Astronomy & spaceflight
Supernovae & black holes
When giant stars explode, they scatter the building blocks of life and sometimes leave behind the strangest object in the universe.
The iron in your blood, the calcium in your bones, the oxygen in every breath: none of it existed at the beginning of the universe. All these atoms were assembled inside stars and hurled into space by tremendous explosions. You are quite literally made of stardust. And the same explosions that give us the building blocks of life sometimes also create the most puzzling object known to physics: a black hole.
The explosion of a giant
A star with more than about eight times the Sun's mass lives fast and dies dramatically. When its fuel is used up, the pressure that balanced suddenly vanishes. The core collapses within a fraction of a second, the outer layers crash onto it and are blasted into space in a supernova. For a few weeks this single star shines brighter than billions of stars combined, as bright as an entire . What is left of the core decides the outcome: if that remnant weighs no more than about two or three solar masses, a neutron star stays behind, a ball only around 20 kilometres across. If it is heavier, it keeps collapsing until a black hole is born.
We are stardust
The produced almost nothing but hydrogen and helium. Everything heavier was forged by stars: carbon, oxygen, silicon, iron. But as long as a star lives, those elements stay trapped inside it. Only the supernova spreads them through space and forges further heavy elements. Most gold, by current understanding, is made even more dramatically, when two neutron stars merge. Later, new stars and planets form from the enriched gas clouds. Our solar system, the Earth and we ourselves are made from the dust of earlier generations of stars.
| t (s) | v (m/s) | s (m) |
|---|---|---|
| 0 | 0.0 | 0.0 |
| 1 | 9.8 | 4.9 |
| 2 | 19.6 | 19.6 |
| 3 | 29.4 | 44.1 |
Without air resistance everything falls at the same rate, no matter how heavy it is.
Black holes do not suck
In films, black holes look like cosmic vacuum cleaners sucking everything in. That is wrong. A black hole is simply a very large mass in a tiny space, and it attracts with perfectly ordinary gravity. If the Sun were replaced by a black hole of the same mass, the Earth would keep orbiting exactly as before, just in the dark. Things only get dangerous if you come extremely close to a black hole, closer than you could get to any other object of the same mass.
The event horizon
To escape any celestial body you need a minimum speed. For Earth it is 11 km per second. The more compact the mass, the higher this value gets. For a black hole there is a boundary where even the is no longer enough: the event horizon. It is not a solid surface but an invisible line in space. Whatever crosses it can never come back, not even light. That is why the hole appears black. As a rule of thumb the horizon grows with mass: about 3 km of radius per solar mass.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
The Sun is suddenly replaced by a black hole of the same mass. What happens to the Earth?
Which stars end as a supernova?
Where does the iron in your blood originally come from?
A giant star dies. Put the steps in the correct order.
- 1The outer layers crash on and are blasted into space as a supernova
- 2Sometimes a black hole is left behind
- 3The giant star uses up its fuel
- 4The balancing pressure vanishes, the core collapses
A star of 20 solar masses explodes. A core of 2 solar masses remains as a neutron star. How many solar masses are hurled into space?
Match each group of elements to where it was formed in the universe.