|
The Composition of Stars
The only elements that were formed during the Big Bang were hydrogen and helium. As the universe slowly expanded and cooled, gas started to collect into massive balls (hundreds of times the mass of our Sun) which eventually formed the first stars. Within these stars, where temperatures and pressures are extremely high, the hydrogen and helium atoms would collide with each other and stick, forming new elements such as carbon and oxygen. The first stars lived short lives and when they died, their explosion sent their material -- including the newly-created elements -- all over the universe. However, like the universe itself, the stellar explosions were uneven, meaning they all kinds and amounts of elements to different regions of the universe. Eventually, these new elements would be swept up in another huge ball of gas and form a star. Instead of being composed of only hydrogen and helium, this star also had carbon and oxygen, which were the seeds for creating larger elements such as silicon, titanium, and iron. As this cycle continued, the rest of the Periodic Table was produced as a direct result of fusion within stellar interiors, stellar collisions, and explosions (or supernovae): 
The Periodic Table of Elements showing the origin(s) of each element, note that nearly all are made within stars.
Measuring Stellar Element Abundances
Astronomers are able to measure the amount of individual elements within stars, or element abundances, by using a technique called spectroscopy. Stars, as we all know, shine. The light that comes out of them originates from their core and eventually leaves through the top layers of the star, known as the photosphere. The composition of the photosphere is not the same as the stellar core, in other words, it does not contain all of the elements that are made deep within the star. Instead, the photosphere is made up of the same elements and material from when the star was originally formed, or the stellar birth cloud. As the light enters the photosphere, it runs into individual atoms and often a bit of the light gets absorbed by the atoms. Since each atom has a different structure, the kind (or wavelength) of light that gets absorbed is unique to the specific atom, say nitrogen. We can see the light that is absorbed by looking at the stellar light across a range of wavelengths (the rainbow diagram in the figure below) and looking for the places where the light has been removed by the nitrogen, shown as dark absorption lines. If the light runs into many nitrogen atoms, then more and more light is absorbed. Converting the rainbow diagram (top) to show us the intensity of the light (bottom), or stellar spectra, we can see that more nitrogen absorption gives us bigger dips in the spectra that are specific to nitrogen -- no other atom can absorb in exactly the same place within the stellar spectra. We can then measure the area of those dips and calculate how many nitrogen atoms must be present, or the abundance of nitrogen, within the star in order to create dips of that size. By looking across a wide wavelength range, we are able to measure the abundances of a huge number of elements within stars. The Hypatia Catalog brings together data from anyone who has measured element abundances within nearby stars -- which anyone can access and use! 
Illustration of spectroscopy, where light interacts with atoms as it leaves the star, creating absorption lines in the stellar spectra from which we can determine the abundance or amount of the elements that are present within the star.
The origin of every element varies -- as shown by the Periodic Table above. But also, every region of the universe -- even the galaxy -- is affected by a variety of phenomena. And since element abundances give us information about the composition of the cloud from which the star originally formed (and not those elements that are being made within the star), studying the patterns within stellar abundances allows us to understand a variety of astrophysical phenomena. For example, when should a supernova have exploded near(ish) the Solar System such that we have the right elements to make the Earth habitable? What about binary systems, where two stars were born at the same time and now orbit each other, can they tell us how the birth cloud was split between two bodies? Is our Sun "normal" or different compared to stars of similar size and temperature? What is "abnormal"? What additional physics affect the element abundances -- for example, stars that rotate very quickly?
Understanding the Composition of Exoplanets
Planets outside of our Solar system, or exoplanets, are very very small compared to their host star -- imagine a raindrop next to an average human. So, measuring the composition of a planet's interior or even its surface is not currently technologically possible. However, understanding the make-up of a planet is so incredibly important to our understanding of planet habitability (I mean, look at Earth and Venus!). Fortunately, stars and planets are formed at the same time and from the same material within the stellar birth cloud. Therefore, we can use the composition of the star as a proxy for what is very likely in the planet. For example, the ratio of iron and magnesium in the Sun, Earth, and Mars are all the same to within ~10%! 
Possible interiors and compositions of exoplanets, which have a big impact on the surface conditions and overall habitability of the planet. Image credit: JPL.
So using the stellar abundances, we can make models of nearby planets to get a better understanding of their composition, internal structure, and overall dynamics -- because Mercury, Venus, Earth, and Mars are all fairly similar in size, but have very dissimilar compositions. From there we can try to understand star and planet formation. For example, since stars and planets are formed out of the same cloud, does a planet "steal" elements from its parent star or does the star need to have enough of particular elements so that a planet can be formed? What is the elemental interaction between stars and planets? What kinds of elements must be present in a planetary system such that they can be habitable for Earth-like life or any kind of life?
Stars are the building blocks of the universe, they make-up both galaxies and planetary systems, therefore they can shed a lot light onto a wide variety of sub-fields within astronomy. My job is to try to understand these interactions a little better. M-dwarf Stars and Their Planets
The smallest stars with fusion in their core are referred to as M-dwarfs, or sometimes "red dwarfs" since the majority of the light that they emit is red or infrared. M-dwarfs are the most common star within the Milky Way, making up ~75% of all stars! Because M-dwarfs are so small and cool, they are the best place to measure really difficult elements within the galaxy -- like chlorine and fluorine. It is also much easier to find Earth-sized planets that orbit M-dwarfs since it's easier to detect a smaller planet around a smaller star. Unfortunately, measuring M-dwarf composition is incredibly difficult from the Earth because our atmosphere -- which has a lot of water and carbon dioxide -- actually blocks most of the (near) infrared light from reaching the Earth's surface. And space-based instruments, like the James Webb Space Telescope, doesn't have enough resolution to view individual atomic absorption lines (it's like having a phone camera with a handful of pixels instead of mega-pixels). As a result, we know very little about the composition of M-dwarf stars...and their planets! For example, only a small fraction of stars within the Hypatia Catalog are M-dwarfs. 
Artist’s rendition of an M-dwarf star surrounded by planets.
Image credit: NASA/JPL-Caltech/MSSS.
In order to better understand these "forgotten stars," I am leading an amazing team to win $20 million from NASA to build a high-resolution spectrograph that we plan on flying on a weather balloon. The balloon will fly at an altitude of ~36km above the Earth's surface, using the circumpolar winds above Antartica to make two full revolutions around the South Pole. By getting above the atmosphere, we will be able observe hundreds of M-dwarf stars in a way that has never been achieved before! Our mission is called: Spectroscopic Abundances to Know the Heritage of M-dwarf Environs through Time, or SAKHMET, after one of the Egyptian goddesses of creation. Check out some of the articles on the Outreach & Media page for more information.
Artist illustration of the SAKHMET balloon mission flying around the Antarctic continent. Image credit: Danielle Harrelson.
Discrepancies in Stellar Abundance Techniques
Through my work, I analyzed elements abundances that were measured within the same star but by different groups using a variety of telescopes over the last +40yrs. Below is an example of how these determinations graphically compare, with respective error bars (Hinkel et al. 2014). The stars and elements were chosen to be representative, simply because they were commonly measured. The obvious discrepancy between not only the five elements (Na, Si, O, ScII, and Al) but for iron as well is cause for serious concern; namely, abundance measurements by different groups do not produce the same result. I worked with a team of international collaborators to analyze the same stellar spectra using varying techniques. The goal is to better understand how the methods, stellar parameters, and line lists affect the resulting abundance measurements, so that results will be more comparable. Our results were published in Hinkel et al. (2016). 
Data from the Hypatia Catalog showing the dispersion in abundance measurements for the same element (Na = green, Si = blue, etc.) in the same star (x-axis) by varying
literature sources (circles). The corresponding [Fe/H] measurements for each of the literature sources are shown as triangles. Individual error bars published by each group are shown -- note that in many cases they do not overlap, indicating there is a lack of consensus between abundance measurement techniques. Figure from Hinkel et al. 2014.
|
| Natalie Hinkel | Research |
