All the stars we can see with the naked eye are part of the Milky Way. The gravitational power of the galaxy’s combined mass binds the stars to the galaxy. But sometimes stars are evicted from the galaxy.
These stars are called hypervelocity stars, and some of them are born from powerful gravitational interactions in globular clusters.
Supernovae are some of the most powerful events in the Universe. They’re extremely energetic, luminous explosions that can light up the sky. Astrophysicists have a pretty good idea how they work, and they’ve organized supernovae into two broad categories: they’re the end state for massive stars that explode near the end of their lives, or they’re white dwarfs that draw gas from a companion which triggers runaway fusion.
Every once in a while, the Milky Way ejects a star. The evicted star is typically ejected from the chaotic area at the center of the galaxy, where our Super Massive Black Hole (SMBH) lives. But at least one of them was ejected from the comparatively calm galactic disk, a discovery that has astronomers rethinking this whole star ejection phenomenon.
Our universe is capable of some truly frightening scenarios, and in this case we have an apparent tragedy: two stars, lifelong companions, decide to move away from the Milky Way galaxy together. But after millions of years of adventure into intergalactic space, one star murders and consumes the other. It now continues its journey through the universe alone, much brighter than before, surrounded by a shell of leftover remnants.
At least, we think. All we have to go on right now is a crime scene.
Faster than a speeding bullet, this supergiant star looks like it might be wearing a red cape. Alpha Camelopardalis, the bright star in the middle of this image, is a runaway star, moving at incredible speeds – astronomers believe could be zooming along at somewhere between 680 and 4,200 kilometers per second (between 1.5 and 9.4 million miles per hour). The speed of this star is so fast, a huge bow shock is being created as the star moves through space. Alpha Cam’s bow shock can’t be seen in visible light, but WISE’s infrared detectors allow us to see this arc of heated gas and dust around the star.
Runaway stars are kicked into motion either through the supernova explosion of a companion star or through gravitational interactions with other stars in a cluster. The WISE team explains the bow shock:
“Because Alpha Cam is a supergiant star, it gives off a very strong wind. The speed of the wind is boosted in the forward direction the star is moving in space. When this fast-moving wind slams into the slower-moving interstellar material, a bow shock is created, similar to the wake in front of the bow of a ship in water. The stellar wind compresses the interstellar gas and dust, causing it to heat up and glow in infrared.”
Just as astronomers aren’t quite sure about the speed Alpha Cam is traveling, its distance is also somewhat uncertain, but it is probably somewhere between 1,600 and 6,900 light-years away. It is located in the constellation Camelopardis, near Ursa Major. (Right ascension: 4h 54m 03.0113s, declination: +66° 20′ 33.641”)
The colors used in this image represent specific wavelengths of infrared light. Stars are seen primarily in blue and cyan (blue-green), because they are emitting light brightly at 3.4 and 4.6 microns. Green represents 12-micron light, primarily emitted by dust. The red of the blow shock represents light emitted at 22 microns.