Astronomers Discover Disc Around Star in Another Galaxy

Illustration of the young stellar body in the Large Magellanic Cloud surrounded by an accretion disk.

The analogy to life is almost unavoidable in the developmental history of stars. Their existence, which begins in dense molecular clouds, is characterized by turbulent growth and various developmental stages before they burn out and die in a more or less spectacular manner. In its infancy, during the growth phase of a young star, it draws on gas and dust that converge in a rotating disk around it. The remnants of this material feast can later contribute to the formation of planets.

Astronomers have now observed the growth process of a star for the first time outside the Milky Way: they discovered an accretion disk around a massive young star in the Large Magellanic Cloud, one of our neighboring galaxies. “When I first saw evidence of a rotating structure in the ALMA data, I couldn’t believe that we had discovered the first extragalactic accretion disk. It was really a special moment,” says Anna McLeod from the UK’s Durham University, who published the discovery with an international research team in the journal Nature. “We know that disks are crucial for the formation of stars and planets in our galaxy, and here we see direct evidence of it in another galaxy for the first time.

Insight into the Neighborhood

The discovery occurred in two steps, initially using the Muse instruments on the Very Large Telescope of the European Southern Observatory (ESO) in Chile. This powerful 3D spectrograph can not only measure distant galaxies but also perform spectral analyses of individual stars. That’s exactly what revealed a jet from a young star deep within a gas cloud of the Large Magellanic Cloud. “We discovered a jet emanating from this young massive star. That was the first sign of ongoing disk accretion,” said McLeod.

To confirm that such a disk actually exists, the team had to measure the movement of dense gas around the star. This was achieved using the Atacama Large Millimeter/Submillimeter Array (Alma), which could detect the dynamics of the disk. When matter is drawn toward a growing star, it doesn’t fall directly into the star but flattens into a rotating disk. Near the center, the disk rotates faster. By measuring this difference in velocity, it was demonstrated that an accretion disk is present.

Unobstructed View into the Distance

The frequency of light changes depending on how quickly the luminous gas is moving toward or away from us,” explained Jonathan Henshaw from Liverpool John Moores University, who was also involved in the study. “This is the same phenomenon that occurs when the pitch of an ambulance siren changes as it passes by. The frequency of the sound shifts from higher to lower.” The frequency measurement confirmed that the system known as HH 1177 is the first observed extragalactic star with an accretion disk.

Looking into a neighboring galaxy can be helpful in studying such phenomena more precisely, the researchers note. Massive stars in the Milky Way are challenging to observe because the dust clouds from which they originate frequently heavily obscure them. In the Large Magellanic Cloud, about 160,000 light-years away, the material for star formation differs from that in our galaxy. Thanks to the lower dust content, HH 1177 is not enveloped in its birth cocoon anymore and is therefore well observable despite the significant distance, said McLeod.

Featured Image: Illustration of the young stellar body in the Large Magellanic Cloud surrounded by an accretion disk, ESO/M. Kornmesser.