Reach for the Stars
Like many of you, I am extremely eager to find those tiny, elusive dips in the TESS lightcurves that reveal the existence of a distant, undiscovered, alien world. However, even though planets are my main focus, they are not the only interesting objects that we are able to find using TESS.
I have recently been talking to astronomers at the KU Leuven, in Belgium, who use the TESS lightcurves to study stars, their spots and pulsations as well as the architecture and behaviour of multiple star systems.
The lighcurves of these systems often boast beautiful patterns! Cole Johnston from the KU Leuven explains some of the systems behind these lightcurves:
Gamma Doradus variables are stars slightly more massive than the Sun with temperatures between 6,500K and 7,500K (the Sun is around 5,777K). You can recognise their lightcurves due their characteristic fluctuations in brightness that last from a few hours to several days. The fluctuations are caused by pulsations known as gravity-mode pulsations, which are waves that behave in the same way as surface waves in the ocean. These waves affect the surface temperature of the stars, changing the brightness that we observe.
Delta Scuti (also known as dwarf Cepheid variables) stars are hot, young (A-F type) stars that are ~1.5 to ~2.5 times the mass of our Sun. These stars can have a single strong pulsation or potentially hundreds of smaller pulsations with periods ranging from a couple of minutes to a few hours. The pulsations are caused by pressure waves, which behave identically to sound waves, that propagate near the surface of the star.
Cepheid Variables and RR Lyrae stars the older and more evolved cousins of delta scuti stars that also pulsate due to pressure waves. These stars are very exciting as they exhibit a pulsation period – luminosity (brightness) relationship, also known as the Leavitt law. This means that we can calculate the distances to these objects by studying the pulsations that we see in their lightcurves. Historically, these stars have helped us to calculate the distance to the Large Magellanic Cloud as well as to the centre of our Galaxy.
Slowly Pulsating B-stars
Slowly Pulsating B-stars (SPB stars) are very similar to Gamma Dor variables but are much more massive (2.5 to ~8 time as massive as the Sun). They are extremely hot with effective temperatures between ~11,000K and ~30,000K and typically show multiple gravity-mode pulsations that range from lasting a couple of hours to several days.
Beta Cephei stars are the high mass (8 to 20 times more massive than the Sun) stars that oscillate due to pressure waves, similar to Delta Scuti stars. The iron interior of these stars reaches extremely high temperatures of 200,000 K. At this temperature, the metal starts to behave strangely, resulting in a build up energy deep within the interior of the star. This causes the star to expand, resulting in an increase in surface area and thus an increase in observed brightness. The expansion of the star, however, uses up the stored energy and eventually it runs out of ‘expansion fuel’. At this point, the star begins to contract again due to gravity, resulting in a decrease in surface area and a decrease in brightness. This cycle repeats resulting in pulsations on time-scales of a couple of hours.
Stars are not simple, and many of them exhibit both pressure and gravity mode pulsations. Those lower mass stars which exhibit both are hybrid delta scuti / gamma dor pulsators, while the higher mass stars which exhibit both are hybrid SPB-Beta Cep pulsators.
Many stars are in binary (double) systems. Heartbeat stars are a class of binary stars that are so eccentric (not circular) that at the point when the stars are closest together, their gravity is so strong that the spherical stars morph into rugby-ball shapes. This increases their visible surface area, and hence increases the total amount of light that we see. Depending on the orientation of the system, we might see either a single brightening, a dip then a brightening, visa versa, or a brightening and eclipse.
We often like to visualise different types of stars on a plot of brightness (luminosity) versus their temperature, an example of which is shown in the figure below.
I love looking through the TESS data and coming across these beautiful light curves of fascinating stars. Maybe some of them even host a planet or two…