In our last post about low-mass stellar evolution, we left off at the AGB. So, what comes next? We said of white dwarfs (WDs) and planetary nebulae (PNe), but these can get somewhat detailed, so they deserve a post of their own. Especially since WDs have a very interesting relationship to a certain event we call a supernova (SN; we adore our acronyms).
After a star gets to the pulsating red giant phase, it eventually gets to the point where it doesn’t have the energy to continue fusion, usually by the C/N/O elements. When this happens there is insufficient radiation pressure outwards to repel the gravity pushing the star inwards, making it condense (sadly we can’t condense all this astronomy information). With that, the outer layers of the star shed off to form a PN. Some may ask why they are called planetary nebulae, since they in fact do not have any planets at all. It comes from the fact that early astronomers couldn’t see them clearly, and with PNe being generally green-blue, they looked similar to the planet Uranus.
Most PNe are indicators of the later stages of a low-mass star. Therefore, they are generally found with Population I stars, younger stars closer to the galactic plane or center. Like other nebulae, they also look awesome and are among the most popular targets for astrophotography. An interesting fact is that they don’t only form as a spherical shell, but they can come in many shapes, such as a bipolar shape, and also rings and ellipses. In general, PNe are distinguished by shape, so another major question is how these shapes form. Many of the different shapes are theorized to be from high-speed stellar winds, but the rings most likely come from an ejection of material that occurred at a different time from the main ejection.
As for their colorful nature, PNe have red emission lines from hydrogen and have green lines from doubly ionized oxygen, or OIII. They are created from the star’s light ionizing atoms, ripping electrons away and then recombining. The lines from a PN are also known as forbidden lines, not because they shouldn’t happen, but because in a lab we could normally only produce these spectra under high density. Space, on the other hand, is an odd, low-density environment.
There are also Fast-Low Ionizing Emission Regions (FLIERs). These are strange red-colored regions of a PN that are exactly what their name says. But sadly their origin is not fully understood. Some say they seem like they should be moving outward, but they could be stationary or even moving inward. So yes, this lovely universe of ours likes to seem to make little sense.
They can have multiple features. They may seem like degenerates for being aged as they are, but really they are fascinating. They can be categorized as many types of stars. They can go ahead and nova or even go SN (for fun, let’s just say that they are little things with quite the Napoleon complex). Yes, my fellow astronomy-lovers, we have gotten to the white dwarf (WD).
WDs are the result of a star collapsing in on itself due to lack of radiation pressure, disrupting hydrostatic equilibrium, and allowing gravity to push inwards. They come from stars that have run out of fuel, and they’re very small, being around the mass of the Sun shrunk down to the size of the Earth. They are made mostly of the C, N, O elements because by that point a medium-mass star can’t fuse much more. The color of the WD comes from the fact that it’s quite hot. It may not seem like it, but WDs can actually be detected in X-ray better than in visible light.
So then, the next question would be why doesn’t the star just continue to collapse? It doesn’t even have fusion occurring to oppose the collapse. Well, WDs actually have stability. They fulfill hydrostatic equilibrium, due to the Pauli exclusion principle, which works independent of temperature. The compaction of the WD produces such a high density that all the atoms are smashed together to the point where electrons start to collide. The Pauli exclusion principle says that two particles cannot occupy the same space with the same energy and spin, since they are effectively identical. So, what happens when you have a lot of electrons in a little space? Some that are in the same state collide. Blasphemy! In actuality a pressure keeps these two electrons apart, which creates pressure that stops total collapse. This state of matter, as mentioned in the light and matter post, is known as “degenerate matter.”
Next, how do we detect them? Well, Friedrich Bessel did manage to spot the white dwarf companion of Sirius in the visible range through careful observation (we’ll call this one Sirius business). WDs can (and often do) form in binary star systems, which is an especially important way to detect any sort of object that’s normally tough to see, because if we cannot see the target object, the companion is still affected by that object.
And yet another interesting part to the WD is the famous discovery of one Subrahmanyan Chandrasekhar. To get to the point, he found that WDs actually have a mass limit. His work was highly debated at the time, but now it is heavily accepted. So just how large can a star be before its mass overcomes even the pressure trying to oppose gravity? The answer is about 1.4 solar masses.
What’s interesting about this is that there are actually multiple types of WDs, because the Chandrasekhar Limit is an upper mass limit. This means smaller stars (even red dwarfs) can still form WDs, which have differing amounts of the elements C, N, O, and Ne. For an analogy of how massive a WD is imagine one teaspoon of WD matter. This is about 16 tons. That is, if we had a beach ball of WD matter, that would still be heavier than an ocean liner.
Ultimately these stars would form into black dwarfs, fading away into another boring death. But does this always happen? Well, since it’s the universe, of course not! In fact, WDs not only have multiple types, but they can even pulsate. This will be discussed in more detail later (see, we like to mimic stars and evolve this blog as we go along). Aside from spectral differences from different elements they also can differ by the presence of magnetic fields.
But the mass limit has even more interesting implications. See, WDs can be in binary systems with a red giant star with unstably held outer layers. Those layers can be ripped off the companion star and accumulate around the WD in a process known as accretion. Aside from our fancy names, accretion allows for material to get close enough to the WD such that fusion can occur. This flash of very high amounts of fusion all at once is what we call a nova. The name comes from the fact that when astronomers first saw these bright flashes, they thought new stars were being made. But the even greater thing is that when a WD gains more than 1.4 solar masses, it produces one of the many magnificent events in the universe, an supernova.
Now, to be specific SNe can be split into two types, and they are further split up from there. The type of SN produced by a WD is known as a Type Ia SN. There is hot debate over what causes it, perhaps at a thermonuclear level! Okay, not that hot, but one theory is the above mentioned accretion. There is also what’s known as the double degenerate progenitor (a progenitor is a fancy word meaning “the thing that created it”), which states that a binary system of two WDs with a total mass greater than 1.4 solar masses collided and, exceeding the limit, exploded. But what is the exact cause of an SN? Well, so much mass is gained in such a little space that runaway thermonuclear fusion occurs when it burns up all at once, releasing a massive amount of energy.
Type Ia SNe are well-known for multiple reasons. They are located almost everywhere, since WDs are just older low-mass main sequence stars. They are thought to be able to fuse carbon/oxygen all the way up to iron/nickel, creating the heavy elements of the universe. The spectra is noted for a lack of Balmer hydrogen lines, and further distinguished by the presence of silicon in their spectra. Another major note is that since there is a mass limit on WDs, the light curves of a Type Ia SN should always be the same. This means they are what’s called a standard candle; that is, they can be used to consistently measure what distance the SN was from earth. The absolute magnitude for a Type Ia is approximately -19.5. This is essential for understanding distances across the cosmos and can be used to calibrate Hubble’s Law.
Lastly, we will show how this is really, really powerful. An initial comparison is that some note that this is enough to equal the entire Sun’s energy emitted over its whole lifetime. But let’s continue to show another way. We said earlier in the apparent/absolute magnitude post that we can relate brightness or luminosity to magnitudes. Let’s apply this now. Given the absolute magnitude of the Sun is +4.83 and a solar luminosity is about 4×1026 W we can now find it:
L of the SN/1 solar luminosity =100(4.83+19.5)/5
=5395106225.15 solar luminosities = 2×1036 W.
Now to put this relative. The global energy consumption per year is about 15 terrawatts. That is, we could run the entire world’s energy supply and then some for oh about 1023 years. I believe it is undebatable to say this is insane. Now, others say that the energy output is about 2×1044 J. So, take this all as you will.
Now to condense (perhaps into a star, or even a WD!) all this information. After we have a red giant star it sheds its loosely held layers to form a PN. This PN can have many different colors and shapes. At the center the remaining core of mass condenses into a white dwarf, composed of degenerate matter. This means it is so condensed electrons repel eachother, creating an electron degeneracy pressure opposing gravity. Beyond that, WDs have a mass limit known as the Chandrasekhar limit, being 1.4 solar masses. WDs can also form in binary systems. If the partner accretes, or accumulates, matter to the WD then the WD can quickly fuse it and form an extremely bright nova. If either this goes past the Chandrasekhar limit OR if a WD combines with another WD and exceeds the limit then the result is a beautiful Type Ia SN. Type Ia SNe are valuable for creating heavier than normal elements, being common, and being standard in peak luminosity. This is useful for calculation of large distances which leads to a better understanding of what we like to call the universe.
Sources and further reading:
Type Ia SNe:
Carroll and Ostlie, An Introduction to Modern Astrophysics, 2nd edition (pg 446-474, 524-529, 557-578)