I Got Sent More Reading Materials

June 5, 2014

I don’t have anything particular to say about them. They are pretty much a technical summary of brown dwarfs found, classification of spectra, proper motion (angle speed in sky), and distance, which were found using the 2MASS survey and WISE telescope. You can read them here, here, and here, if you want to learn more about brown dwarfs. They are pretty technical, though.

Update: You know what? I was mulling things over, and turns out there are a few things I want to comment about. Firstly, there is a spectral classification cooler than the T type, called the Y type, which is characterized by absorption from ammonia. These brown dwarfs are cooler than 600 K. The articles above talk about some of those.

Secondly, the articles above have a large focus on high proper motion brown dwarfs. Higher proper motion implies that the object are more probable to be closer to us than those having low proper motion. Think about it this way, when you are in a car, things that are far away look slower than things that are closer to you. It’s not so much that in reality things that are farther away are slower, as it is the fact that things that are farther away has lesser angular movement because distances look smaller when farther away.

How are proper motion found? Well, turns out that the 2MASS survey took observations a decade previous to WISE telescope. So, you look at the pictures from 2MASS, and you look at one from WISE, and see how much the position has changed in angle. The objects are likely to have constant speed, so just divide the angle difference by the year difference.

What is proper motion helpful for? Well, it let us know that an object is likely to be close to the solar system. This is supposed to help bridge the gap in our knowledge of the solar system neighborhood. The distance itself can later be taken using parallax. Aside from that, finding close brown dwarfs is also helpful because it will help us study the atmosphere better. We still have an imperfect model on the process behind condensation and cloud formation in the brown dwarf atmosphere, and the more of them we find, the better we will know the details behind it.


Brown Dwarf Pics

June 3, 2014

This is what an image for brown dwarf looks like, just a faint dot. As you can see in the link, each of those four images represent the different wavelength of infrared the pictures were taken. Obviously WISE band 1 and 2 (WISE is the name of the telescope) are best for this sort of stuff. Oh, and fun fact, this one was discovered by my prof.

 


The Point of My Research

June 2, 2014

I talked with my prof and got further information on what the point of the project is. It has to do with the Kepler space telescope. If you haven’t heard, two of its four wheels, which are used to point the telescope to a location, are broken, and so the telescope can’t maintain its sight to a position in the sky. Not only is there the fact that it is rotating around the Earth, the light from the sun has momentum. The light will push the telescope, and the irregularity of the telescope’s shape causes it to torque. The only way for the telescope to not be perturbed is to lie perpendicular to the sun. Unfortunately, that means that it can only observe in the plane of ecliptic, and it can’t maintain the same field of view throughout the year as the telescope has to maintain perpendicularity to the sun as the Earth orbits the sun. Nevertheless, useful science can be done. The telescope will observe certain fields of view, and when time is up, it will rotate again to another field of view that will maintain perpendicularity.

The point where my research comes in has to do with the way the above procedure means that the antenna is not facing the Earth properly. That means in order for them to continually observe an area in space, they will have to keep the information in the hard drive, and then send it back to Earth once the observation period is over. That means they have a limited amount of data they can store, and so the mission will have to be picky in which data they store. Looking for brown dwarfs to observe is supposed to help out Kepler in keeping . There are areas where not much brown dwarfs discovered, so what I am doing is helping that process out.

For now, I am just installing the astropy library for Python language. l will be looking at some picture of brown dwarfs, download them, and hopefully astropy can take those pictures and present them to me.

Source

http://keplerscience.arc.nasa.gov/K2/MissionConcept.shtml


Introduction to my Research: Brown Dwarfs

June 2, 2014

So, I got accepted into a summer research program. They are going to pay me and everything, yay! Which is why I thought I should blog about my research.

What I am doing this summer is basically sift through infrared data and looking for object called brown dwarfs and classifying them. Brown dwarfs are in a way objects that represent the transitions between planets and stars. They are much more massive than planets, enough to have fused deuterium (hydrogen, but with a neutron) for a short while, but they are not massive enough to fuse regular hydrogen. The mass may range from 13 to 80 Jupiters. After the short lived deuterium fusion phase, they just start cooling down. They tend to emit most of the light in the near infrared zone. Size wise, they are not too different from Jupiter because the interior pressure is governed by electron degeneracy. Basically, the pressure of electrons due to them not being able to occupy the same energy states is what holds the object from further collapse. In a pressure dominated by degenerate matter, adding more mass doesn’t cause an object to grow much.

How does one empirically distinguish a brown dwarf from a low mass star? There is a simple test called the Lithium test. If one can detect lithium, it is probably a brown dwarf. Stars are hot enough to be able to fuse lithium, unlike brown dwarfs, and lithium fuses at a similar temperature to hydrogen. That said, the highest mass brown dwarfs can burn lithium, so at the high end range of brown dwarf mass, they are indistinguishable from stars. Well, there is another way, but it is harder to do. Basically, measure the temperature and mass of a brown dwarf binary. The theory is that an object of certain mass will have a different temperature curve as it ages than one of a different mass. If an object is massive enough to be a star, the temperature will stabilize due to sustained fusion, if not it keeps cooling down.

Before we go on, I would like to address how we can know what compounds such a distant object have. We look at the spectrum of light from the object, and see which wavelengths are missing. Certain compounds absorbs light at certain specific frequencies because of the way the electrons are configurated energy wise. Electrons can only occupy certain energies, and so they will absorb only light of certain energies.  So, when you look at the spectrum of light coming off the planet, there will be chunks of light that will be missing corresponding to the energy absorbed. Imagine that the rainbow has various thin strips on it missing, that is what it is like.

Brown dwarfs come in two spectral type, the L type and the T type, with L being the hotter, more luminous type. The temperature itself is correlated with luminosity by the fourth power, so measuring luminosity gives you the temperature. The spectral types are related to what compounds one detects in the atmosphere. The L types have the temperature range of 1300 K to 2500 K. The hottest L type show vanadium oxide and titanium oxide absorption signatures in the spectra, and as the brown dwarf gets cooler, this transitions to a spectra with absorption due to metal hydride (FeH, MgH, CaH, etc) and Alkali metal compounds (NaI, KI, etc), while the metal oxide absorption disappears. The T type are objects below 1500 K, and the spectra contains absorptions from methane, water, ammonia, and CO. Does this mean that the transition of  spectra represents another good way to gauge the temperature of the object? Yes for early L and late T. Not so much for the transition between late L to early T. In that section, the temperature doesn’t change very much while the spectra changes. A point of note is that the temperature/spectra correlation works much better with optical than infrared.

There is a reasonable explanation for the transitions of spectra a brown dwarf undergoes. As the brown dwarf cools, the metal oxides condense and fall off deeper into the atmosphere. This uncovers the spectra from the alkali metals and hydride metals. As the temperature goes lower to the T type level, clouds become an important component. This explains the rapid change in spectra with the slower change in temperature as this transition of dusty atmosphere occurs around a short range around 1400 K.  At temperatures lower than 1400 K, the dust sinks and the metal hydrides condense while conditions are right for the reaction of CO+H2=H2O+CH4 occurs, meaning carbon monoxide with hydrogen reacts to for water and methane. This explains the methane absorption that dominates in the T brown dwarfs.

Finally, I would like to point out the model pointed out above for the brown dwarfs differs with metallicity. Metals in astronomy are all elements with higher atomic number than helium. Brown dwarfs are allowed to be more massive and hotter with less metal.

This is a basic summary of what we know about brown dwarfs. If you want to know more about them, you can of course go ahead and read what I linked below. There has also been some advances, like the discovery of below freezing brown dwarfs. And there are mysteries to be solved, like the fact that there are three scenarios for condensations and dusts that plays a key role in the late L to early T spectrum.

I have done all this preliminary reading. Hopefully I know what I will be doing. Granted, there will be some fumbling around as this is my first legit science project ever.

Source and reading materials

http://www.annualreviews.org/doi/abs/10.1146/annurev.astro.42.053102.134017?journalCode=astro

http://web.archive.org/web/20060928065124/http://www.carnegieinstitution.org/News4-3,2001.html (brown dwarf)

https://en.wikipedia.org/wiki/Brown_dwarf


A Little Something on Triton

May 28, 2014

Here is a video on Neptune’s largest moon Triton, with a cute Triton thumbnail. It is a great summary of the basic facts regarding this object:


Star Endings: Pair Instability and Photodisintegration

May 27, 2014

Population III Stars

For the third part in the series of posts on star deaths, we look at deaths at the most extreme ends. They happen to enormously massive stars that were only common during the beginning of the universe, called the population III stars. Population III stars are a class of hypothetical first generation, never observed stars, that are believed to be over a hundred times more massive than the sun, and have none or almost no metal ( in astronomy any element of higher atomic number than helium).  Now, why they are called population III instead of I, don’t ask me, but turns out it is all reversed. Population II is the metal poor stars, and population I is metal rich stars like the sun.

While stars of 100+ solar mass exists in today’s universe (as seen here), they are extremely rare. In terms of probability, there is the fact that this kind of massive stars are extremely short lived compared to other stars, less than 10 million years compared to billions of years like the sun. In terms of the time scale of the universe, that is a blink of an eye, so more likely than not, you will miss it. Physically, huge stars form more easily in metal poor condition. The presence of metals allow for further cooling of gas to takes place, which inhibits the formation of huge stars. The temperature is important because the hotter the gas, the higher the gas pressure is, which counteracts the force of gravity. So, cooler gas contracts further, which gets fragmented in the process, creating smaller stars. In that way, metallicity is once again shown to be a powerful factor in determining the life and times of a star.

So, considering the hypothetical/theoretical nature of these objects and events, should we take all of this with a grain of salt? Well, yes and no. We do believe the theories are correct because they have worked in other situations. At the same time, that doesn’t mean we know all the details, and those details do matter when formulating accurate theories. Furthermore, there are the issues of rotation. The way I think of it is, the general picture is likely to be right, but if new observations come that contradict certain aspects of the models, then of course, I change my mind. That said, these are issues that are already being probed, with the search of population III stars being a high priority, and hey look, pair instability supernovas are already looking very likely.

Pair Instability End

Pair instability happens to stars beyond 140 solar mass. The energy in the core is such that the photons in there create electron-positron pairs. This happens because other forms of energy can be transformed into matter, and due to conservation principles, of charge (+1 for positive charge, -1 for negative charge) and particle number(+1 for matter, -1 for antimatter). Enough energy is taken away in the process that gravity dominates and the core starts contracting. Now, pair instability does occur in lower mass stars, around 100 to 140 solar masses, but it isn’t  enough to cause a runaway nuclear explosion. Instead, it will increase nuclear reaction until gravity and pressure balances out. This pulsation goes on until a star loses enough mass so pair instability doesn’t happen. Otherwise, the collapse of the core causes an uncontrollable reaction of oxygen and silicon to begin. While turning a huge amount of mass into Nickel 56, the core bounces back from collapse. What ends up happening is an extremely energetic explosion that leaves no remnant of the star behind. The star has pretty much been completely obliterated. Well, I guess you count count the gas and dust left by the star as remnant if you want…

Photodisintegration End

At the 250 solar mass point, a star dies by photodisintegration instability. At the energy generated by this star, a significant portion of the photons gets used up splitting atoms apart. The core collapses without the energy needed to hold up the star from its own gravity, and a black hole forms. This is pretty much core collapse, as in previous post, but with a different reason for the instability to happen, so some of the detail from the previous post about degeneracy and black holes applies here. I am not exactly sure at which part of the chain of nuclear reactions this happens at, though.

I wish I could say that there have been this kind of supernova observed. As far as I can see, I haven’t found any article regarding any observation of this type of supernova. So, for now you can put this entirely in the hypothetical area in your mind.

Conclusion

With this, I conclude the three series of posts on how stars die. Now you know the general process of how stars die, with some of the gory details displayed. If they felt dense, I feel for you. Even I learned a lot on the nuances of these processes, and had one hell of a time researching a lot of these facts. Even so, if these posts have informed you and made you interested in astronomy, well then I am happy. If it made you want to vomit, I am okay with that too, just tell me what went wrong!

Sources and Reading Materials

http://www.universetoday.com/24776/what-were-the-first-stars/

http://www.astro.yale.edu/larson/papers/Bromm02.pdf

http://scitation.aip.org/content/aip/magazine/physicstoday/article/64/4/10.1063/1.3580493

http://www.solstation.com/x-objects/first.htm

http://arxiv.org/pdf/astro-ph/0107037v2.pdf

http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.0929v1.pdf

http://www.skyandtelescope.com/astronomy-news/a-super-duper-supernova/

http://news.nationalgeographic.com/news/2009/12/091202-biggest-star-explosion-supernova/

http://iopscience.iop.org/0004-637X/550/1/372/pdf/52134.web.pdf

http://arxiv.org/pdf/astro-ph/0212469v1.pdf


The Power of Scientific Modeling

May 10, 2014

Scientific modeling is a powerful tool. Using the laws of physics, and computers if you want things to be much more convenient, you can predict how nature will behave and how it has behaved in the past. Recently, astronomers modeled the universe starting from 12 million years after the Big Bang, and let it run to present day, 13.7 billion years later. The result is what I would call beautiful. A great match between observation and theory. Here is a video by Nature journal explaining this:

As explained in the video, the simulated properties of the universe matches a lot with the properties of a real life universe, so we know that the laws of physics we have developed is in the ball park area of correctness. It is also explained that it is not perfect, and that is because even though we got a lot of it right, our knowledge of the universe is not perfect. That doesn’t mean science is wrong, it means it is incomplete, and we have to do more detective work in order to work out the rules of nature.

Here is another great match between data and models, this time between real life yearly average temperature and two models crunched by computers using the laws of physics. One of the models is temperature with human forcing of CO2, and the other one only includes natural factors. As you can see, real world data matches the one with human forcing, and it also “predicts” past temperatures very closely (shout out to badatronomy’s great post on climate change):

Another point of note is, all scientific models and measurements have uncertainties in them, but the data itself remains most of the time within the boundaries of the errors, so it is a good fit.

One of my favorites is the modeling of the ENSO without taking account of it. Meaning, you model the Earth with the ocean and atmosphere, plug in the laws of physics and various conditions, and ENSO will naturally occur in the simulation even though we don’t the exact mechanism for it. That is the predictive power of science.


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