Massive Star Seen Forming in Real Time

On the York Universe radio show this past Monday evening on astronomy.fm, I was having a discussion with another host about how so many things in Astronomy can take Millions or even Billions of years, yet there are still all kinds of phenomena that happen in seconds, hours, weeks, years, or on the scale of human lifetimes.  Stars live for hundreds of Millions of years at the low end, yet the intense brightening from the Supernova death of a massive star lasts for only a few days or weeks.  It’s as if the Universe is piquing our interest with short timescale events that are spectacular to watch, and then we begin looking closer and we see that these short lived events bookend Millions of years of preparation time.

Now that we have had a series of powerful observational tools operating for the past few decades, such as the Hubble Space Telescope (reaching its 25th anniversary later this month), we can look at the evolution of different astronomical objects over this time scale, and we have new opportunities to learn more about their past and future.  A good example of this is the formation of massive stars, around 8 solar masses or larger, which form in a different way than lower mass stars like our Sun.  We have a solid understanding of how sun-like stars form, but the exact processes that form these larger stars still elude astronomers.

Artist’s conception of the development of W75N(B)-VLA-2.. At left, a hot wind from the young star expands nearly spherically, as seen in 1996. At right, as seen in 2014, the hot wind has been shaped by encountering a dusty, donut-shaped torus around the star and appears elongated. Credit: Bill Saxton, NRAO/AUI/NSF

Using the Karl G. Jansky Very Large Array (VLA) in New Mexico, astronomers looked at two images taken 18 years apart to study the formation of a young star about 8 times as massive as the Sun.  The star, W75N(B)-VLA-2, 4500  light years from Earth, is forming in a dense, gaseous environment, and experiences periods where it ejects hot winds in a spherical shell for years at a time.

As the spherical ejection spread outward, it encountered a torus shaped ring of dust surrounding the star.  As the hot gas hit the torus it slowed down, while the outflow continued to expand radially, giving it an elongated shape.  Theoretical models exist to explain why massive stars have spherical outflows, while smaller sun-like stars have narrow, beam-like outflows at similar stages of development.  But since the full picture of massive star formation has yet to be developed, the long term study of massive stars such as W75N(B)-VLA-2 can give astronomers clues to the processes that are key to their development.

 

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