Cosmicinsightz - Within The Cosmos

The Red Spider Planetary Nebula : Oh what a tangled web a planetary nebula can weave. The Red Spider Planetary Nebula shows the complex structure that can result when a normal star ejects its outer gases and becomes a white dwarf star. Officially tagged NGC 6537, this two-lobed symmetric planetary nebula houses one of the hottest white dwarfs ever observed, probably as part of a binary star system. Internal winds emanating from the central stars, visible in the center, have been measured in excess of 1000 kilometers per second. These winds expand the nebula, flow along the nebulas walls, and cause waves of hot gas and dust to collide. Atoms caught in these colliding shocks radiate light shown in the above representative-color picture by the Hubble Space Telescope. The Red Spider Nebula lies toward the constellation of the Archer . Its distance is not well known but has been estimated by some to be about 4,000 light-years. via NASA

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A View Toward M106

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.

The wings of the Twin Jet Nebula are the remnants of a dying star at the nebula’s centre, flung off by jets of gas.  The nebula is 2,100 light-years from Earth, and a white dwarf star orbits the dying star.

Picture of NGC 7635 captured in narrowband by amateur astronomer Luca Moretti

NGC 1097, Galaxy Foraged in the Furnace