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Map of local interstellar clouds just outside Earth’s solar system, with blue arrows showing which directions these clouds are moving. The yellow arrow indicates the direction of the sun’s own movement. | Credit: NASA/Adler/U. Chicago/Wesleyan
Astronomers have discovered that the Sun had a close encounter with two extremely hot massive stars about 4.4 million years ago. The discovery was made thanks to a “scar” left by the event in swirling clouds of gas and dust just beyond the solar system. Not only does this research reveal more about the immediate celestial environment of the solar system, but it could also shed light on how the surrounding features in that environment played a role in the evolution of life on Earth.
To make this discovery, the team of astronomers had to take into account the motions of these “local interstellar clouds,” which span about 30 light-years, the sun, and the intruding stars, which now reside 400 light-years from Earth, in the front and back “legs” of the constellation. Canis Major (The Great Dog). It is difficult because SUN Only, it rockets through space at 58,000 miles per hour (93,000 km/h), or about 75 times faster than the speed of sound at sea level here on Earth.
“It’s kind of a puzzle where all the different pieces are moving,” team leader Michael Shull of the University of Colorado Boulder said in a statement. “The sun is moving. The stars are moving away from us. The clouds are moving away.”
Beyond the local interstellar clouds and their wispy clumps of hydrogen and helium atoms in gas and dust, the solar system lies in a region of the Milky Way that is relatively free of such matter, called the “local hot bubble.”
Understanding these regions could be important to understanding how life was given the conditions it needed to thrive on Earth.
“The fact that the sun is inside this set of clouds that can shield us from ionizing radiation may be an important part of what makes Earth habitable today,” Shull explained.
To investigate this influence, Shull and colleagues began to model the forces that shaped our region of the Milky Way. This involved looking closely at two stars in Canis Major known as Epsilon Canis Majoris or Adhara and Beta Canis Majoris or Mirzam. The team found that these two stars are likely to have passed the sun about 4.4 million years ago, coming within 30 light-years of our star. While this is a huge distance in terrestrial terms, the equivalent of about 175 trillion miles (281 trillion km), it is a close pass in cosmic terms and in a galaxy that is 105,700 light years across.
Such a close pass would have made these stars quite visible from Earth, scientists say. “If you think back 4.4 million years, these two stars would have been anywhere from four to six times brighter than Sirius is today, by far the brightest stars in the sky,” Shull said.
These stars are each much larger than the sun, about 13 times more massive than our star. They are also much hotter than the sun, with temperatures of up to 45,000 degrees Fahrenheit (25,000 degrees Celsius), making the sun’s 10,000 degrees Fahrenheit (5,500 degrees Celsius) seem relatively calm. When these massive, powerful but short-lived stars passed through our cosmic backyard, they emitted powerful ultraviolet radiation that stripped electrons from atoms in local interstellar clouds, a process called “ionization.” Stripping away the negatively charged electrons left those hydrogen and helium atoms with a positive charge — the “scar” the team was able to detect.
The team’s research solves a long-standing mystery about the local interstellar clouds, which arose when astronomers previously discovered that 20 percent of the hydrogen atoms and 40 percent of the helium atoms in these clumps of gas and dust were ionized, an unusually high level of ionization, especially for helium.
The local hot bubble a void of gas and dust in the Milky Way where the sun sits | Credit: CfA, Leah Hustak (STScI)
The team claims that these stars had assistance in ionizing these clouds from at least four other sources of ultraviolet radiation. These include three white dwarf stars, the type of stellar remnant left when sun-sized stars die, and the local hot bubble itself.
This is because this underdense region of gas and dust is thought to have been cleaned up by the explosive supernova death of between 10 and 20 stars. Those supernovae heated the gas, causing the local hot bubble to emit ionizing radiation in the form of X-rays and ultraviolet radiation, frying the local interstellar clouds around the solar system.
The ionization of these clouds will not last forever, disappearing as the hydrogen and helium atoms regain their neutral electrical charge by picking up free electrons. This process could take around several million years.
Epsilon and Beta Canis Majoris are also living on borrowed time. While the 4.6 billion-year-old sun will live around another 5 billion years before it pulverizes as a white dwarf, massive stars like these burn through their nuclear fusion fuel much more quickly. Both Epsilon and Beta Canis Majoris are likely to go supernova within the next few million years.
Although they are too distant to pose any risk to Earth, the explosive deaths of these stars could provide a spectacular spectacle for any remaining life forms on Earth. “A supernova exploding this close will light up the sky,” Shull said. “It will be very, very bright, but far enough away that it won’t be lethal.”
The team’s research was published in late November in The Astrophysical Journal.