Astronomers uncover the secret of giant star formation: Hot ammonia and TG盗号系统VIP免杀技术accretion disks

May 7, 2025  22:45

An international team of astronomers has revealed how massive stars, destined to become supernovae, are born. Their study, published in the journal Astronomy & Astrophysics (A&A), shows that the star HW2in the constellation Cepheus A, located 2,300 light-years from Earth, grows through an accretion disk of hot ammonia. This discovery settles long-standing debates about the formation mechanism of giant stars and opens new frontiers in astrophysics. Here’s how the scientists reached their conclusions and why it matters.

Star HW2: A Giant in Cepheus A

HW2is a young star that has already reached a mass 16 times that of the Sun. It resides in an active star-forming region in Cepheus A, about 2,300 light-years away. Such massive stars are rare, making up less than 1% of all stars in the galaxy, but their impact is immense—they produce heavy elements and end their lives in supernova explosions.

Using the Very Large Array (VLA)radio telescopes in the U.S., scientists discovered an accretion disk of hot ammonia (NH₃) around HW2, with a radius of 200–700 astronomical units (30–105 billion km). This disk—a rotating structure of gas and dust—feeds the star with new material, enabling its growth.

Hot Ammonia: The Key to Growth

The pivotal finding is a powerful gas stream falling onto the star at a rate of 0.002 solar masses per year (roughly 3.98 sextillion tons, or 6.7 billion times Earth’s mass). This is a record-breaking rate for massive stars observed directly. The hot ammonia in the disk, heated to hundreds of kelvins, serves as a marker: its radiation allowed scientists to track the gas’s movement.

“We’ve obtained direct evidence that massive stars form through disk accretion,” said the study’s lead, Dr. Alberto Sanna. “This puts an end to debates about how such giants gain mass.”

Analysis revealed the disk’s asymmetry: one side is denser, suggesting external gas flows feeding the system, like a river pouring into a lake to maintain its level. Computer simulations confirmed the observations, showing a balance between gravitational collapse (gas compression) and disk rotation, which prevents its destruction.

Why It Matters

The discovery resolves a long-standing astrophysical mystery: how do stars tens of times more massive than the Sun accumulate mass in their short lifetimes (just a few million years)? Previously, some scientists suggested that such giants form through chaotic mergers of smaller stars rather than orderly accretion. The new data prove that accretion disks are a universal mechanism, even for the most massive objects.

Other key takeaways:

• Role of Ammonia: Hot ammonia (NH₃) is a rare indicator in accretion disks, as it emits in the radio spectrum, penetrating dust that obscures visible light.
• Supernovae and Elements: Massive stars like HW2 produce carbon, oxygen, and iron, which, after a supernova explosion, become building blocks for planets and life.
• Cosmic Evolution: Studying HW2 helps understand star formation in other galaxies, such as the Large Magellanic Cloud, where an accretion disk was also found around a young star.

What’s Next?

The scientists plan to:

• Use the ALMAtelescope to map HW2’s disk in detail, studying its chemical composition and dynamics.
• Observe other massive stars in Cepheus A to confirm whether ammonia disks are common.
• Model HW2’s evolution to predict when it will become a supernova (roughly in 100,000–1 million years).

These findings will help refine the initial mass function (IMF), which describes how many stars of different masses are born in a galaxy.

Conclusion

The discovery of a hot ammonia accretion disk around HW2 confirms that even the most massive stars form through an orderly process, not chaos. This not only settles debates about star formation but also brings us closer to understanding how galaxies produce the elements of life. HW2 is a cosmic laboratory showing how giants are born. What would you name such a star? Share in the comments!