Stellar materialStellar collision High-speed stellar collisionBlue straggler starStar/black hole collisionAccretion disk (hot gas)Hydrogen gas68 ASTRONOMY • MAY 2020
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
EVERY TIME I READ ABOUT THE
ANDROMEDA GALAXY COLLIDING
WITH THE MILKY WAY, SOMEONE POINTS
OUT HOW UNLIKELY IT IS FOR TWO
STARS TO COLLIDE BECAUSE OF IT. BUT
WHAT WOULD HAPPEN IF TWO STARS
DID COLLIDE?
Jeremy Strzynski
Aurora, IndianaAI
It’s rare, but stars do collide in the densest parts
of our galaxy: near the center and in massive
globular star clusters. The outcome of the collision
depends on how fast the stars are moving relative to
each other, rather like a car accident. In star clusters,
the stars are moving relatively slowly, and so the “fender
bender” results in the two stars merging into one new,
more massive star that we call a blue straggler. We can
identify these stars fairly easily, since they are hotter
and brighter than the other stars in the cluster.
The center of the galaxy is more like the interstate,
and the stars are moving very quickly. A collision there
is much more destructive, and often the aftermath is
just “star bits” (that is, mostly hydrogen gas) spread out
all over interstellar space. The most exciting collisionsStellar
collisions
happen when a star runs into the cen-
tral black hole in our galaxy. The star
won’t survive, of course, but it goes
out in a blaze of glory called a tidal
disruption event. Some of the star’s
material gets thrown away, but the rest
falls into the black hole and forms a
hot disk of gas before it is consumed.
Alison Sills
Professor, Department of Physics &
Astronomy, McMaster University,
Hamilton, OntarioQI
SINCE SURFACE
ATMOSPHERIC
PRESSURE ON MARS IS
ROUGHLY EQUIVALENT
TO WHAT EXISTS WELL
INTO THE STRATOSPHERE
ON EARTH, HOW WOULD
A HELICOPTER BE ABLE
TO GENERATE SUFFICIENT LIFT TO GET
OFF THE GROUND?
John A. Ferko
Colorado Springs, ColoradoAI
Flying a helicopter on Mars, such as the Mars
Helicopter that will be traveling to the Red
Planet with the Mars 2020 mission, is a bit of a trade-off.
The air pressure at the martian surface is equivalent to
the air pressure at about 100,000 feet (30,500 meters)
on Earth, but the current height record for helicopters
on Earth is roughly 40,000 feet (12,200 m). According
to staff at NASA’s Jet Propulsion Laboratory (JPL), the
softball-sized, 4-pound (1.8 kilograms) Mars Helicopter
will compensate for the lower atmospheric density on
Mars by using more power and rotating its blades at a
higher speed than would be required to lift the craftStars rarely collide,
but when they do, the
result depends on
factors like mass and
speed. When two
stars merge slowly,
they can create a new,
brighter star called a
blue straggler. If two
stars traveling at a
fast pace hit, they’ll
likely leave behind
only hydrogen gas.
Stars that collide with
a black hole are
ultimately consumed.
ASTRONOMY: ROEN KELLY
The Mars Helicopter’s small size and fast rotor rotation will allow
it to fly on Mars. This photo shows the helicopter’s flight model,
as well as the base, crossbeam, and side posts that will protect
its delicate legs and attach it to the Mars 2020 rover. NASA/JPL-CALTECHWHEN STARS COLLIDE