also orbit Sgr A*. When two of
these objects have a close encoun-
ter, their gravity perturbs each
other’s orbits and they head out on
new, altered trajectories. Most of
these orbits remain stable, or per-
haps f ling the star outward from
the center of the galaxy. But on
rare occasions, a star’s new orbit
sends it inward on a collision
course with disaster.
As the doomed star approaches
the supermassive black hole, it
begins to experience tidal forces:
Because gravity is stronger closer
to an object, the black hole pulls
more strongly on the star’s near
side than its far side. Eventually,
when the star reaches a certain
distance from the black hole —
the tidal radius — the difference
in force from one side to the other
becomes greater than the star’s
self-gravity holding it together.
When this happens, “the star
gets stretched along its direction
of motion,” explains Enrico
Ramirez-Ruiz, an astrophysicist at
the University of California, Santa
Cruz, who specializes in TDE the-
ory. The star deforms from its
usual sphere into an oval, and
then into a long, thin stream. This
process is called spaghettification.
As it occurs, the star’s density
decreases and fusion at its core
stops altogether. Though a star
may take millions of years to form
and shine for billions more, this
final unraveling takes just a few
hours.
The hunt for TDEs
What happens next? “Half the
star’s material falls in and forms
an accretion disk around the star,”
explains Ramirez-Ruiz, “and half
gets ejected.” The material in the
disk falls onto the black hole and
feeds it, powering a luminous f lare
that can be seen at vast distances
before slipping past the black hole’s
event horizon (where light can no
longer escape).
At first glance, these dazzling
events can resemble a supernova
— a massive star that explodes at
the end of its life when its fuel is
exhausted. To distinguish TDEs
from supernovae, astronomers
keep an eye out for two things.
First, they look for a bright f lare at
the center of a galaxy whose super-
massive black hole was previously
dormant. Then, they break down
the light by wavelength and study
its spectrum to see what elements
it contains. Unlike in a supernova,
the elements observed in a TDE
f lare are similar to those in main
sequence stars that are still burn-
ing strong. If your f lare fits both
criteria, you have a potential TDE
on your hands!
Astronomers spotted the first
TDE candidates in the 1990s. In
recent years, finding them has
gotten easier thanks to automatic
sky surveys that scan the night
sky for transient objects — signals
that change in the sky over time
instead of remaining constant.
Still, to date, we have onlyobserved about 100 TDEs.
That’s because TDEs are rare.
Astronomers estimate that a gal-
axy like the Milky Way has a TDE
no more than once every 100,000
years. Supernovae, on the other
hand, occur in a galaxy our size
roughly once a century.An incredible picture
Arguably the most famous
TDE to date occurred in 2011,
when NASA’s Neil Gehrels Swift
Observatory detected a strange
burst of radiation from the center
of a galaxy 3.8 billion light-years
away. Swift was launched in 2004ABOVE: The tidal
forces that act upon
a star near a black
hole rip it apart in a
process called
spaghettification,
depicted in this
simulation. As
matter is pulled off
the star, it forms
dramatic tidal tails
and, eventually, an
accretion disk (at
bottom left).
J. GUILLOCHON AND E.
RAMIREZ-RUIZLEFT: This
simulated view
shows the density
in the accretion
disk, with denser
regions in red and
less-dense regions
in blue. J. LAW-SMITH AND
E. RAMIREZ-RUIZWWW.ASTRONOMY.COM 27