Category: 7. Science

• Apep is a multiple star system. Surrounding it is an intricate, swirling nebula that has intrigued scientists.
• The James Webb Space Telescope has taken a new image of Apep. The image gives astronomers an even better view of what is happening within this star system.
• Two Wolf-Rayet stars lie at the heart of this system, along with a third companion. And the third star is taking a bite out of their dust shells.

By Benjamin Pope, Macquarie University

The twisted world of Apep

The day before my thesis examination, my friend and radio astronomer Joe Callingham showed me an image we’d been awaiting for five long years. It was an infrared photo of two dying stars we’d requested from the Very Large Telescope in Chile.

I gasped. The stars were wreathed in a huge spiral of dust, like a snake eating its own tail.

We named it Apep, for the Egyptian serpent god of destruction. Now, our team has finally been lucky to use NASA’s James Webb Space Telescope to look at Apep.

If anything could top the first shock of seeing its beautiful spiral nebula, it’s this breathtaking new image. The new Webb data are now analyzed in two papers on arXiv.

Violent star deaths

Right before they die as supernovas, the universe’s most massive stars violently shed their outer hydrogen layers, leaving their heavy cores exposed.

These are Wolf-Rayet stars, named after their discoverers Charles Wolf and Georges Rayet. Wolf and Rayet noticed powerful streams of gas blasting out from these objects, much stronger than the stellar wind from our sun. The Wolf-Rayet stage lasts only millennia – a blink of the eye in cosmic time scales – before they violently explode.

Unlike our sun, many stars in the universe exist in pairs known as binaries. This is especially true of the most massive stars, such as Wolf-Rayets.

When the fierce gales from a Wolf-Rayet star clash with their weaker companion’s wind, they compress each other. In the eye of this storm forms a dense, cool environment in which the carbon-rich winds can condense into dust. The earliest carbon dust in the cosmos – the first of the material making up our own bodies – was made this way.

The dust from the Wolf-Rayet blows out in almost a straight line. And the orbital motion of the stars wraps it into a spiral-shaped nebula. So it appears exactly like water from a sprinkler when viewed from above.

We expected Apep to look like one of these elegant pinwheel nebulas, discovered by our colleague and co-author Peter Tuthill. To our surprise, it did not.

Equal rivals

Webb’s infrared camera took the new image. The camera is like the thermal cameras that hunters or the military use. The image represents hot material as blue and colder material in green to red.

It turns out Apep isn’t just one powerful star blasting a weaker companion, but two Wolf-Rayet stars. The rivals have near-equal strength winds, and the dust spreads out in a wide cone, wrapping into a wind-sock shape.

When we originally described Apep in 2018, we noted a third, more distant star, speculating whether it was also part of the system or a chance interloper along the line of sight.

The dust appeared to be moving much slower than the winds, which was hard to explain. We suggested the dust might be carried on a slow, thick wind from the equator of a fast-spinning star, rare today but common in the early universe.

The new, much more detailed data from Webb reveals three more dust shells zooming farther out, each cooler and fainter than the last and spaced perfectly evenly, against a background of swirling dust.

New data on Apep, new knowledge

Researchers have now published the Webb data, interpreted in a pair of papers. One is led by Caltech astronomer Yinuo Han, and the other by Macquarie University Masters student Ryan White.

Han’s paper reveals how the nebula’s dust cools, links the background dust to the foreground stars and suggests the stars are farther away from Earth than we thought. This implies they are extraordinarily bright, but weakens our original claim about the slow winds and rapid rotation.

In White’s paper, he develops a fast computer model for the shape of the nebula and uses this to decode the orbit of the inner stars very precisely.

He also noticed there’s a “bite” taken out of the dust shells, exactly where the wind of the third star would be chewing into them. This proves the Apep family isn’t just a pair of twins … they have a third sibling.

Understanding systems like Apep tells us more about star deaths and the origins of carbon dust. But these systems also have a fascinating beauty that emerges from their seemingly simple geometry.

The violence of stellar death carves puzzles that would make sense to Newton and Archimedes, and it is a scientific joy to solve them and share them.

Benjamin Pope, Associate Professor, School of Mathematical and Physical Sciences, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: The James Webb Space Telescope has captured an image of Apep, which includes the swirling nebula around two dying stars. A scientist explains what we know about it now.

Bacteria strains

The 59 strains of E. faecalis, 5 strains of E. faecium, 2 strains of Staphylococcus aureus, 2 strains of Listeria monocytogenes, and 2 strains of Escherichia coli used in this study (for details, see Table 2) were conserved by Institute of Microbe & Host Health, Linyi University (Linyi, Shandong province, China). VR-Efs V583 was provided by the Utrecht University (Netherlands) and stored by the Institute of Microbe & Host Health (Linyi, Shandong province, China).

Isolation and purification of phage

Phage A155 was isolated from a sewage sample from a farm (Minggang Farm, Linyi) using VR-Efs V583 through the modified enrichment technique [14]. In brief, 15 mL of pre-settled sewage supernatant was centrifuged at 2876 ×g for 5 min (Pingke-165–6N). Subsequently, 2.5 mL of clarified supernatant was mixed with 2.5 mL of 2× Brain Heart Infusion (BHI) broth. The mixtures were incubated overnight at 37 °C with shaking(200 r/min). Then, 1.0 mL of the culture was centrifuged at 11,586 ×g for 5 min (ALLSheng-mini-15k), and the supernatant was filtered with a 0.22 μm membrane filter(Jinteng, China), using the double-layer plate method [15] to screen phage presence.

For purification, individual plaques were excised using sterile pipette tips, soaked in 200 µL SM buffer overnight, and subjected to serial dilution and repeated with the double-layer plate method. The purification cycle was iterated until homogeneous plaque morphology was achieved. The phage A155 was cultured and stored at 4℃ and − 80℃ in 25% glycerol.

Phage morphology

The purified phage was amplified using PEG-8000. After overnight treatment at 4 °C, the supernatant was discarded by centrifugation at 6236 ×g for 10 min (Eppendorf-5810R-SL142). The precipitate was resuspended in 2.0 mL of SM buffer (NaCl 5.8 g/L, MgSO₄ 1.5 g/L, Tris 6.06 g/L, 1 M HCl, gum Aladdin 0.1 g/L, pH 7.5), followed by extraction with an equal volume of chloroform to obtain the phage concentrate. The concentrate was mixed with 1.5 g CsCl in a 15-mL centrifuge tube and centrifuged at 6236 ×g for 20 min at 4℃ using a centrifuge machine (Eppendorf-5810R-SL142). The phage band was collected with a syringe and dialysed overnight in phosphate-buffered saline (PBS) to obtain purified phage particles. The samples were stained using the phosphotungstic acid negative staining method [16] and observed under a transmission electron microscope at 80 kV after 15 min.

Optimal MOI

The multiplicity of infection (MOI) is the ratio of phage to host bacteria used for phage amplification. VR-Efs V583 was cultured to the logarithmic growth phase (OD₆₀₀≈0.7), and the bacterial concentration was adjusted to 1 × 10⁹CFU/mL. The phage was serially diluted and mixed with VR-Efs V583 and BHI broth at MOIs of 10, 1, 0.1, 0.01, and 0.001. The mixtures were incubated at 37 °C with shaking at 200 r/min for 6 h. Phage potency in each mixture was determined using the double-layer plate method, and the mixture with the highest potency was identified as the optimal MOI for the phage.

One-step growth curve

VR-Efs V583 was cultured to the log-phase growth (OD₆₀₀≈0.7), and 1.0 mL of the culture was centrifuged at 2200 ×g for 5 min at 4 °C (Eppendorf-5418R). Following centrifugation, the collected precipitate was subjected to two rounds of PBS resuspension and centrifugation under the same conditions to remove residual impurities. The supernatant was discarded, and the pellet was resuspended in 1.0 mL of sterile BHI broth. Phage was added at the optimal MOI (MOI = 0.001) and incubated at 37 °C for 15 min. The mixture was then centrifuged at 4827 ×g for 5 min (AllSheng-mini-15k), the supernatant was discarded, and the pellet was resuspended in 10 mL of sterile BHI broth. The suspension was incubated at 37 °C with shaking at 200 r/min. Samples (50 µL) were collected every 5 min for the first 30 min and every 10 min for the next 90 min (total 2 h) for phage titer determination. Three replicates were performed at each time point, and the average values were used. The one-step growth curve was plotted with sampling time on the x-axis and the logarithm of phage titer on the y-axis.

Temperature and pH stability

Difference from previous assay [13], phage A155 (10⁹ PFU/mL) was incubated in a thermostatic water bath at 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C for 30 min and 60 min. After incubation, 100 µL of the phage solution was serially diluted 10-fold, and the phage titer was determined using the double-layer plate method. Three replicates were performed at each temperature, and the average values were used to analyze the changes in phage titer.

Phage A155 (10⁹ PFU/mL) was incubated at pH levels ranging from 2.0 to 14.0 (in increments of 1.0) for 60 min at 37 °C. After incubation, 100 µL of the phage solution was serially diluted 10-fold, and the phage titer was determined using the double-layer plate method. Three replicates were performed, and the average values were used to analyze the changes in phage titer under different pH conditions.

Sequencing and bioinformatics analysis of phage A155 genome

Following the instructions of the TIANamp Virus DNA/RNA Kit (Tiangen Bio-Tek Inc., Beijing, China), phage A155 was amplified, and concentrated, and its nucleic acid was extracted. Whole genome sequencing was conducted by Shanghai Personal Biotechnology Co., Ltd., following the method described by Han [17]. Briefly, using the Illumina TruSeq Nano DNA LT protocol (Illumina TruSeq DNA Sample Preparation Guide) to custom 2 × 250 bp paired-end DNA library, the average fragment length was 400 bp. Raw sequencing data quality was assessed by FastQC v0.11.7, followed by adapter trimming. The genome sequence was assembled using A5-MiSeq [18] and SPAdes [19]. Viral genome identification was performed by extracting high-depth sequences and conducting BLASTn alignment against the NCBI NT database [20]. Synteny analysis and gap closure were conducted by MUMmer [21], with subsequent error correction performed via Pilon v1.24 [22]. Protein-coding genes were predicted via GeneMarkS [23]. The genome data were uploaded to the National Center for Biotechnology Information (NCBI) database, GenBank accession number: PQ093903, and a comparative gene circle map was made by BRIG.

Phylogenetic tree construction

Based on the whole sequences, phylogenetic tree analysis was performed comparing phage A155 with multiple Enterococcus faecalis phages. Sequence alignment and phylogenetic tree construction were conducted using VICTOR [24].

Efficiency of plating (EOP)

Following a previously described method with certain modifications [25], the phage host range was quantitatively assessed through the efficiency of plating (EOP). Briefly, phage A155 solution was serially diluted 10-fold. Phage titers were determined in triplicate for each bacterial strain using the double-layer agar method. EOP values were calculated as (mean PFU on target bacteria / mean PFU on host bacteria). An EOP value of 0.5 or higher was classified as “high production”, indicating that at least 50% of PFUs were produced in target bacteria compared to host bacteria. An EOP value between 0.1 and 0.5 was classified as “Medium production.” An EOP value between 0.001 and 0.1 was considered “low production,” and an EOP less than 0.001 was referred to as inefficient.

Inhibitory effect of phage A155 on VR-Efs V583 in vitro

VR-Efs V583 was cultured to the stationary phase, harvested, and resuspended in fresh BHI broth. The suspension was distributed into 12-well plates, adjusted to a final bacterial concentration of 10⁷ CFU/mL(1/100 of the original bacterial liquid volume), and supplemented with BHI broth. Phage A155 was added at MOI of 0.001, 0.01, 0.1, 1, or 10, with a final volume of 2.5 mL per well. Bacterial growth without phage served as the control. All experiments were performed in triplicate. The plates were incubated in a Bacterial Growth Curve Instrument (Scientz MGC-200, Ningbo, China), and OD₆₀₀ was measured every 30 min for 16 h to assess the lytic efficiency of phage A155 in vitro.

Phage therapy in the murine bacteremia model

Female BALB/c mice aged 6–8 weeks were purchased from a commercial supplier (Pengyue, Shandong, China). They were provided with water and standard mouse chow ad libitum and monitored daily.

An intestinal model of E. faecalis infection in mice was established as previously described [26] (Fig. 7a). Sixteen SPF BALB/c mice (6–8 weeks old) were randomly divided into two groups (n = 8). After one week of acclimatisation, antibiotics were administered orally to deplete the intestinal flora and create more ecological niches for E. faecalis colonization [27]. Mice received a mixture of antibiotics (vancomycin 10 mg, neomycin 10 mg, ampicillin 10 mg, metronidazole 10 mg) by gavage for 3 days, followed by the same antibiotics in drinking water (vancomycin 500 mg/L, neomycin 500 mg/L, ampicillin 500 mg/L, metronidazole 500 mg/L) for 7 days. Faecal bacterial counts, particularly Enterococci, were monitored using the plate-counting method during this period. After 2 days, 100 µL of VR-Efs V583 suspension (1 × 10⁹ CFU/mL) was administered to both groups to simulate enterococcal proliferation. Faecal Enterococci levels were monitored using PSE agar.

To assess the effect of phage treatment, a single dose of phage A155 (2.4 × 10⁸ PFU/mouse) was administered orally 4 days post-bacterial challenge. The control group received an equivalent volume of BHI broth. Faecal samples were collected daily, and Enterococci counts on selective media were used to evaluate the efficacy of phage treatment in reducing E. faecalis colonization.

Data analysis

GraphPad Prism 8.3.0 was used to analyse the experimental data using statistics. Data were analyzed using one-way ANOVA. Results are expressed as standard deviation (SD). Error bars indicate the standard deviation of the mean, and the p-value was used to indicate the statistical significance of the data.

Extreme “wind droughts” that reduce power output from turbines for extended periods could become 15% longer by the end of the century across much of the northern hemisphere under a moderate warming scenario.

That is according to a new study in Nature Climate Change, which explores how climate change could impact the length and frequency of prolonged low-wind events around the world.

According to the study, “prominent” wind droughts have already been documented in Europe, the US, northeastern China, Japan and India.

As the planet warms, wind droughts will become longer in the northern hemisphere and mid-latitudes – especially across the US, northeastern China, Russia and much of Europe – the paper says.

The study – which focuses on onshore wind – warns that “prolonged” wind droughts could “threaten global wind power security”.

However, they add that research into the effects of climate change on wind supply can help “prepare for and mitigate the adverse impacts” of these prolonged low-wind events.

Combining wind power with other energy technologies – such as solar, hydro, nuclear power and energy storage – can help reduce the impact of wind droughts on global energy supply, the study says.

One expert not involved in the research tells Carbon Brief that the findings do not “spell doom for the wind industry”.

Instead, he says the study is a “navigation tool” which could help the energy industry to “counteract” future challenges. 

Wind drought

Wind power is one of the fastest-growing sources of energy in the world and currently makes up around 8% of global electricity supply. It is also playing a crucial role in the decarbonisation of many countries’ energy systems. 

Wind is the result of air moving from areas of high pressure to areas of low pressure. These differences in air pressure are often due to the Earth’s surface being heated unevenly. 

Human-caused climate change is warming the planet’s atmosphere and oceans. However, different regions are heating at different rates, resulting in a shift in global wind patterns. The IPCC finds that global average wind speeds (excluding Australia) slowed down slightly over 1979-2018.

There have already been dozens of recorded instances of prolonged low-wind events, known as wind droughts, which can drive down power production from wind turbines.

Dr Iain Staffell is an associate professor at the Centre for Environmental Policy at Imperial College London who was not involved in the study. He tells Carbon Brief that wind droughts often “push up power prices” as countries turn to more expensive alternative energy supplies, such as fossil fuels.

For example, Staffell tells Carbon Brief that, in the winter of 2024-25, Germany saw an “extended cold-calm spell which sent power prices to record highs”. (In German, this type of weather event is referred to as a “dunkelflaute”, often translated as “dark doldrums”.) He adds:

“It’s important to note that I’m not aware of anywhere in the world that has suffered a blackout because of a wind drought.”

Capacity factor

The productivity of wind power sites is often measured by their “capacity factor” – the amount of electricity that is actually generated over a period of time, relative to the maximum amount that could have been generated in theory.

A capacity factor of one indicates that wind turbines are generating the maximum possible amount of electricity, while zero indicates that they are not producing any power. 

The authors define a wind drought as the 20th percentile in each grid cell – in other words, winds ranking in the slowest bottom fifth of winds typically recorded in the region.

They look at the frequency of prolonged wind droughts and how that might change as the world warms.

The map below shows regions’ average capacity factor at 100 metres above the ground level, derived from the ERA5 reanalysis data over 1980-2022, where darker shading indicates a higher capacity factor.

It also shows 19 wind droughts recorded since the year 2000 across Europe, the US, northeastern China, Japan and India. Wind droughts are indicated by yellow triangles for local events and hashed areas for larger-scale events.

The map also shows that the darker shading for “abundant wind resources” is typically found in the mid-latitudes near “major storm tracks”, including the central US, northern Africa, northwestern Europe, northern Russia, northeastern China and Australia.

Modelling wind

To assess the severity of past and future wind droughts, the authors consider both the frequency and duration of these low-wind events.

To calculate wind drought duration, the authors use reanalysis data and models from the  sixth Coupled Model Intercomparison Project (CMIP6) – the international modelling effort that feeds into the influential assessment reports from the Intergovernmental Panel on Climate Change (IPCC). 

The authors then look at how wind drought conditions may change in the future, by modelling wind speeds over 2015-2100 under a range of future warming scenarios.

They find that wind drought frequency and duration will both increase in the northern hemisphere and mid-latitudes by the end of the century. The authors identify “particularly notable increases” in wind drought frequency in the US, northeastern China, Russia and much of Europe.

In the northern mid-latitudes, there will be a one-to-two hour increase in average wind drought duration by the end of the century under the moderate SSP2-4.5 scenario, according to the study. This is a 5-15% increase compared to today’s levels.

The authors also assess “extreme long-duration events” by looking at the longest-lasting wind drought that could happen once every 25 years.

The study projects roughly a 10%, 15% and 20% “elongation” in these long-duration wind droughts across “much of the northern mid-latitude regions” under the low, moderate and very high warming scenarios, by the end of the century.

However, the authors find “strong asymmetric changes” in their results, projecting a decrease in wind drought frequency and intensity in the southern hemisphere.

The authors suggest that the increase in wind droughts in the northern hemisphere is partly because of Arctic amplification – the phenomenon whereby the Arctic warms more quickly than the rest of the planet.

Accelerated warming in the Arctic narrows the temperature gap between the north pole and the equator and alters atmosphere-ocean interactions, which reduces wind speeds in the northern hemisphere. 

Conversely, the authors suggest that increasing wind speeds in the southern hemisphere are caused by the land warming faster than the ocean, resulting in a greater difference in temperature between the land and the sea.

Record-breaking wind droughts

Finally, the authors also investigate the risk of “record-breaking wind droughts” – extreme events that would only be expected once every 1,000 years under the current climate. 

They use CMIP6 models, based on historical data over 1980-2014, to assess how long-lasting such an event would be in different regions of the world. These results are shown on the map below, where darker brown indicates longer-duration wind droughts.

These 1,000-year record-breaking wind droughts typically last for 150-350 hours (6-15 days), occasionally reaching up to 400 hours in regions such as India, East Russia, east Africa and east Brazil, the paper says.

The authors go on to assess the risk of record-breaking wind droughts for existing wind turbines under different warming scenarios. 

The plot below shows the fraction of the CMIP6 models used in this study that project record-breaking wind droughts for onshore wind turbines.

Blue bars show the percentage of wind turbines that face a “weak” risk of exposure, meaning that fewer than 25% of models predict that the turbine will be exposed to record-breaking wind droughts by the year 2100. Green bars indicate a “moderate” risk of 25-50% and brown bars denote “severe” risk of greater than 50%.

Each panel shows a different region of the world, with results for low (left) moderate (middle) and very high (right) warming scenarios.

The study finds that, globally, around 15% of wind turbines will face “severe” risk from record-breaking wind droughts by the end of the century, regardless of the future warming scenario. However, different parts of the globe are expected to face different trends.

In North America, the percentage of turbines facing a “severe” risk from such extended wind droughts in the year 2100 rises from 14% in a low warming scenario to 39% in a very high warming scenario. Europe also faces a higher risk to its wind turbines under higher emissions scenarios.

However, the trends vary across the world. In south-east Asia, for example, the percentage of wind turbines at “severe” risk of the longest wind droughts drops from 18% under a low warming scenario to 11% under a very high warming scenario. 

Energy security

The planet currently has 1,136GW of wind capacity. The authors say that, according to a report by the International Renewable Energy Agency, “wind power capacity is projected to grow substantially as the world pursues decarbonisation, aiming for 6,000GW by 2050”.

The paper sets out a number of ways that energy suppliers could reduce their exposure to record-breaking wind droughts.

The authors say that developers can avoid building new turbines in areas that are prone to frequent wind droughts. They add:

“Other effective mitigation measures include complementing wind power with other renewable energy sources, such as solar, hydro, nuclear power and energy storage.”

Staffell tells Carbon Brief the study provides helpful insights for how the world’s power supply could be made less vulnerable to prolonged low-wind events:

“I don’t see this study as spelling doom for the wind industry, instead it’s a navigation tool, telling us where to expect challenges in future so that we can counteract them.”

Staffell argues that there are “many solutions” for combatting wind droughts – including building the infrastructure to enable “more interconnection” between countries’ power grids. 

For example, he says the UK could benefit from connecting its grid to Spain’s, noting that “wind droughts in the UK tend to coincide with [periods of] higher wind production in Spain”.

He adds:

“Increasing flexibility and diversity in power systems is a way to insure ourselves against extreme weather and cheaper than panic-buying gas whenever the wind drops.”

Similarly, Dr Enrico Antonini, a senior energy system modeller at Open Energy Transition, who was not involved in the study, tells Carbon Brief that wind droughts “do not necessarily threaten the viability of wind power”. He continues:

“Areas more exposed to these events can enhance their resilience by diversifying energy sources, strengthening grid connections over large distances and investing in energy storage solutions.”

In a news and views piece about the new study, Dr Sue Ellen Haupt, director of the weather systems assessment programme at the University of Colorado, praises the “robust” analysis.

She says the work “would ideally be accomplished with higher-resolution simulations that better resolve terrain, land-water boundaries and smaller-scale processes”, but acknowledges that “such datasets are not yet available on the global scale”. 

Meanwhile, Dr Frank Kaspar is the head of hydrometeorology at Germany’s national meteorological service. He tells Carbon Brief how additions to this study could further help energy system planning in Germany. 

Kaspar tells Carbon Brief it would be helpful to know how climate change will affect seasonal trends in wind drought, noting that in Germany, wind power “dominat[es] in winter” while solar plays a larger role in the energy mix in summer. [The UK sees a similar pattern.]

He adds that the study does not address offshore wind – a component of Germany’s energy mix that is “important” for the country.