Stars close to the supermassive black hole (SMBH) at the center of our galaxy live in constant danger, but many of them seize the opportunity to look younger than their true age anyway. That can be achieved by eating neighboring stars.
Astronomers are sometimes puzzled by stars where different ways to estimate their ages produce conflicting results. The most common explanation is that the star ate a companion, causing it to look younger than it really is. Out in the quiet galactic suburbs in which we dwell, such behavior is rare – there’s plenty of space between stars and little opportunity for cannibal snacking except by compact objects like neutron stars.
Things are different close to the galactic center, where stars are packed so closely, and accelerated to immense speeds by the gravity of the SMBH known as Sagittarius A*. Dr Sanaea Rose of Northwestern University has been simulating the behavior of 1,000 stars close to Sagittarius A* and has revealed just how dangerous a place it can be.
“The region around the central black hole is dense with stars moving at extremely high speeds,” Rose said in a statement. “It’s a bit like running through an incredibly crowded subway station in New York City during rush hour. If you aren’t colliding into other people, then you are passing very closely by them. For stars, these near collisions still cause them to interact gravitationally. We wanted to explore what these collisions and interactions mean for the stellar population and characterize their outcomes.”
“The closest star to our sun is about four light-years away,” Rose added. “Within that same distance near the supermassive black hole, there are more than a million stars.”
Rose and colleagues modeled how often stars collide under these conditions, taking into account the density with which they are packed, their mass, orbit speed, and the gravitational influence of Sagittarius A*.
The most important factor, the team found, was the distance to the SMBH. Once you get down to distances of 0.03 light years, encounters are common. However, these are seldom direct hits. Instead, they undergo what Rose calls “violent high fives”. The pair pass each other, but their outer layers are torn off, with the depth of the losses depending on the star’s relative velocities and the closeness of the collision. “These destructive collisions result in a population of strange, stripped down, low-mass stars,” Rose said.
Within this region, some stars lose so much of themselves they cease to be able to fuse, leading to a halving of the population of low-mass stars in this area.
A little further out, at distances till tiny on the scale of the rest of the galaxy, collisions are rarer – but a star of the Sun’s mass will still experience at least one in its lifetime. When this happens, stars lack the energy to escape each other. Instead, they combine to become larger, younger-looking stars. For some that are close to Sagittarius A* (but not too close), this can happen many times, forming giants with masses more than 10 times that of the Sun. Rose and co-authors calculate there are about 100 such multi-cannibal giants lurking in this region.
“A few stars win the collision lottery,” Rose said. “Through collisions and mergers, these stars collect more hydrogen. Although they were formed from an older population, they masquerade as rejuvenated, young-looking stars. They are like zombie stars; they eat their neighbors.”
Greater star mass means a shorter life, since the rate at which stars fuse their hydrogen increases faster than the mass itself. Consequently, a youthful appearance comes with low life expectancy. That doesn’t always matter because the stars will get close enough to Sagittarius A* to become part of its accretion disk before dying of what might be considered natural causes. The bigger a star is, however, the more likely it is to turn into a supernova before being eaten by a black hole.
“It’s an environment unlike any other,” Rose said. “Stars, which are under the influence of a supermassive black hole in a very crowded region, are unlike anything we will ever see in our own solar neighborhood. But if we can learn about these stellar populations, then we might be able to learn something new about how the galactic center was assembled. At the very least, it certainly provides a point of contrast for the neighborhood where we live.”
Rose presented the work at the American Physical Society April meeting, based on previously published papers in The Astrophysical Journal Letters and The Astrophysical Journal, for both of which she was first author.
Source Link: Zombie-Like Stars Arise From Stellar Cannibalism
