Sky & Telescope - June 2024 - 18

Stellar Smashups
Observe, Model, Repeat
The more mergers we find, the better we can understand
what happens physically during the crash. But to really have
a handle on what's going on, we need to watch mergers up
close. Smashing two stars together in a lab is out of reach.
The next best thing is smashing simulated ones together on a
computer. My own research uses this sort of computer modeling
to try to interpret and reconstruct the signals we see.
Compared with a star's lifetime, all of these splashy
stellar mergers seem incredibly sudden: A star goes through
its immensely long life, only to collide with its companion
over the course of a few days. To find out what's causing the
sudden brightness changes, I built computer models of the
very beginning of a stellar merger - the phase during which
the pair just starts to fall together. These models reveal that
the progressive pre-outburst brightening starts when the
stars are still separate but spiraling closer together, spilling
out some of their gas into their surroundings as their
orbits tighten. The flashy outbursts themselves come from
the moment of splashdown, when the smaller star first falls
inside its companion.
With that general picture in mind, we're now working
hard to understand the growing population of events and
their differences. Right now, our best predictions for the light
a merger event should produce don't quite match the real
thing. Eventually, through a mix of crime-scene reconstruction
and trial and error, we hope to improve our computer
BLUE STRAGGLERS IN M67 Near right:
As stars age, they first stop fusing hydrogen in
their cores, then swell and redden. Plotting a
cluster's stars by color and brightness reveals
this turnoff point (rightward kink). Because a
star's lifetime depends on its mass, astronomers
can tell how old a cluster is based on
which stars have stopped fusing hydrogen
and moved rightward on the diagram. Here,
the upper left of the diagram for the cluster
M67 ought to be empty, because all the most
massive stars have already evolved rightward.
Yet some " blue stragglers " remain. These stars
have been rejuvenated by binary interactions.
Far right: The blue stragglers of M67 are
circled. Blue circles mark blue stragglers with
detected binary companions. Pink circles mark
blue stragglers that don't have companions -
these are potentially merger-made. North is up.
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models to the point that we're able to see the light from a
merger in a distant galaxy and use the models to infer the
nature of the colliding system, including the stars' masses,
radii, and other properties.
Old Stars Made Young Again
Despite all of the advances, merging stars are rare enough
that we're still not often lucky enough to catch them in the
act. Instead, we often have to look for telltale signs that
reveal a star was created by a merger. If Mason and Tylenda
are merger hunters, other astronomers are merger archaeologists,
searching for traces of past stellar mergers in populations
of stars.
For a short time, merged stars remain surrounded by
evidence of the act. So, if not quite caught red-handed,
they're at least easier to identify. One such example is the star
TYC 2597-735-1 and its surrounding nebula in the constellation
Hercules. This object probably merged a few thousand
years ago. That is long enough that the literal and metaphorical
dust is starting to clear, letting us see through the ejected
gas to the newly remade star. The star itself is fading slowly
as the extra heat deposited by the most dramatic moments of
merger leaks out.
When mergers have occurred further back in a star's history,
the traces are still there but are perhaps more subtle.
Merger-made objects, by virtue of the fact that they are two
stars stuck together, tend to be more massive than those
binary blue stragglers
single blue stragglers
Red giants
Main-sequence stars
0.00
0.25 0.50 0.75 1.00 1.25 1.50
Bluer
Color (B-V bands)
18 JUNE 2024 * SKY & TELESCOPE
Redder
1.75
Magnitude (V band)
PLOT: THE AUTHOR
Sky & Telescope - June 2024

Table of Contents for the Digital Edition of Sky & Telescope - June 2024
