"We think that the loss of those hormones at menopause renders those neurons more vulnerable to age-related cellular decline and pathologies like Alzheimer's," said Karyn Frick, a neuroscientist and professor of psychological and brain sciences at the University of Wisconsin-Milwaukee.

So, if falling hormone levels are a risk factor for dementia, could replacing those hormones help stave off the disease?

While the idea sounds logical on its face, studies that have looked at hormone replacement therapy (HRT) for dementia prevention have had mixed results. There are early hints that restoring estrogen during early menopause may protect brain health, but ultimately, more gold-standard clinical trials are needed to know for sure, experts told Live Science.

Support for estrogen protecting the brain

The idea that estrogen could help ward off dementia is plausible. Women are more likely to develop Alzheimer's disease than men are, and scientists think the loss of estrogen during menopause may be partly to blame, said Jennifer Bruno, an instructor of psychiatry and behavioral sciences at Stanford University.

Lab and animal studies buttress the idea that estrogen could protect brain health.

In the brain, estrogens act as neuromodulators, or chemical messengers that alter how neurons behave. Estrogen also helps neurons keep their shape. Estrogen loss in female mice triggers a drop in the number of dendritic spines ‪—‬ the small, bud-like bumps on neurons that receive chemical messages.

Researchers think that estrogen may help prevent the formation of the abnormal proteins — including one called amyloid beta — that accumulate, disrupt communication between neurons and ultimately kill cells in Alzheimer's disease. Amyloid beta is produced when a larger protein is cut the "wrong" way by an enzyme, Frick explained. Estrogen promotes the "normal" cutting of this protein, thus reducing amyloid beta, studies in lab mice show.

And estrogen may limit the buildup of a second Alzheimer's-related protein, called tau, by preventing the chemical processes that cause the tau proteins to clump together. This helps prevent the formation of tau tangles, a hallmark of Alzheimer's disease.

The hormone may also support brain health indirectly by improving sleep quality.

"It's well documented that menopausal women have difficulty sleeping, whether it's due to hot flashes or insomnia," Frick said. Estrogen can ease those nighttime hot flashes, thus helping menopausal women get more, less-fragmented sleep. Deep rest may help the brain clean out toxic proteins, while research shows sleep deficiency is linked to Alzheimer's disease.

Mixed results with estrogen-only HRT

HRT for menopause comes in two main forms: treatments that include only estrogen, and combined therapies with both estrogen and progesterone. The treatment has a complicated history.

HRT was popular in the 1990s but fell out of favor after results from the Women's Health Initiative study were published in 2002. That study identified various risks tied to both estrogen-only and combined HRT options, some of which have since been overturned by newer data.

That said, the Women's Health Initiative study found that estrogen-only HRT came with an increased risk of endometrial cancers, and later analyses have backed up this idea. That's why today, clinical guidelines recommend that women who have not gotten a hysterectomy take combined HRT, while estrogen-only options are reserved for those who have had their uterus removed. (The added progesterone counteracts estrogen's thickening effect on the uterine lining.)

For the subset of women who take estrogen-only HRT, observational data published in August showed that this form of the treatment was linked to a reduced risk of dementia. The study, published in the journal Neurology, found that women who took estrogen-only HRT were less likely to show Alzheimer's-related changes in the brain than women who didn't take any HRT.

Estrogen-only HRT was also tied to better clinical outcomes: Treated women were less likely to be diagnosed with dementia, were less likely to show clinical and functional cognitive decline, and had better scores on memory tests, on average, compared with untreated women. These findings drew on two large, separate groups of older women, reporting data from 21,400 people in total, but they didn't include data on when each woman started HRT.

Whereas earlier studies of HRT and dementia risk relied on doctors' ratings of patients' symptoms, the Neurology study measured levels of Alzheimer's-related proteins in the brain after death. That's a more "definitive" marker of disease, said Bruno, a co-author of the study. However, the research was observational, so it can't say whether estrogen-only HRT directly causes the better brain outcomes the team observed, she stressed.

Although the Neurology study's findings are promising, they don't necessarily match the results of other studies of estrogen-only HRT and dementia. For example, a slightly larger observational study, published in 2023, found that estrogen-only HRT was associated with an increased dementia rate.

In that research, most of the women took a type of estrogen called estradiol, while Bruno's study included a greater variety of estrogens. That may have been the difference in the two studies' results, but teasing that out would require further study. Meanwhile, the conflicting findings muddy the waters of what estrogen-only HRT can do for brain health.

Does timing matter?

One big finding from the Women's Health Initiative study was that starting HRT — either estrogen-only or combined — after age 65 was associated with worse cognitive function and increased dementia risk, said Yuko Hara, director of aging and Alzheimer's prevention at the Alzheimer's Drug Discovery Foundation.

The same may not be true for people who start HRT earlier. For context, menopause starts around age 51 to 52, on average, and it's preceded by perimenopause, a transitional period when hormone levels begin to drop, typically around the mid-40s.

An observational study published in August in the journal Alzheimer's & Dementia found that starting HRT between ages 46 and 56 was linked to a reduced risk of dementia. That study did not differentiate between estrogen-only and combined treatment; it lumped all of the data together.

The protective link was especially strong in three groups: women with "surgical menopause," triggered by the removal of one or both ovaries; carriers of a certain variant of a gene called APOE that raises Alzheimer's risk; and women who had a relatively short time between their first period and menopause.

A 2024 meta-analysis of randomized controlled trials also found that HRT had a cognitive benefit for women with surgical menopause; that finding was tied mostly to estrogen-only therapies.

However, outside of that specific group, the idea of early HRT protecting brain health is far from settled.

Who might benefit, if anyone?

When it comes to using hormone replacement to protect brain health, the evidence isn't yet strong enough to recommend any kind of HRT as a way to protect the brain in all women, Hara said. Women who had one or both of their ovaries surgically removed are the exception to this rule.

"This is a distinct clinical population because the procedure can cause an abrupt, early loss of estrogen, rather than the gradual hormonal decline that occurs with natural menopause," Hara said.

For most women who have gone through natural menopause, "there is not enough evidence to recommend [HRT] specifically for preventing cognitive decline or dementia," she said.

HRT is not currently prescribed to prevent dementia — it is prescribed to ease menopause symptoms, such as hot flashes, night sweats and sleep disturbances. To settle the science on HRT and the brain, what's really needed is a well-designed clinical trial that follows women from perimenopause to postmenopause, Bruno said. That would reveal the effects of HRT on brain health and dementia risk more reliably than the studies done so far.

This article is for informational purposes only and is not meant to offer medical advice.

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'> Does estrogen protect against dementia? And if so, who should take it?

Fall is also an excellent time for casual stargazers, with the earlier evenings and cooler temperatures offering a comfortable compromise before we head into those freezing winter nights when we can't feel our toes. Don't worry if you're just starting out and don't have any gear yet, as many of this season's celestial objects are visible with the naked eye — and some are actually best viewed that way.

This September, October and November are great times to view the distant planets at opposition, catch several meteor showers and photograph far-flung galaxies and nebulas with your smart telescope. Plus, November brings the first full supermoon of the year. So, grab your telescope, your headlamp and a cozy sweater, and let's see what lies ahead.

Lunar & Planetary events

Neptune at opposition

Neptune reaches opposition on September 26 at 02:00 UTC, which is the best time to view it at its brightest, magnitude +7.8. Unfortunately, the full moon will somewhat ruin the party, and Neptune isn't particularly easy to spot at the best of times. That said, the opposition window lasts for several weeks, so waiting a couple of weeks to observe during the new moon week won't make any noticeable difference.

You'll need a good telescope to spot the big blue planet — at least a 6-inch aperture, although an 8-inch or bigger will give you the best chance. Make sure you have a good quality, high-powered eyepiece (something like a 6mm or 9mm) or a good Barlow lens to boost the magnification of your existing eyepieces.

But even with high magnification, Neptune will only appear as a small disc with a bluish hue — you won't see any cloud bands or surface features, because it’s just too far away. Still, it's worth trying to get a glimpse anyway.

Saturn at opposition

A week later, on October 4 at 12:00 UTC, Saturn reaches opposition, shining at magnitude +0.3. It will be easily visible to the naked eye, although to see Saturn's rings or any of its largest moons, you'll need a telescope.

The moon will be just below 50% illuminated and doesn't rise until past midnight, so you'll have a good portion of the late evening to observe Saturn while it's high in the sky before the moonlight affects visibility. Even though the rings are only partially tilted, you can still make out a lovely golden oval around the planet, and in larger telescopes you'll be able to spot the Cassini division. We recommend a telescope with at least 4 or 5 inches of aperture, and a good quality, high-powered eyepiece to observe the finer details.

Full supermoon

November 24 brings the first full supermoon of fall/winter, when the bright Beaver Moon will light up the skies, shining around 15% brighter than a normal full moon.

You can enjoy this moon without any gear whatsoever, but observing with binoculars or a telescope, or capturing its magnificence with a camera, makes the experience much more enjoyable.

Until then, you can catch the other full moons of the season on September 26 and October 26.

Uranus at opposition

The third planetary opposition of the fall belongs to Uranus on November 25 at 23:00 UTC. But like Neptune in September, the almost full moon will dampen its visibility, so we'd recommend waiting until the new moon week.

Like the other outer planets, you'll need a larger telescope with an aperture of at least 6 or 8 inches to view Uranus, alongside a good quality, high-powered eyepiece for the best views. These larger scopes gather enough light to distinguish Uranus's turquoise color, although you'll need to go even bigger if you want to see its brightest moons.

Meteor showers

Draconids

The first meteor shower of the season — and unlike most showers, which involve a lot of caffeine and an all-nighter, the Draconids are best seen in the late evening instead of the dead of night.

This meteor shower is active between October 6-10, peaking on the night of October 8-9. With the radiant point in Draco being highest at nightfall and the thin crescent moon not causing any interference, the best viewing hours are in the late evening before midnight.

These meteors are leftover debris from comet 21P/Giacobini-Zinner, and you can expect to see around 6-10 meteors per hour under ideal conditions.

Orionids

The Orionid meteor shower happens every October, when Earth passes through the trail of debris from Halley's Comet. In 2026, it's active between October 2 and November 7, with the peak occurring on the night of October 21-22. This shower brings between 10 and 20 meteors per hour under dark skies, radiating from the Orion constellation.

The waxing gibbous moon during the peak will add a fair bit of light pollution, but the moon will set between 03:12 am and 04:15 am depending on where in the States you're watching from, so you'll have a few hours before dawn to catch the darkest skies. You just need your eyes and clear, dark skies to see them, and a good astrophotography camera equipped with a wide-angle lens is perfect for capturing photos.

Taurids

The Taurids meteor shower comes in two parts. The first is the Southern Taurids, active between September 10 and November 20, peaking on the night of November 4-5 (during a thin waning crescent moon). The Northern Taurids come slightly later and are active between October 20 and December 10, peaking on November 12 (during an even smaller waxing crescent moon).

The hit rate is one of the lowest of all the meteor showers, with only around 5 meteors per hour. However, the Taurids are known for producing bright, slower-moving fireballs that linger in the sky — making them a particularly appealing astrophotography target.

Peak viewing time will be around midnight, when the Taurus constellation reaches its highest point in the sky.

Leonids

Radiating from the Leo constellation, the Leonids come from the dust and debris left behind by comet 55P/Tempel–Tuttle, which orbits the Sun every 33 years. They produce between 10 and 15 meteors per hour, and the shower period runs from November 6 to November 30, with the peak falling on the night of November 17-18.

The first quarter moon will set around midnight, making way for the best of the shower between midnight and 4 am in totally dark skies.

Star clusters and constellations

Pleiades (and Hyades)

A favorite among many stargazers, the Pleiades (M45), an open cluster known as the "seven sisters" in the constellation of Taurus, slowly returns to the night sky this fall. By October, it rises in the east a couple of hours after sunset and is visible from dusk to dawn from November. Although it's about 440 light-years from Earth, you can see it with the naked eye under dark conditions — even though it's usually just a faint, fuzzy patch — but binoculars or a telescope provide a much more impressive view.

As Greek mythology has it, the sisters — named Maia, Alcyone, Asterope, Celaeno, Taygete, Electra and Merope — were the daughters of the Titan Atlas and Pleione. The story is that Zeus transformed them into a cluster of stars to escape the relentless pursuit of the giant hunter Orion, who fell in love with the sisters and chased them for seven years. It seems he couldn't take the hint.

Look below the Pleiades, and you'll find their five half-sisters, the Hyades — an open cluster which makes up the V-shaped face of Taurus, the Bull.

The Pleiades are a fantastic target to capture with a smart telescope. If you want to stack multiple sessions across fall and winter, start in late November when they're high in the sky. You want an altitude of 50 degrees or higher, and ideally 60 degrees or more. The cluster itself is easy to capture, but the less atmosphere you're looking through, the more you can pull out that delicate blue nebulosity, which is what takes an image of the Pleiades from good to spectacular.

Another enemy of that lovely blue glow is the moon, so make sure you're imaging under completely dark skies with no more than around 25% moonlight. If you get a clear, moonless night, aim for 2-3 hours minimum to capture the faint nebulosity, and go for 3-5 hours for a really impressive image.

Pegasus

The best time to see the Great Square of Pegasus is during the fall evenings between September and January, with October offering the clearest and highest views in the night sky.

This asterism is a large, prominent square shape made up of four bright stars in the eastern sky — Markab, Scheat, Algenib and Alpheratz — with the latter belonging to the neighboring constellation of Andromeda. The square is said to make up the body of the winged horse, and the four stars range from about 133 to 670 light-years away from Earth. Point a pair of binoculars at them, and they'll reveal many more stars that are not visible to the naked eye.

Cassiopeia

Cassiopeia is one of the Northern Hemisphere's circumpolar constellations, so although it's technically visible all year, it's best seen during the autumn and winter months. Throughout the fall, it climbs higher and higher in the sky, reaching its upper culmination in mid-November, when it is best viewed between 9 pm and midnight.

Depending on its rotation in the sky, Cassiopeia appears as a "W" or "M" shape made up of five bright stars ranging from 19 to 425 light-years away and visible with the naked eye.

Cassiopeia was a vain, arrogant queen in Greek mythology, and after boasting that she and her daughter, Andromeda, were more beautiful than the Nereids, the gods placed her in the sky tied to her throne, where she spends half of the year upside down as punishment for her vanity.

The Owl Cluster

In addition to the constellation itself, there are some fun hidden objects within Cassiopeia that require binoculars or a telescope. The Owl Cluster (NGC 457) is a bright, open star cluster near the bottom-left star of the W shape, containing around 150 stars (although only 60 are identified as true cluster members), and is roughly 8,000 light-years from Earth. The blue and yellow "eyes" consist of the stars Phi Cassiopeiae (magnitude +5.0) and HD 7902 (magnitude +7.0).

You'll need a pair of astronomy binoculars to see it — go for 15x70 for a decent view, or 20x80 for an even better observation.

Caroline's Rose

Caroline's Rose (NGC 7789) is another open cluster in Cassiopeia, situated close to the last of the five bright stars in the "W" shape. Astronomer Caroline Herschel discovered the cluster in 1783 and it contains over 1,000 confirmed member stars, with estimates suggesting up to 3,000 potential stars.

It was named "Caroline's Rose" or the "White Rose" because its winding loops of stars and dark empty lanes resemble the petals of a cosmic rose, and it is estimated to be around 1.4 to 1.7 billion years old.

You can see it with a pair of binoculars under reasonably dark skies, but the best views will come from a telescope.

Deep-sky nebulas & galaxies

Andromeda galaxy

Our closest neighboring galaxy, Andromeda (M31), is about 2.5 million light-years from Earth and may one day collide with the Milky Way ("one day" being a 50/50 chance around 10 billion years from now, according to new research). But until then, we can easily make do with observing from afar with binoculars and telescopes.

October and November are prime evening viewing time, with the galaxy reaching its highest point between 7 pm and 11 pm. Under clear, dark skies, it's the most distant object visible to the naked eye, appearing as a very faint smudge — provided you know where to look. Binoculars provide a marginally better view of a somewhat larger, slightly brighter faint smudge — we've managed to see it with a pair of 8x25s under dark skies — but for the best views, you want a telescope.

It's a great first target for beginners using a smart telescope, with the prominent dust lanes revealing themselves after 30-60 minutes. If you want more detailed images of the faint outer arms, leave it for 3-4 hours. Moonlight can wash out the faint galactic arms and dust lanes, so if possible, choose a dark night with minimal light pollution.

In Greek mythology, Andromeda was the daughter of the aforementioned Cassiopeia. After angering the sea god Poseidon, Cassiopeia (who was not exactly Mother of the Year) chained Andromeda to a rock as a human sacrifice to the sea monster, Cetus, that Poseidon had sent to ravage their kingdom's coast. Andromeda was saved by Perseus, who she then married, and after her death, Athena placed her in the stars as a constellation.

Helix Nebula

The Helix Nebula (NGC 7293) is a large planetary nebula around 650 light-years away in the constellation Aquarius. Often called the "Eye of God", it has a striking eye-like appearance, and it features thousands of comet-like radial filaments and complex gas rings.

It stays relatively low in the southern sky, with September being the best time to view or photograph before it gradually sinks over the coming months. Moonlight will completely wash out its faint outer gaseous ring, so it's best to shoot during a new moon (or when the moon hasn't yet risen) to get the best image possible.

The best hours to capture it are between about 10:30 am and 2 am, when it's either rising or at its highest. This should get you a pretty decent standard image, but if you want more detail, try to capture it over several nights to pull in the faint outer nebulosity and stack the images.

Triangulum Galaxy

The Triangulum Galaxy (M33) is the third-largest in our local group of galaxies, behind the Milky Way and Andromeda. This stunning spiral galaxy — not to be confused with the Pinwheel Galaxy (M101), which is best seen in the spring — sits in the small Triangulum constellation about 2.7 million light-years from Earth and is a great astrophotography target.

Its peak viewing time is October to December, when its highest and clearest position is after dark in the late evening. Because it's so faint, you need a moonless night away from light pollution. The sweet spot for smart telescopes is around 90 minutes, but if you can shoot for longer or want to split your shoot across multiple nights, 2.5 to 4+ hours can bring out more detail in the core and spiral arm.

Heart and Soul Nebula

The Heart and Soul Nebula is a nebula complex consisting of two massive emission nebulas located about 6,000 to 7,500 light-years from Earth in the constellation Cassiopeia. They are primarily made of glowing ionized hydrogen gas, darker cosmic dust lanes and trace amounts of ionized oxygen and sulfur.

Like most other nebulas, they aren't visible to the naked eye, so you'll need a telescope or a long-exposure astrophotography setup to see them. The Heart Nebula (IC 1805) is, unsurprisingly, named after its heart-like shape, while its neighbor, the Soul Nebula (IC 1848), has a more irregular shape with large cavities made by stellar winds.

If you're imaging the Heart and Soul with a camera or smart telescope, a total integration time of 6 to 12 hours will provide you with the cleanest details and most contrast in the faint gas structures. That said, 2 to 4 hours will give you a good basic stacked image.

As Cassiopeia is at its highest in the fall, this is the best time to see and photograph it with the least amount of atmospheric interference.

Ghost of Cassiopeia

The Ghost of Cassiopeia, often referred to as the Ghost Nebula (IC 63), is a mixed reflection and emission nebula around 550 light-years away in Cassiopeia. Its eerie red glow is caused by its proximity to the nearby blue giant star, Gamma Cassiopeiae, which sits right in the middle of the constellation's distinctive 'W' shape. The star’s powerful ultraviolet radiation causes the hydrogen to emit a soft red glow, while the surrounding dust reflects the star's light in fainter blue hues.

Like the other celestial sights in Cassiopeia, autumn and winter evenings are the best times to see and photograph it due to its high altitude in the sky. As the nebula is so faint and elusive to capture, you can only see it with a smart telescope or a long-exposure astrophotography setup.

When it comes to integration time, the longer the better. Aim for 3-5 hours minimum, ideally 6+ hours if the skies allow, and avoid shooting during moonlight if you can.

'> Fall stargazing 2026: The best skywatching sights in autumn (and how to see them) auroras on Earth and defines the extent of the solar system, these stars' powerful winds blast material into space. Meanwhile, the most massive of these stars eventually explode as supernovas, pushing even more gas aside. The collective radiation from both these old and young stars carves the nebula's giant cavity.

So what happened to that expelled gas? Most of it has been pushed outward and compressed into a dense shell of gas and dust surrounding the superbubble. Within that shell, new stars are forming ‪—‬ so the same massive stars that destroy the gas cloud that created them also may help to produce their replacements.

This is known as "stellar feedback" — when massive stars heat, ionize and disperse the gas and dust around them, making star formation impossible in their vicinity. Meanwhile, their winds can squeeze neighboring gas clouds until they become dense enough to collapse under their own gravity and make new stars.

This Hubble image of N44 contains almost half a million stars, including those in the nebula and some lying along the same line of sight. About 30,000 of them are young stars yet to fuse hydrogen in their cores — the reaction that powers stars like the sun. With so many young protostars not yet hot and dense enough to become full-fledged main-sequence stars, N44 gives astronomers a rare opportunity to attempt an answer to what may seem like a very simple question: how long does it take to make a star?

'> Haunting Hubble telescope image captures cosmic cycle of destruction and creation —‬ Space photo of the week

But is that really the case? Are there hidden, more complex reasons for why cats lick each other?

While research into allogrooming is ongoing, scientists have suggested different explanations for why cats lick each other, including some social reasons and some more practical ones.

Social bonding or social tension?

The historical view, based on observations of free-roaming-cat colonies, was that allogrooming is a sign of affection and helps cats create and maintain bonds with other cats in their social group, perhaps by creating a shared group smell.

But a 1998 study of 25 cats living together in an indoor-outdoor enclosure poked holes in this theory. The study found that aggressive behavior showed up in about 35% of allogrooming sessions, which suggested that allogrooming might not be about friendship. Instead, the authors proposed, it may help to defuse a tense situation in a confined space, where an actual fight would be too risky.

To investigate, Morgane Van Belle, a cat behavior scientist at Ghent University in Belgium, and her colleagues analyzed videos of allogrooming from 53 two-cat homes. They concluded that allogrooming may have different social functions, depending on the situation.

The team noticed that the cats sometimes licked each other while huddling together and copying one another's body postures. In these moments, allogrooming probably helps strengthen social bonds, the researchers proposed. Most licking was directed toward the ears; this may help cats relax because the skin there is rich in sensory nerve endings, Van Belle said. Allogrooming produces a relaxing effect in other animals too. For example, it lowers the heart rate in horses and triggers the release of feel-good hormones in monkeys.

They also noted that allogrooming sometimes led to friendly wrestling, in which the cats grabbed each other with their front legs and tumbled around. This suggests that allogrooming may serve as a signal to start play.

In other situations, however, the researchers noticed that allogrooming was followed by behaviors linked to conflict, such as turning the ears backward, pawing, and biting. In these cases, one cat would stand or lean over the other, and both cats showed subtle signs of stress like lip licking, head shaking or self-scratching.

Based on these observations, Van Belle's team think that allogrooming may be an "appeasement signal" in fraught moments that could lead to conflict or injury, for example, when one cat wants to take over a sleeping spot already occupied by another. They think it possible that the licking helps to calm things down and keeps a spat from turning into a real fight. Similar behavior has been seen in meerkats, where subordinate individuals groom dominant ones to placate them.

Yet because this licking often targets the neck — the same area cats bite during fights — it also may signal a subtle aggressive threat, the researchers suggested.

Hygiene and grooming

Van Belle and her colleagues think that, beyond sending social signals, allogrooming may help with hygiene. After all, it's often aimed at the head and neck ‪—‬ the areas that are "least accessible for the cat itself during grooming," Van Belle said. Allogrooming is thought to have a hygienic function in other species too. For example, ants achieve colony-level hygiene by grooming heavily infected individuals more, and primates concentrate their grooming on body parts the recipient can't easily reach. When it comes to cats, however, more research is needed. Van Belle's study did not measure parasites, infections or skin cleanliness to see whether such grooming actually reduces dirt or bugs.

Dr. Terry Curtis, a veterinary behaviorist who has studied how often allogrooming happens, also thinks cats might groom each other for hygiene reasons. She wonders whether feral cats, who likely carry more fleas than house cats, might have a different allogrooming pattern.

The explanations proposed by Van Belle and colleagues are reasonable but not definitive, Curtis said. Because so many factors can shape a cat's behavior, she's skeptical that researchers will ever be able to say with real confidence exactly why one cat grooms another.

After working as a veterinary behaviorist for 25 years, Curtis has concluded it is often impossible to understand what's going on in a cat's mind.

"Congratulations, you got a cat," she said. "Let the mystery begin.”

How much of a cat fan are you? Find out by taking our cat quiz!

'> Why do cats lick each other?

It's odd to think of diamond, the hardest natural material on Earth, melting — but melt it does when blasted with extremely powerful lasers under the right conditions. Understanding how diamond responds to shock waves from lasers is an important part of developing nuclear fusion, the process that powers stars. Nuclear fusion is also a potential energy source for the future, so researchers have put a lot of effort into developing models that describe and predict how diamond behaves.

However, diamond is weird. Although the experimental data and theoretical models match up pretty well most of the time, there have been some strange discrepancies scientists haven't been able to explain. The biggest one is the 2,240 F (1,244 C) ‪—‬ roughly 20% ‪—‬ difference between previous experimental data and model-predicted melting temperatures of diamond. There's also been some debate about whether diamond reorganizes its atoms into a different kind of solid carbon before turning into a liquid at the end of the melting process.

Researchers have struggled to explain these discrepancies because the conditions diamond melts at are so extreme that it's extraordinarily difficult to measure it in labs on Earth. However, new experiments may finally offer the solution that scientists have pursued for two decades.

In a study published Aug. 13 in the journal Nature Physics, scientists zapped tiny plates of synthetic diamond with an ultraviolet laser, creating shock waves that were so powerful that as they passed through the samples, the diamond changed from transparent to mirror-like. The strong increase in reflectivity is one indication the diamond melted. By combining this change with measurements of how brightly the diamonds glowed while being zapped, the researchers mapped the melting temperature with great precision.

"We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity," study co-author Marius Millot, a research scientist at Lawrence Livermore National Laboratory in California, said in a statement.

The team found that the diamond sample's melting temperature was more than 1,300 F lower than previously thought — putting the melting point in line with theoretical predictions and finally explaining the long-held discrepancy.

The team also measured the samples' atomic structure with X-ray diffraction and saw that the diamond didn't transition to a different kind of solid carbon before melting, possibly because the energy required to rearrange the atoms was too large, the researchers wrote.

However, they also hypothesized that multiple shocks could be powerful enough for this transition to occur and that the way the shocks are applied to the diamond might affect how it changes phase. Understanding this is important for nuclear fusion research, as certain types of experiments involve lasers melting and crushing a diamond capsule to put the capsule’s contents, solid deuterium and tritium, under more than 30 petapascals of pressure and temperatures higher than 180 million F (100 million C), the requisite conditions for a fusion chain reaction to occur.

The researchers found that between about 660 and 1,060 gigapascals of pressure and at around 12,140 F (6,727 C), diamond exists as solid chunks floating in liquid carbon. As the pressure increases, more diamond transitions into liquid carbon, which is thought to be a very strange material. Unlike most forms carbon takes on Earth ‪—‬ like coal, graphite and diamond ‪—‬ liquid carbon is metallic, so it conducts electricity. It's also denser than diamond. So hypothetically, if you somehow were to put liquid carbon in a cup without instantly vaporizing it, a chunk of solid diamond could happily bob around in it like an ice cube in a glass of water.

Knowing how diamond behaves under such extreme conditions is also important for understanding the ice giant planets Uranus and Neptune. Based on measurements from the Voyager 2 spacecraft in the late 1980s and lab experiments on Earth, scientists think it literally rains huge chunks of diamond inside these planets and that their mantles may have liquid carbon oceans with diamonds floating around like icebergs. The new research means scientists can make better predictions about the planets’ interiors and their carbon cycles.

See how much you know about gemstones with our gold and gems quiz!

'> Scientists got diamond's melting point wrong by more than 1,000 degrees, crushing new laser experiment reveals