On December 8, 2023
Introduction
No one has ever seen a new star feeding on its protostellar disk anywhere outside our galaxy – until now. Star formation is a complex process. Although the process takes much longer than a human lifespan, we have studied its various stages in the star nurseries scattered throughout our galaxy enough to gain a good general understanding of how it works. The process generally begins with a massive cloud of gas and cosmic dust – like the Orion Nebula, which currently adorns the winter sky. Movements within the cloud can lead to the formation of fragile clumps of material. If these clumps grow large enough, they can gain the gravity needed to collapse and become denser, drawing in more material from the surrounding cloud at the same time.
Star Formation and Feeding
As this collapsing mass gathers, the material inside enhances any rotational motion in the gas, causing the clump to spin and flatten into a disk containing a promising star glowing at the center. This protostar becomes hotter and more massive as it feeds on the inflowing gas from the disk. Eventually, it gains enough mass to compress hydrogen atoms together in a tightly packed, high-pressure core until they fuse into helium, releasing vast amounts of energy. At this point, the star is literally born.
Discovery of the Protostellar Disk
Although the central sun is the “star” of this show, the disk that feeds it material plays a crucial supporting role – both in star birth and the emergence of accompanying planets. We have seen such disks around many stars still in the formation stage in our Milky Way galaxy but have not observed them outside – until now.
Discovery of a Protostellar Disk Outside the Milky Way
Astronomers have for the first time discovered a rotating disk of material around a very young star in another galaxy, a discovery that offers new insights into how stars form under different cosmic conditions. The results were published in the journal “Nature.”
The Galaxy in Question
The galaxy in question is the Large Magellanic Cloud (LMC), a small companion to the Milky Way galaxy located about 160,000 light-years from Earth. This nearby cosmic companion is visible to the naked eye in the Southern Hemisphere but does not rise above the celestial horizon of the night sky in most northern latitudes. A few years ago, astronomers examined the gas cloud LH 117 (also known as NGC 2122), a stunning star factory in the LMC filled with hundreds of stars, and found that one of these stars stood out due to long jets of material erupting from it. Such jets are common around young stars.
Formation of Protostellar Disks
Although the details of how these jets arise are still unclear, magnetic fields in the disk must somehow be involved. The gas in the disk is extremely hot – hot enough to strip electrons from its parent atoms in a process called ionization. The ionized gas, or plasma, creates an internal magnetic field as it moves, causing plasma directed towards the central star of the disk to acquire an increasingly strong magnetic field. The rapid circular rotational motion of the plasma also winds around this strong magnetic field like spaghetti around a twisted fork. At the center, very close to the star itself, the magnetic field bursts outward – up and down in relation to the disk – in a swirling motion that pulls material along with it. These stellar vortices create jets and can carry enormous energy as they expel the material within at very high speeds, sometimes exceeding 300,000 kilometers per hour. These objects are known as Herbig-Haro objects, or HH objects.
Discovery
The Proto-planetary Disc of Stars
The tightly wound magnetic field holds a concentration of objects, so they often extend over long distances. The star that caught the attention of astronomers, named HH 1177, has objects that stretch over a distance of 33 light-years from end to end. We can even tell the direction these objects are pointing in space; the light of one particle is moving towards the blue end of the spectrum, with its wavelength compressed and shortened due to the motion of its source towards the observer. This particle is heading towards us. The other particle is moving away from us, causing its emitted light wavelength to stretch and become longer.
Significance of the Discovery
The polar directional nature of the particle beams suggests that there is a rotating disc at its source that focuses and feeds the star. Hints of such a disc were evident in the original images from the Very Large Telescope in Chile. To obtain evidence, astronomers turned to the Atacama Large Millimeter/submillimeter Array (ALMA), which is also located in the high desert of Chile. ALMA can create high-resolution maps of the spatial distribution of gases such as carbon monoxide and sulfur monoxide (commonly observed around young stars). It can also measure the precise wavelength of light emitted by such molecules, which can reveal their motion towards or away from us through blue and redshift.
Discovery of the Proto-planetary Disc of Stars
What the team found is compelling evidence, or at least proof, of a rotating disc very close to the star, at the base of the objects, with gas compressed towards us on one side and redshifted gas on the other moving away from us. Our view of HH 1177 is thus akin to standing in front of a whirlpool and watching it spin clockwise: the brightly colored plastic horses on the left are moving towards you, while those on the right are moving away from you. The gas in the HH 1177 disc exhibits precisely this kind of motion.
Importance of the Discovery
This discovery beyond the galaxy is not just a new record for the farthest star-forming disc ever observed. It is also a beautiful example of star formation to compare with what we see in our own galaxy. The star at the heart of HH 1177 is massive, likely double the mass of our Sun. In the Milky Way, such massive stars are typically surrounded by dense clouds of mysterious dust, making their direct study challenging.
Differences in the Large Magellanic Cloud
But the Large Magellanic Cloud (LMC) is different. Its gas and stars are relatively poor in heavy elements like carbon and iron compared to the material in the Milky Way, altering the appearance and behavior of this small galaxy. In particular, since dust is made up of heavier elements like carbon and silicon, there is less of it in the LMC than in our galaxy, which gives us a clearer view of massive star formation there. HH 1177 is the first massive star observed by astronomers unobstructed at this stage of its stellar evolution.
Differences in the Proto-planetary Disc of Stars
The disc is also different from its counterparts in the Milky Way. It is massive, with two to four times the mass of the Sun itself, and in our galaxy, dense discs tend to fragment and break apart. The disc around HH 1177 appears stable; however, its discovery suggests that this stability is also due to the LMC’s lower abundance of heavy elements. Stars with fewer heavy elements often emit more ultraviolet radiation, which can heat the surrounding gas more efficiently. This may be the case here. The hotter gas in the disc means that the disc has greater internal pressure to resist the gravitational pull of the disc itself, making the disc stable like a strong and inflated bicycle tire.
Similarity
The Stars in the Large Magellanic Cloud
Despite this, HH 1177 is very similar to the group of massive young stars in our galaxy at the same evolutionary stage. This similarity suggests that stars in other galaxies form in the same way they do here in the Milky Way – but as we’ve seen, there may be differences that appear in the details.
The Importance of the Discovery
This is crucial for our understanding of the dynamic complexity of how stars and planets are born from disks. We use gravity, radiation physics, gas dynamics, magnetism, and more to predict how such objects behave. By observing how the process forms under different conditions, we can push the limits of our models to see how they perform under stress. If they remain intact, our confidence in their accuracy holds, and if broken, there are significant gaps that we must continue to account for in our calculations of star birth.
The Importance of the Discovery
Star-forming regions rich in gas are distributed throughout the Large Magellanic Cloud. HH 1177 is the first disk we see directly there, but it will not be the last. Every time we find a new disk, it will be another step toward understanding how stars are born – and how we all came to be here.
Copyright and Permissions: Phil Plait is a professional astronomer and science communicator in Virginia. He writes Bad Astronomy. Follow him on Substack.
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