The Milky Way is blowing a star-forming bubble, and we’re in the middle of it.
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Star formation in the 500 light-years around Earth is being
driven by a cosmic bubble known as the Local bubble, as seen in this artist's
concept.
Think “bubbles,” and you may think “soap” or “gum.”
But not Catherine Zucker, currently a Hubble Fellow at the
Space Telescope Science Institute and a former researcher with the
Harvard-Smithsonian Center for Astrophysics. Zucker’s interest in bubbles is
cosmic. And she and her collaborators have found new insights about a bubble in
which our solar system sits.
Astronomers have long known about the 1,000-light-year-wide
Local Bubble. In a new paper published Jan. 12 in Nature, Zucker and her
co-authors describe it as “a cavity of low-density, high-temperature plasma
surrounded by a shell of cold, neutral gas and dust.” But for years,
astronomers were in the dark beyond that. The history of the Local Bubble, even
its size, remained unknown.
Not anymore. Zucker and her team became accidental
historians when, starting work on a different project, they found instead a
kind of creation story of our local stellar neighborhood, and provided robust
confirmation of the assumption that supernovae — the explosions of dying stars
— lead to the birth of other stars. This happens when the blown-out materials
recombine elsewhere due to the force of gravity.
Bubbly beginnings
What Zucker’s team found, according to their paper, was “that nearly all of the star-forming complexes in the solar vicinity lie on the surface of the Local Bubble and that their young stars show outward expansion mainly perpendicular to the bubble’s surface.” She calls it a “eureka moment.”
In other words, the young stars in our galactic neighborhood
are almost all due to the expansive shock waves of a series of supernovae and
that process of blown-out remains recombining to birth new suns and new solar
systems. The bubble - which is actually shaped more like a piece of pipe
cutting through the plane of the Milky Way – seems to have formed 14 million
years ago from some 15 supernovae, and the triggered star formation that is
still happening today.
The last such supernova took place about 2 million years
ago, according to Zucker’s research — a finding that matches nicely with the
previously reported deposition of cosmic iron in the Earth’s crust.
Zucker presented her team’s work virtually this week at a
drastically scaled-back gathering of the American Astronomical Society, which
was to have met in-person in Salt Lake City. The ongoing COVID-19 pandemic
upended those plans.
Zucker told Astronomy that while there are “tens of millions
of ‘old’ stars [those that are more than 14 million years old] inside the Local
Bubble,” there are “on the order of thousands of ‘young’ stars … on its surface
that have been birthed by the supernovae.”
It just happens that the Sun and our solar system currently
sit inside this bubble. According to the team, the Sun rolled into the Local
Bubble about 5 million years ago — but it likely sat in other bubbles at other
times.
“This work is most useful for providing the ‘big picture’
context for star and planet formation,” Zucker says. “One takeaway that might
have been missed is that this study is really the tip of the iceberg. The Local
Bubble is just the first bubble whose history we have mapped out — it's the
easiest one to understand first, since it's the bubble in which our Sun
currently resides. However, we have clues that not just single superbubbles,
but the interactions of many superbubbles, are driving the formation of young
stars near our Sun.”
Zucker compares the process to plowing snow. If one or more
superbubbles is “piling up gas in the same region of space … we should get even
more enhanced star formation at those intersecting surfaces.” And, in fact, one
such bubble, called Perseus-Taurus, is interacting with our Local Bubble “at
the site of the Taurus molecular cloud” — home to known protoplanetary disks.
The European Space Agency’s Gaia star-mapping mission was
crucial in providing the precise data needed to discover the star-formation
nuances of the Local Bubble — what Zucker has been calling an “origin story.”
Luckily, you don’t need access to high-end data to connect
to this work. “The two clusters of stars that hosted the supernovae are still
around and they are about 15 to 16 million years old,” says Zucker. “They
currently lie near the edge of the Local Bubble's shell.” (At the time, the
supernovae were getting underway, these clusters were in the thick of the
action.)
You can point your telescope toward those local star-forming
regions. One is in Taurus, the other is in Ophiuchus, home of the Ophiuchus
Nebula. Looking into those areas gives you a chance to bear witness to the
history and continuance of star birth in our Local Bubble.
Interested readers can also find cool data visualizations
and more information at the team’s dedicated website.