The Nearby Star Clusters Come from Only Three Places

The most well-known open cluster is probably the Pleiades, or Seven Sisters. The Japanese call it the Subaru cluster, and keen observers might recognize its pattern on the Subaru automobile logo. New research shows that the Pleiades and more than 150 other star clusters all originated in only three star-forming regions. Image: By NASA, ESA, AURA/Caltech, Palomar Observatory. http://hubblesite.org/newscenter/archive/releases/2004/20/image/a/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7805481

Many astronomy-interested people know of the Hyades and the Pleiades. They’re star clusters in the Taurus constellation. They’re two out of a handful of star clusters that are visible to the unaided eye under dark sky conditions.

It turns out that these clusters, along with more than 150 other nearby clusters, all originated in only three massive star-forming regions.

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Was Earth’s Climate Affected by Interstellar Clouds?

About two million years ago, the Solar System may have passed through a dense cloud of hydrogen and radioactive material. It compressed the heliosphere, the protective cocoon the Sun provides for Earth and the other planets. It's shown here as the dark gray bubble over the backdrop of interstellar space. This could have exposed Earth to high levels of radiation and influenced the climate, and possible human evolution. Photo courtesy of Opher, et al., Nature Astronomy

Scientists scour the Earth and the sky for clues to our planet’s climate history. Powerful and sustained volcanic eruptions can alter the climate for long periods of time, and the Sun’s output can shift Earth’s climate over millions of years.

But what about interstellar hydrogen clouds? Can these regions of gas and dust change Earth’s climate when the planet encounters them?

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Nearby Supernovae Exploded Just a few Million Years Ago, Leading to a Wave of Star Formation Around the Sun

Artist's illustration of the Local Bubble with star formation occurring on the bubble's surface. Scientists have now shown how a chain of events beginning 14 million years ago with a set of powerful supernovae led to the creation of the vast bubble, responsible for the formation of all young stars within 500 light years of the Sun and Earth. Credit: Leah Hustak (STScI)

The Sun isn’t the only star in this galactic neighbourhood. Other stars also call this neighbourhood home. But what’s the neighbourhood’s history? What triggered the birth of all those stars?

A team of astronomers say they’ve pieced the history together and identified the trigger: a series of supernovae explosions that began about 14 million years ago.

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A Supernova 2.6 Million Years Ago Could Have Wiped Out the Ocean’s Large Animals

Artist's impression of a Type II supernova. Credit: ESO
Artist's impression of a Type II supernova. Credit: ESO

For many years, scientists have been studying how supernovae could affect life on Earth. Supernovae are extremely powerful events, and depending on how close they are to Earth, they could have consequences ranging from the cataclysmic to the inconsequential. But now, the scientists behind a new paper say they have specific evidence linking one or more supernova to an extinction event 2.6 million years ago.

About 2.6 million years ago, one or more supernovae exploded about 50 parsecs, or about 160 light years, away from Earth. At that same time, there was also an extinction event on Earth, called the Pliocene marine megafauna extinction. Up to a third of the large marine species on Earth were wiped out at the time, most of them living in shallow coastal waters.

“This time, it’s different. We have evidence of nearby events at a specific time.” – Dr. Adrian Melott, University of Kansas.

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Nearby Supernovas Showered Earth With Iron

Visible, infrared, and X-ray light image of Kepler's supernova remnant (SN 1604) located about 13,000 light-years away. Credit: NASA, ESA, R. Sankrit and W. Blair (Johns Hopkins University).

We all know that we are “made of star-stuff,” with all of the elements necessary for the formation of planets and even life itself having originated inside generations of massive stars, which over billions of years have blasted their creations out into the galaxy at the explosive ends of their lives. Supernovas are some of the most powerful and energetic events in the known Universe, and when a dying star finally explodes you wouldn’t want to be anywhere nearby—fresh elements are nice and all but the energy and radiation from a supernova would roast any planets within tens if not hundreds of light-years in all directions. Luckily for us we’re not in an unsafe range of any supernovas in the foreseeable future, but there was a time geologically not very long ago that these stellar explosions are thought to have occurred in nearby space… and scientists have recently found the “smoking gun” evidence at the bottom of the ocean.

Two independent teams of “deep-sea astronomers”—one led by Dieter Breitschwerdt from the Berlin Institute of Technology and the other by Anton Wallner from the Australian National University—have investigated sediment samples taken from the floors of the Pacific, Atlantic, and Indian oceans. The sediments were found to contain relatively high levels of iron-60, an unstable isotope specifically created during supernovas.

The Local Bubble is a 300-light-year long region that was carved out of the interstellar medium by supernovas (Source: Science@NASA)
The Local Bubble is a 300-light-year long region that was carved out of the interstellar medium by supernovas (Source: Science@NASA)

Watch: How Quickly Does a Supernova Happen?

The teams found that the ages of the iron-60 concentrations (the determination of which was recently perfected by Wallner) centered around two time periods, 1.7 to 3.2 million years ago and 6.5 to 8.7 million years ago. Based on this and the fact that our Solar System currently resides within a peanut-shaped region virtually empty of interstellar gas known as the Local Bubble, the researchers are confident that this provides further evidence that supernovas exploded within a mere 330 light-years of Earth, sending their elemental fallout our way.

“This research essentially proves that certain events happened in the not-too-distant past,” said Adrian Melott, an astrophysicist and professor at the University of Kansas who was not directly involved with the research but published his take on the findings in a letter in Nature. (Source)

The researchers think that two supernova events in particular were responsible for nearly half of the iron-60 concentrations now observed. These are thought to have taken place among a a nearby group of stars known as the Scorpius–Centaurus Association, some 2.3 and 1.5 million years ago. At those same time frames Earth was entering a phase of repeated global glaciation, the end of the last of which led to the rise of modern human civilization.

While supernovas of those sizes and distances wouldn’t have been a direct danger to life here on Earth, could they have played a part in changing the climate?

Read more: Could a Faraway Supernova Threaten Earth?

“Our local research group is working on figuring out what the effects were likely to have been,” Melott said. “We really don’t know. The events weren’t close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either. We’re trying to decide if we should expect to have seen any effects on the ground on the Earth.”

Regardless of the correlation, if any, between ice ages and supernovas, it’s important to learn how these events do affect Earth and realize that they may have played an important and perhaps overlooked role in the history of life on our planet.

“Over the past 500 million years there must have been supernovae very nearby with disastrous consequences,” said Melott. “There have been a lot of mass extinctions, but at this point we don’t have enough information to tease out the role of supernovae in them.”

You can find the teams’ papers in Nature here and here.

Sources: IOP PhysicsWorld and the University of Kansas

 

UPDATE 4/14/16: The presence of iron-60 from the same time periods as those mentioned above has also been found on the Moon by research teams in Germany and the U.S. Read more here.