Twinkle twinkle little star - how I wonder what you are?
Stars are the fundamental unit of astrophysics. They are the objects around which exoplanets orbit, the engines that drive the dynamical and chemical evolution of galaxies and the Universe, and the progenitors of high energy phenomena that illuminate the Cosmos, such as novae, supernovae, and gravitational wave events. Thus, it is crucial that we understand how stars are formed, how they evolve, and how they die since stellar structure and evolution calculations serve as input for every other field of astrophysics. While the theory of stellar structure and evolution is generally successful in explaining the broad strokes of stellar evolution, when confronted with high precision data, evolution models clearly demonstrate an inability to explain the observations. My research is focused on improving modern stellar evolution models through combining the fields of data science, binary star physics, and asteroseismology.
What is a star?
According to the International Astronomical Union, a star is a big ball of plasma that is held together by its own gravity, and prevented from collapse by inner pressure that is the consequence of nuclear fusion processes in the star's core regions. While this is a nice technical definition (that has sparked many lively discussion at workshops and conferences), what does it mean?
Consider our Sun. We see it as a big yellow ball in the sky, and we literally feel the heat coming from it, despite being extremely far away. This heat, or radiation, is a hint as to what is happening deep inside the star. At its surface, the Sun is 5778 K (5505 C or 9941 F). However, as you delve deep into the interior, it gets much denser and much hotter. The intense gravity of the Sun that holds it together also causes it to get very hot and very dense at the center of the star. The temperatures and densities are so high here that the elements in the stellar center undergo nuclear fusion - which generates a large amount of heat and energy. This heat and energy pushes back against the inward pull of gravity, causes a star to enter a sort of equilibrium.
Why are stars difference colours?
Consider the burning match you see here - what is the hottest part of the flame? The blue part at the bottom! Why is that? Well stars radiate light away approximately as a blackbody - this means that they continuously emit light at all wavelengths, with the maximum output being determined by their surface temperature. As shown in the image below, the temperature of a star determines the profile of the light that it emits. So a star like our Sun (5778 K) will produce most of its light in the visible range, and will appear yellow. A hotter star produces more UV light at shorter wavelengths, and therefore appears blue to us, while a cooler star will produce most of its light in the Infrared range, making it appear red to our eyes.
Credit: https://webbtelescope.org/ & STScI
Why does this matter?
The fact that stars have a distinct colour according to their temperature allows us to sort them very neatly. Take a look at the video below to see why.
So what?
The surface temperature of a star is very strongly linked to its overall mass and its evolutionary state, while the size of a star is directly related to its brightness and its evolutionary state - therefore, just by placing stars in the diagram based on their brightness and colour (temperature) we have a way of categorising the mass and evolutionary stage of a star! This is an immensely useful diagnostic for astrophysicists, and is actually the starting point for all of my research!
