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.

White Dwarfs

White dwarfs are the stellar remnants of low- to medium-mass main sequence stars.




Typically, once these stars can no longer fuse hydrogen in their cores, gravity will take over, compress the core, and increasing its temperature. This, in turn, allows the fusion of helium into carbon via the triple-alpha process. As the heat from the core increases, it pushes the outer layers of the star out, making the star a Class III (giant) star. When the helium in the core stops fusing, the outer layers collapse again and recompress the core. The temperature of the core will not get hot enough to fuse the carbon (and possibly nitrogen and oxygen) into anything heavier. Instead, something else will occur.




The outer layers will be pushed out and off the star but the rebound of the core. What is left are the naked core (the white dwarf) and the outer layers of the star which becomes a planetary nebula. The planetary nebula is named such because when they were first discovered, they were thought to be the beginning of a solar system. After initial observations, the first planetary nebulae were discovered to have a white dwarf at the center.

Helix Nebula. Note the white dot at the center. This is the white dwarf

Image Credit:

NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

White dwarfs, themselves, are basically dying stars. Fusion no longer takes place on or in white dwarfs. All that happens is that the white dwarf is slowly cooling down. Since the white dwarf is the core of a star, the white dwarf is initially very hot, which is why they are on the left side of the HR Diagram. But they are also very small, so extremely dim. White dwarfs are mostly carbon, since that was the last material that was created in the core. Also, white dwarfs are very dense. They have a radius similar to Earth's radius, but with a mass comparable to the Sun. A teaspoon of white dwarf material weighs several tons.


What makes the white dwarf stable is something called electron degeneracy. Electron degeneracy is the electromagnetic force from electrons in a confined space counteracts the gravity wanting to squash the white dwarf. However, there is a limit to the mass that will be stable with electron degeneracy. This limit is called the Chandrasekhar limit and is around 1.4 solar masses. Any higher, and the white dwarf will collapse farther and lead to a Type I supernova (later topic). Any main sequence star with an initial mass of 10 solar masses or lower will end up as a white dwarf.

.Sirius A and B (with Sun for comparison). Sirius B is the closest known white dwarf to the Sun

Image Credit:

Astro Bob

Luminosity Classes of Stars

Stars are not only described by their temperatures, but also by their brightness, or luminosity. These luminosity classes can tell us what kind of star the star is and what its past was like and what its future may hold.


Luminosity Class Ia and Ib - Supergiants:

• Typically, Supergiants are stars that had have evolved off the main sequence. These stars are very bright because they have a large radius. There may be some that are blue, but the majority of them are red and lie in the upper right corner of the HR Diagram. Betelgeuse in Orion is an example of a supergiant. They come from massive stars that can no longer fuse hydrogen in their core and will one day supernova, leaving behind either a neutron star or a black hole.

Luminosity Class II - Bright Giants:

• Giants that are brighter than "normal" giants, these are evolved medium mass stars (7 times or more heavier than the Sun). When they go supernova, they will most likely leave behind a neutron star.

Luminosity Class III - Giants:

• Evolved average mass stars (like the Sun). They will burn until they no longer can fuse elements in the core into heavier material. The Sun will become a giant once it stops fusing hydrogen in the core. These stars will have radii on the order of the Venus' orbit (0.7 AU) to Mars' orbit (1.4 AU). They become white dwarfs once fusion ends.

Luminosity Class IV - Subgiants:

• Likely the next stage of evolution for low mass stars (M type dwarf stars). Too small to burn anything heavier than helium in their cores and may be too cool to even fuse helium. Probably become low mass white dwarfs.

Luminosity Class V - Dwarfs or Main Sequence Stars:

• The luminosity class with the highest population. The majority of stars we see in the sky are class V. These are stars that fuse hydrogen into helium in their cores via the proton-proton chain or the CNO cycle (using carbon, nitrogen, and oxygen as catalysts to fuse hydrogen into helium). Depending on their masses, these stars can spend a few million years in this stage or billions of years (higher the mass, shorter the main sequence stage). Class V stars will evolve into the above classes.

Luminosity Class VI - Subdwarfs:

• Stars that are too small to fuse hydrogen into helium. Probably brown dwarfs, but are still hot from formation.

Luminosity Class D - White Dwarfs (do not confuse with Dwarfs):

• The final stage of a low mass star. No fusion occurs. The star is just slowly cooling off. These stars are found in the lower left corner. Very hot, but very due to being very small. This topic will be expanded on.

There are two other types of stellar remnants as mentioned in the supergiant category: neutron stars and black holes. These two topics will be explored more in detail in future posts.

Spectral Types Part 2

There is still a little more to discuss when looking at spectral types of stars. Last time, we learned that based on the surface temperature of the stars, the spectral types break down in this way:

• O
• B
• A
• F
• G
• K
• M
• L
• T
• Y

In astronomy terms, we say that O stars are earlier type stars compared to M stars because they are hotter. The idea of calling hotter stars early-type stars and cooler stars late-type stars, evolved from the early days of spectroscopic study of stars when astronomers thought that hotter stars evolved into the cooler stars, i.e. hot stars were in the early stages of their formation while cooler stars were in the later stages of their lives.


Astronomy can even break down stars further in each spectral type. The hottest stars in each spectral type are the 0 (zero) stars and the coolest stars are the 9 stars. For example, G stars that have a 6000 K surface temperature are the G0 stars while stars with a surface temperature of 5200 K are G9 stars. The Sun has a surface temperature of around 5800 so is a G2 star (some even break it down farther into a G2.5 star.

Next time, we will break stars up into their different Luminosity Classes (the y-axis on the HR Diagram).

Spectral Types of Stars

When we describe stars, we can divide them based on their temperature. However, when the science of spectroscopy was new, astronomers looked the spectra of stars and decided to break them down into the strength of the hydrogen lines.


Initially, Edward Pickering broke them down in this way:

• A stars: strong hydrogen lines
• B stars: strong, but a little weaker than A stars
• C stars
• D stars
• E stars
• F stars: medium strength
• G stars: weaker than F stars
• H stars
• I stars
• J stars
• K stars: really weak hydrogen lines
• L stars
• M stars: very weak lines and very dim stars
• N stars
• O stars: non-existent hydrogen lines, but very, very bright stars

Later on, Annie Jump Cannon took these spectral types and discovered that it would be easier to classify the stars by the temperature of the star, which could be gotten from the spectrum. She reclassified them in the following way:

• O stars: blue stars, very hot, >30,000 K surface temperature
• B stars: blueish white, hot, between 10,000 K and 30,000 K
• A stars: white, between 7500 K and 10,000 K (Sirius is an A star)
• F stars: whitish-yellow, 6000 K to 7500 K
• G stars: yellow, 5200 K to 6000 K (our Sun is a G star)
• K stars: orange, 3700 K to 5200 K
• M stars: red, 2400 K to 3700 K (the most abundant of main sequence stars)
• L, T, and Y stars: brown, < 2400 K (are brown dwarfs, failed stars that cannot fuse hydrogen in the core)

While before, the stars were classified by the strength of their hydrogen lines, Annie Jump Cannon's reordered makes more sense to base it on their temperatures. An easy way to remember the order is to use the pneumonic that I learned when I was a kid:

Oh, Be A Fine Girl, Kiss Me

This helps with the main sequence stars, but does not incorporate the brown dwarfs. You can come up with your own to help you remember.

The spectral types of the stars can actually tell us a lot about the star's past, present, and future. Generally, the hotter the star, the more mass it has. A hotter star also reaches the main sequence quicker than a cooler star, but also ages much more rapidly. To overcome the gravity the hot, massive star experiences, the star must burn hydrogen faster to keep the star in hydrostatic equilibrium.

This also helps us to determine what type of stars to look at when looking for planets that might support life. We don't want a star that is too hot, or the radiation from the star would sterilize any planet orbiting it, and that star would not last too long on the main sequence to have any life to evolve.