Astronomy

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WWW.ASTRONOMY.COM 71

Mars will be in the future. This
orbit requires the least energy
(and thus the least fuel) and
allows the spacecraft to arrive
within seven to nine months.
But you can’t just decide to
pick up and go. Mars and
Earth are in their best position
for interplanetary travel only
every 26 months. A launch
outside that window can dra-
matically increase how long the
spaceship — and any astro-
nauts — spend in space.
NASA’s Orion spacecraft
will carry crew members to
Mars on top of the Space
Launch System (SLS) rocket,
which will be more powerful
than the Saturn V rocket that
carried astronauts to the
Moon. The agency plans to
test the pair with Exploration
Mission-1 (EM-1), an uncrewed
journey around the Moon and
back to Earth, in 2019.
Nola Taylor Redd
Freelance science journalist
and Astronomy contributor

Q: HOW DO YOU MEASURE
THE ROTATIONAL SPEED
OF A GALAXY, TAKING
INTO CONSIDERATION
THE MOTION OF OUR
GALAXY, SOLAR SYSTEM,
PLANET, ETC.?
Chris Mathews
Plano, Texas

A: Almost all measurements of
motion in astronomy make use
of a law of physics called the
Doppler effect. This change in
the wavelength (or frequency,
color, or pitch) of a wave emit-
ted by a moving source was
first described by physicist
Christian Doppler in 1842. It is
familiar to most of us: I’m sure
you’ve noticed that the siren of
an ambulance changes pitch as
it passes you, going from higher
(as it moves toward you) to
lower (as it passes and moves
away). This same effect hap-
pens to the light emitted by

stars and gas in galaxies. With

light waves, even large motions

create only a tiny shift in color,

but we can still measure it

using an instrument called a

spectrograph, which divides

light into its component wave-

lengths, allowing astronomers

to pick out specific features

caused by atoms in stars or gas.

One of the most famous

— and prevalent — of these

features is Hydrogen-alpha

(or Hα), which lies at precisely

656.28 nanometers (for a non-

moving source). To measure

the rotational speed of a galaxy,

we map out a line like Hα

across the galaxy and compare

it to the value from a source at

rest. If we can see that on one

side of the galaxy the line is

blueshifted (moving toward

us), and on the other redshifted

(moving away) relative to the

central redshift of the galaxy,

we know the galaxy is rotating,

and the amount of shift of

either line tells us how much. It

is common to do this using a

long-slit spectroscope, which

measures the shifts in a single

spectral line across the galaxy.

Alternatively, resolved spec-

troscopy of entire galaxies has

become possible in more recent

years, so now we often get full

spectral maps.

Another technique uses a

radio telescope to measure the

21-centimeter emission line of

hydrogen, which also reveals

galaxy rotation. The 21 cm line

shows us where the hydrogen

in a galaxy lies, and as that

hydrogen either rotates toward

or away from us relative to the

central redshift of the galaxy,

the resulting Doppler shift

broadens the single emission

line into a line with two peaks,

each associated with motion in

one direction or the other.

As you note, we must indeed

take into consideration the

average shift of light from the

motion of our galaxy. This will

almost always result in a net

redshift, as it includes the

expansion of the universe, and

also our solar system’s motion

toward or away from the gal-

axy we are observing (the rota-

tion of our planet, our orbit

around the Sun, the Sun’s

motion around the galaxy, and

the galaxy moving through the

universe). These are known

quantities, and any extragalac-

tic measurements are done

relative to them.

Karen Masters

Associate Professor, Department

of Physics and Astronomy,

Haverford College, Pennsylvania

Send us your

questions

Send your astronomy

questions via email to

[email protected],

or write to Ask Astro,

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you live. Unfortunately, we

cannot answer all questions

submitted.

As a galaxy rotates, the material moving away from us shows a redshift in the wavelength of any emitted

light (red arrow). Material moving toward us shows a blueshift (blue arrow). By measuring these shifts

across a galaxy, astronomers can determine its rotation. ASTRONOMY: ROEN KELLY

Measuring a galaxy’s rotation