June 6th: Astronomy Word of the Week : Redshift

By on June 6, 2012 in

Date: June 6th, 2012

Title: Astronomy Word of the Week : Redshift

Podcaster: Dr. Christopher Crockett

Organization: United States Naval Observatory

Link: http://christophercrockett.com

Description: Unraveling the motion of the cosmos depends on the ability to measure subtle changes in the color of starlight. The astronomy word of the week is “redshift”.

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.


How do astronomers track the rotation of our galaxy, tease out the subtle tug of a distant planet on its parent star, and measure the expansion rate of the universe?  The same way a police officer catches you when you’re speeding. They all rely on the ability to detect miniscule changes in the color of light.

Police and astronomers rely on a principle called the Doppler shift.  This is something you’ve experienced while standing near a passing train.  As the train approaches, you hear the horn blowing at a particular pitch.  Suddenly, as the train passes, the pitch drops.  But, why does the horn pitch depend on where the train is?

Sound can only move so fast through the air – about 1200 km/hr.  As the train rushes forward and blows its horn, the sound waves in front of the train get squished together.  Meanwhile, the sound waves behind the train get spread out.  This means the frequency of the sound waves is now higher ahead of the train and lower behind it. Our brains interpret changes in the frequency of sound as changes in pitch.  To a person on the ground, the horn starts off high as the train approaches and then goes low as the train recedes.

Light, like sound, is also a wave stuck at a fixed speed – one billion km/hr – and therefore plays by the same rules.  Except, in the case of light, we perceive changes in frequency as changes in color.  If a lightbulb moves very rapidly through space, the light appears blue as it approaches you and then becomes red after it passes.

Measuring these slight changes in the frequency of light lets astronomers measure the speed of everything in the universe!

Of course, making these measurements is little trickier than just saying “that star looks redder than it should be”.  Instead, astronomers make use of markers in the spectrum of star light.  If you shine a flashlight beam through a prism, a rainbow comes out the other side.  But if you place a clear container filled with hydrogen gas between the flashlight and the prism, the rainbow changes!  Gaps appear in the smooth continuum of colors – places where the light literally goes missing.

The hydrogen atoms are tuned to absorb very specific frequencies of light.  When light consisting of many colors tries to pass through the gas, those frequencies get removed from the beam.  The rainbow becomes littered with what astronomers call “absorption lines”.  Replace the hydrogen with helium and you get a completely different pattern of absorption lines.  Every atom and molecule has a distinct absorption “fingerprint” that allows astronomers to tease out the chemical makeup of distant stars and galaxies.

When we pass starlight through a prism (or similar device), we see a forest of absorption lines from hydrogen, helium, sodium, and so on. However, if that star is hurtling away from us, all those absorption lines undergo a Doppler shift and move towards the red part of the rainbow – a process called redshifting. If the star turns around and now comes flying towards us, the opposite happens. This is called, not surprisingly, blueshifting.

By measuring how far the pattern of lines moves from where it’s supposed to be, astronomers can precisely calculate the speed of the star relative to Earth!  With this tool, the motion of the universe is revealed and a host of new questions can be investigated.

Take the case where the absorption lines of a star regularly alternate between blueshift and redshift. This implies the star is moving towards us and away from us – over and over and over. It tells us the star is wobbling back and forth in space! This could only happen if something unseen was pulling the star around.  By carefully measuring how far the absorption lines shift, an astronomer can determine the mass of the invisible companion and its distance from the star. And that’s how astronomers have found nearly 95% of the known planets orbiting other stars!

In addition to finding roughly 750 other worlds, redshifts also led to one of the most important discoveries of the 20th century.  In the 1910s, astronomers at Lowell Observatory and elsewhere noticed that the light from nearly every galaxy was redshifted.  For some reason, most galaxies in the universe were racing away from us!  In 1929, American astronomer Edwin Hubble matched up these redshifts with distance estimates to these galaxies and uncovered something remarkable: the farther away a galaxy, the faster it’s receding.  Hubble had stumbled upon a startling truth: the universe was uniformly expanding!  What came to be known as the cosmological redshift was the first piece of the Big Bang theory – and ultimately a description of the origin of our universe.

Redshifts, the subtle movement of tiny dark lines in a star’s spectrum, are a fundamental part of the astronomer’s toolkit.  Isn’t it remarkable that the principle behind something as mundane as the changing pitch of a passing train horn underlies our ability to watch galaxies spin, find hidden worlds, and piece together the entire history of the cosmos?

[train horn]
[tori – beauty of speed] 1:00 – 1:10
traveling w/o moving
seein red

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.

About Christopher Crockett

Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

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