Dead Stars Are Dense Stars

This week is is full of groundbreaking stories about neutron stars. The public media have had their eyes on these delicious objects since Monday, when ESO announced the discovery of gravitational waves and light from a neutron star merger called GW170817 in a galaxy far, far away. More than 3,000 scientists around the globe collaborated on this fantastic observation, which confirmed astronomical theories that until August this year had only lived on paper among the piles of speculation at the scientists’ desks. Or simply: theories that never before had been confirmed by observation.

These days neutron stars are big in the media, partially because they are such extreme objects. But what exactly is a neutron star – and how do you put something so extreme into context?

Let’s try.

It Is Cool To Be Compact

Stars live for millions or even billions of years. They are born out of interstellar clouds of gas and dust. Interstellar implies that the cloud is floating in the empty space between stars in the galaxy. Stars start their lives by burning nuclear fuel – and they do this until physics no longer allows it. Depending on the pressure and temperature in the stellar core, it can take millions or billions of years before a star dies. And it will die eventually, but the death depends on the mass of the star.

Stars that burn nuclear fuel, i.e. stars the ‘live’, can be divided into multiple categories depending on who you ask. I prefer to use the Morgan-Keenan spectral classification system, because the name is long enough to sound scientific and easy enough to remember in public lectures. That system has more than 7 categories for living stars – but a dead star can only be one of three types. Those three are the ones we are going to take a closer look at here.

Compact Objects

Dead stars are called compact objects, because they are the densest objects in the Universe. Why call them objects and not stars? Because black holes are included in this category and early on in the discussion some people called them black stars and others called them black holes. So by naming the combined group of stellar mortality ‘objects’ you are sure to have included all three of them.

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The three types are shown above: white dwarfs, neutron stars and black holes. Let us dwell for a moment at the simplicity and harmony of this spectrum. The color and the density go hand in hand making it that much easier to distinguish what is what. Most of us remember that black holes are merciless enough to eat everything around them and massive enough to keep anything from escaping. By remembering this simple fact, you can quickly infer that the white dwarf must therefore be the least dense – and in between the two lies the gray neutron star.

The rule of thumb is that the more dense a dead star is, the more violent was its death.

White Dwarf

A white dwarf is the remaining product, when a star similar to the Sun dies. Stars of that size (up to around 8 solar masses) die a fairly reasonable way by shredding off the outer layer of gas while the central core contracts until all atoms are as ‘small’ as physics allows atoms to be (exactly how small was formulated by Wolfgang Pauli around 100 years ago).

The central core of a solar-type star is mostly carbon and oxygen. Have you ever heard what happens when carbon is squeezed together really tightly? If you are thinking of diamonds, you are right. The center of a white dwarf is essentially just a big clump of diamond wrapped in hydrogen and helium hanging on the night sky.

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Neutron Star

Neutron stars are as suggested by their name made entirely out of neutrons. Maybe you remember that neutrons are the neutral particles in the center of an atom. Neutron stars are made when living stars with masses above roughly 8 solar masses die. This process is a very violent explosion called a supernova, which is triggered when nuclear fusion in the core comes to a halt. There can be nothing to balance the inwards gravity if the outwards pressure from fusion is ended. The core therefore contracts until around 1.5 solar masses. The rest of this massive star is blown away in an explosion beyond imagination. The pressure is so high when the central core is contracting, that electrons bound in atoms are actually shot into the protons and hereby creating neutrons. This happens everywhere in the core and the result is a large ball of neutrons that effectively touch each other.

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The density of the star is as high as the density of neutrons. This means that the neutrons lie side by side and as a result the star has a radius of 10 km. The mass of 1.5 suns squeezed into a ball with a radius of 10 km. That is amazing! In the center of the star the density is still thought to be higher, which leads to speculations if some sort of exotic matter or other strange particles could exist in there.

Black Hole

These you have probably heard of. Their size (at least the big ones) is beyond comprehension. Perhaps more their density than their mass. After all, the Milky Way galaxy also have a large mass, but it is not difficult to imagine the stars distributed through space in the beautiful spiral galaxy we live in. The thing with black holes is, that the large amounts of mass are located in tiny areas. Take for instance the black hole in the center of our own galaxy (because there IS one and it is called Sagittarius A-star). This black hole has the mass of 4 million Suns squeezed into an area only 31 times larger than the sun. It means our black hole is 4 million times more massive than the Sun – but only 31 times broader. Imagine you were 4 million times heavier, but only 31 times taller!

Black holes are created when highmass stars die. In that sense, they are identical to neutron stars. The difference is, that if the central collapsing core has a mass higher than roughly 1.5 solar masses, the density becomes too high and the system collapses to a black hole, which mathematically speaking is a singularity, i.e. a place where math stops making any sense (and usually math makes sense in physics). Black holes are black because not even light can escape! That is how strong gravity is on that thing.

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Black holes are black holes because of their density – not their mass. You can make a black hole out of any mass, if you just squeeze it together tightly enough. In fact, our own Earth would become a black hole, if it was squeezed into the size of a sugar cube.

Headerphoto credit: ESO/L. Calçada

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