Andromeda, Our Sister 

Earth and Luna from the ISS

Abell 39 is a planetary nebula formed by the ejected outer layers of a sun-like star collapsing into a white dwarf. It sits 7000 light years distant in the constellation of Hercules.

What Happens When Planets, Stars, And Black Holes Collide?

“Brown dwarf collisions. Want to make a star, but you didn’t accumulate enough mass to get there when the gas cloud that created you first collapsed? There’s a second chance available to you! Brown dwarfs are like very massive gas giants, more than a dozen times as massive as Jupiter, that experience strong enough temperatures (about 1,000,000 K) and pressures at their centers to ignite deuterium fusion, but not hydrogen fusion. They produce their own light, they remain relatively cool, and they aren’t quite true stars. Ranging in mass from about 1% to 7.5% of the Sun’s mass, they are the failed stars of the Universe.

But if you have two in a binary system, or two in disparate systems that collide by chance, all of that can change in a flash.”

Nothing in the Universe exists in total isolation. Planets and stars all have a common origin inside of star clusters; galaxies clump and cluster together and are the homes for the smaller masses in the Universe. In an environment such as this, collisions between objects are all but inevitable. We think of space as being extremely sparse, but gravity is always attractive and the Universe sticks around for a long time. Eventually, collisions will occur between planets, stars, stellar remnants, and black holes.

What happens when they run into one another? Unbelievably, we not only know, we have the evidence to back it up!

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White Dwarfs and Neutron Stars

What happens to a star after it dies depends entirely on the mass it contains. If the star has a low to medium mass (anything less than 8 solar masses) then at the end of its life it will transform into a white dwarf. If a star is massive (8-20 solar masses) then it will turn into a neutron star.

When a red giant starts to fuse helium to carbon and oxygen but lacks the mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at the core. Towards the end of the stars nuclear fusion stage, it will shed its outer layers in the form of ionized gas forming a planetary nebula. The core that is left behind is the white dwarf typically about the size of earth. This is made up of electron degenerate matter which forms because the white dwarf lacks its previous ability to create an internal pressure meaning gravity squashes the mass much closer together. The reason this is happens is because under normal circumstances electrons with the same spin can’t occupy the same energy level, and there’s only two ways an electron can spin (this is known as the Pauli Exclusion Principle). In a normal gas this isn’t a problem because there aren’t enough electrons to fill the energy levels. In a “degenerate” gas however, all its energy levels filled. For a white dwarf to be forced smaller by gravity, it would have to make electrons go where they couldn’t go thus white dwarfs survive through quantum mechanical principles that prevent their collapse further. There are other unusual properties as well, white dwarfs with greater masses are actually smaller because gravity has to force the electrons closer together to maintain the outward pressure. However the limit to how much mass they can have is about 1.4 solar masses.

Neutron stars are incredibly dense (and one of my favourite things ever) with a typical one being about 20km and containing 1.4 solar masses. A teaspoon would weigh about a billion tonnes on a neutron star, that’s how dense they are. They are also composed entirely of neutrons as the force of gravity is so great that it has caused the electrons and protons to fuse into neutrons. The power from the resulting supernova that created the star causes it to spin up to 43,000 times a minute, gradually slowing over time. The neutron stars which are still spinning emit electromagnetic radiation that we can detect when it’s pointing towards earth (much like a lighthouse). These are known as pulsars. The magnetic axis of a pulsar is what determines the direction the beam will fire off in. However this is not necessarily the same as the rotational axis and this misalignment is what causes some to appear to pulse. There are currently three different types of pulsars that astronomers are aware of. The first is rotation powered pulsars which the radiation given off is caused by a slowing down of the rotation of the star. Accretion powered pulsars occur when the gravitational potential energy that falls onto the neutron star causes X-Ray’s that can be received from Earth. Finally there is magnetars where the radiation is caused by an extremely strong magnetic field losing energy.

Large Magellanic Cloud & Comet 252P/Linear

Remnant of supernova toward the constellation of Vela, which exploded 11,000 years ago. 

Image credit: NASA / Chandra x-ray Observatory