It’s a good question. When we think of galaxies we think of the millions and billions of stars that make up the huge body. But could a star roam out of the galaxy? Could a star form outside a galaxy?
Well, it is believed that as many as half of all stars in the universe are found in the vast emptiness of space between galaxies. These rogue stars manage to escape the gravitational pull of the galaxy that keeps it together, and escape at great velocities out of the galaxy. As of 2012, astronomers have identified 16 of these ‘hypervelocity’ stars.
So where has this huge velocity come from? Astronomers estimate that the black hole at the centre of the Milky Way has a mass of four million solar masses. They calculate that the gravitational field surrounding such a supermassive black hole is strong enough to accelerate stars to hypervelocities.
The classic case of this would involve a binary pair of stars that encounter the black hole. As one of the stars spirals towards the black hole, its pair is flung outwards at a huge velocity, a ‘hypervelocity’.
So yes, a star could leave its galaxy. But could a star be formed outside a galaxy? The simple answer is probably not. For a star to form the clouds of gas, known as molecular clouds, need to collapse. Usually this is done by some kind of compression, perhaps a nearby star going supernova or if the star is in a spiral galaxy and it travels through one of the spiral arms which acts as a density sound wave, causing compression. Outside of a galaxy these events don’t really occur, so the molecular clouds stay as molecular clouds.
Well, it is believed that as many as half of all stars in the universe are found in the vast emptiness of space between galaxies. These rogue stars manage to escape the gravitational pull of the galaxy that keeps it together, and escape at great velocities out of the galaxy. As of 2012, astronomers have identified 16 of these ‘hypervelocity’ stars.
So where has this huge velocity come from? Astronomers estimate that the black hole at the centre of the Milky Way has a mass of four million solar masses. They calculate that the gravitational field surrounding such a supermassive black hole is strong enough to accelerate stars to hypervelocities.
The classic case of this would involve a binary pair of stars that encounter the black hole. As one of the stars spirals towards the black hole, its pair is flung outwards at a huge velocity, a ‘hypervelocity’.
So yes, a star could leave its galaxy. But could a star be formed outside a galaxy? The simple answer is probably not. For a star to form the clouds of gas, known as molecular clouds, need to collapse. Usually this is done by some kind of compression, perhaps a nearby star going supernova or if the star is in a spiral galaxy and it travels through one of the spiral arms which acts as a density sound wave, causing compression. Outside of a galaxy these events don’t really occur, so the molecular clouds stay as molecular clouds.