learn more about the universe.

Units at a Glance

We will take a journey through space and time from the beginning to the end of the universe. Can you think of anything larger or more expansive than the universe? How was the universe created? How is the universe changing? What exactly is our universe made from? These are all questions that scientists have been trying to answer since the idea of a universe was formed in the minds of our earliest cosmologists. Astronomers and other scientists have since accumulated a great deal of knowledge about what has happened—and what is currently happening—since the inception of the universe.

What will you learn in this unit?

  • Describe the study of the cosmos.
  • Discuss the theory of the origin of the universe.
  • Analyze the evidence that supports the Big Bang theory.
  • Examine the composition of matter and how it is distributed within the universe.
  • Describe the theories of evolution and fate of the universe.

Unit 2: Techniques and Tools of the Trade: Studying the Universe

At one point or another, you have probably looked up at the sky at night and thought about how big the universe really is. At times, space can appear like an empty vacuum, and other times, we realize that the universe is filled with such a wide variety of materials, substances, and celestial bodies that it seems more than overwhelming. Scientists have been studying the universe for thousands of years in various ways using many different processes and tools. Today, astronomers follow the scientific method and utilize several types of astronomical tools including binoculars, telescopes, and even software that can replicate the night sky! How will you apply these techniques and tools to study our universe?

What will you learn in this unit?

  • Distinguish science from pseudoscience
  • Discuss the impact of scientific research on our society
  • Follow the steps of the scientific method to conduct an astronomy investigation
  • Choose proper tools and follow safety procedures in the field

Unit 3: The Earth, Moon, and Sun Systems

Day turns into night, and summer turns into fall. Why do we experience these predictable changes on Earth? In this unit, we will explore the systems and interactions between the sun, Earth, and moon. You will learn how the Earth’s motion in space causes us to experience days, nights, and seasons in a cyclic pattern. We will discuss the properties of gravity and how gravity affects the relationships between orbiting bodies in space. You will discover how solar and lunar eclipses occur and examine the characteristics, origin, and phases of the moon.

What will you learn in this unit?

  • Learn about the movements of celestial bodies in the sky.
  • Describe how the motion of the Earth causes seasons and night-day cycles.
  • Identify the characteristics and phases of the moon.
  • Explore how the moon’s gravitational pull manipulates tides on Earth.
  • Distinguish between a lunar eclipse and a solar eclipse.

What are stars? Where did they come from? Will stars evolve with time? In this unit you will discover the secrets behind the stars in our night sky. We will solve the mystery behind why and how stars shine. We will explore the characteristics and composition of stars. You will learn how astronomers classify types of stars using the H-R diagram and how stars are identified within the celestial sphere. Finally, we will examine the evolution, or life cycle, of a star from conception to death.

What will you learn in this unit?

  • Describe the composition and characteristics of stars.
  • Learn how astronomers identify and describe constellations such as Ursa Major, Ursa Minor, Orion, and Cassiopeia.
  • Analyze and characterize stars by their physical and chemical properties.
  • Explain the use of diagrams and models in obtaining physical data on stars.
  • Examine the evolution of stars.

Galaxies are beautiful, majestic, and mysterious places within our universe. Our home in the Milky Way galaxy is a galactic suburb, far from other galaxies. Our Sun is just one of approximately 500 billion stars in our galaxy, meaning that there could possibly be up to 500 billion solar systems, maybe like our own, in the universe. In addition, the Milky Way galaxy is only one of the 50 billion to one trillion galaxies that are thought to exist in our observable universe. Compared with the whole universe, our home, Earth, is like a speck of sand in the largest desert imaginable.

In this unit, we will examine and describe the evolution, organization, distribution, and differences among types of galaxies. You will be able to characterize the movement of galaxies within the universe and describe the properties of our own galaxy, the Milky Way. Finally, we will discover the incredibly mysterious and dark forces that shift and shape galaxies.

What will you learn in this unit?

  • Differentiate and describe the types of galaxies within the universe.
  • Characterize the Milky Way.
  • Identify how galaxies are organized and distributed within the universe.
  • Describe the evolution of galaxies.
  • Examine the forces that shape galaxies of stars.

You have just traveled through the universe, exploring the different galaxies that make up outer space. Now, it’s time to return to our own galaxy: the Milky Way. The Milky Way galaxy is what houses the solar system within which our planet Earth resides. Just how old is the Milky Way? And what kind of tools do scientists use to understand our galaxy? It’s time to drive a little deeper into our home galaxy of the Milky Way.

What will you learn in this unit?

  • Find ways to determine the age of the Milky Way
  • Discover the oldest planet located in the Milky Way
  • Decipher why there are more younger stars than older stars in the galaxy
  • Understand Gaia Mapping and how it is used today

Possibly no celestial object has captured the attention and imagination of scientists and lay people the way black holes have. Even before they were officially “discovered,” people noticed areas of darkness in the night sky. Now that we know more about black holes, it seems the questions just keep coming. Their power and force are only beginning to be understood by scientists.

What will you learn in this unit?

  • Define black holes and understand why they are important
  • Trace the history of black holes
  • Answer questions about how we detect black holes, how they form, and how big and strong they are
  • Discuss what happens at the event horizon and singularity of a black hole
  • Investigate time travel options that black holes might offer

Unit 8: Becoming a Space Professional

Now that we’ve discussed what the universe is, how it is studied, and where we fit in, you may be eager to explore ways that you can continue learning even more about space! In this unit we’ll explore careers in astronomy from astronauts who literally travel through the stars to the crew that supports them on the ground—and much more. Whether you have more technical and mathematical skills, love writing and communication, or want to “stay in school” forever, there is likely a career for you in the aerospace industry.


  1. Start or join a meeting with the web client.
  2. Click View in the top-right corner, and then select Speaker or Gallery .

    Note: Gallery view on the web client is currently limited to 9 videos per page. This is due to technical limitations of the web client and maintaining consistency across all supported web browsers.

Star Clusters and the Shape of the Milky Way

In the lesson on star clusters, you learned that these objects are excellent tracers of the size and shape of our galaxy. What this means is that since these groups of stars are part of our galaxy, their distribution in space helps define the boundaries of our galaxy. Star clusters are big and bright, so they stand out above the background, making them easy to spot even at large distances. An analogy in this case may be to think of them like the tallest skyscrapers in a city. From a distance, you can see these tall buildings very easily, allowing you to determine roughly your distance from the city. You can also estimate how big the city is by how many skyscrapers it contains and how spread out they are.

In 1917, Harlow Shapley used the globular clusters in the Milky Way to gain a better understanding of the Milky Way Galaxy. He measured their positions and distances and plotted their locations on a two-dimensional chart. I have reproduced his work in the two-dimensional plot below, but using modern data for the distances and locations of all known globular clusters.

In this figure, the X and Y axes are in units of distances in kiloparsecs (1 kiloparsec = 1,000 parsecs). The hatched region represents the plane of the Milky Way (that is, roughly the part of the sky where the Milky Way is visible to the eye), and the X located at (0,0) marks the location of the Sun in the plane of the Milky Way. The Galaxy fills a 3D region in space, so this 2D plot only shows a slice from the top to the bottom through the plane of the Milky Way.

Shapley's data was not as extensive as in the plot shown above, nor was it as accurate. Similar to Herschel and Kapteyn, dust extinction and reddening affected his distance measurements to the clusters, and thus his conclusions as well. Because dust makes stars appear fainter than they truly are, if you do not account for the amount of extinction, you will overestimate the distances to these objects. This is just what Shapley did. However, the data he did have allowed him to make two very important discoveries:

  1. The globular clusters trace out a roughly spherical region in space. (Remember, a slice through a sphere is a circle.)
  2. The Sun is significantly offset from the center of this distribution.

There appears to be a significant discrepancy between the work of Shapley using the globular clusters and much of our early discussion about the appearance of the Milky Way in the sky. Why are the globular clusters tracing out a round, spherical distribution on the sky if the Milky Way itself appears to be a flattened plane? The answer is that the Milky Way is actually a little of both!

If we use a different type of object to trace the structure of the Milky Way, we find a different size and shape. If we use open clusters as tracers, they do match well the shape of the visible band of the Milky Way. Open clusters are predominantly young objects, so if we select other objects that also trace out the regions in the Milky Way that contain newly forming or newly formed stars (e.g., giant molecular clouds, O and B stars, emission nebulae), they also show that the Milky Way is a flattened, disk-shaped object. So the globular clusters (which contain very old stars) reveal a spherical component of the Milky Way, while the open clusters and other young stars and star-forming regions reveal a disk-shaped component of the Milky Way.

Again, we can use modern data and plot the locations of both open clusters and globular clusters to compare and contrast their locations. Below are two different views of this data. The first is a top-down view and the second is an edge-on view. In both images, the green dots represent open clusters, and the yellow dots represent the globular clusters. The scale is approximately the same as the plot above.

Viewing the Milky Way from the top down in the plot below, you can see that the open clusters and the globular clusters are not observed in the same location. This is misleading because it is a selection effect. What this means is that there are more open clusters in the Milky Way than the ones shown in green, we just are not able to observe the more distant ones given the current state of telescope and detector technology. In this image, the Sun is located in the center of the dense group of open clusters, and like with Herschel's map of the Milky Way, the only reason the Sun is in the center is because that is the location from where we are observing and we are prevented from observing the objects far beyond the edge of that group. In the case of the globular clusters, you can see the Sun is offset from their center (just as you can in the plot above), and in this case, we are able to observe the majority of these objects in the Milky Way, so that offset is real.

If you take the top view image above and picture rotating it by 90 degrees, you will get the edge-on view of the Milky Way below:

Now when we look at the Milky Way edge-on, we can see how the globular clusters still describe a circular region, but the open clusters do not. This shows you how you can use the distribution of the open clusters to measure the thickness of the Milky Way, but the globular clusters to measure its radius.

Big Data is Transforming How Astronomers Make Discoveries

Earlier this year, astronomers stumbled upon a fascinating finding: Thousands of black holes likely exist near the center of our galaxy.

The X-ray images that enabled this discovery weren’t from some state-of-the-art new telescope. Nor were they even recently taken—some of the data was collected nearly 20 years ago.

No, the researchers discovered the black holes by digging through old, long-archived data.

Discoveries like this will only become more common, as the era of “big data” changes how science is done. Astronomers are gathering an exponentially greater amount of data every day – so much that it will take years to uncover all the hidden signals buried in the archives.

Sixty years ago, the typical astronomer worked largely alone or in a small team. They likely had access to a respectably large ground-based optical telescope at their home institution.

Their observations were largely confined to optical wavelengths—more or less what the eye can see. That meant they missed signals from a host of astrophysical sources, which can emit non-visible radiation from very low-frequency radio all the way up to high-energy gamma rays. For the most part, if you wanted to do astronomy, you had to be an academic or eccentric rich person with access to a good telescope.

Old data was stored in the form of photographic plates or published catalogs. But accessing archives from other observatories could be difficult—and it was virtually impossible for amateur astronomers.

Today, there are observatories that cover the entire electromagnetic spectrum. No longer operated by single institutions, these state-of-the-art observatories are usually launched by space agencies and are often joint efforts involving many countries.

With the coming of the digital age, almost all data are publicly available shortly after they are obtained. This makes astronomy very democratic—anyone who wants to can reanalyze almost any data set that makes the news. (You too can look at the Chandra data that led to the discovery of thousands of black holes!)

The Hubble Space Telescope (NASA)

These observatories generate a staggering amount of data. For example, the Hubble Space Telescope, operating since 1990, has made over 1.3 million observations and transmits around 20 GB of raw data every week, which is impressive for a telescope first designed in the 1970s. The Atacama Large Millimeter Array in Chile now anticipates adding 2 TB of data to its archives every day.

The archives of astronomical data are already impressively large. But things are about to explode.

Each generation of observatories are usually at least 10 times more sensitive than the previous, either because of improved technology or because the mission is simply larger. Depending on how long a new mission runs, it can detect hundreds of times more astronomical sources than previous missions at that wavelength.

For example, compare the early EGRET gamma ray observatory, which flew in the 1990s, to NASA’s flagship mission Fermi, which turns 10 this year. EGRET detected only about 190 gamma ray sources in the sky. Fermi has seen over 5,000.

The Large Synoptic Survey Telescope, an optical telescope currently under construction in Chile, will image the entire sky every few nights. It will be so sensitive that it will generate 10 million alerts per night on new or transient sources, leading to a catalog of over 15 petabytes after 10 years.

The Square Kilometre Array, when completed in 2020, will be the most sensitive telescope in the world, capable of detecting airport radar stations of alien civilizations up to 50 light-years away. In just one year of activity, it will generate more data than the entire internet.

These ambitious projects will test scientists’ ability to handle data. Images will need to be automatically processed—meaning that the data will need to be reduced down to a manageable size or transformed into a finished product. The new observatories are pushing the envelope of computational power, requiring facilities capable of processing hundreds of terabytes per day.

The resulting archives—all publicly searchable—will contain 1 million times more information that what can be stored on your typical 1 TB backup disk.

The data deluge will make astronomy become a more collaborative and open science than ever before. Thanks to internet archives, robust learning communities and new outreach initiatives, citizens can now participate in science. For example, with the computer program [email protected], anyone can use their computer’s idle time to help search for gravitational waves from colliding black holes.

It’s an exciting time for scientists, too. Astronomers like myself often study physical phenomena on timescales so wildly beyond the typical human lifetime that watching them in real-time just isn’t going to happen. Events like a typical galaxy merger—which is exactly what it sounds like—can take hundreds of millions of years. All we can capture is a snapshot, like a single still frame from a video of a car accident.

However, there are some phenomena that occur on shorter timescales, taking just a few decades, years or even seconds. That’s how scientists discovered those thousands of black holes in the new study. It’s also how they recently realized that the X-ray emission from the center of a nearby dwarf galaxy has been fading since first detected in the 1990s. These new discoveries suggest that more will be found in archival data spanning decades.

A black-hole-powered jet of hot gas in the giant elliptical galaxy M87. (NASA, ESA, E. Meyer, W. Sparks, J. Biretta, J. Anderson, S.T. Sohn, and R. van der Marel (STScI), C. Norman (Johns Hopkins University), and M. Nakamura (Academia Sinica))

In my own work, I use Hubble archives to make movies of “jets,” high-speed plasma ejected in beams from black holes. I used over 400 raw images spanning 13 years to make a movie of the jet in nearby galaxy M87. That movie showed, for the first time, the twisting motions of the plasma, suggesting that the jet has a helical structure.

This kind of work was only possible because other observers, for other purposes, just happened to capture images of the source I was interested in, back when I was in kindergarten. As astronomical images become larger, higher resolution and ever more sensitive, this kind of research will become the norm.

Eileen Meyer, Assistant Professor of Physics, University of Maryland, Baltimore County