For more than seven decades, fusion researchers have relied on a mathematical rule to determine whether plasma can remain sufficiently hot and dense, for a long enough period, to sustain fusion without additional external heating. Known as the Lawson criterion, this equation defines the conditions required for ignition. However, it does not explain the most efficient way to achieve them.

Now, PPPL physicists Luis Delgado-Aparicio, Masayuki Ono, and Jonathan Menard have developed a more comprehensive version of the criterion. By incorporating four additional factors that influence fusion performance, they have mapped a promising route to ignition that could require substantially less energy than alternative approaches.

Their findings were published in the journal Physical Review Letters.

The research focuses on plasma, often called the fourth state of matter. Plasma forms when a gas becomes so energetic that its particles carry electrical charges. In fusion reactors, this extremely hot material contains the atomic nuclei that researchers hope to combine, releasing energy through nuclear fusion.

The ultimate goal is to produce a burning plasma, in which fusion reactions generate enough heat to sustain the process without continued external heating. Reaching this state efficiently remains one of the most important challenges in fusion energy research.

Scientists Find a Shortcut to Fusion Ignition

Delgado-Aparicio compares the discovery to finding a more efficient route through a mountain range.

Imagine that the energy requirements for fusion ignition form a landscape dominated by a towering mountain. The destination lies on the other side, but there are several possible routes to reach it.

Many proposed fusion strategies effectively attempt to climb directly over the mountain. They begin by increasing the density of the plasma and then supplying the enormous amount of heat needed to reach ignition.

The new calculations suggest that a different sequence could be considerably more efficient. Instead of increasing density first, researchers could heat the plasma before making it denser. This alternative route avoids the highest energy requirements while still reaching the desired conditions.

“A lot of companies want to climb the mountain head-on and spend enormous energy to get there,” Delgado-Aparicio said. “Go around the peak instead. You reach the same place in a much smarter way, and you use far less energy.”

The Hidden Mountain Pass in Fusion Physics

The researchers identified a particularly important region of this mathematical landscape called the Cordey saddle.

In geographical terms, a saddle is a low passage between higher elevations. In fusion physics, the Cordey saddle represents a relatively accessible point along the boundary separating plasma that still needs outside heating from plasma capable of sustaining its own fusion reactions.

To evaluate different routes, the scientists used a measurement called Q, which compares the power produced by fusion reactions with the external heating power supplied to the plasma.

For example, a Q value of 5 means the fusion reactions produce five times as much power as the external heating system delivers.

In an idealized plasma containing only pure fusion fuel, the Cordey saddle occurs at approximately Q = 5. However, actual fusion systems are much more complicated. Contaminants within the plasma and extremely strong magnetic fields can shift the location of this passage, increasing the Q value needed to reach it.

Understanding how these conditions change is essential for determining whether a proposed fusion reactor can realistically achieve ignition.

A More Realistic Roadmap for Future Fusion Reactors

The findings could influence the design of two major types of magnetic fusion devices: tokamaks and stellarators.

Both approaches use powerful magnetic fields to confine plasma while it is heated to the extraordinary temperatures needed for fusion. Tokamaks, which have a characteristic doughnut shape, are among the leading technologies being investigated for future fusion power plants.

Research institutions and private companies are pursuing several different ways to generate electricity from fusion and eventually deliver that power to the electrical grid. Identifying an efficient route to ignition could help engineers design more practical systems while avoiding costly mistakes.

For PPPL, this work is part of a broader effort to address the scientific and technological challenges that must be overcome before commercial fusion power becomes possible, including the development of future fusion pilot plants.

One of the study’s major advances is that it evaluates several important physical processes together rather than examining them separately, as much previous research has done.

The researchers incorporated four additional factors that can influence whether a plasma reaches and maintains ignition:

1. Helium ash buildup: Fusion reactions produce helium as a byproduct. As this spent fuel accumulates, it can dilute the remaining fusion fuel and reduce performance.
2. Plasma contamination: Light and heavy impurities can enter the plasma from materials lining the inside of the reactor, interfering with fusion conditions.
3. Synchrotron radiation: Electrons traveling through magnetic fields emit radiation, carrying energy away from the plasma.
4. Heat loss: Energy continuously escapes from the plasma, and these losses can become greater as its temperature rises.

Accounting for these effects produces a more realistic picture of the conditions future reactors must achieve.

“When you leave these effects out, you say the design will work fine,” Ono said. “When you put them in, the picture changes, and it becomes quite important.”

According to Ono, the goal is to establish a more reliable, widely applicable model that scientists can use to evaluate fusion reactor designs before construction begins.

Treating the Lawson criterion as a single unchanging threshold overlooks physical processes that can substantially alter the requirements for ignition. Including those effects early in the design process could prevent expensive problems later.

“Fusion experiments cost a great deal of money, and you do not want to make mistakes you could have caught beforehand,” he said.

Tiny Amounts of Tungsten Could Make Fusion Much Harder

One particularly striking finding concerns tungsten, an extremely heat-resistant metal selected for the interior walls of more than a dozen next-generation fusion machines.

Tungsten is attractive because fusion reactors must withstand conditions involving plasma temperatures greater than those found at the center of the sun. However, the material also presents a significant challenge if even tiny quantities enter the plasma.

The team’s calculations show that tungsten contamination at a concentration of just one part in 10,000 can approximately double the pressure required to achieve fusion ignition.

That is a substantial increase caused by an exceptionally small amount of material.

The calculations were initially developed in two dimensions. When the researchers extended their analysis to three dimensions, they found that the pressure needed for ignition could exceed the limits at which the plasma remains stable.

This suggests that seemingly minor contamination could have major consequences for the feasibility of certain fusion reactor designs.

How Energy Losses Could Help Stabilize Fusion

Surprisingly, some of the processes that make ignition more difficult may also help fusion reactors operate more safely and consistently.

Scientists have long investigated the possibility of thermal “runaway” instability. This occurs when heat from fusion reactions increases the reaction rate, producing still more heat and potentially creating a self-reinforcing cycle.

The researchers found that energy losses within the plasma can naturally counteract this effect.

Although those losses increase the difficulty of reaching ignition, they also provide a balancing influence that could allow a burning plasma to maintain a stable operating state without constant adjustments.

This balance could be valuable for future reactor designs.

(Unlike a fission power plant, a fusion system holds only seconds of fuel at a time, so losing control means the plasma cools and the burn stops rather than anything worse.)

Liquid Lithium and Advanced Fuel Could Improve Fusion Efficiency

The study also identifies technologies that might help overcome some of the obstacles revealed by the calculations.

One possibility involves coating the inside of fusion reactors with liquid lithium, a technique PPPL researchers have investigated for years.

These coatings could help prevent tungsten from entering the plasma while reducing heat losses, making it easier to maintain the extreme conditions necessary for fusion.

Another promising approach uses spin-polarized fuel. This involves preparing the atomic nuclei so that their spins are aligned before fusion occurs, which can increase the fusion reaction rate.

Together, these strategies could improve the performance of future reactors and make the path to ignition more achievable.

What Comes Next for the New Fusion Strategy?

Despite the promising findings, the proposed heat-first approach has not yet been demonstrated experimentally.

The research is theoretical and relies on mathematical calculations rather than direct measurements. Existing fusion experiments have not reached the temperatures associated with the Cordey saddle, preventing scientists from testing the predicted route under those conditions.

The PPPL team plans to investigate the approach further through digital experiments, using computer-based simulations to explore whether heating plasma before increasing its density can deliver the expected advantages.

If future studies confirm the calculations, the findings could provide an important foundation for designing more efficient fusion systems and help researchers make better decisions about reactor construction.

“While more study is needed, we are excited by these results, and they suggest a clear path forward for future research in this area,” said Menard.

The work was supported by the DOE Office of Science, Office of Fusion Energy Sciences, under contract DE-AC02-09CH11466. Additional support came through a 2015 Fusion Energy Sciences Early Career Award, a 2018 Diagnostic Innovation and Development award, and the 2021 and 2025 Long-Pulse Tokamak Research Programs.

Most planets, including Earth, travel around their stars in relatively stable orbits. But these peaceful arrangements can sometimes end violently. If a planet ventures too close to its host star, the star’s gravity can drag it inward and eventually engulf it.

Scientists have observed many planetary systems with stable orbits, along with a small number of stars caught rapidly swallowing their planetary companions. Now, researchers have identified something that falls between those two extremes.

In a study published October 5 in Nature Astronomy, an international team led by scientists at MIT describes a star that is slowly feeding on a brown dwarf, an object more massive than a typical planet but not massive enough to sustain the hydrogen fusion that powers ordinary stars.

The system, designated ZTF J0440+2325, lies within our Milky Way galaxy approximately 300 light-years away. It is the first observed example of a low-mass star steadily drawing material from another low-mass object.

Even more remarkably, the transfer of material appears to be happening so slowly that the star could continue feeding on its companion for hundreds of thousands of years, potentially even billions.

“When we think of stars interacting with planets or brown dwarfs, the picture is always that the star eventually swallows the other thing,” says Kevin Burdge, assistant professor of physics at MIT. “This is what will happen to the Earth when the sun becomes a red giant. But here, we’ve found an alternative: Instead of swallowing the thing up, the star can gradually eat it, for billions of years.”

The MIT research team also includes Aaron Householder, Kaitlyn Shin, Saul Rappaport, Joheen Chakraborty, and Emma Chickles. Additional collaborators contributed from Caltech, the University of Hawaii, the Instituto de Astrofísica de Canarias and Universidad de La Laguna in Spain, and the Harvard-Smithsonian Center for Astrophysics.

A Mysterious Signal That Puzzled Astronomers for Years

The first clue to this unusual stellar relationship came from the Zwicky Transient Facility (ZTF), an astronomical survey that searches for objects whose brightness changes over time.

Operating with a telescope at Palomar Observatory in California, ZTF repeatedly photographs large sections of the night sky. Its sensitive camera can detect sudden or unusual changes in light that may indicate powerful cosmic events, including exploding stars called supernovae, gamma-ray bursts, and collisions between neutron stars.

Several years ago, while examining observations collected by ZTF, Burdge noticed a particularly strange pattern.

Astronomers study changes in an object’s brightness using a graph called a light curve. For some supernovae, these graphs form a rounded, bell-like shape as the exploding star grows brighter and then gradually fades.

The signal Burdge encountered looked completely different. Instead of a smooth rise and fall, the brightness repeatedly traced something resembling a triangle.

“I remember first looking at this and thinking, stars don’t make triangular waveforms like this,” he recalls.

At the time, Burdge and his colleagues were investigating another unusual signal that turned out to originate from a system known as a “black widow binary.”

These systems contain a neutron star, the extraordinarily dense remnant of a massive star, that spins rapidly while stripping material from a much smaller companion. They are named after black widow spiders because of the way the neutron star gradually destroys its partner.

Burdge initially considered whether the repeating triangular signal might come from another black widow system. However, the observations did not match what astronomers normally expect.

In a typical black widow binary, the strong contrast between the masses of the two objects produces a distinctive motion. The smaller companion moves rapidly around the much heavier neutron star, creating detectable changes in the light reaching Earth.

But the mysterious triangular signal showed no evidence of the same pronounced motion.

“We weren’t seeing that whipping back and forth here,” Burdge says. “It didn’t make any sense. We couldn’t explain what this was.”

That discrepancy led the researchers to consider another possibility. Perhaps the system did not contain one extremely massive object paired with a much lighter companion. Instead, both objects might have relatively small masses, allowing them to orbit each other with much less dramatic motion.

“If you have less mass in the system overall, things can gently orbit each other without whipping back and forth,” Burdge says. “That was the idea. But we never had any proof. And this weird triangle just sat for years.”

A Brown Dwarf That Orbits Its Star Every 87 Minutes

Eventually, Burdge and Aaron Householder, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, returned to the unexplained signal.

Using the original ZTF observations, they identified its source as an object roughly 300 light-years away in the Milky Way. They then directed several telescopes toward the system, ZTF J0440+2325, to investigate its properties in greater detail.

A key measurement involved determining how much the system’s objects move in response to their mutual gravitational attraction. Although the researchers detected some wobbling, it was far smaller than the motion typically associated with black widow binaries.

“That was the real clincher for this system,” Householder says. “When we measured that wobble, we found we were not seeing a black widow. This was a low-mass star that’s orbited by a brown dwarf. The wobble was too small in amplitude to be anything else.”

The observations revealed two unusually compact objects orbiting at an extremely close distance.

The central star has approximately 85 times the mass of Jupiter, while the brown dwarf weighs about 25 Jupiter masses. For comparison, the Sun is more than 1,000 times as massive as Jupiter.

The brown dwarf completes a full revolution around its host star in just 87 minutes. Its entire orbit is so small that it would fit within the diameter of the Sun.

That extraordinarily close arrangement suggested that gravity might be doing more than simply keeping the objects in orbit. The researchers suspected that the star could be actively pulling matter away from the brown dwarf.

Scientists Discover a New Form of Stellar Cannibalism

When one astronomical object draws gas or other material from another, astronomers call the process accretion.

Accretion is especially familiar in systems involving black holes and neutron stars. Although these objects can be extremely massive, their physical dimensions are relatively small. Material falling toward a black hole, for example, often forms a swirling disk around it before moving inward.

But ZTF J0440+2325 appears to work differently.

Rather than a black hole or neutron star collecting matter, an ordinary low-mass star is receiving material from a nearby brown dwarf. Because the star has a much larger physical surface than a compact object such as a black hole, the incoming material can strike it directly.

“The difference here is, the thing absorbing matter is not a tiny black hole but a star, which is relatively big in size,” Burdge explains. “So matter just pummels directly onto the surface, at very high speeds, like an asteroid hitting the Moon.”

To investigate whether this was actually happening, the researchers created computer simulations of the system.

They modeled particles originating from the brown dwarf and calculated how those particles would move under the gravitational influence of both objects. The simulations incorporated the measured properties of the star and its companion, along with the physical equations governing orbital motion.

The results supported the team’s suspicion. Material leaving the brown dwarf followed trajectories that carried it directly into the star.

“When we track those test particles, we see they indeed fall right onto the surface of the star,” Householder says. “This is the first time we’ve caught a low-mass star actively accreting from another low-mass object.”

A Cosmic Feast That Could Continue for Billions of Years

The team also estimated how quickly the brown dwarf is losing material.

According to their calculations, the star is consuming approximately one hundred-thousandth of Earth’s mass every year.

That amount may sound insignificant, but it represents an enormous quantity of material. The researchers compare it to approximately 40 million dump trucks’ worth of matter, or about 1.3 trillion one-pound burritos every second.

Despite those staggering figures, the brown dwarf is so massive that this material loss represents only a tiny portion of its total mass.

In astronomical terms, the feeding process is remarkably gradual.

Based on the properties of the system and the estimated rate of mass transfer, researchers believe the star could continue drawing matter from its companion for billions of years.

Instead of a sudden, catastrophic collision that destroys the smaller object, the star appears to be sustaining a relatively steady flow of material from its neighbor.

That realization also helped explain the strange triangular light pattern that first attracted Burdge’s attention.

As matter streams from the brown dwarf toward the star, it strikes the stellar surface at high speeds, generating an intensely heated region.

This bright hotspot acts like a persistent fireball on the star.

As the brown dwarf moves through its 87-minute orbit, the hotspot repeatedly rotates into and out of the observer’s view. Those changes in visibility produce the distinctive pattern of brightening and dimming that the astronomers had struggled to understand.

“It’s like you’ve got this continuous fireball onto one of the objects, and as one orbits the other, that hotspot comes in and out of view, and the peak of the triangle signal is when you’re looking right at the fireball,” Burdge explains.

A Strange Star System Opens a New Window Into Planetary Evolution

With the triangular signal finally explained, the researchers are now interested in identifying other stellar systems that behave in similar ways.

Until now, observations of close interactions between stars and their planetary companions have largely emphasized either stable orbits or relatively rapid engulfment events.

ZTF J0440+2325 demonstrates that another possibility exists. Under the right conditions, a small star can steadily extract matter from a nearby brown dwarf without immediately destroying it.

Finding additional examples could help astronomers determine how common this process is, how long it can persist, and what ultimately happens to objects trapped in such extraordinarily close relationships.

The discovery may also provide new insights into the evolution of planets and brown dwarfs that orbit near their host stars.

“It’s inspiring a lot of new searches on our part,” Householder says. “I think we’re going to learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars.”

This research received partial support from the National Science Foundation.

Researchers at the University of Helsinki investigated this question and uncovered a striking finding: people with the same depression diagnosis can have dramatically different patterns of brain activity. In some cases, communication between brain regions was unusually strong, while in others, it was weaker than normal.

Depression is a major global health concern. According to the World Health Organization (WHO, 2025), approximately 332 million (5.2%) adults worldwide are affected. In Finland, depression is the leading individual cause of both extended sick leave and disability pensions.

Brain Study Reveals Five Distinct Depression Profiles

To investigate the biological differences associated with depression, researchers examined brain activity in 263 people diagnosed with major depressive disorder and 75 healthy participants who served as a comparison group.

The analysis revealed five distinct groups based on differences in functional connectivity, a measure of how closely activity in different brain regions is coordinated. When two regions show synchronized activity, it suggests they are working together as part of a larger brain network. However, stronger connectivity does not necessarily mean the brain is functioning better.

“What was particularly interesting was the contrasting patterns of brain activity found under the umbrella of the same depression diagnoses. In some individuals, the functional connectivity between brain regions was stronger than usual, while in others it was weaker,” says Director Satu Palva of the Neuroscience Center, University of Helsinki.

These differences were also associated with particular symptoms. Participants in one group showed especially strong connections between brain regions, along with more pronounced substance abuse problems. Another group displayed weaker connections and a greater prominence of post-traumatic stress disorder (PTSD) symptoms.

The contrasting results could help explain why earlier studies examining the brains of people with depression have sometimes reached conflicting conclusions. If researchers study different combinations of patients, they may observe very different patterns, even though everyone involved has received the same diagnosis.

The findings add to growing evidence that depression may not have a single, consistent biological signature. Instead, the condition could involve several distinct patterns of brain function.

The Five Brain Patterns Linked to Depression

The researchers classified participants with depression into five groups based on how strongly different brain areas communicated and the frequencies at which their activity was coordinated.

Each group had a characteristic pattern of brain connectivity and associated symptoms:

Group 1: Strong connectivity and severe symptoms

Participants generally showed relatively strong connections between brain regions. This profile was associated with more severe symptoms across several areas, including depression, anxiety, rumination, and reduced ability to function in daily life. Rumination refers to repeatedly dwelling on distressing thoughts, worries, or negative experiences.

Group 2: Weaker connectivity and milder depression

This group was characterized by relatively weak communication between brain regions. Compared with the other groups, participants generally experienced milder symptoms.

Group 3: Widespread weak connectivity and trauma symptoms

People in this group showed reduced connectivity across substantial portions of the brain. Symptoms associated with post-traumatic stress disorder (PTSD) were particularly prominent.

Group 4: Mixed connectivity and multiple challenges

This profile combined unusually strong connections in some brain regions with weaker connections in others. Participants tended to experience more severe depression, greater substance abuse problems, and lower overall well-being.

Group 5: The strongest connectivity and substance abuse symptoms

Participants in the fifth group had the strongest connections between brain regions of any group identified in the study. Substance abuse was a particularly prominent issue, while trauma related symptoms were less pronounced than in the other groups.

Importantly, all five groups also showed differences from the healthy participants. These distinctions extended beyond whether brain connections were strong or weak. The groups also differed in which brain regions were involved and the frequencies at which their activity was synchronized.

Together, the patterns suggest that people diagnosed with depression may experience substantially different changes in how their brain networks operate.

Measuring Brain Activity With Millisecond Precision

To capture these differences, the scientists used magnetoencephalography (MEG), a brain imaging technique that detects extremely faint magnetic fields produced by electrical activity in the brain.

Unlike methods that track slower changes associated with brain activity, MEG can follow electrical signals as they unfold over extremely short periods. This makes it particularly useful for investigating the timing and coordination of communication between brain regions.

Using MEG, the researchers recorded brain activity in all 263 participants with depression and the 75 healthy control subjects.

“MEG enabled us to monitor electrical brain activity with millisecond precision, helping us get closer to what actually happens in the brain at any given moment. Previously, depression phenotypes have been studied using methods with slower responses,” Palva notes.

This high level of temporal precision allowed the team to examine rapid changes in brain activity that might be difficult to distinguish using slower measurement techniques.

By studying these fast signals, the researchers were able to identify differences not only in the strength of connectivity but also in the frequencies at which brain regions appeared to work together.

Could Brain Activity Lead to More Personalized Depression Treatments?

The discovery raises an important possibility for the future of mental health care: treatments might eventually be selected partly according to a person’s individual pattern of brain activity.

Today, finding an effective depression treatment can involve trying different medications or therapies and evaluating how well each works. Because patients can respond differently to the same treatment, identifying the most suitable approach is not always straightforward.

Brain activity measurements could eventually provide additional information to help guide these decisions. However, the researchers emphasize that their findings are not yet ready for clinical use.

“We’re not yet at the point where brain measurements can be used to choose the right treatment for patients, but the study does show one possible route,” Palva says.

A major long-term goal is to better understand how particular symptoms relate to the way brain networks function. Connecting these biological patterns with patients’ experiences could help researchers develop more precise ways to identify and characterize depression.

Eventually, such knowledge might allow doctors to determine which therapies are most likely to help individual patients, potentially reducing the amount of trial and error involved in treatment.

For now, the five brain profiles offer a different way to understand depression. Rather than assuming that one diagnosis corresponds to one underlying pattern of brain activity, the findings suggest that depression can involve markedly different biological changes, sometimes even in opposite directions.

The research was carried out in collaboration with Aalto University and Helsinki and Uusimaa Hospital District (HUS).