Creating Black Holes

stellar mass black hole
An artist's conception of a stellar-mass black hole (in blue) hat likely formed when a supermassive star collapsed, feeding from material ejected by a nearby star. ESA, NASA and Felix Mirabel)

One of the questions that astronomers hear a lot is "How does a black hole form?" The answer takes you through some advanced astrophysics and astronomy, where you learn something about stellar evolution and the different ways that some stars end their lives.

The short answer to the question about making black holes lies in stars that are many times the mass of the Sun. The standard scenario is that when the star begins to fuse iron in its core, a catastrophic set of events gets set in motion. The core collapses, the upper layers of the star collapse onto THAT, and then rebound out in a titanic explosion called a Type II supernova. What's left collapses to become a black hole, an object with such a gravitational pull that nothing (not even light) can escape it. That's the bare-bones story of creating a stellar-mass black hole.

Supermassive black holes are real monsters. They're found in the cores of galaxies, and their formation stories are still being figured out by astronomers. Generally, however, they can get larger by merging with other black holes and by eating whatever happens to stray by them in the galactic core.

Finding a Magnetar Where a Black Hole Should Be

Not all massive stars collapse to become black holes. Some become neutron stars or something even weirder. Let's take a look at one possiblity, in a star cluster called Westerlund 1, It lies roughly 16,000 light-years away and contains some of the most massive main-sequence stars in the universe. Some of these giants have radii that would reach to Saturn's orbit, while others are as luminous as a million Suns.

Needless to say, the stars in this cluster are quite extraordinary. With all of them having masses in excess of 30 - 40 times the mass of the Sun, it also makes the cluster quite young. (More massive stars age more quickly.) But this also implies that stars that have already left the main sequence contained at least 30 solar masses, otherwise they would still be burning their hydrogen cores.

Finding an star cluster full of massive stars, while interesting, is not terribly unusual or unexpected. However, with such massive stars, one would expect any stellar remnants (that is, stars that have left the main sequence and exploded in a supernova) to become black holes. This is where things get interesting. Buried in the bowels of the super cluster is a magnetar.

A Rare Discovery

A magnetar is a highly magnetized neutron star, and there are few of them known to exist in the Milky Way. Neutron stars usually form when a 10 - 25 solar-mass star leaves the main sequence and dies in a massive supernova. However, with all the stars in Westerlund 1 having formed at nearly the same time (and considering mass is the key factor in the aging rate) the magnetar must have had an initial mass much greater than 40 solar masses.

This magnetar is one of the few known to exist in the Milky Way, so is a rare find in of itself. But to find one that was born from such impressive mass is another thing entirely.

The Westerlund 1 super cluster is not a new discovery. On the contrary, it was first detected nearly five decades ago. So why are we only now making this discovery? Simply, the cluster is shrouded in layers of gas and dust, that make it difficult to observe the stars in the inner core. So it takes incredible amounts of observational data, to get a clear picture of the region.

How Does This Change our Understanding of Black Holes?

What scientists must now answer is why the star didn't collapse into a black hole? One theory is that a companion star interacted with the evolving star and caused it to expend much of its energy prematurely. The result is that much of the mass escaped through this exchange of energy, leaving too little mass behind to fully evolve into a black hole. However, there is no companion detected. Of course the companion star could have been destroyed during the energetic interactions with the magnetar's progenitor. But this itself isn't clear.

Ultimately, we are faced with a question that we can not readily answer. Should we question our understanding of black hole formation? Or is there another solution to the problem that, as yet, goes unseen. The solution lies in collecting more data. If we can find another occurrence of this phenomenon, then perhaps we can shed some light on the true nature of stellar evolution.

Edited and updated by Carolyn Collins Petersen.