Definition
A supermassive black hole is a giant black hole hundreds of thousands to tens of billions of times heavier than the Sun. It sits at the center of most large galaxies, including our own. By pulling in nearby matter and blasting out powerful energy, it has a decisive effect on how a whole galaxy forms and evolves.
A Giant Sitting at the Heart of a Galaxy
Look closely at the center of a galaxy in the night sky with a special telescope, and you will see the stars there whirling around an invisible point. In that seemingly empty spot hides an object with an almost unimaginable amount of mass.
The ordinary black holes that form when a star dies and collapses are only a few to a few dozen times the Sun's mass. A supermassive black hole, though, packs the weight of millions to tens of billions of Suns into one object. At the center of our own galaxy sits one called 'Sagittarius A*,' about 4 million times the mass of the Sun, quietly holding its place.
Thanks to this huge mass, it bends the space and time around it to an extreme degree. Its strong gravity makes the stars near the galactic center orbit at high speed. What holds the galaxy as a whole together is the galaxy itself and dark matter. But in the central region, this giant black hole acts as a powerful anchor point.
How Did It Get So Huge?
Packing hundreds of millions of Sun-sized stars into one place is never easy, even in the universe. Scientists suspect that in the early universe, either a giant cloud of gas collapsed directly, or many early black holes crashed into each other and merged to grow bigger.
When gas and dust get pulled in around a supermassive black hole, they form a huge spinning disk called an accretion disk. As the material rubs and collides, the light it gives off from friction heat can become hundreds of times brighter than all the starlight in a galaxy combined. This is called a quasar.
To be more precise, giant supermassive black holes have already been found in the very early universe, not long after it began. How they grew so large in such a short time is one of the most exciting puzzles modern astronomy is trying to solve.
Is It Just a Monster That Destroys Everything?
When people hear 'black hole,' they often picture a scary destroyer that swallows every nearby star and planet like a vacuum cleaner. But a supermassive black hole is not a simple monster that only swallows things.
When a black hole takes in matter, the energy spilling out shoots off from both poles at nearly the speed of light. This is called a 'jet.' The powerful jets and radiation spread across the galaxy, stirring up cold clouds of gas and pushing them away.
This can keep gas from clumping together too much, which stops stars from forming wildly. In other cases, it squeezes gas far away and helps new stars to be born. In that way, it is a giant regulator in the cosmic ecosystem, tuning how a galaxy grows.
Common misconceptions
A supermassive black hole holds every star in the whole galaxy in place with its gravity.
A supermassive black hole's gravitational reach is limited to the galaxy's central region. What binds the hundreds of billions of stars in the whole galaxy is the gravity of the galaxy's own mass and dark matter.
Where you meet it
'Sagittarius A*' at the center of our galaxy is a supermassive black hole about 4 million times the mass of the Sun.
The black hole at the center of the galaxy M87, the first one humans ever observed directly and photographed, has as much as 6.5 billion times the Sun's mass.
In one sentence
A supermassive black hole is a giant object at a galaxy's center, millions to tens of billions of times heavier than the Sun, whose powerful energy helps tune how the galaxy forms and how stars are born.