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.

If Our Sun was a Black Hole

Back in 1994, Soundgarden released a song off their album Superunknown called "Black Hole Sun". Since I want to talk about what would happen if our Sun became a black hole, it gives me an excuse to post this video.

from Soundgarden's Official YouTube Channel

 

Remember, the Sun will never become a black hole as it is too small, but for fun, let's find out what would happen if it did.

First, we should determine what the Schwarzschild radius of the Sun would theoretically be. When a black hole evolves, not all the mass of the main sequence star is in the black hole, but for our purposes, we will assume that somehow the Sun we see shining right now will have all its mass converted into the black hole. We will need to know two things: the Sun's mass in kilograms and the Sun's radius in meters (we do everything in metric).

• Sun's Mass (M₈) = 1.99x10³⁰ kg (we will say 2x10³⁰ kg for simplicity)
• Sun’s Radius (R₈) = 696,342 km = 6.96x10⁸m

Note: the symbol 8 refers to the Sun. When that subscript follows M or R, it just means the unit is in solar masses or solar radii.

Recall that the Schwarzschild radius is this:

Plugging in the values for the Sun's mass, G (6.67x10^-11 m³kg^-1s^-2), and c (3x10⁸ m/s), we find that if all the mass of the Sun became a black hole, the Schwarzschild radius would be 2,960 meters or about 3 km.  Compare that to the actual radius of the Sun and you can see how much smaller it would be.

So what would it mean for the Earth if the Sun became a black hole? In a couple of words, not much. The Earth would still be orbiting the same mass, so our orbital radius and speed would not change. However, life on Earth would die out because we would not be receiving the same radiation from the Sun to which we have been accustomed. No visible light from the Sun warming us, heck, no radiation of any kind. Black holes do not give off radiation in that manner. Due to Hawking radiation, particles near the event horizon of the black hole (the theoretical distance where light could escape from the black hole) would evaporate off. Some might hit Earth, but there wouldn't be anything to keep our planet warm.

Observation of a Black Hole

As mentioned in the last post, black holes are black because light cannot escape them. In fact, black holes do not emit any radiation at all because of their extreme density. So exactly how can we observe a black hole?




There are a couple of ways. The first way is too look at the gas and dust that surround a black hole. This gas and dust are in what is called an accretion disk and as the accretion disk orbits the black hole (just outside the Schwarzschild radius), it does two things.

1. It falls into the black hole, in turn making the black hole slightly bigger. This is where the term accretion comes from as the material accretes onto the black hole.
2. The inner edge of the disk is very hot because it's moving very fast. The gas and dust gets so hot that the material begins to radiate energy and give off energy in the form of jets. We see these coming from the "poles" of the black hole and know that there is a black hole there.

Accretion Disk around a Black Hole with jets

Image Credit:

Christopher Wanjek/NASA

Also, if you observe the gas and dust, you can measure the orbital velocity of the gas. You can also determine the radial distance of the gas and dust from the central body. By using these two values, the mass of the central body can be measured. For any radius, a mass above a certain value for that radius will tell you that the body is a black hole using the Schwarzschild radius equation.




Another strange thing about black holes is that black holes are not permanent. Hawking radiation predicts that due to quantum effects, particles of the black hole near the Schwarzschild radius can escape. Over time, as more and more particles escape, the black hole can shrink in size. See more about Hawking radiation here.

Black Holes

Black holes may be one of the most strange features in the universe. These objects are so dense, light cannot even escape from them!





Amazingly enough, almost all black holes started out as a hot, bright, large star. These stars go through the same evolution that leads to a neutron star, but unlike a neutron star, these objects are so massive, even neutron degeneracy is not enough to overcome gravity. Gravity compresses the star so much that instead of having a small core remnant after the supernova explosion, all that's left is what is called a singularity. Singularities are thought to be an object so small but so massive that it has an infinite density. In reality, this isn't true, but a naked black hole has never been observed, so the assumption is accurate for now.





There are a couple of things that define a black hole. One is its Schwarschild (pronounced Schwartz-Shield), which is the theoretical distance from the center of the black hole where the escape velocity from the black hole equals the speed of light. The speed of light is approximately 300,000 kilometers per second, or fast enough that a beam of light can circle the Earth 12 times at the equator. The escape velocity is given by the square root of 2 times G (the gravitational constant ~ 6.67*10^-11 m³/kg*s²) times the mass of the star or planet divided by the radius of the planet squared.





Or in equation form:

If you change V_esc to the speed of light, c, you can solve for R, and solve for the Schwarschild radius, R_s.

where M_Star is the mass of the Star, or in this case a black hole.

Next time, we will learn more about black holes, like how they are observed and what else we know about them.

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.