Tanner Leighton here. During the summer of 2017, I became involved in stellar astrophysics research at Northwestern University in Evanston, IL, working under the direction of Aaron Geller and Adam Miller. Our research focuses on variable star classification for the upcoming Large Synoptic Survey Telescope (LSST). This telescope is going to revolutionize astronomy and astrophysics in many ways and discover millions of stellar variables and eclipsing binaries!
We are creating a citizen project on Zooniverse as means of an efficient and accurate way to classify millions of variable stars from LSST data. From LSST and our citizen science project, we will generate a census of the variable stars in the Milky Way galaxy. This will lead to a better understanding of variable star populations and inform theories of star formation and stellar evolution! We also expect to double the number of known instances for the existing variable star classes. This will inform future studies and be of particular utility for classes with fewer known instances!
"One millennium ago, the few astronomers working on Earth––in Asia (particularly in China), in the Middle East, and in Mesoamerica––knew of only six of the nine planets that orbit the Sun. Although they studied the stars, they did not understand that these points of light were as mighty as our own Sun, nor could they imagine the vast distances that separate these stars from Earth. One millennium later humanity's astronomical horizons, enlarged by observations made from every part of our planet and above it, had expanded to include the entire [observable] universe. Today we know that the Sun is but one of 300 billion stars in the Milky Way Galaxy, which itself is but one of trillions of galaxies within the visible universe. By peering billions of light-years into space, telescopes look billions of years into the past to observe the cosmos when it was young. Astronomers can now interpret what they see within the framework of a well- tested model, called the inflationary Big Bang theory. This theory describes how the cosmos has evolved since the first 10^–36 second of cosmic time, the moment of the Big Bang that began the universe. The universe has been expanding ever since that moment. During the first billion years after the Big Bang, galaxies and galaxy clusters began to emerge from a relatively featureless cosmos. Most of the matter in the universe exists in the form of dark matter, whose nature remains a mystery but whose existence is convincingly deduced from its gravitational effect on visible matter. Startling new observational evidence points to an even more mysterious dark energy that pervades the universe, driving the expansion to ever-greater velocities as time goes on" (National).
So although we understand baryonic (regular) matter quite well, we do not (yet) understand the nature of dark matter and dark energy––this is an active area of research. Their effect on the evolution of the universe ties into a fundamental question in astronomy and astrophysics (see question #1 below). Our research effort focuses most on question #3.
Five Fundamental Questions for Astronomy and Astrophysics:
(1) How did the universe begin, how did it evolve from a primordial soup of elementary particles into the complex structures we see today, and what fate lies in store for the cosmos?
(2) How do galaxies first arise and mature?
(3) How are stars born and how do they live and die?
(4) How do planets form and change as they age?
(5) Are we alone? That is, does life exist elsewhere in the universe?
This current decade, much work focuses in five particular areas:
(1) Determining the large-scale properties of the universe: its age, the types of matter and energy that it contains, and the history of its expansion
(2) Studying the dawn of the modern universe, when the first stars and galaxies formed
(3) Understanding the formation and growth of black holes of all sizes
(4) Studying the formation of stars and their planetary systems, including the birth and evolution of giant and terrestrial planets
(5) Understanding the effects of the astronomical environment on Earth
Every ten years, leading astronomers sit down and take stock of the status of the astronomy and astrophysics community. The above is what they came up with (National)!
Again, our research focuses on classifying the millions of variable stars that will be found by LSST. But just what are variable stars and why do we care about classifying them?
Variable stars are stars whose brightness varies with time. Stars can be intrinsically variable––variable because of some change within the star itself (e.g., pulsating variables, cataclysmic variables, etc.)––or extrinsically variable, implying their mechanism of variability is due to extrinsic effects, such as a binary companion passing in front and blocking part of the stars light for a period of time. Studying variable stars provides information about stellar properties, such as mass, radius, luminosity, temperature, internal and external structure, composition, and evolution.
Variable stars have been key players in astronomy and astrophysics, drastically improving our understanding of the universe––by determining the age of the universe, providing evidence that the universe is expanding (evidence for the big bang!) and that this expansion is accelerating (supernovae), and in determining distances to far-away galaxies (Cepheids and other standard candles!), among many other triumphs!
Further study and classification of variable stars will almost certainly lead to deeper insight about the universe and exciting discoveries. For example, LSST will likely find odd stars, which could reveal new classes of variability and rare instances of existing classes. When these stars are subjected to deeper scrutiny... Nobel Prize!
Classifying millions of variable stars into the many available classes is not something that computers can easily do (on their own). It’s also not something that one or even a large group of researcher can do. We are simpling dealing with too much data! One approach to overcome this data deluge problem is to create a Zooniverse project and enlist thousands of citizen scientists to help us classify variable stars! And fortunate for our research effort, many citizens are hungry to contribute to modern science; the days where only an elite group of scientists made all the contributions are behind us.
Our project will present data from the Palomar Transient Factory (PTF) database and, later on, LSST data to volunteer citizen scientists on Zooniverse. Our goal is to boil down the complexities of variable star classification into something that citizen scientists will be able to understand and be excited about. We have developed software in python to parse the data from the PTF database and produce light curve plots to visualize the data––both raw data light curves and Lomb-Scargle phase-folded light curves, which are much more informative for periodic sources. We are presenting our Zooniverse users light curve plots and best-fit periods (as well as instructions and background information about variable star classification).
Here is some of the software I have developed for this purpose:
I have also been combing the astrophysical literature to find coordinates (i.e., RA and DEC values) for variable stars of the many classes to instruct our Zooniverse users. I have been obtaining stellar data from the Caltech PTF database and running it though my software to produce light curve plots that we can show the citizen scientist as prototypical examples for each of the classes of variable stars!
Below are light curves for RR Lyrae (type RRab and RRc), an Eclipsing Binary system, a Mira variable, and a Quasar (AGN):
As a result of our project, we expect to approximately double the number of known instances of variable stars for each of the existing classes. This will inform future studies and be particularly valuable for classes with fewer known instances. With a larger sample size, future studies can do more stringent tests (e.g., of the so far very successful theory of stellar evolution). Our project will also almost certainly find a sizable number of odd stars, which could reveal new classes of variability and rare instances of existing classes! Furthermore, the census of variable stars in the Milky Way galaxy that this project will generate will lead to greater understanding about variable star populations. This information can be used to determine how likely certain stellar births are, and we can compare that with our current understanding of how stars are born!
Note: I will provide an update on this website for those interested when it is determined the date my group will be presenting if awarded the Nobel Prize for this effort! :)
