Mystery Of Runaway Stars Revealed

Do you know there are stars in the universe that are racing through the sky at a staggering high speed that is in the order of 1000 km/second? This speed is 10 or more times the speed of normal stars that move at 100 km/second. The Milky Way galaxy's fastest hypervelocity star, US 708, runs at the speed of 1200 km/second while an ordinary star like our sun has the speed of only 200 km/second.

"But how is this possible?"

"Which mechanism causes such a massive body to move at this high speed?"

What Is A Runway Star? 

A runaway star is a star that is moving through interstellar space at tremendously high speed. We know these stars for several decades but we are still unable to find out the reason for their exceptional speed. In 2005, astronomers found that these stars travel so fast that even the gravity of the galaxy can not stop them. In an attempt to calculate its orbit, they realized that it had come from somewhere near the center of the galaxy. The star was heading straight out of it.

It was believed that the star once belonged to a binary system that went too close to the supermassive black hole at the galactic center. The gravitational pull from the black hole disrupted the binary system and rocketed it with a speed that surpasses the escape velocity of the galaxy.

Later, it was discovered that a few of these stars were not coming from the galactic center. Instead, they seemed to come from a large Magellanic cloud, one of our galactic neighbors. But there was something strange about the Magellanic cloud: "It does not have a black hole at its center!" It is a region of young stars where star formation takes place. This means that the reason for the high speed of the stars was different for some stars.

Reason For Their High Speed 

"So, how did Astronomers come across the reason for their high speed?"

Well, this has something to do with multiple-star systems that got messed up somehow. There are two scenarios by which a runaway star gains its peculiar velocity: Binary Supernova Ejections(BSE) and Dynamical Ejections from star clusters(DE).

Binary Supernova Ejection 

Zeta Ophiuchi was once in close orbit with another star, before being ejected when this companion was destroyed in a supernova explosion. Credit: NASA/ CXC/ University of Cambridge/ Sisk-Reynés et al/ NSF/ NRAO/VLA/ PanStarrs

In this scenario, one of the two stars(mostly the heaviest of the two) bound in a strong binary system explodes as a supernova and accelerates the other star. 

 

If something makes a red giant loses its shell, it becomes a hot subdwarf(just the helium core). When a dense star like a White dwarf forms a binary with a red giant, the high gravitational pull of the white dwarf(or the denser star) sucks in the material of the other star. Sometimes, it becomes so much dumped with helium that it results in a supernova explosion. 

The gravitational attraction of the primary star reduces as its supernova shell passes the secondary(smaller) star. As a result, the star gets ejected and starts to move through space at a speed similar to its original orbital speed. After the explosion, the star either becomes a neutron star or a black hole.

Dynamical Ejections from Star Clusters

This three-frame illustration shows how a grouping of stars can break apart, flinging the members into space. Credits: NASA, ESA, and Z.Levy(STScl)

In this system, Runaway stars are ejected through gravitational interactions between stars and binaries in compact clusters of stars. 

When two stars orbit each other in a binary system, they are held together by their mutual gravitational forces. A third star enters this system and forms a tight binary with one of the stars. This event releases tremendous gravitational energy which propels the other star outward with ridiculously high speed. An outflow of material is also produced during this event. The star then leaves its orbit and continues in a straight line just like cutting a swinging rope with a stone attached to it. This star then begins its journey through an interstellar spat at a high speed.

Hypervelocity Stars 

People often confuse hypervelocity stars with runaway stars. But they are different. Hypervelocity stars are extreme runaway stars that are kicked out of the galaxy at exceptionally high velocities. 

Runaway stars that are formed in the Milky Way are not fast enough to be hypervelocity stars. The fastest runaways are formed in the Large Magellanic Cloud(LMC) because it has 10% of the mass of the Milky Way and, hence, has a less gravitational pull. This makes it easy for the stars to escape the gravity of LMC. The speed of the Large Magellanic Cloud itself is 250 miles/second and the orbital speed of the runaway star is added to it. This results in a star with a speed so high that it can be called a hypervelocity star.

Why Are These Stars Important?

Besides wanting to better understand the reason for the ridiculously high velocity of Runaway stars, scientists' objective is to explore the valuable information they provide about the Milky Way. We may have a greater insight into the mass distribution of our galaxy by measuring the three-dimensional motions of hypervelocity stars. Do you know what this information can do for us? It can reveal the shape of the Milky Way's dark matter halo.

In addition to this, runaway stars play a vital role in driving galactic outflows. While traveling through the low-density interstellar medium, the star affects various large-scale properties of the galaxy like outflow. Runaway states cause a large number of supernova explosions which results in more efficient heating of inter-arm regions. This boosts strong galactic winds and mass loading factors up to one order of magnitude.