The Solar
System floats in the middle of a peculiarly empty region of space.
This region
of low-density, high-temperature plasma, about 1,000 light-years across, is
surrounded by a shell of cooler, denser neutral gas and dust. It's called the
Local Bubble, and precisely how and why it came to exist, with the Solar System
floating in the middle, has been a challenge to explain.
A team of
astronomers led by the Harvard & Smithsonian Center for Astrophysics (CfA)
has now mapped the Local Bubble with the highest precision yet – and found that
the Local Bubble was likely carved out of the interstellar medium by a series
of supernova explosions millions of years ago. This is consistent with previous
studies, with an additional sting in the tail: the still-expanding Local
Bubble is responsible for regions of heightened star formation at its
perimeter.
"This is really an origin story; for the first time we can explain how all nearby star formation began," says astronomer Catherine Zucker of the Space Telescope Science Institute, who conducted the research while at the CfA.
The Local
Bubble was only discovered relatively recently, in the 1970s and 1980s, through
a combination of optical, radio, and X-ray astronomy. Gradually, these surveys
and observations revealed a huge region about 10 times less dense than the
average interstellar medium in the Milky Way galaxy. Since we know supernovae
can carve out cavities in space, sweeping up gas and dust as they expand
outwards, this seemed a reasonable explanation for the Local Bubble.
But piecing
together how and when was more tricky. It's hard, for one thing, to measure the
dimensions of a region of space when you're inside it; and doubly hard to
measure a void when you're surrounded by bright stars and other cosmic objects.
Zucker and her team used data from the most recent Gaia data
release – an ongoing project to map the positions and motions of stars
in the Milky Way with the highest precision yet – to map the gas and young
stars within 200 parsecs (around 650 light-years) of the Sun.
They found
that all young stars and star-forming regions are on the "surface" of
the Local Bubble. This makes sense; as a supernova expands outwards, it shocks
and compresses the material it expands into. This creates dense knots in
molecular gas floating in the interstellar medium that collapse under their own
gravity to form
baby stars.
Next, the
researchers conducted simulations and tracebacks of the motions of star-forming
regions to model the Bubble's expansion. This allowed them to reconstruct its
history, matching the outcomes of their calculations against their map of the
bubble. They found that the Bubble's history started about 14.4 million years
ago, first with a period of stellar birth, followed by the supernovae of
massive, short-lived stars.
"We've calculated that about 15 supernovae have gone off over millions of years to form the Local Bubble that we see today," Zucker explains.
It's
currently about 165 parsecs (538 light-years) in radius, and it's still
expanding outwards, although relatively slowly, at a rate of about 6.7
kilometers (4 miles) per second. So why is the Solar System in the middle?
Well, that's purely a coincidence.
"When the first supernovae that created the Local Bubble went off, our Sun was far away from the action," says physicist and astronomer João Alves of the University of Vienna in Austria. "But about five million years ago, the Sun's path through the galaxy took it right into the bubble, and now the Sun sits – just by luck – almost right in the Bubble's center."
According to
the researchers, this suggests that the Milky Way is likely full of similar
bubbles since the likelihood of this happening is very low if the bubbles are
rare. The idea evokes a Milky Way structured similarly to a sea sponge, or
perhaps a flattened wheel of Swiss cheese. The next step along this line of
inquiry is to try and find and map the other bubbles. Their locations, sizes,
shapes, and how they interact with each other could be clues that help us
better understand the star formation and evolutionary history of the Milky Way.
The next Gaia data release, due to drop later this year, should
prove extremely useful for this.
"This is an incredible detective story, driven by both data and theory," says astronomer Alyssa Goodman of Harvard University. "We can piece together the history of star formation around us using a wide variety of independent clues: supernova models, stellar motions, and exquisite new 3D maps of the material surrounding the Local Bubble."
The research has been published in Nature.