Post-main-sequence evolution of low mass stars

Understanding the post-main-sequence evolution of low mass stars is imperative when it comes to understanding the entire process of the birth and death of a star. In this blog post, we will exclusively focus on the post-main-sequence evolution of low mass stars. However, before delving into the subject, it is important to go over a life cycle of a low mass star first. As shown in below image, the low mass stars go through different phases than the high mass stars. Even though the image shows the same number of steps of stellar evolution for both high and low mass stars, their processes are essentially different in detail.

source: https://www.khanacademy.org/partner-content/big-history-project/stars-and-elements/how-were-stars-formed/a/infographic-life-cycles-of-the-stars

As you can see from above image, low mass stars have following life cycle steps:[1]

1. Protostar: A star system forms when a cloud of interstellar gas collapses under gravity. Typical giant molecular clouds are roughly estimated to be 100 light-years across, and they contain up to 6,000,000 solar masses! When the giant molecular clouds collapse, it breaks into small fragments. In each of these, the collapsing gas emits gravitational potential energy as heat energy. Through this process, the temperature and pressure are destined to increase, and a garment goes through a condensing phase and becomes a rotating sphere of extremely hot gas called protostar.[2]

2. Yellow main-sequence star: In the core of a low-mass star, four hydrogen nuclei fuse into a single helium nucleus by the series of reactions known as the proton-proton chain.

3. Red giant star: The post-main-sequence evolution stage starts from this third stage where the core hydrogen is exhausted, and the core shrinks and heats. Hydrogen fusion starts around the inert helium core, which causes the star to become a red giant.[3] Once the star used up the core hydrogen, the gravitational core collapses and it causes the core to be heated. It is also worth to know that low mass stars go through the entire life cycle slower than high-mass stars, including post-main-sequence steps.

4. Helium core-fusion star: After this stage, Helium fusion starts in the core when it heats enough to fuse Helium into Carbon. This triggers the core to expand and cause the speed of hydrogen fusion rate to decrease. This allows the star’s outer layers to shrink and the size of the star decreases as well.

5. Double shell-fusion red giant: In the next stage, the Helium fusion begins around the inert carbon core soon after the core helium is exhausted. This brings the star into the second red giant phase that has a hydrogen shell and a helium shell fusion.

6. Planetary nebula: The sixth step is called Planetary nebula that the low-mass star becomes a dying star that expels the surface layers in a planetary nebula with the exposed inert core left behind.

7. White Dwarf: The star eventually becomes a white dwarf, which is made mostly of carbon and oxygen. This is due to the low heat in the core of the low-mass star that never gets hot enough to make heavier elements.

Of course, the process of the stellar evolution cannot be studied based on observing the life of a single star since the process occurs too slowly to be detected within the span of our civilization. Thus, astrophysicists use computer models to simulate the process of this stellar evolution, including the post-main-sequence evolution, to understand the universe. Even though it takes much effort to understand the stellar evolution, it is still an interesting subject of study since we are star-species as Carl Sagan once said, and our sun will eventually face the same fate as just any other stars in the far future. However, we wouldn’t exist on Earth then!

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[1] Bennett, Jeffrey O. The Essential Cosmic Perspective, Boston: Addison-Wesley, 2012, pg. 354-355

[2] Prialnik, Dina(2000) An Introduction to the Theory of Stellar Structure and Evolution, Cambridge University Press, ISBN 0-421-65065-8

[3] http://www.ucolick.org/~woosley/ay112-14/lectures/lecture14.4x.pdf

Structure of The Milky Way Galaxy

This is an artist's rendition of our Milky Way Galaxy, our galactic home.

Image Credit: https://www.nasa.gov/mission_pages/GLAST/science/milky_way_galaxy.html

Beautiful, isn't it?

Let's take a look at the structure of our glorious galaxy.

The Galaxy From the Inside Out

Much like how the Earth is not the center of our solar system, our solar system is NOT the center of our galaxy. So what is at the center of our galaxy?
Spinning like a glorious ballerina of darkness at the center of the galaxy is a supermassive black hole. While we cannot observe our black hole directly, we have observed a compact radio wave source that most likely is powered by the accretion disk of materials encircling our black hole. This, and the black hole are known as Sagittarius A* (pronounced Sagittarius A Star). Our black hole has a mass about 4.5 billion times larger than the sun. The x-ray closeup in the image below, taken at the Chandra Observatory, shows the accretion disk of our galaxy's black hole.

Image Credit: https://www.nasa.gov/mission_pages/chandra/multimedia/black-hole-SagittariusA.html

Here's a link to an interesting article about a particularly large flare up in Sagittarius A*'s activity last year: http://www.space.com/28193-monster-black-hole-largest-flare-ever.html

The black hole is also surrounded by a bar in the center of our galactic neighborhood. This bar is full of stars that orbit quickly about the center of the galaxy. For a visual interpretation, take a look at the first picture in this post. The galactic bar is the section seen in the center with a more yellowish hue.
This bar is contained within the bulge at the center of our galaxy, visible as the yellowish blob in the center of the picture below.

Image Credit: http://www.space.com/18199-milky-way-core-star-photos.html

As we move further out, we can see the spiral arms of our galaxy. The spiral arms are comprised of stars and the interstellar medium: clouds of Hydrogen, other gasses, and dust that allow for star formation to occur. The arms are density waves, meaning that they are longstanding structures that orbit as a structure about the center of the galaxy, much like the way that a record rotates as it is played. This means that the farthest reaches of the arms are moving at faster speeds than the center of the arms. Stars, however, orbit at similar rates of speed throughout all sections of the spiral arms. This means that stars that are formed in the arms flow out towards the ends of the arms and may wind up between arms or in a new arm entirely before their stellar deaths.
Our solar system is contained within a spur between two arms called the Orion Spur and our sun orbits the center of the galaxy at a distance of approximately 26,000 light years. The entirety of the disk of our galaxy is approximately 100,000 light years in diameter.

Image Credit: https://www.nasa.gov/jpl/charting-the-milky-way-from-the-inside-out

Given our position within the galaxy, observations from Earth show the Milky Way as a thick band of stars and dust that forms a circle about the sky. This is because we view the galaxy edge-on, looking towards the spiral arms that are around us. 

Image Credit: https://www.nasa.gov/topics/earth/features/2012-alignment.html

Due to this positioning, we can't actually take an image of our entire galaxy, although the work of talented artists gives us a good idea of what it most likely looks like.

Beyond the spiral arms of the galaxy exists a less dense, spherical cloud of stars and other materials we also consider as part of the galaxy. We call this the galactic halo. The halo extends to a radius of approximately 300,000 lightyears from the center of the galaxy. The prevalence of globular clusters in the halo, along with the absence of new star formation leads many astronomers to believe that the halo is an older structure than the disk of our galaxy.

Image Credit: http://www.cefns.nau.edu/geology/naml/Meteorite/Book-GlossaryG.html

 Much like how the study of our solar system allowed for the creation of the solar nebula theory which explains both our sun's formation and the formation of other stars, this study of our galaxy provides insights to the formation of our galaxy as well as other spiral galaxies like our own.

Interstellar Medium, Star Formation, and Gas Recycling

The Interstellar Medium, Star-formation, and Gas Recycling

In order to truly understand some of the most important astronomical phenomena, it is necessary to be familiar with the interstellar medium, (ISM).  The interstellar medium is essentially what makes up the space between stars, planets, etc.  The interstellar medium is comprised of 99% gas and only 1% dust. This dust is unbelievably small and is made of Carbon, iron compounds, ice, or a variety of silicates. Approximately 75% of the gas is Hydrogen compared with 25% Helium and is a mix between uncharged atoms and positively or negatively charged particles. Most of the interstellar gas is neutral hydrogen. However, since it is neutral and is not excited like ionized particles and giving off light, it is difficult to observe. Because of Hydrogen's occasional electron spin flip, this neutral it is possible to view using radio waves when when looking through the 21cm bands these transitions emit. Except in the cases of nebulae, the interstellar medium makes up only a miniscule fraction of the density of the air on Earth. In nebulae, the ISM is either in the form of freezing clouds or, when close to stars, incredibly hot ionized particles of Hydrogen since the particles are hit with mass amounts of UV radiation as new stars form. The ISM creates stunning displays in space as light from stars radiates off the amorphous clouds as seen below.

Picture taken from wiki

But how did all this material get there? The Hydrogen and Helium were formed from the big bang. The dust, however, comes from the deaths of stars.  As a star approaches the end of its life, Hydrogen to Helium fusion comes to an abrupt end in the core.  Extremely large stars go through a series of fusion processes that heat it until iron is created and cannot be fused to give off any more energy so the star dies and ejects much of its matter into space. Once smaller stars are no longer fusing H to He in their cores they go through a chain of events and end their lives as white dwarfs.  At this point the star has shed its outer layers and they spread adding dust to the ISM as we know it today. This dust is necessary in the ISM because it provides a base upon which hydrogen in from the medium may accumulate and form molecules.

Areas where these molecules are more densely packed are known as nebulae. There are three types of nebulae. The first, emission nebulae, a clouds of ionized gas. As UV radiation is given off by nearby stars it excites the electrons in the gas and when they are excited, they emit photons which produce beautiful pink light displays such as in the image seen below of the eagle nebula.

Picture taken from wiki 

The second kind of nebulae is known as dark nebulae. In these areas, the gas and dust is so densely packed together that light from stars or other objects cannot pass through. An example of this is the horsehead nebula as seen below.

The last type of nebula is known as reflection nebula. In these regions, light from nearby stars reflects off of gas particles of the ISM and as it is reflected, has a bluish hue. These nebulae are also sites for star creation. Both the Pleiades and the lower left region of the horsehead nebula where blue light is seen are excellent examples of reflection nebulae.


Picture taken from wiki

Areas of the ISM where star formation happens in ares known as molecular clouds. The matter that makes-up these freezing cold clouds is pulled closer together under its own gravity.  As these particles come closer to each other, they heat up, giving off more energy and dart around at higher speeds.  At these increased temperatures they are able to collide and stick together forming more massive objects through a process known as accretion.  The cloud contracts more at an ever increasing rate as gravity becomes stronger each second and forms a truly dense object.  The entire time, the temperature of these gases increases, and when it hits 10 million degrees Kelvin, Hydrogen fusion begins and this massive object is now officially a star on the main sequence.  After billions of years this star dies and disperses its matter across the universe to one day undergo the same processes that led to its creation.  This continuum of star death creating a massive cloud of dust and gas which will one day form a new star is known as gas recycling since the gas is constantly being reused to create an endless number of stars throughout the existence of the universe.  

Works Cited

"Interstellar Gas Cloud | COSMOS." Interstellar Gas Cloud | COSMOS. Swinburne University of Technology, n.d. Web. 05 Dec. 2016. <http://astronomy.swin.edu.au/cosmos/I/interstellar+gas+cloud>.

Smith, Gene. "University of California, San Diego Center for Astrophysics & Space Sciences." The Interstellar Medium. CASS/UCSD, n.d. Web. 05 Dec. 2016. <http://casswww.ucsd.edu/archive/public/tutorial/ISM.html>.

"What Is the Interstellar Medium?" What Is the Interstellar Medium? N.p., n.d. Web. 28 Nov. 2016.

Black Holes -- Daunting and Dramatic, But How Do They Work?

Black holes are an oft-discussed element of astronomy. That is to say that most people understand them as a dramatic, dark emptiness in space. It’s likely, though, that many of these people can’t explain how black holes work or what their implications are. Black holes are anything but “empty space,” for example. It's not so simple.

Let’s begin with a basic definition of black holes. A black hole is a region of space where gravity wins out over, well, everything -- even light. That's where we get the “black” concept. Humans can't actually see black holes.

Get too close to a black hole, and nothing can escape its gravity. The point that defines “too close” is called the event horizon. Cross this threshold, and there's no escaping. The escape velocity equals the speed of light.

A visual explanation of the different features of a black hole region.
Image Credit: made myself!

Another critical point is the range of black holes. Black holes are not all huge as we understand it. Some can be as small as about 10 km in radius. Even at this size though, it's vital to understand that a black hole is still always massive.
The Schwarzschild radius is the radius of the event horizon of a black hole, found by equating escape velocity to the speed of light). Larger mass stars create larger black holes, but stars as small as ~25x the mass of the Sun can also become black holes. Some theorize that black holes can be created from sources other than stars or can be even smaller, but there is no evidence for any of this yet, so they remain theories.

The above image is a computer-simulated rendering of a supermassive black hole found at the center of a galaxy. You can see in the image the way the black hole’s intense gravity distorts a ring of space around it.

Image credit: NASA

A computer-generated video of a star being ripped apart the tidal forces around a black hole.

Video Credit: NASA

So what are the implications of black holes? The video above depicts the interaction of a star with a black hole. If a star gets too close to the gravitational pull of a black hole, it will be torn apart by tidal forces. Tidal forces refer to a difference in the gravitational strength between two points. This same concept causes the tides on Earth. When it comes to black holes, however, tidal forces are a whole different story.

Black holes literally crush matter out of existence, which can be a difficult concept to understand or visualize. The terms "noodle effect" or "spaghettification" are attempts to clarify just how brutal it would be to slip inside the event horizon of a black hole (if that wasn't already clear from the video above).

What is a singularity?
Once something collapses into a black hole, we can't know exactly what happens. Scientists predict, based on the laws of physics, that "the matter that forms a black hole should ultimately be crushed to an infinitely tiny and dense point in the black hole's center. We call this point a singularity" (Essential Cosmic Perspective 368).

As I mentioned earlier when discussing the size of black holes, it can be difficult to understand black holes because there’s still a lot that has either yet to be discovered or will remain undiscoverable about them for the foreseeable future (or forever). Scientists are still actively studying black holes, and there's so much we still don't know. It's also, simply put, really really hard to think about what goes on in black holes. To understand what happens inside a black hole, we need both quantam mechanic and gravity, and these areas of astronomy don't exactly coincide perfectly. Quantam mechanics is concerned with the small scale: atomic level interactions. On the other hand, gravity depends on mass. So black holes are a perfect nightmare -- a miniscule point in space but also so, so massive. No simple scenario for even the most advanced astronomers.

To refine your understanding of stellar remnants in general, and also black holes, check out Natalie's post here. 

Thanks for reading!

Kylie

Bibliography

Bennett, Jeffrey O., Megan O. Donahue, Nicholas Schneider, and Mark Voit. The Essential Cosmic Perspective. 7th ed. Boston: Addison-Wesley, 2012. Print.

Clark, Stuart. "What Is a Black Hole?" The Guardian. Guardian News and Media, 1 Feb. 2016. Web. 14 Nov. 2016.

Wheeler, J. Craig. Cosmic Catastrophes: Exploding Stars, Black Holes, and Mapping the Universe. Cambridge: Cambridge UP, 2007. Print. 

Compact Stars: A Pretty Stellar Story

Stellar Remnants

If I were to immediately define “stellar remnants,” I would be beginning at the end. Instead, let’s begin at the beginning: the life cycle of a star, or stellar evolution.

Credit: http://www.schoolsobservatory.org.uk/astro/stars/lifecycle

After formation in a nebula, stars begin to live via nuclear fusion in their core. But eventually, the atoms needed for nuclear fusion -- hydrogen -- run out, and the death of the star begins. This death involves the fusion of new elements, resulting in either a large supernova explosion or a planetary nebula. After either comes our subject: compact stars, or stellar remnants.

Stellar remnants are the final stage of the star life cycle and are made up of the final material leftover at the death of a star. This material can take the form of a white dwarf, a neutron star, or a black hole, based upon its mass (see the Chandrasekhar limit).

Credit: https://www.spaceanswers.com/deep-space/five-amazing-facts-about-white-dwarfs/

A white dwarf, or degenerate dwarf, shown in the above image, is the result of the death of an average sized star. That is, someday our own Sun will become a white dwarf. It is very bright and very hot (as seen above), and composed mostly of helium, carbon, and oxygen. White dwarfs are also called degenerate dwarfs because they are made up of degenerate matter. Degenerate matter exists due to the Pauli Exclusion Principle, which states that electrons cannot occupy the same space. Therefore, electrons are forced to make up the energy state of an atom, and end up filling all energy levels of the atom, making it degenerate. This degeneracy halts the collapse of the white dwarf and its ability to perform nuclear fusion. Afterwards, white dwarfs continue to contract due to the force of gravity, which is why it is so hot! However, this heat is released by the white dwarf shining over time, and the white dwarf is not hot enough to replace that energy with nuclear reactions. Thus over time white dwarfs cool down and become black dwarfs, but this can take trillions of years -- longer than the current age of the Universe -- and so none exist today. White dwarfs are also capable of becoming supernovae if they consume a nearby star, often one that's part of a binary system. This can happen due to the white dwarf's extreme gravitational pull due to its large density -- a density so large that only a teaspoon of white dwarf matter would weigh 5.5 tons -- as much as an elephant here on Earth~

Credit: http://www.nbcnews.com/id/20360861/ns/technology_and_science-space/t/astronomers-spot-nearest-neutron-star/

The above image displays an artist's rendition of a neutron star -- actual images neutron stars are difficult to capture due to their small size. A neutron star occurs when the mass of the dying star exceeds the Chandrasekhar limit. Electron degeneracy can’t support the star’s mass, so collapses both protons and electrons into neutrons (hence the name!). Neutron stars are special in a couple of ways, and their existence actually affects human beings directly. Due to their high spin rate and electromagnetic radiation, the pulsing light emission of neutron stars can be seen from earth. The first observation of this was in 1967 by a woman named Jocelyn Bell. Though she suspected something complex was happening, her thesis advisor didn’t recognize the pulses (now known as Pulsars) and mistook the light as attempted contact from alien civilization. The pulses of light are actually electromagnetic radiation. Some neutron stars are the most magnetic objects in the Universe -- these especially magnetic stars are called Magnetars. Magnetars are so strong because the star's collapse and subsequent compression of magnetic field lines trap radiation in all but breaks at the magnetic poles. Like Earth, these magnetic poles are not directly aligned with the physical North and South poles.

Credit: http://www.pbs.org/wgbh/nova/next/physics/black-holes-could-turn-you-into-a-hologram-and-you-wouldnt-even-notice/

Pictured above is perhaps the most (subjectively) mythic of stellar systems -- the black hole. Black holes are the results of the death of the largest stars discussed in this article, and as a result have an immense density. They can be difficult to discover because black holes can’t be seen. However, we are able to see the accretion disks surrounding black holes. These are made up of a spiral of hot gas from a nearby star being pulled towards the black hole by its gravity, and can be seen at X-ray wavelengths. The binary star system of the star and the black hole is called an X-ray binary. The incredible density of the black hole contributes to such a large gravitational pull that the speed needed to escape is faster than the speed of light -- thus, not even light can escape a black hole. This impossibility of escape means that very little exploration of black holes can be done, but what is observed from the outside is strange enough to increase curiosity. For example, around the Event Horizon of a black hole time appears to slow. If you were to sit at the event horizon of a black hole, you would be able to use it as a time machine to see back through the entire evolution of the universe. You could even witness the future! Black holes are also infinitely dense, and because of this, the surrounding spacetime continuum is flexible. So flexible, in fact, that black holes can bend light from distant objects into a circle, which is called an Einstein ring.

TL;DR -- Even post mortem, stars continue to do some pretty fascinating things. We still have a lot to explore!

Hawking Radiation and evaporation of black holes

                Many theorize that there is nothing within a Black Hole, or more clearly no temperature or light. Within the Event Horizon, the point within a black hole where not even light can escape; the gravitational forces are so strong that anything passing the Event Horizon, including light, will never be able to escape. This theory that nothing can escape a black hole is theoretically sound because there is nothing faster than the speed of light. The only way matter could escape the gravitational pull beyond the event horizon would be if it were faster than the speed of light. While scientist have not been able to see a Black Hole close enough to determine whether or not it gives off blackbody radiation, Stephen  Hawking believes that a Black Hole can give off radiation and energy. This theory is known as Hawking Radiation. Hawking Radiation as well helps with the theory that Black Holes can evaporate by giving off energy.

Image credit: https://qph.ec.quoracdn.net/main-qimg-527e8a63b6a99759434c841f1f8bf28b-c?convert_to_webp=true

What is Hawking Radiation?

                 Due to the principles of Quantum Mechanics allows some form of energy to escape the event horizon and the black hole[1]. While classical mechanics does not allow this idea of energy escape.  If a black hole gives off a blackbody radiation what is creating this energy when everything that a black hole absorbs cannot escape? The energy that the black hole is giving off is radiating right before the event horizon. This energy is in the form of particles and antiparticle know as Virtual Particles. Their interaction with each other create this energy. 

Image credit: http://casa.colorado.edu/~ajsh/hawk.html

What do the Virtual Particles do and how they relate to Hawking radiation and energy?

                While Hawking Radiation is not proved to be correct, Stephen Hawking theorized that this energy escapes the Event Horizon as Virtual Particles. Virtual Particles are these theoretical particles move at such a fast speed that they cannot be seen, or virtually nonexistent. A virtual particle is composed of both a particle and an antiparticle the two separate, but then are attracted to each other and collide annihilating both. Virtual Particles in every section of space, only existing and annihilating at such a speed they cannot be seen.  This phenomenon occurs at such a fast rate that they cannot be seen, everywhere except on the event horizon of a black hole. On the Event Horizon, this separation can be seen because of the gravitational pull of the event horizon. While one particle is pulled within the black hole and disappears, the other is shot out of the event horizon.

Image credit: http://ircamera.as.arizona.edu/Astr2016/text/extplaydice.htm

 Evaporation of a black hole

              Only occurring after all matter surrounding the black hole has been absorbed. Virtual particles directly cause black holes to lose mass. This separation of Virtual particles causes a loss of a minutely small amount of energy due to the kinetic energy used when the particles separate. This loss of kinetic energy is because the particle and antiparticle are attracted to each other, for them to separate the black hole must use energy to separate them and thus lose mass. The idea of particles tunneling can also describe the loss of energy in a black hole. The Heisenberg uncertainty principle determines that the wavelength of a particle is an uncertainty. Because momentum and position cannot be precise for a particle, this uncertainty allows particles to exist inside or near the event horizon. The particle can then tunnel out of the black hole and cause it to lose a minute amount of mass.  While the energy loss is not even significant, over a long period this loss of energy will become significant, and the black hole would shrink and evaporate. This shrinking and evaporation of the Black hole cause it to become hotter as mass is inversely proportional to mass in a black hole as well decreases entropy.  This process would take over the life of the universe to take place, but it would evaporate.

 Conclusion

 For us to prove or disprove the idea of Hawking radiation and evaporation of black holes in nature rather than just in a laboratory could only be possible with the existence of micro black holes. The reason why they could be observed and help prove this theory is because they are so small that they could potentially evaporate right now.

In case you want to hear the same relative idea but through the voice of Morgan Freeman, click on this video below.

https://www.youtube.com/watch?v=1wHdtP0U6JA

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[1].Hamilton, Andrew. "Hawking Radiation." casa.colorado.edu. Last modified April 19, 1998. Accessed November 8, 2016. http://casa.colorado.edu/~ajsh/hawk.html.

Sources used:

Hamilton, Andrew. "Hawking Radiation." casa.colorado.edu. Last modified April 19, 1998. Accessed November 8, 2016. http://casa.colorado.edu/~ajsh/hawk.html.

Baez, John. "Hawking Radiation." math.ucr, University of California Riverside, 1994, math.ucr.edu/ 

     home/baez/physics/Relativity/BlackHoles/hawking.html. Accessed 1997.