A Type Ia Supernova in an Unusual Place

Observations of Type Ia supernovae underpin our measurements of distances to other galaxies and even our understanding of the expansion of our universe. There’s just one problem: researchers still don’t know exactly how Type Ia supernovae happen. New research explores what a Type Ia supernova in an unusual environment could tell us about the source of these explosions.

Studying Supernovae

Illustration of a white dwarf accreting gas from a red giant companion star

Accretion from a companion star onto a white dwarf is one way to trigger a supernova. [Still image from an animation by NASA’s Goddard Space Flight Center Conceptual Image Lab]

Type Ia supernovae happen in binary systems containing at least one white dwarf: the crystallized core of a low- to intermediate-mass star. In theory, Type Ia supernovae happen when a white dwarf grows larger than 1.4 solar masses and explodes.

But research increasingly points to more than one way for white dwarfs to reach this limiting mass — two white dwarfs in a binary system could collide, or a white dwarf could bulk up by accreting matter from a stellar companion. The exploding white dwarf may not even need to reach 1.4 solar masses: if a thin shell of helium siphoned from a companion star ignites on the surface of a white dwarf, it could trigger a second, catastrophic detonation in the white dwarf’s core, regardless of the white dwarf’s mass. Pinpointing these different scenarios is critical to interpreting Type Ia supernova observations.

How can we examine the progenitor of a Type Ia supernova explosion? Researchers can’t just sift through archival observations after a Type Ia supernova to see what caused it, since white dwarfs in other galaxies are too faint to see — and we haven’t spotted a Type Ia supernova in our own galaxy since 1604. We need a different approach to uncover the origins of these explosions.

Cluster Candidate

In a recent article, Joel Bregman (University of Michigan) and collaborators proposed a path forward: find a Type Ia supernova in a globular cluster. Globular clusters are roughly spherical collections of tens of thousands to millions of stars that orbit within a galaxy’s halo. Because all stars in a globular cluster have roughly the same age and composition, the age and composition of the stars involved in the explosion could also be known — even if we can’t observe the stars directly.

Hubble Space Telescope image of supernova SN 2019ein

The location of the supernova SN 2019ein relative to the globular cluster candidate. Click to enlarge. [Bregman et al. 2024]

Bregman’s team searched for records of a Type Ia supernova occurring within about 100 million light-years in a galaxy with globular clusters. The team pinpointed one promising candidate: SN 2019ein, which was discovered in May 2019 on the outskirts of the elliptical galaxy NGC 5353. The supernova popped up far from the center of this galaxy but close to a faint object that may be a globular cluster.

Detonation Scenarios

Hubble observations show that the supernova happened about 192 light-years from the globular cluster candidate. Statistical tests suggested that the supernova is likely associated with the cluster — the authors put the odds of a chance alignment at 3% — allowing researchers to use the age and composition of the cluster to constrain the properties of the supernova progenitor system.

There’s another interesting wrinkle that could further constrain the identity of the progenitor: given the large separation between the supernova and the candidate cluster, the star going supernova may not have been bound to the cluster when the explosion happened. In this scenario, dynamical interactions between binary systems could have kicked the supernova progenitor system out of the cluster, which is more likely if both stars in the binary were white dwarfs. Then, after careening through space for 3–10 million years, 1) the white dwarfs collided or 2) one white dwarf stole a thin layer of helium from the other, leading to a two-stage detonation on its surface and in its core.

The proposed scenario is rare but not impossible, but there’s not yet enough data to draw firm conclusions. This study highlights the challenge of studying the origins of Type Ia supernovae and shows what can be gained by seeking supernovae in unusual places.


“A Type Ia Supernova near a Globular Cluster in the Early-Type Galaxy NGC 5353,” Joel N. Bregman et al 2024 ApJL 968 L6. doi:10.3847/2041-8213/ad498f