Poor, Poor Moon :(

Einstein ... thank.

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Herr Einstein…

Yeah, Mercury did kinda kick Newton in the balls, didn’t it?

Guess that’s why it’s my favorite planet

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Ah yes, the science

THE LIFE OF A STAR: WHAT GOES AROUND, COMES AROUND

Previously on The Life of a Star, Chapter 6 ...

"But what happens after the shell is fused? We'll get back to that in Chapter 7, where we'll discuss White Dwarfs and Planetary Nebulae."

        After a low-mass star loses its hydrogen core, it becomes a mighty Red Giant - the star contracts and then heats up again, igniting hydrogen shell fusion and swelling the star to epic proportions. That is, until the hydrogen shell and the helium core and all fused up, in which the helium shell will begin to fuse. Remember the last chapter, when I said that these stars don't have enough pressure to fuse the results of the triple-alpha process? Well, I wasn't lying.

        And unlike the end of hydrogen fusion - where low-mass stars have a "2nd life" and continue fusing the elements - this means the end for our star.  Now, due to the build-up of carbon and oxygen in the core (and the lack of enough pressure to fuse these elements), the star has run out of fuel. This cancels out gas pressure, which breaks the hydrostatic equilibrium. Gravity wins the constant battle within the star, and the core collapses.

        The leftover core - tiny and hot - is called a Wolf-Rayet type star and squeezed into a volume one-millionth the size of the original star (Harvard). Now, why does the star stop here? If gravity overpowers the pressure inside the star, why does it not completely collapse into a black hole? Well, that's due to a little thing called electron degeneracy pressure.  Basically, the Pauli exclusion principle states that "no two electrons with the same spin can occupy the same energy state in the same volume." Due to the core collapse, electrons are forced together. The Pauli exclusion principle predicts that these electrons, once having filled a lower energy state, will move to a higher one and begin to speed up. This creates pressure and prevents the core from further collapse. However, at a certain mass, this becomes impossible to maintain. White dwarfs have something called the Chandrasekhar limit, which states that white dwarfs cannot exist if their original mass is over 1.44 times the mass of the Sun. This is due to mass-radius relationships, something we'll discuss in the next chapter.

        One of my favorite things about stars is the fact that they're a cycle - the death of some stars causes the birth of others. White dwarfs do this, too, by creating something we talked about in Chapter 3: Planetary Nebulae.

        The collapsed Wolf-Rayet type star is extremely small, with high density and temperature. Streams of photons/energy/heat - stellar winds - push out the cooler outer layers of the dead star (Astronomy Notes). The core emits UV radiation, which ionizes the hydrogen and causes it to emit light, forming fluorescent and spherical clouds of gas and dust surrounding the hot white dwarf. These are Planetary Nebulae, which can later be clumped by gravity and spun to create a new star. The cycle continues (Uoregon).

        The leftover core, the White Dwarf, is characterized by a low luminosity (due to the lack of new photons, which the star will start to lose by radiation) and a mass under about 1.44 times that of the Sun.

        Due to the intense gravity, the White Dwarf (despite being very large in mass) has a radius comparable to that of the Earth. If you consult the density equation (d=m/v, which basically means that if you enlarge or shrink either the mass or the volume that the density will increase), White Dwarfs have enormous densities. The core is a compact of carbon and oxygen. Because the star is unable to fuse these elements, they kind of just ... sit there. Surrounding this is a shell of helium and a small hydrogen envelope. Some even have a very thin layer of carbon (Britannica).

        However, the White Dwarf isn't the end for the star. There's one more stage for the star to go through before completely "dying": becoming a Black Dwarf.

        After the core is left behind, there Is no fuel left to burn. That means no new energy production. However, the leftover heat from the contraction remains, and the star will begin to cool down. Higher mass White Dwarfs, due to having a smaller radius, radiate this away slower than the low-mass ones. There are two types of cooling: radiative and neutrino. Radiative cooling is simple: as the star gives off light and energy outward, it loses heat. Neutrino cooling is a bit more complex: at extremely hot temperatures, gamma radiation passes electrons, and this reaction creates a pair of neutrinos. Because neutrinos interact very weakly with matter, they escape the White Dwarf quickly, taking energy with them. It's also possible to have a hunch of crystal in the center of a Black Dwarf: "On the other hand, as a white dwarf cools, the ions can arrange themselves in an organized lattice structure when their temperature falls below a certain point. This is called crystallization and will release energy that delays the cooling time up to 30%." (Uoregon).

        The White Dwarf will become a Black Dwarf after it radiates away all of its heat and becomes a cold, dark shell of its former self. Because it's radiated away all of its heat, it emits no light, hence the name. However, according to theoretical physics, there isn't a single Black Dwarf in the universe. Why? Because it should take at least a hundred million, billion years for a White Dwarf to cool down into a Black Dwarf. Because the universe is predicted to be around 13.7 billion years old, there hasn't been enough time for a single White Dwarf to completely cool down (space.com).

        However, there's one last thing that can happen to a White Dwarf. And that's where things in this book will start to get explosive.

        White Dwarfs in binary star systems (where two stars orbit around a center of mass, we'll touch on it more in Additional Topics) can undergo a Classical Nova. These supernovae occur in systems with one White Dwarf and one main-sequence star. If they orbit close enough, the White Dwarf will begin to pull the hydrogen and helium from the other star in what is called an Accretion Disk, what is to say a disk of plasma and particles which spiral inwards due to gravity and feeds one body off of another. The accretion of this plasma onto the surface of the White Dwarf increases pressure and temperature so much that fusion reactions spark and the outburst of energy ejects the shell in a burst of light - a nova (Cosmos).

        This process doesn't end, however. It can repeat itself again and again in what is called a Recurrent Nova. We know the existence of these based on pictures of the same star system with expanding shells, the aftermath of recurrent novae. Because White Dwarfs are the most common star death in the universe, and most stars are in binary or multiple star systems, novae are fairly common (Uoregon).

        Our discussion of novae will be an excellent transition into our next topic: supernovae! This will be the beginning of the end for the High-mass stars we talked about in Chapter 6, and we’ll even talk a little bit more about White Dwarf collisions and how they are related to supernovae, neutron stars, and more!

        From here on out, stars are going to become much more dramatic - and all the cooler (well, not really)!

First -  Chapter 1: An Introduction

Previous -  Chapter 6: The End (But Not Really)

Next - Chapter 8: Why We’re Literally Made of Star-stuff (unpublished)

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My favorite YouTube video as of now (I know this doesn’t seem like it’s related to space - but it has a nice discussion about black holes and hawking radiation, which is I love it so much)

Remember kids: be cautious of bouncy castles!

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Ooo, that’s pretty cool

Also - a nice little teaser - we’ll be covering brown dwarfs in the next chapter of the Life of a Star! 

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ASTRONOMERS FIND JUPITER-LIKE CLOUD BANDS ON CLOSEST BROWN DWARF

A team of astronomers has discovered that the closest known brown dwarf, Luhman 16A, shows signs of cloud bands similar to those seen on Jupiter and Saturn. This is the first time scientists have used the technique of polarimetry to determine the properties of atmospheric clouds outside of the solar system, or exoclouds.

Brown dwarfs are objects heavier than planets but lighter than stars, and typically have 13 to 80 times the mass of Jupiter. Luhman 16A is part of a binary system containing a second brown dwarf, Luhman 16B. At a distance of 6.5 light-years, it’s the third closest system to our Sun after Alpha Centauri and Barnard’s Star. Both brown dwarfs weigh about 30 times as much as Jupiter.

Despite the fact that Luhman 16A and 16B have similar masses and temperatures (about 1,900°F, or 1,000°C), and presumably formed at the same time, they show markedly different weather. Luhman 16B shows no sign of stationary cloud bands, instead exhibiting evidence of more irregular, patchy clouds. Luhman 16B therefore has noticeable brightness variations as a result of its cloudy features, unlike Luhman 16A.

“Like Earth and Venus, these objects are twins with very different weather,” said Julien Girard of the Space Telescope Science Institute in Baltimore, Maryland, a member of the discovery team. “It can rain things like silicates or ammonia. It’s pretty awful weather, actually.”

The researchers used an instrument on the Very Large Telescope in Chile to study polarized light from the Luhman 16 system. Polarization is a property of light that represents the direction that the light wave oscillates. Polarized sunglasses block out one direction of polarization to reduce glare and improve contrast.

“Instead of trying to block out that glare, we’re trying to measure it,” explained lead author Max Millar-Blanchaer of the California Institute of Technology (Caltech) in Pasadena, California.

When light is reflected off of particles, such as cloud droplets, it can favor a certain angle of polarization. By measuring the preferred polarization of light from a distant system, astronomers can deduce the presence of clouds without directly resolving either brown dwarf’s cloud structure.

“Even from light-years away, we can use polarization to determine what the light encountered along its path,” added Girard.

“To determine what the light encountered on its way we compared observations against models with different properties: brown dwarf atmospheres with solid cloud decks, striped cloud bands, and even brown dwarfs that are oblate due to their fast rotation. We found that only models of atmospheres with cloud bands could match our observations of Luhman 16A,” explained Theodora Karalidi of the University of Central Florida in Orlando, Florida, a member of the discovery team.

The polarimetry technique isn’t limited to brown dwarfs. It can also be applied to exoplanets orbiting distant stars. The atmospheres of hot, gas giant exoplanets are similar to those of brown dwarfs. Although measuring a polarization signal from exoplanets will be more challenging, due to their relative faintness and proximity to their star, the information gained from brown dwarfs can potentially inform those future studies.

NASA’s upcoming James Webb Space Telescope would be able to study systems like Luhman 16 to look for signs of brightness variations in infrared light that are indicative of cloud features. NASA’s Wide Field Infrared Survey Telescope (WFIRST) will be equipped with a coronagraph instrument that can conduct polarimetry, and may be able to detect giant exoplanets in reflected light and eventual signs of clouds in their atmospheres.

IMAGE….Astronomers have found evidence for a striped pattern of clouds on the brown dwarf called Luhman 16A, as illustrated here in this artist’s concept. The bands of clouds were inferred using a technique called polarimetry, in which polarized light is measured from an astrophysical object much like polarized sunglasses are used to block out glare. This is the first time that polarimetry has been used to measure cloud patterns on a brown dwarf. The red object in the background is Luhman 16B, the partner brown dwarf to Luhman 16A. Together, this pair is the closest brown dwarf system to Earth at 6.5 light-years away. CREDITS: Caltech/R. Hurt (IPAC)

Just so you guys know, the Galilean Moons aren’t Jupiter’s only moons. It has 79 confirmed moons.

Hey, at least they’re not all lonely.

Also, fun-fact, the Moon Europa is a big candidate for a “2nd Earth,” having an ocean under a shell of ice.

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Over the span of a few hours, I collected images of Jupiter and its Galilean Moons. I labelled each one and you can see them move in their orbits! 🪐🪐🪐

Taken by me (Michelle Park) using the Slooh Canary Two telescope on June 3rd, 2020.

Dark matter is one of my favorite mysteries in Astrophysics, oh I would just love to study it. Some are using particle accelerators to try to study DM and figure out what it is - and it’s so so exciting!!!

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I love this meme format

THE LIFE OF A STAR: A DAY IN THE LIFE

Stars are born, and then they live. If a body is large enough and has enough pressure in its core, it will squeeze to fuse hydrogen. The hydrogen in a star's core fuses into helium, releasing photons and fueling the star. The heat created in this process attempts to expand the star, but as their gravity is so strong which threatens to collapse them (making it a problem once fusion stops - we'll get to this later!), this creates an equilibrium. And while stars have some things in common, they do have unique qualities of their own.

        Here are the properties that all main sequence stars share: hydrogen fusion, hydrostatic equilibrium ("the inward acting force, gravity, is balanced by outward acting forces of gas pressure and the radiation pressure"), the mass-luminosity relationship (in other words, the more massive a star, the brighter it is), it is the stage where stars spend the most of their lives, and a composition made almost entirely of hydrogen and helium (ATNF - Australia Telescope National Facility).

        Like the planets and our sun, stars have structure. The layers of a star are as follows, from the innermost to the outermost: the core, the radiative and convective zones, the photosphere, the chromosphere, and the corona. The structure of our Sun is illustrated above.

        The core of a star undergoes fusion in order to maintain hydrostatic equilibrium, and prevent the star from collapsing in on itself. As such, the core is the hottest and most dense region of a star (Universe Today). Thermonuclear energy spreads from the core through convection, the process by which heat moves: heat moves up and cold moves down because cold has a higher density than hot (Britannica: convection). Furthermore, some stars are fully convective, while others just have regions of convection. "The location of convection zones is strongly dependent on the star’s mass. Cool and low-mass stars are fully convective ... Stars slightly more massive and warmer than the Sun, also form a convective core." (Stellar Convection). I'll touch on this in the next chapter, where small stars such as Red Dwarfs are fully convective and are able to avoid the Red Giant phase, due to a lack of build-up of particles in their cores.

        In radiative zones, this energy is carried by radiation. In convective zones, it is carried by convection. These zones are not hot or dense enough to undergo nuclear fusion. The photosphere is the surface of a star, then the inner atmosphere (colored red due to the abundance of hydrogen) is the chromosphere, and the outermost atmosphere is the corona (space.com).

        In terms of stellar composition, they are mainly composed of hydrogen and helium (which also happen to be some of the most abundant elements in the universe and are the fuel behind a star's nuclear fusion), but also include heavier elements (such as carbon and oxygen). As observed by spectrums and other observations, stars with a greater amount of heavier elements are typically younger because older stars give these elements off due to mass-loss (ATNF - Australia Telescope National Facility).

        Stars also undergo atomic and molecular processes internally to maintain their hydrostatic equilibrium:

The Proton-Proton Cycle is the main source of energy for cool main-sequence stars, such as the Sun. This cycle fuses four hydrogen nuclei (aka, protons) into one helium nucleus and two neutrinos (some of the original mass is converted into heat energy). Two hydrogen nuclei combine and emit a positron (a positively charged electron) and a neutrino. The hydrogen-2 nucleus captures a proton to become hydrogen-3 and emit a gamma-ray. There are multiple paths after which, but it always results in the same (Britannica: proton-proton cycle).

The CNO Cycle (aka the Carbon-Nitrogen-Oxygen Cycle) is the main source of energy for warmer main-sequence stars. This cycle has the same resultants but the process is much different. *SKIP AHEAD TO AVOID MY NERD RANT* It fuses a carbon-12 nucleus with a hydrogen nucleus to form a nitrogen-13 nucleus and a gamma-ray emission.  The nitrogen-13 emits a positron and becomes carbon-13, which captures another proton/hydrogen nucleus and becomes nitrogen-14 and another gamma-ray. The nitrogen-14 captures a proton to form oxygen-15 and then ejects a positron and becomes nitrogen-15. This, of course, captures another proton and then breaks down into a carbon-12 nucleus and a helium nucleus (an alpha particle). *JUST IN CASE YOU SKIPPED AHEAD* TLDR, it ends up as helium. Nuclear fusion, folks, it's weird (Britannica: CNO cycle).

        The products of these processes aren't just automatically transferred and radiated away from the star. No, first they must make their way through the radiative and convective zones. Neutrinos travel almost at the speed of light, and so are the least affected. Photons also lose some energy during the journey, due to interactions with other particles. This energy heats up the surrounding plasma and keeps it flowing, in turn the convection currents transport energy to the surface (ATNF - Australia Telescope National Facility).

        Even though a star spends most of its life in the main-sequence stage, this cycle of processes and equilibrium ends eventually. In the next chapter, we'll be talking about what happens after a star runs out of hydrogen to fuse - Giant and Super-Giant Stars. 

        The rate at which a star runs through its hydrogen is proportional to its mass: the greater the mass, the faster it runs through hydrogen,  and vice versa (Britannica: star). Then the star will begin to fuse the heavier elements until it meets its match: iron. Then things get real ... explosive.

First -  Chapter 1: An Introduction

Previous -  Chapter 4: A Star is Born

Next -  Chapter 6: The End (But Not Really)

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Well TECHNICALLY it’s a helium-4 nucleus

I guess I can see where the confusion comes from

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first post on Reddit lets go

In a sense cosmology contains all subjects because it is the story of everything, including biology, psychology and human history.

Peter Theodore Landsberg

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