Tag Archives: stellar wind

Massive Black Hole Could Challenge Stellar Evolution Theories

Astronomers have used the Very Long Baseline Array (VLBA) to discover that the first black hole ever detected is actually much larger than previously believed. So large, at 21 times the mass of the Sun, that it challenges existing theories about the evolution of stars and how they form black holes. These constraints should limit stellar black holes in binary systems to about 15 solar masses.

Cygnus X-1 is a Milky Way binary system that contains a black hole and a supergiant companion star feeding it gas and other material. First discovered in 1964, the binary system has gone on to become one of the most intensely studied objects in astronomy. Yet, our familiarity with Cygnus X-1 doesn’t mean it can’t still deliver a surprise or two.

An artist’s impression of the Cygnus X-1 system. A stellar-mass black hole orbits with a
companion star located 7,200 light-years from Earth. (ICRAR)

In addition to finding the black hole is 50% more massive than prior estimates, near 21 solar masses as opposed to 15 solar masses, the team also discovered that the companion star also has a greater mass than previous measurements had revealed. The system as a whole is 20% further away than previously calculated–7,200 light-years from Earth as opposed to 6,100 light-years.

“We know that Cygnus X-1 hosts a black hole that is 21 times the mass of the Sun. We also learned that the supergiant companion star in Cygnus X-1 is also more massive than we had thought, with a mass of about 40 times the mass of the Sun,” Professor James Miller-Jones, the International Centre for Radio Astronomy Research (ICRAR), Curtin University, Australia, tells ZME Science. “The revised masses and distances also lead to an updated orbital separation between the star and the black hole — they orbit each other at a separation of one-quarter the distance from the Earth to the Sun.”

Cygnus X-1: New findings show the binary system is more massive and further away than previous estimates implied. (ICRAR)

The finding, published in the latest edition of the journal Science, means that Cygnus X-1 contains the largest black hole created through the collapse of a star alone, that has ever been detected with traditional electromagnetic astronomy without the use of gravitational waves. Larger black holes do of course exist, but these are formed through other mechanisms such as mergers between smaller black holes after that initial stellar collapse.

Miller-Jones, the study’s lead researcher, goes on to explain that the team also learned that the black hole is spinning very rapidly , close to its maximum possible speed. 

“With all this new information, we were able to propose a likely scenario for how this system formed, which can explain its observed properties.”

Professor James Miller-Jones, ICRAR, Curtin University

The finding doesn’t conform to current theories about black hole formation and stellar development in binary systems as its mass is greater than the limit imposed on such an object. 

How Cygnus X-1 Challenges Theories of Stellar Evolution

The team chanced on their finding whilst conducting an ambitious project to observe Cygnus X-1 almost continuously over a full 5.6-day orbit with the network of radio telescopes that comprise VLBA and X-ray telescopes. The aim of the research was to better understand how gas being fed into a black hole from a binary partner via a spiraling accretion disc connects to powerful jets of material that launch out from near the central region at near light speed.

“We had not originally aimed to refine the distance and the mass of the black hole but realised that our data would allow us to do so, by accounting properly for the effects of the black hole orbit.  But there is still a wealth of data from this rich observing campaign that we are looking to analyse more fully.”

Professor James Miller-Jones, ICRAR, Curtin University

“Black holes form from the deaths of the most massive stars when they run out of fuel and gravity takes over,” says Miller-Jones. “The mass of the resulting black hole is set by the initial mass of the star from which it formed — which we call the progenitor star — the amount of mass that star lost in winds over its lifetime, and any interactions with a nearby companion star.” 

Miller-Jones continues, saying massive stars launch very powerful winds from their surfaces, which leads to significant mass loss over their few-million year lifetimes. Some of the later phases of star’s evolution have particularly strong winds — determined by the abundance of elements heavier than helium in the gas from which the star was formed. More heavy elements mean stronger winds, and ultimately, a less massive star immediately before gravitational collapse. 

While some stars can also lose further mass in supernova explosions as they collapse to form a black hole, the evidence suggests that in Cygnus X-1, there was no explosion, and the star collapsed directly into a black hole,” says Miller-Jones. “The stronger the stellar winds during the late evolutionary phases of the star, the less massive we would have expected the black hole to be.”

An artist’s impression of the Cygnus X-1 system. This system contains the most massive stellar-mass black hole ever detected without the use of gravitational waves, weighing in at 21 times the mass of the Sun. (ICRAR)

At first, the team wasn’t totally aware of just how significant their discovery of mass disparities in the Cygnus X-1 binary system was. “I think that our biggest surprise was when we appreciated the full implications of our measurements,” Miller-Jones says. “As observational astronomers, my team and I had already found that we could revise the source distance and the black hole mass. However, it was not until I visited a colleague, Professor Ilya Mandel of Monash University, who is a theoretical astronomer, that we realised how important this actually was.”

Mandel–co-author on the resulting paper– realised that a 21-solar mass black hole was too massive to form in the Milky Way with the constraints in place due to the current prevailing estimates of the amount of mass lost by massive stars in stellar winds.

“The existence of such a massive black hole in our own Milky Way galaxy has shown us that the most massive stars blow less mass off their surface in winds than we had previously estimated. This improves our knowledge of how black holes form from the most massive stars.”

Professor James Miller-Jones, ICRAR, Curtin University

Cygnus X-1: No Stranger to Contraversy

The team’s findings have allowed them to put forward a scenario that would allow the formation of a 21 solar mass black hole in a binary system. “We suggest that the star that eventually collapsed into a black hole began its life a few million years ago with a mass of 55-75 times the mass of the Sun,” Miller-Jones tells ZME.  “Over its lifetime, it was close enough to its companion–the current supergiant–that gas from its surface was transferred onto its companion.  This removed the outer layers of the black hole progenitor and caused it to rotate more rapidly because the two stars were always keeping the same face towards one another. 

“Eventually, possibly as recently as a few tens of thousands of years ago the progenitor star collapsed directly into a black hole–of close to its current mass of 21 times the mass of the Sun–without a supernova explosion.”

Professor James Miller-Jones, ICRAR, Curtin University



Additionally, as well as gaining an insight into the black hole’s birth, Miller-Jones believes the team’s results could also indicate how the system could end its life. “Finally, we considered the eventual fate of this system,” the paper’s lead author says. “While the current companion star may eventually form a black hole, the separation of the two stars is such that the two black holes are unlikely to merge on a timescale comparable to the age of the Universe.”   

A companion paper appearing at the same time in the Astrophysical Journal will delve deeper into these elements of the research.

A closer look at the massive star in the Cygnus X-1 binary (ICRAR)

This isn’t the first time that Cygnus X-1, and more specifically its black hole, has sparked discussion in the fields of astronomy and cosmology. As speculation grew during that the intense X-ray source in the region was the result of a black hole, renowned physicist Stephen Hawking bet fellow scientist Kip Thorne–well known for his black hole work–in 1974 that Cygnus X-1 did not contain a black hole.

“This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse.”

Stephen Hawking, A Brief History of Time

Hawking lost the bet, conceding by breaking into Thorne’s office whilst he was on a trip to Russia and signing the framed bet.

The team now intend to apply the technique that led them to this finding to investigate further black holes. This should enable them to better understand how massive stars lose mass through stellar winds. With that said, as Cygnus X-1 is relatively unique in the Milky Way–as one of the few black holes so far detected in orbit with a massive companion star– Miller-Jones believes that they are unlikely to find any more binary systems in which the masses of the constituent star and black hole diverge so drastically from current estimates.

“Most excitingly for me, the advent of cutting-edge new telescopes such as the Square Kilometre Array radio telescope (SKA) will allow us to detect many more black holes, and study their properties, including how matter flows into and away from them, in more detail than ever before,” concludes Professor Miller-Jones. “It’s an exciting time to be in this field!”

Sources

Miller-Jones. J., Orosz. J. A., Mandel. I., ‘Cygnus X-1 contains a 21-solar mass black hole – implications for massive star winds,’ Science, [2021], [https://science.sciencemag.org/lookup/doi/10.1126/science.abb3363]

Gallery of stellar winds around cool ageing stars, showing a variety of morphologies, including disks, cones, and spirals. The blue colour represents material that is coming towards you, red is material that is moving away from you. (L. Decin, ESO/ALMA)

How Stellar Winds of Dying Stars Are Shaped

New observations have revealed that stellar winds are not spherical as previously believed, but instead come in a variety of shapes that resemble those of planetary nebulae — created when a dying star explosively sheds its outer layers, which by a weird naming quirk actually have nothing to do with planets.  In fact, those winds could mark out the ‘molds’ by which planetary nebulae are shaped.

The discovery comes as a result of research conducted by a team of astronomers including Leen Decin, from the Institute of Astronomy, KU Leuven, and is detailed in a paper published today in the journal Science. “We noticed these winds are anything but symmetrical or round,” Decin says. “Some of them are actually quite similar in shape to planetary nebulae.”

Gallery of stellar winds around cool ageing stars, showing a variety of morphologies, including disks, cones, and spirals. The blue colour represents material that is coming towards you, red is material that is moving away from you. (L. Decin, ESO/ALMA)
Winds of red giant stars observed around Gamma Aquilae
[Credit: Decin et al., Science (2020)] (L. Decin, ESO/ALMA)

The team believes that this variety in stellar winds and planetary nebulae shape around dying stars are connected and a result of interactions with companion stars in binary pairings, or even from exoplanets in orbit around the stars. “The Sun — which will ultimately become a red giant — is as round as a billiard ball,” Decin explains. “So we wondered; how can such a star produce all these different shapes?”

The findings collected by the team could explain a long-standing mystery of planetary nebulae around stellar remnants like red dwarfs come in a variety of close-but-not-quite-spherical shapes. 

Planetary nebulae display such a wide range of complex shapes and structures that although the influence of binary companions has been suggested as a possible cause of this diverse range of asymmetric forms, the fact they can arise around stars with spherically symmetric stellar winds has, until now, remained unexplained.

The answer found by the team is that these winds aren’t symmetric at all and that the shape of the winds directly informs the shape of planetary nebulae. 

Dying Stars’ Companions are a Bad Influence

The observations of the stellar winds of 14 AGB stars using the Atacama Large Millimeter/submillimeter Array made by the team were so-detailed that they actually allowed the team to categorize the shapes of the stellar winds and planetary nebula. Some were disc-shaped, some contained spirals, and some were conical — a good indication that the shapes were not created randomly — but, none had spherical symmetry.

Gallery of stellar winds around cool ageing stars, showing a variety of morphologies,
including disks, cones, and spirals. The blue colour
represents
material that is coming towards you, red
is material that is moving away from you. (L. Decin, ESO/ALMA)

In fact, the team realized it was the presence of other low-mass stars or exoplanets in the vicinity of the primary star that was shaping the stellar wind and planetary nebula. Professor Decin is on hand to provide a useful and colorful analogy: “Just like how a spoon that you stir in a cup of coffee with some milk can create a spiral pattern, the companion sucks material towards it as it revolves around the star and shapes the stellar wind.”

Stellar winds are important to astronomers as they account for one of the main mechanisms by which stars lose mass. This mechanism becomes even more critical when attempting to understand the death throes of stars of similar sizes to the Sun and as their cores contract and the outer layers swell creating planetary nebulae — the other major contributor to mass-loss in aging stars. Discovering the role played by stellar companions in such a process is a surprise, to say the least. 

The stellar wind of R Aquilae resembles the structure of rose petals. (L. Decin, ESO/ALMA)

“All our observations can be explained by the fact that the stars have a companion,” says Decin. “Our findings change a lot. Since the complexity of stellar winds was not accounted for in the past, any previous mass-loss rate estimate of old stars could be wrong by up to a factor of 10.”

Following this discovery, the team will now research how it impacts other crucial characteristics involved in the life and death stars like the Sun. In the process, the team believes that their research will add more depth to our view of stars.

The Stellar winds around R Hydrae take a more conical shape (L. Decin, ESO/ALMA)

“We were very excited when we explored the first images,” adds co-author Miguel Montargès, also from KU Leuven. “Each star, which was only a number before, became an individual by itself. Now, to us, they have their own identity. “This is the magic of having high-precision observations: stars are no longer just points anymore.”

But, whilst we are on the subject of the future, the team says their findings have particular ramifications for the end of our own star.

Death Spiral: How the Sun Dies and What it Leaves Behind

The Sun is roughly halfway through its lifetime, with half its core hydrogen exhausted, meaning that in approximately 5 billion years it will start to die. For a star of the Sun’s mass, this means undergoing the transformation into a red giant.

For stars with masses greater than the Sun, the collapse of their core will spark a new lease of life, with the fusion of helium into heavier elements being kick-started by tremendous gravitational pressure, providing an outward force that halts the collapse.

The Sun, in contrast, will fade as its core cools, the planetary nebula will continue to expand outwards, ultimately resulting in a white dwarf surrounded by diffuse material that was once its outer layers. 

The team’s research gives us an idea of just what shape this planetary nebula will take, and how it will be crafted by the solar system’s largest planets. “Jupiter or even Saturn — because they have such a big mass — are going to influence whether the Sun spends its last millennia at the heart of a spiral, a butterfly, or any of the other entrancing shapes we see in planetary nebulae today,” Decin notes. 

“Our calculations now indicate that a weak spiral will form in the stellar wind of the old dying Sun.”