MACHOs

Massive Compact Halo Objects, or MACHOs, could be one of the missing pieces of the puzzle that astronomers call dark matter. These objects, as the name suggests, are found in the halo of the galaxy and could help explain why some bodies in the halo orbit faster than the amount of light would tell us.




MACHOs are normal baryonic matter, i.e. they are made up of baryons*

• Baryons are subatomic particles made up of quarks. Well-known examples would be protons and neutrons. Baryonic matter is matter made up of ordinary atoms.

Non-baryonic matter would be something like free electrons and other leptons (which I will not get into here) or neutrinos (which we be discussed in a future post).


So what are MACHOs? A few ideas exist of what they could be. They include, but are not limited to, the following:

• White dwarfs, especially older white dwarfs. After a white dwarf is no longer hot, it will cool down and be called a black dwarf since it no longer is radiating energy. They are thought to exist, but we have never found one...yet.
• Neutron stars that have had their supernova remnants dissipate or are not pulsars.
• Solar mass black holes
• Brown dwarfs which form the same way a star does, but cannot sustain fusion in its core. There will be more on these in a later post.
• Planets. Again, this will be discussed in a future post.

We know that MACHOs are not the only explanation for dark matter as they are only found in the halo of the galaxy. We know from the mass curve of the galaxy that dark matter is found within the disk as well. The five above objects can also be found in the disk as well.

Pulsars

As mentioned in the last post, pulsars are a specific type of neutron stars. Neutron stars have very strong magnetic fields, that are not necessarily lined up with their axes of rotation. Pulsars give off radiation as energy is directed along the axis of the magnetic poles and "beamed" towards Earth. These beams or jets are very energetic and can be seen for thousand of light years. When the jet of energy is pointed in the direction of Earth, we see them as a pulsar.

A very badly drawn schematic of a pulsar

A pulsar acts kind of a like a lighthouse in that the beam sweeps around as the light source rotates. Unlike a lighthouse, however, a pulsar's period can be extremely short, on the scale of milliseconds. The faster pulsar discovered has a period of about 3 milliseconds, and for something as large as a neutron star (about 10 km in diameter), the pulsar has to be rotating very fast.

We are unsure whether or not all neutron stars are pulsars, though we do know that most pulsars are neutron stars. Since pulsars can only be detected if the jet of energy is pointed directly towards Earth, we don't know if all neutron stars do this.

Neutron Stars

Neutron stars are the final stage of a medium-high mass star. They come about after a medium-high mass star ends in a violent Supernova Type II explosion. What exactly are neutron stars?

Neutron stars are a second type of degenerate stars, much like white dwarfs. However, unlike white dwarfs, the degeneracy comes from neutrons, not electrons, hence the name. But neutron degeneracy only arises when the neutrons are much more closely packed than the electrons. Whereas a white dwarf is on the scale on the Earth, in terms of radius, neutron stars are only 10 km in diameter, about as big as the size of Manhattan Island. Masses for neutron stars are larger than the Chandrasekhar limit, but only goes up to two (2) solar masses. Anything larger than that, gravity breaks the neutron degeneracy, and the neutron star will collapse into a black hole, which is ever wackier.

Neutron stars can also spin rapidly, and if they are aimed correctly, they can beam jets of energy along magnetic field lines towards Earth. When we see a neutron star rotating in this manner and we detect the beam of energy, the neutron star is called a pulsar, even though the star does not pulsate. The beams of energy are similar to a lighthouse shining light out to the ocean at regular intervals. However, pulsars rotate so fast that they can have periods on the scale of milliseconds (rotating almost a thousand times a second). We will talk more about pulsars in the next post.

Supernova Type II

Supernova Type II Remnant

Image Credit:

NASA/CXC/Penn State/S.Park et al.; Optical: Pal.Obs. DSS

 

Let's give a quick explanation of how neutron stars and black holes form. Neutron stars start out as medium-high mass main sequence stars around Spectral Type B and A stars and black holes start out as heavy stars at Spectral Types O and B. Once the hydrogen in the core is exhausted, the outer layers fall onto the core, heating it up and allowing the core to fuse helium into carbon, nitrogen, and oxygen via the triple alpha process. Surrounding the core is an envelope of hot hydrogen, still fusing into helium via the CNO cycle. Once the helium in the core runs out, again the star collapses onto the core, heating it up, allowing the heavier elements in the core to, fuse into even heavier material, with a layer of burning helium surrounding the core, followed by a layer of burning hydrogen. These processes and cycles continue until the core is mostly iron. Iron is a unique element in such that in order to furs it, energy must be added and no temperature can cause iron to fuse into anything heavier. When the outer layers collapse onto the core in this case, the temperature rises dramatically, and the outer layers explode into space via a supernova explosion.

A cool thing about supernovae is that all natural elements heavier than iron are created in supernovae. All the heavy elements found naturally on Earth, formed in a supernova. For that matter, any element heavier than helium was also formed in a star. So we are all made up of star stuff.

This is called a Supernova Type II because it is created via a core collapse of a medium-high to high mass star. Supernova Type I are caused by the accretion of material onto a white dwarf and can come from white dwarfs in binary (or multiple-) star systems. Type II supernova have hydrogen present in their spectra, while Type I lack hydrogen.

Supernova Type II remnants can be identified by two properties: hydrogen in their spectra and a stellar object at the center of the supernova remnant. Depending on the mass of the star, the remaining stellar object will either be a neutron star or a black hole. The next two posts will talk about both these remnants.