WWW.ASTRONOMY.COM 71
SEND US YOUR
QUESTIONS
Send your
astronomy questions
via email to askastro@
astronomy.com, or
write to Ask Astro,
P.O. Box 1612,
Waukesha, WI 53187.
Be sure to tell us
your full name and
where you live.
Unfortunately, we
cannot answer all
questions submitted.the star. Furthermore, astronomers believe that, based
on factors such as the star’s magnetic field and its rota-
tion during the red supergiant phase, more material
piled up around its equator and in regions around its
poles. When faster winds crashed into those regions
later, it formed three distinct rings.
When the star finally exploded, the energy from the
blast moved outward in all directions. The rings we now
see are caused by shocks that occur as that energy
smacks into the existing rings, exciting the gas and light-
ing them up. The other areas around the now-dead star
aren’t glowing in a sphere simply because there isn’t
much material there, since it’s mostly concentrated in
t he r i ngs t hat were lef t beh i nd f rom before t he ex plosion.
Over time, these rings have appeared to change and
expand as the shocks move through them, illuminating
new regions as the material closer to the star fades once
the shock has passed.
To complicate matters, astronomers suspect the
supernova explosion itself was possibly not symmetric,
but instead sent more energy in one direction than oth-
ers, exaggerating the nonspherical shape of the remnant
we see.
Alison Klesman
Senior Associate Editor
QI
WHY DO TWO COLLIDING SPIRAL
GALAXIES FORM AN ELLIPTICALGALAXY RATHER THAN ONE LARGER
SPIRAL GALAXY?
Bill Dellinges
Apache Junction, ArizonaAI
When two spiral galaxies collide, gravity is the
main force that comes into play. As the galax-ies approach each other, gravitational forces start to
pull the stars, gas, and dust of the spiral arms out of
their original orbits. This is what causes the warped
shape of famously interacting pairs such as the Mice
(NGC 4676) and the Antennae Galaxies (NGC 4038
and NGC 4039). It is also what ultimately destroys the
initial spiral patterns of the two galaxies and results in
an elliptica l ga la x y once the merger is complete.
Galaxy collisions aren’t typically one-and-done
events — galaxies don’t approach each other, smack
together, and stick. Instead, depending on their initial
t r ajec tor ie s a s t he y approach one a not her, t wo ga la x ie s
may pass by or even through each other several times
over the course of billions of years before they finally
merge. This repeated interaction pulls the spirals apart
a bit like stretching taffy, yanking at the stars that once
serenely circled the center of their home galaxy in an
orderly fashion. During the merger, the stars become
scattered and their orbits become random. This is what
results in a single, football-shaped elliptical galaxy with
stars orbiting the center in all planes, rather than the
single plane of a f lat spiral galaxy.
Elliptical galaxies are also known for their dearth of
star-forming material. This is because when spirals —
which typically contain plentiful gas with which to
make new stars — collide, gravity also triggers that gas
to quickly condense and form stars all at once in a single
big burst. Such starbursts, as astronomers call them,
use up the gas so that by the time the merger has fin-
ished, it has little material for new stars. What gas
remains is too hot and spread out to condense into new
stars or clump together into new spiral arms. Instead,
the elliptical galaxy remains puffed up and its compo-
nents keep their randomized orbits, never again settling
into a spiral shape.
Alison Klesman
Senior Associate EditorEach image in this
series depicts a unique
galaxy pair in a
different stage of a
galactic merger. As
time progresses from
1 to 6, the galaxies’
spiral shapes become
stretched out and are
ultimately destroyed.
NASA, ESA, THE HUBBLE HERITAGE
TEAM (STSCI/AURA)-ESA/HUBBLE
COLLABORATION AND A. EVANS
(UNIVERSITY OF VIRGINIA,
CHARLOTTESVILLE/NRAO/STONY
BROOK UNIVERSITY), K. NOLL (STSCI),
AND J. WESTPHAL (CALTECH)1
4
2
5
3
6