In my work as an astrophysicist, I focus on understanding the stories of binary star systems – two stars that orbit around one another – and am specifically drawn to stories where a star in a binary system is fundamentally changed by the presence of its companion. Stars in pairs can change each other’s lives.
Our sky is full of binary stars. By some estimates, half of the stars in the sky are actually two stars – these stars are just so far from us they look like a single point of light to our eyes. This means the sky is actually a dynamic place of connection and interaction, of change and transformation.
My collaborations with other astrophysicists are crucially important to understand the stories of these binary systems. We each bring our own expertise, perspectives, and inquisitive minds to our work, and together can accomplish more than any of us could on our own. My collaborative relationships make it possible to study the relationships between stars. In this way, my research practice is one of relational astronomy.
Blue Stragglers and Blue Lurkers
We understand how stars like our Sun age. If we know a star’s mass and age, we can predict its temperature and brightness. Conversely, if we know a star’s temperature and brightness (and how far away it is), we can predict its age.
In the 1950s, the first color-magnitude diagrams of globular clusters revealed a population of stars that looked out of place. These stars appeared younger and bluer than the rest of the stars in the cluster, even though cluster stars are all formed at roughly the same time. Because these stars are bluer than expected, and are straggling behind their expected evolution, they were coined “blue straggler stars”. But how do blue stragglers form?
Our work:
The main theories for blue straggler formation all involve adding mass to an existing star. By adding mass the star will appear bluer and brighter than it was before, creating the younger appearance we observe. Using the Hubble Space Telescope, we determined that the majority of blue stragglers in open clusters are formed through mass transfer in binary systems. By finding the remnant core from this mass transfer process – what we call a white dwarf – we not only can definitely say that mass transfer occurred, we can even tell how long ago it happened.
My collaborator Emily Leiner discovered that not all stars who go through this process become massive enough to be blue stragglers. If the final mass is lower, these stars can remain hidden among the rest of the stellar population. She coined these stars “blue lurkers”. We recently found the first spectroscopically confirmed white dwarf companion of a blue lurker, confirming they have a similar formation history as their blue straggler cousins.
Papers:
Sub-subgiant Stars
Sometimes binary stars can change each other without ever coming into physical contact. Because stars interact gravitationally, they can impact each other’s motion and rotation from a distance. And because stars like our Sun have a magnetic field, changing a star’s rotation can also change its magnetic activity.
This is true for a group of stars called “sub-subgiant” stars. Subgiant stars have run out of their hydrogen fuel and are on their way to becoming red giant stars. Sub-subgiant stars are subgiants that are a bit too faint and a bit redder than expected for this evolutionary stage. These stars were first discovered in open and globular clusters, and similar to blue straggler stars, they raised the question – what happened here?
Our work:
We determined that sub-subgiants are almost all in close binary systems that are causing increased magnetic activity. These stronger magnetic fields change the interior structure and the surface of the star, creating the fainter and redder subgiant we observe.
But why doesn’t every subgiant become a magnetically active sub-subgiant? As a star expands, it naturally slows down its rotation to conserve angular momentum. But a subgiant star with a close companion experiences tidal forces, which can keep the star spinning rapidly despite its expansion. Sub-subgiants become magnetically active because of their close companion.
This magnetic activity causes the surface of sub-subgiants to be 30-40% covered in spots! Since we can’t see the inside of the star, we can use the amount of the surface covered with spots as a proxy for the total magnetic activity. Our observed constraints of the surface of sub-subgiants are important tests for theoretical models of magnetically active stars.