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Some facts about Wolf-Rayet Stars

Wolf-Rayet stars are named the ‘most massive and brightest stars known’. This is because their temperatures start at 30,000 degrees celsius. They also have strong stellar winds which blow away their outer atmospheres, revealing the stars’ inner layers. Blowing at over ten million miles per hour, the stars shed about 2 thousand billion billion tons of material every year. That’s equivalent to the mass of three Earths! Since these stars have trouble holding themselves together, they don’t last very long, burning up their fuel quickly and blasting mass into space, eventually tearing themselves apart. They are usually members of binary stars with O or B stars (types of stars classed by temperature) as companions.vd

The discovery

The French astronomers Charles Wolf (1827-1918) and Georges Rayet (1839-1906) co-discovered this type of unusual, hot star, which are now named after them. They discovered Wolf-Rayet stars by using the Paris Observatory’s 40cm Foucault telescope in 1867 to observe three stars whose spectra had strong, broad emission lines, but few absorption lines, which is unusual for stars. 

For decades, the reason for these emission bands remained a mystery. Eventually, it was discovered that these lines resulted from the presence of helium, which was discovered in 1868, one year after the original observation. E.C.Pickering also compared Wolf-Rayet spectra with nebula spectra, and noticed similarities between them. This led to his discovery that some or all Wolf-Rayet stars are in the centre of nebulae.

In 1929, doppler broadening (the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules) was being used to explain the width of the emission bands. It was concluded that the gas surrounding Wolf-Rayet stars must be moving with velocities of 300-2400 km/s, and therefore they are continually ejecting gas into space. This produces an expanding envelope, or bubble, of nebulous gas. The force that ejects this gas was then discovered to be radiation pressure.

Later, Rayet became Director of the Bordeaux Observatory, and to this day, we have discovered over 500 Wolf-Rayet stars in our galaxy. 

About the most massive star, R136a1, which is a Wolf-Rayet star

R136a1 is the most massive star known, which is a Wolf-Rayet star. It is 163,000 light years away from the Sun, and is located in the Large Magellanic Cloud. It is also part of the R136 super star cluster, and has the mass of 315 suns. Despite being the most massive star known and shining 9 million times brighter than the sun, it requires a telescope to see. An interesting fact about R136a1 is that it defies what scientists know about how stars form. A popular hypothesis among scientists is that R136a1 did not form directly from the collapse of a molecular hydrogen cloud, but rather from two massive stars colliding. 

About WR 124, a Wolf-Rayet star

WR 124 is 15,000 light years away from the Sun. Its spectral type is WN, meaning it falls into the hottest ‘O’ spectral type of stars, but is referred to as ‘W’ from Wolf-Rayet. The ‘N’ means it shows strong emission lines of nitrogen. WR 124 is the glowing star in the centre of a huge, fiery nebula. WR 124 has a surface temperature of around 50,000 degrees celsius, and is one of the hottest known Wolf-Rayet stars. It is a massive, unstable star which is blowing itself apart. Its material is travelling at up to 150,000kph. The nebula that surrounds the star, M1-67, consists of vast arcs of glowing gas which is violently expanding outwards into space. M1-67 is quite young, only 10,000 years old, and it contains clumps of material within it with masses 30 times the mass of Earth and diameters of 150 billion km.

WR 7, another Wolf-Rayet star

WR 7 is also 15,000 light years away from the Sun. It produced the emission nebula NGC 2359, which is also known as Thor’s Helmet because it looks like a helmet with wings. The nebula has a diameter of around 30 light years, and has WR 7 at its centre. Its surface temperature is between 30,000 degrees celsius and 50,000 degrees celsius, which is 6 to 10 times the temperature of the sun. It is an incredibly unstable star, ejecting stellar material into the interstellar medium at speeds which approach 7.2 million kph! Even though it is a massive star, it loses the mass of the Sun every thousand years. Material ejected from the star is done so in a spherical manner, which produces a bubble of material. This bubble has been shaped further by its interactions with the surrounding interstellar medium. WR 7 lies at the edge of a dense, warm molecular cloud, which is unusual.

NASA’s Webb Telescope Captures Rarely Seen Prelude to Supernova

A massive star on the cusp of death ejected huge volumes of hot gas into space. Webb’s Mid-Infrared Instrument revealed the structure of the material now surrounding the star.

The rare sight of a Wolf-Rayet star – among the most luminous, most massive, and most briefly detectable stars known – was one of the first observations made by NASA’s James Webb Space Telescope in June 2022. Webb shows the star, WR 124, in unprecedented detail with its powerful infrared instruments. The star is 15,000 light-years away in the constellation Sagittarius.

Massive stars race through their lifecycles, and only some of them go through a brief Wolf-Rayet phase before going supernova, making Webb’s detailed observations of this rare phase valuable to astronomers. Wolf-Rayet stars are in the process of casting off their outer layers, resulting in their characteristic halos of gas and dust. The star WR 124 is 30 times the mass of the Sun and has shed 10 Suns’ worth of material – so far. As the ejected gas moves away from the star and cools, cosmic dust forms and glows in the infrared light detectable by Webb.

The origin of cosmic dust that can survive a supernova blast and contribute to the universe’s overall “dust budget” is of great interest to astronomers for multiple reasons. Dust is integral to the workings of the universe: It shelters forming stars, gathers together to help form planets, and serves as a platform for molecules to form and clump together – including the building blocks of life on Earth. Despite the many essential roles that dust plays, there is still more dust in the universe than astronomers’ current dust-formation theories can explain. The universe is operating with a dust budget surplus.

Webb opens up new possibilities for studying details in cosmic dust, which is best observed in infrared wavelengths of light. Webb’s Near-Infrared Camera (NIRCam) balances the brightness of WR 124’s stellar core and the knotty details in the fainter surrounding gas. The telescope’s Mid-Infrared Instrument (MIRI) reveals the clumpy structure of the gas and dust nebula of the ejected material now surrounding the star. Before Webb, dust-loving astronomers simply did not have enough detailed information to explore questions of dust production in environments like WR 124 and whether the dust grains were large and bountiful enough to survive the supernova and become a significant contribution to the overall dust budget. Now those questions can be investigated with real data.

Stars like WR 124 also serve as an analog to help astronomers understand a crucial period in the early history of the universe. Similar dying stars first seeded the young universe with heavy elements forged in their cores – elements that are now common in the current era, including on Earth.

Webb’s detailed image of WR 124 preserves forever a brief, turbulent time of transformation, and promises future discoveries that will reveal the long-shrouded mysteries of cosmic dust.

More About the Mission

The James Webb Space Telescope is the world’s premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency), and CSA (Canadian Space Agency).

MIRI was developed through a 50-50 partnership between NASA and ESA. NASA’s Jet Propulsion Laboratory led the U.S. efforts for MIRI, and a multinational consortium of European astronomical institutes contributes for ESA. George Rieke with the University of Arizona is the MIRI science team lead. Gillian Wright is the MIRI European principal investigator. Alistair Glasse with UK ATC is the MIRI instrument scientist, and Michael Ressler is the U.S. project scientist at JPL. Laszlo Tamas with UK ATC manages the European Consortium. The MIRI cryocooler development was led and managed by JPL, in collaboration with Northrop Grumman in Redondo Beach, California, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Caltech manages JPL for NASA.

2023 February 8

Stellar Wind-Shaped Nebula RCW 58 Image Credit & Copyright: Mike Selby & Mark Hanson; Text: Natalia Lewandowska (SUNY Oswego)

Explanation: Imagine traveling to a star about 100 times as massive as our Sun, a million times more luminous, and with 30 times the surface temperature. Such stars exist, and some are known as Wolf Rayet (WR) stars, named after French astronomers Charles Wolf and Georges Rayet. The central star in this image is WR 40 which is located toward the constellation of Carina. Stars like WR 40 live fast and die young in comparison with the Sun. They quickly exhaust their core hydrogen supply, move on to fusing heavier core elements, and expand while ejecting their outer layers via high stellar winds. In this case, the central star WR 40 ejects the atmosphere at a speed of nearly 100 kilometers per second, and these outer layers have become the expanding oval-shaped nebula RCW 58.

If my Lord asks me, “Have you shyness in disobeying me?

وتخفي الذنب عن خلقي وبالعصيان تأتيني

You conceal your sins from my creation – and with sins you come to me.”

فكيف أجيبُ يا ويحي ومن ذا سوف يحميني؟

So how will I answer? O woe to me – and who shall protect me?

أسُلي النفس بالآمالِ من حينٍ الى حيني

I keep averting my soul with thoughts of hope – from time to time.

وأنسى ما وراءُ الموت ماذا بعد تكفيني

And I forget what is to come after death – and what is to come after I am shrouded.

كأني قد ضّمنتُ العيش ليس الموت يأتيني

As if I am guaranteed life (eternally) – and that death will not come to me.

وجائت سكرة الموتُ الشديدة من سيحميني

And when the severe stupor of death overtakes me – who will protect me?

نظرتُ الى الوُجوهِ أليـس منُهم من سيفدينـــي

I looked at the faces; is there not from amongst them who will ransom me?

سأسأل ما الذي قدمت في دنياي ينجيني

I will be asked regarding what I have prepared in my life to save me (on the Day of Judgement).

فكيف إجابتي من بعد ما فرطت في ديني

Then how will I answer – after I have neglected my religion.

ويا ويحي ألــــم أسمع كلام الله يدعوني

Woe to me! Did I not hear the Speech of Allāh inviting me?

ألــــم أسمع لما قد جاء في قاف ويسِ

Did I not hear what came in (the chapters of) Qāf and Yā-Sīn?

ألـــم أسمع بيوم الحشر يوم الجمع و الديني

Did I not hear about the Day of Gathering, the Day of Assemble and the Day of Judgement?

ألـــم أسمع مُنادي الموت يدعوني يناديني

Did I not hear the crier of death inviting me, calling me?

فيا ربــــاه عبدُ تــائبُ من ذا سيؤويني

So O my Lord, a slave (turning to you) I have repented – so who then shall shelter me?

سوى رب غفور واسعُ للحقِ يهديني

Except a Lord extensive in forgiveness – to the truth He will guide me.

أتيتُ إليكَ فارحمني وثقــّـل في موازيني

I have come to you (in repentance) – so have mercy on me, and make heavy my scales (with good deeds).

وخفَفَ في جزائي أنتَ أرجـى من يجازيني

And lighten my account – You are the best of who will bring me to account.