The Flaming Star Nebula (IC405) ranks as one of the showpiece sights in the northern constellation Auriga. This glowing emission nebula gains its energy from the star AE Aurigae, a 6th-magnitude massive blue-white star about 1,500 light years away. This brilliant star, which outshines our Sun by some 30,000 times, blasts out ultraviolet light that ionizes the cloud of hydrogen gas around the star. As the hydrogen atoms reassemble, they emit light at signature wavelengths of red and green light that make these nebulae so beautiful. Most such nebulae are energized by stars that formed within their densest and most opaque regions. But that’s not the case with the Flaming Star Nebula. AE Aurigae did not originate here; it’s just passing by chance through a cold cloud of hydrogen as it hurtles through the Milky Way, far removed from the place it was born.
AE Aurigae is an example of a runaway star, one that gained kinetic energy from a chance interaction with another star long ago. By measuring its speed and direction, astronomers have determined this star is on a beeline through the Milky Way at a speed of 100 km/s, and its traveling directly away from another famous showpiece object, the Orion Nebula, and likely formed within this nebula about 2.5 million years ago.
So how did AE Aurigae get from Orion to Auriga?
The answer, it seems, is that it had a chance gravitational encounter with a pair of other massive stars shortly after it was formed. AE Aurigae likely had a companion star before this encounter, and both stars passed close to another binary or multiple star system. In a complex gravitational dosey doe, these multiple star systems swapped and traded gravitational energy such that two stars became bound to each other in a long elliptical orbit and remained in place, while at least two others were slingshotted out of the Orion Nebulae. One was AE Aurigae, and the other was the runaway star Mu Columbae in the southern constellation Columba. The paths of both these stars trace back to the Trapezium Cluster, a pack of massive blue-white stars embedded in the Orion Nebula.
At its current speed, AE Aurigae will pass through the gas cloud that it’s currently energizing in about 20,000 years, which means the lovely Flaming Star Nebula will fade from view. In the meantime, this part of the sky remains a favorite of astrophotographers. Even visual observers can see this nebula and AE Aur itself with a modest telescope in dark sky. It’s well worth the effort to see the nebula and the runaway star that was born in a completely different part of the sky.
A Game of Chance – and Gravity
The interactions that propelled AE Aurigae and Mu Columbae across the sky are felt to a lesser degree by most stars sometime in their lives. Our own Sun was born in a big galactic star cluster about 5 billion years ago. Every star in our ‘home cluster’ was eventually nudged by gravity out into the wider Milky Way. We owe our existence, in part, to dozens of chance encounters between the Sun and other stars, any one of which might have sent our solar system into a different part of the galaxy, possibly into the neighborhood of massive stars destined to blow up as supernovae and irradiate our planet with gamma rays that make it impossible for life to form. So we count ourselves lucky.
Also during the Sun’s long life, other stars likely passed through or near the Oort Cloud at the outer edge of our solar system, likely sending a blizzard of comets into the inner solar system, some of which may have hit the Earth and seeded it with the building blocks of life. This is not something we expect (or want) to happen again in the near future, but such events may have accelerated the development of life on Earth.
Escaping the Milky Way
Which brings us to the so-called hypervelocity stars.
Runaway stars like AE Aurigae are rare, but stars that gain enough speed to eventually escape from our galaxy are even more unlikely and interesting to consider. In our part of the Milky Way, a star that gets whipsawed by gravity to a speed of more than 500-600 km/s has enough energy to leave the galaxy. But do such stars exist?
The answer, it seems, is yes. That’s the fate of the star US708, for example, that’s moving about 1,200 km/s, more than twice the speed necessary to escape the Milky Way. The ESA’s GAIA satellite, which maps the speed and position of millions of nearby stars, found a few dozen similar stars on their way out of the galaxy. Some of these stars may have wandered too close to a massive black hole near the center of the galaxy and gained enough energy to reach escape velocity. Others may have found themselves in a tight orbit with a companion star that exploded as a supernova. As the star’s companion suddenly lost most of its mass in the explosion, the orbital potential energy of the remaining star was converted to kinetic energy, like a ball being swung around on a string that suddenly breaks, and the remaining star suddenly picks up enough speed to escape the galaxy.
Beyond the Milky Way, these runaway stars are by no means rare it seems, especially in large galaxy clusters. Astronomers have observed hundreds of billions of escaped stars in the Virgo cluster. Perhaps 10% of the mass of the Virgo cluster is composed of stars that are wandering freely in intergalactic space.
In most cases, any planets around these stars will come along for the ride since the interactions that propelled them to escape speed are not sufficiently strong to strip them away. If such a planet harbored biological life, however, it would likely be eradicated by the supernova that propelled its star into intergalactic space.
However, it might be possible for a planet’s biosphere to start over if the raw ingredients for life remained. If such life evolved into complex and intelligent beings over the ensuing billions of years, they would experience a night sky quite unlike our own. They might see other planets in their own star system, and moons if they are present. But they would see no other individual stars, no constellations, no nebulae or star clusters, just dim smudges of distant galaxies. The idea of a sky full of stars, even clouds of stars flecked with clusters and nebulae and lanes of stardust as we see on a dark night, would be unknown to them, conjured perhaps only by alien science fiction writers and artists who to try to image what it would be like to be embedded in the dim and diffuse conglomerations of stars they see so far away.
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