What is a black hole?
A black hole is a concentration of matter so dense that its gravity prevents anything – even light – from escaping. In principle, black holes can be any size: even a tiny amount of matter can form a black hole if it is compressed into a small enough region. But astronomical black holes typically come in two varieties: black holes ten or so times as massive as the Sun and black holes millions or billions of times more massive than the Sun. The first type forms after stars run out of fuel; the origin of the latter is unknown, but they appear to be common near the centers of galaxies. (Our own Milky Way has a black hole at its center that is about four million times more massive than the Sun.)
Image: Artist concept illustrates a quasar, or feeding black hole
Credit: JPL
How did the first black holes form?
The origin of the first black holes is not yet known - after all, these sources have not yet been observed! But, because the first stars were very likely many times more massive than the Sun, many of them likely collapsed into black holes when they consumed their fuel. Black holes could have formed in other ways, though: in exceptional circumstances, early gas clouds may not have collapsed rapidly into stars but may have condensed much more slowly - and directly - into black holes up to a million times more massive than the Sun. Such black holes were likely very rare, but they may still have had enormous impacts on their surroundings.
Image: A depiction of the gas collapsing in the time around the first stars, created with supercomputers.
Credit: Tom Abel and Ralf Kähler
How do the first black holes grow?
Black holes are very simple objects: in an astronomical environment, their only qualities are their total mass and their spin. Because they still exert gravity, they will continue to accrete any normal matter that comes too close. This matter cannot escape, so the black hole grows – making its gravity even stronger! However, black holes can only grow if they are surrounded by gas – the rates at which most large black holes grow likely varies strongly over time, depending on their local environment, and the fraction of time that these very early black holes can accrete gas and grow is currently unknown.
Image: This artist's conception shows a supermassive black hole at the center of a remote galaxy digesting the remnants of a star.
Credit: JPL
Why do the environments of black holes radiate so much energy?
We have already seen that no matter or energy can escape black holes, so it may seem surprising that they are amongst the most luminous objects in the Universe! The key is that, although light cannot escape the black hole itself, it can escape the incredibly hot environments of some black holes. As a gas stream falls toward a black hole, it collides with other streams. The friction of such collisions heats up the gas, which can reach millions of degrees Celsius. The gas eventually collects into a disk, where it slowly spirals into the black hole. The hot, dense gas in the disk radiates an enormous amount of energy, including not just visible and ultraviolet light but even X-rays. These X-rays may have important implications for the surrounding Universe, but their prevalence will depend upon the fraction of time that black holes spend accreting gas.
Image: Artist's conception of a bright black hole accretion disk.
Credit: JPL
