How Do You Find Something You Cannot See? Two Articles on Black Holes are ideal for Middle School Science reading.
News Spotlight · Space Science · Middle School and Up
How can anything that gives off no light be found, weighed, and even photographed? Two articles by the same astronomer, one written for curious kids and one for curious adults, show how scientists answer that question and what they still do not know.
As you read or listen, ask yourself
- If a black hole gives off no light, what evidence tells us it is there?
- Why would astronomers care about a black hole’s neighborhood and not only the black hole itself?
- Which statements in each article are well established, and which are still open questions?
Teacher notes
Suggested use. Read Part 1 aloud or together on day one, then run the Word Hunt. Assign Part 2 on day two as the stretch text, then use the timeline and discussion questions. Both parts work as a two-text comparison: same author, different audience, different level of certainty.
Where the articles simplify. These are republished unchanged, so these points are worth a quick class conversation:
- Part 1 says the black hole at the center of the Milky Way is “safely millions of miles away.” It is about 26,000 light-years away, which is on the order of 150 quadrillion miles. A good prompt for a scale conversation.
- Part 1 credits Einstein and Schwarzschild with pitching the idea of a stellar black hole formed by a collapsing star. Schwarzschild found the mathematical solution in 1916, and Einstein himself doubted such objects were real. The collapse of massive stars was worked out later, in 1939, by Oppenheimer and Snyder.
- “Only a few times bigger than our Sun” understates it. Stellar mass black holes typically range from a few to several dozen times the Sun’s mass.
- The Event Horizon Telescope image of M87 was released in 2019. The observations were made in 2017.
Images and licensing. Both articles are republished unchanged under their Creative Commons license, with credits and links to the originals. The closing “send us your question” invitation in Part 1 was left out of this page, because our pages do not invite students to share personal information. Two images in Part 2 carry a non-commercial license, and the lead image in Part 1 is credited through AP.
Word Hunt
Preview the vocabulary before you read. Tap a word to see its meaning and mark it found. Each word shows which part it appears in.
0 words found
Part 1: Curious Kids
Start here. This is a question from two students in Abu Dhabi, answered by an astronomer who studies the first supermassive black holes.
Read aloud note: This article works well read aloud, in class or at home. Pause after each heading and ask what the class expects to come next.
Why are some black holes bigger than others? An astronomer explains how these celestial vacuums grow
Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.
Why are there small and big black holes? Also, why are some black holes invisible and others have white outlines? – Sedra and Humaid, Abu Dhabi, United Arab Emirates
Black holes are dense astronomical objects with gravity so strong that nothing, not even light, can escape. Anything that crosses the boundary of a black hole’s gravitational influence, called the event horizon, will fall into the black hole. Inside this deep, dense pit, it is never to be seen again.
Black holes litter the universe. Some smaller black holes are sprinkled randomly throughout galaxies like our Milky Way. Other gigantic ones, called “supermassive” black holes, lie at the centers of galaxies. Those can weigh anywhere between a million to a billion times the mass of our Sun. So you might be wondering: How can astronomers possibly see something so dark and so big?
I am an astronomer who studies the very first supermassive black holes that formed in our universe. I want to understand how black holes form and what kinds of astrophysical neighborhoods they grow up in.
Types of black holes
Let’s talk about how black holes begin their lives. Two famous scientists, Albert Einstein and Karl Schwarzchild, first pitched the idea of a black hole. They thought that when a large star dies, its core might shrink and shrink until it collapses under its own weight. This is what we astronomers call a “stellar mass black hole,” which is just another way of saying it’s comparatively very small.
Stellar mass black holes are only a few times bigger than our Sun. Supermassive black holes are more of a mystery, though. They are many millions of times heavier than our Sun, and they are packed into a small area that’s about the size of our solar system. Some scientists think supermassive black holes might form by many stars colliding and collapsing at once, while others think they might have already started growing several billion years ago.
Growing black holes
What do black holes look like? Most of the time, they are not actively growing, so they are invisible. But we can tell they’re there because stars can still orbit around them, just like Earth around the Sun.
When something is orbiting an invisible object at high speeds, scientists know there must be a massive black hole in the middle. This is the case for the closest supermassive black hole to us, which lies at the center of the Milky Way – safely millions of miles away from you.
Meanwhile, when a hungry black hole is eating up gas in a galaxy, it heats that gas up until you can see a glowing ring of X-rays, optical light and infrared light around the black hole. Once it exhausts all of the fuel near the event horizon, the light dies down once again and it becomes invisible.
Outlines around black holes
One of the most famous “white outlines” is the image of a black hole from the movie “Interstellar.” In that movie, they were trying to show the white-hot, glowing ring of gases that are falling into the actively growing black hole.
In real life, we don’t get such a close-up view. The best image of the ring around a real black hole comes from the Event Horizon Telescope, showing scientists the supermassive black hole at the center of a galaxy called M87. It might look blurry to you, but this doughnut is actually the sharpest image ever taken of something so far away.
There are lots of types of black holes out there in the universe. Some are small and invisible, and some grow to gigantic proportions by eating up stuff inside a galaxy and shining bright. But don’t worry, black holes can’t just keep sucking in everything in the universe – eventually there is nothing close enough to the black hole to fall in, and it will become invisible again. So you are safe to keep asking questions about black holes.
Jaclyn Champagne, JASPER Postdoctoral Researcher, University of Arizona
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Part 2: Growing Up in a Neighborhood
Now the stretch text. The same astronomer explains an open puzzle: some giant black holes grew far faster than theory predicts. Did they have help from their surroundings?
Powerful black holes might grow up in bustling galactic neighborhoods
As people, we are all shaped by the neighborhoods we grew up in, whether it was a bustling urban center or the quiet countryside. Objects in distant outer space are no different.
As an astronomer at the University of Arizona, I like to think of myself as a cosmic historian, tracking how supermassive black holes grew up.
Like you, every supermassive black hole lives in a home – its host galaxy – and a neighborhood – its local group of other galaxies. A supermassive black hole grows by consuming gas already inside its host galaxy, sometimes reaching a billion times heavier than our Sun.
Theoretical physics predicts that black holes should take billions of years to grow into quasars, which are extra bright and powerful objects powered by black holes. Yet astronomers know that many quasars have formed in only a few hundred million years.
I’m fascinated by this peculiar problem of faster-than-expected black hole growth and am working to solve it by zooming out and examining the space around these black holes. Maybe the most massive quasars are city slickers, forming in hubs of tens or hundreds of other galaxies. Or maybe quasars can grow to huge proportions even in the most desolate regions of the universe.
Galaxy protoclusters
The largest object that can form in the universe is a galaxy cluster, containing hundreds of galaxies pulled by gravity to a common center. Before these grouped galaxies collapse into a single object, astronomers call them protoclusters. In these dense galaxy neighborhoods, astronomers see colliding galaxies, growing black holes and great swarms of gas that will eventually become the next generation of stars.
These protocluster structures grow much faster than we thought, too, so we have a second cosmic problem to solve – how do quasars and protoclusters evolve so quickly? Are they connected?
To look at protoclusters, astronomers ideally obtain images, which show the galaxy’s shape, size and color, and a spectrum, which shows the galaxy’s distance from Earth through specific wavelengths of light, for each galaxy in the protocluster.
With telescopes like the James Webb Space Telescope, astronomers can see galaxies and black holes as they were billions of years ago, since the light emitted from distant objects must travel billions of light-years to reach its detectors. We can then look at protoclusters’ and quasars’ baby pictures to see how they evolved at early times.
It is only after looking at spectra that astronomers determine whether the galaxies and quasars are actually close together in three-dimensional space. But getting spectra for every galaxy one at a time can take many more hours than any astronomer has, and images can show galaxies that look closer together than they actually are.
So, for a long time, it was only a prediction that massive quasars might be evolving at the centers of vast galactic cities.
An unprecedented view of quasar environments
Now, Webb has completely revolutionized the search for galaxy neighborhoods because of an instrument called a wide-field slitless spectrograph.
This instrument takes spectra of every galaxy in its field of view simultaneously so astronomers can map out an entire cosmic city at once. It encodes the critical information about galaxies’ 3D locations by capturing the light emitted from gas at specific wavelengths – and in only a few hours of observing time.
The first Webb projects are hoping to look at quasar environments focused on a period about 800 million years after the Big Bang. This time period is a sweet spot in which astronomers can view these monster quasars and their neighbors using the light emitted by hydrogen and oxygen. The wavelengths of these light features show where the objects emitting them are along our line of sight, allowing astronomers to complete the census of where galaxies live relative to bright quasars.
One such ongoing project is led by the ASPIRE team at the University of Arizona’s Steward Observatory. In an early paper, they found a protocluster around an extremely bright quasar and confirmed it with 12 galaxies’ spectra.
Another study detected over a hundred galaxies, looking toward the single most luminous quasar known in the early universe. Twenty-four of those galaxies were close to the quasar or in its neighborhood.
In ongoing work, my team is learning more details about mini galaxy cities like these. We want to figure out if individual galaxies show high rates of new star formation, if they contain large masses of old stars or if they are merging with one another. All these metrics would indicate that these galaxies are still actively evolving but had already formed millions of years before we observed them.
Once my team has a list of the properties of the galaxies in an area, we’ll compare these properties with a control sample of random galaxies in the universe, far away from any quasar. If these metrics are different enough from the control, we’ll have good evidence that quasars do grow up in special neighborhoods – ones developing much faster than the more sparse regions of the universe.
While astronomers still need more than a handful of quasars to prove this hypothesis on a larger scale, Webb has already opened a window into a bright future of discovery in glorious, high-resolution detail.
Jaclyn Champagne, JASPER Postdoctoral Researcher, University of Arizona
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Timeline: How We Came to Know Black Holes
Open each event. Every one ends with a question to think about.
1915 to 1916: A mathematical idea appears
Einstein publishes his theory of general relativity in 1915. In 1916 Karl Schwarzschild works out an exact solution to its equations that describes a region of space from which nothing can escape. Einstein himself doubted that such objects could exist in nature.
Think about it: Part 1 says these two scientists “pitched the idea.” What is the difference between a mathematical solution and a discovery?
1939: A dying star can collapse
J. Robert Oppenheimer and Hartland Snyder calculate that a massive star that has used up its fuel could collapse completely, which is the physical route to a stellar mass black hole.
Think about it: Part 1 describes a star’s core shrinking “until it collapses under its own weight.” Which part of that story needed this calculation?
2019: The first picture
The Event Horizon Telescope collaboration releases the first image of a black hole, the supermassive one at the center of the galaxy M87. The data were collected in 2017 and took two years to process.
Think about it: Part 1 calls the image blurry but also the sharpest ever taken of something so far away. How can both be true?
2020: A Nobel Prize for black hole science
The Nobel Prize in Physics is shared by Roger Penrose, for theory, and by Reinhard Genzel and Andrea Ghez, who tracked stars orbiting an invisible object at the center of the Milky Way. The video in Part 1 comes from Ghez’s work.
Think about it: Which clue did Ghez and Genzel rely on, a glow or a motion? Why does that matter for an invisible object?
2022: Our own black hole
The Event Horizon Telescope collaboration releases an image of Sagittarius A*, the supermassive black hole at the center of the Milky Way, which confirms what the orbiting stars had already shown.
Think about it: Why might a black hole in our own galaxy be harder to photograph than one in M87, even though it is much closer?
2022: Webb begins observing
The James Webb Space Telescope releases its first full-color images in July 2022. Its infrared instruments let astronomers study some of the earliest galaxies and quasars.
Think about it: Part 2 says Webb lets astronomers see objects “as they were billions of years ago.” What makes that possible?
2023: Mapping a quasar’s neighbors
Astronomers on the ASPIRE team use Webb’s slitless spectrograph to map galaxies around some of the earliest bright quasars, including a protocluster confirmed with 12 galaxy spectra, as described in Part 2.
Think about it: The author says astronomers still need “more than a handful of quasars.” What would more examples add?
Discussion Questions
Open a question when your group is ready to compare answers with the text.
1. How do astronomers find a black hole they cannot see?
Use Part 1 to name two kinds of evidence, one based on motion and one based on glowing gas. Which is available when a black hole is not feeding?
2. Why does the Interstellar image look different from the real one?
Compare the film’s glowing ring with the Event Horizon Telescope image of M87. What does the author’s newspaper-in-Paris comparison tell you about distance?
3. Do black holes “suck in” everything?
Part 1 says a black hole eventually runs out of material close enough to fall in. How does that change the common picture of a cosmic vacuum cleaner?
4. Why does Webb’s slitless spectrograph save so much time?
Part 2 contrasts one-galaxy-at-a-time spectra with taking every spectrum in the field at once. Explain the difference in your own words.
5. Why can an image alone mislead?
Part 2 says galaxies can look closer together than they really are. Sketch two galaxies that line up in a picture but are far apart in space.
6. What is the control sample for?
If the team found that galaxies near quasars look just like random galaxies far from any quasar, what would that mean for the hypothesis?
7. Compare the two articles.
Same author, different audience. Find one sentence in each that shows how she adjusts vocabulary, and one sentence in Part 2 that signals uncertainty (look for words like “maybe” or “might”).
8. Where does the neighborhood metaphor help, and where does it mislead?
Part 2 compares galaxy groups to cities and the countryside. List one way the comparison clarifies the science and one way it could confuse a reader.
Standards connections (verify against your state’s adopted standards): NGSS MS-ESS1-2 (gravity and the motions within galaxies and the solar system); HS-ESS1-2 (extension: evidence from light spectra); CCSS.ELA-LITERACY.RI.6.7 and RST.6-8.9 (integrating information from words, images, and video).
Curated by Dr. Katherine M. Schlatter, Chief Editor and CEO of This Month™
