Measuring what cool temperature stars far from Earth are made of

The last four weeks of my life have been dedicated to writing my senior thesis and defending it in a one hour oral exam. I figured I would give a small summary of my work considering it has resulted in a 32 page thesis, which I will soon post to the site where many undergraduate theses accumulate each year.

For close to two years now I have been studying cool temperature stars, which are known as M stars. Their temperatures are far cooler than that of our Sun. M stars typically range anywhere from 2500 to 4000 Kelvin, whereas our Sun is around 6,000K. M stars can range from very dim dwarf stars to very bright giants. This depends on what stage of stellar evolution the star is in. (Stars, like people, change with age. They don’t grow wrinkles or get grey hair, but they do experience significant changes in their temperature, brightness, and chemical composition. Different stages signify how close a star is to dying, and also what kind of death the star will have. The entire process from the first to last stage is known as stellar evolution because the star evolves through its lifetime.)

The halo of the Milky Way is labeled on the right figure.

I am studying M stars that are very far away, therefore I am most interested in the bright M stars because they can be seen a large distances. These bright M stars are known (originally enough) as M giants. Now, what is so interesting about really cool (no pun intended) stars that are far away? Well, these stars happen to lie in the Galactic halo, which is a large sphere that surrounds our galaxy’s disk and central bulge and extends to the outer far outer reaches of the Milky Way. The halo is thought to made from the remnants of smaller galaxies the Milky Way has absorbed. The Milky Way is a pretty hefty (massive) galaxy, so it has a strong gravitational influence on its environment. Particularly, much less massive galaxies that travel too close will be ripped apart by the gravitational influence and their remains, mainly stars, become the material that comprises the halo.

So, by looking at the chemical make-up of these stars we can get a good idea of what the chemical make-up of their deceased-host galaxies. This can give us information about the history of how the halo formed and how small galaxies are created. In fact, it is thought that massive galaxies, such as the Milky Way, formed from collisions between small galaxies. So, we can gather a good idea of merging history between galaxies by studying their chemical compositions.

There are two types of small galaxies; those that are metal-poor and those that are metal-rich. Metal-poor means that these galaxies are primarily made of the basic two elements hydrogen and helium with very little of anything else. Metal-rich galaxies have a higher abundance of heavy elements besdies hydrogen and helium. Now, this does not mean that metal-rich galaxies have more heavy elements than hydrogen and helium; far from accurate. Hydrogen and helium constitute 98% of all real matter in the universe (excluding dark matter), so the remaining 2% consists of everything else.

My job is to look at metal-rich stars to get a better idea of the exact metal abundance found in metal-rich, low-mass galaxies. I’m doing this by looking at M Giants because it is thought that M giants represent some of the most metal-rich stars in the Milky Way Galaxy’s halo. (Actually, they represent the most metal-rich stars anywhere, but I’m looking at M giants in the halo specifically because the halo is where remnants of these low mass galaxies are found.) Now, the whole point of my research is finding a way to measure the abundance of elements heavier than hydrogen and helium in these stars. You would think that this would be a relatively easy process, and in fact it is pretty easy for the hotter stars.

In stars like our sun and hotter you will see a lot of single atoms such as hydrogen, helium, carbon, and oxygen. But, when you get to low temperatures, like those of M stars, these single atoms tend to get close and personal and actually bond to each other. Chemistry 101: any two or more combined atoms are known as a molecule. Therefore, you can see molecules in M stars, whereas any star with a temperature of approximately 4000K or greater will not have molecules. So, what’s the problem with molecules?

A spectrum of one of my M giants. Those large, wide dips are due to molecules such as titanium oxide.

When scientists study stars, they tend to look at what is called a spectrum. This basically provides information about the chemical make-up and temperature of a star. Molecules make it extremely difficult to measure the abundance of heavy elements in stellar spectra. Therefore, calculating the abundance of these heavy elements in M stars is difficult and no consistent method exists. My research is strictly concerned with finding a method that all astronomers can use to calculate the abundance of these heavy elements in M stars.

My research adviser and myself want to make it as practical as possible, so we intend to create this method by taking information directly from the M giants’ stellar spectra. We want to be able to measure a couple of features in the spectrum to get the abundance. I will continue to work on this project throughout the summer, and hopefully I will get some encouraging results. So far, my preliminary attempt at producing a plot that basically provides us a reference for measuring abundance has not given me the results I want. But, there’s still time and still hope, so keeping my fingers crossed I aim to get something useful before I leave for Texas A&M University towards the end of August.

A link to the full thesis can be found on my Publications page under “Measuring Metallicity of M Giants in the Galactic Halo Using Molecular Bands”.

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