Stellar Astrophysics

In the simplest terms, the field of stellar astrophysics is the general study of stars. However, it is a broad field that includes a lot of topics. Stars produce energy in their cores and radiate it out to the environment around them. They are full of magnetic fields and nuclear reactions. They blast out stellar winds and flares. They are born, they evolve, and then they die (explosively!). Stars are very complex objects, and it takes a lot of study to understand all of their various processes.

Know thy star, know thy planet

An artistic representation of the star K2-33 and its exoplanet. (Credit: NASA/JPL-Caltech)

Even though scientists have been studying stars long before anyone ever discovered an exoplanet, the two disciplines are now very closely linked. There is a saying amongst exoplanet researchers: “Know thy star, know thy planet.” This means to really understand an exoplanet, we first need to understand its host star. For example, many of the quantities we try to measure for an exoplanet (like its size or mass) are only known in relation to the star it orbits. Discovering that a planet is 10% the size and 0.1% the mass of its star is only helpful if we have a good idea how big and massive that star is.

In addition, most exoplanet detection methods are indirect and only tell us about the exoplanet through signals in the star’s light. This can sometimes make it hard to disentangle a planet’s signal from its star’s signal. Stars are not perfectly static objects. They have hot and cool spots that change over time, the side of the star that faces us changes as it rotates, and they sometimes pulsate in irregular patterns or belch hot material from their surfaces. These processes can all subtly change the star’s brightness and spectrum, making it a challenge to figure out which of the observed features are caused by the orbiting planet and which are from the star. If we can understand what is happening with a star, it becomes far easier to distinguish the effects of the planet.

In pursuing this stellar understanding, some of our researchers at iREx study stars full time. Using the SPIRou instrument, our team has made incredibly detailed measurements of the nearby red dwarf called Barnard’s Star. The data show more features than even the best computer simulations, helping us understand how to make better models and understand stars more fully.

Young stars and stellar associations

A map of some of the young star associations in the Milky Way. (Credit: M. Thévenot/J. Gagné)

Other iREx scientists are analysing how stars group and move together through our Galaxy. These “stellar associations” can be incredibly useful once they are identified because the stars in an association were likely formed at the same time and built out of the same mix of gasses and dust. This helps to determine how old the stars are, which also tells us how old any planets around them are. Knowing the age of exoplanets aids researchers in figuring out how planets form and evolve.

Age is a particularly important piece of information when we take direct images of exoplanets. Exoplanets are so small and far away that we don’t really “see” a picture of the planet the way we can with planets in our own Solar System, but sometimes we can detect the speck of light coming directly from the planet. A major limitation with this direct imaging method is that one normally can’t tell the difference between a big planet and a hot planet. Both qualities make the exoplanet brighter. However, if we know the age of the planet, we can estimate how much it has cooled down since it formed, finally allowing a determination of its size.

Effects on habitability

The habitable zone around a star is related to the temperature range where liquid water can exist. But that’s not the whole story. There are many other properties of a star that may enhance or ruin a planet’s chance at habitability.

The biggest, hottest, brightest stars have extremely broad habitable zones that could fit several planets in them, but these stars are very rare and don’t live long (relatively speaking) before violently exploding in a supernova. Even if a planet did form in its habitable zone, such hot stars emit a huge amount of high-energy radiation that would be very harmful to Earth-like life and could even strip atmospheres away from planets.

On the other hand, small, cool red dwarfs are by far the most common type of star in our Galaxy. Their small sizes and low masses actually makes detecting small planets around them far easier, especially since there are so many to search through. Many astronomers still think these tiny stars may be the best place to look for habitable worlds.

Stellar astrophysics at iREx

Because understanding how stars behave is so important to exoplanet science, iREx includes many stellar astrophysics experts. By combining knowledge from these complementary fields, both areas of research benefit and we learn more about stars and the planets orbiting them. To learn more, we invite you to read their profiles: