IYL2015

How SDSS Talked about Light for #IYL2015

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This is a re-posting of the wrap-up article which appeared on the IYL2015 main blog.

 

2015 has been the International Year of Light.

As astronomers, here at the Sloan Digital Sky Survey everything we do is based on collecting light from cosmic objects. So we have been pleased to celebrate the International Year of Light, and especially the Cosmic Light Theme, supported by the IAU.cosmiclight_color_whitebgAs a small contribution to this celebration, every month in 2015 SDSS had a special blog post talking about the different ways we use light. Here’s a roundup of what we talked about through out the year.

In January we talked about How SDSS Uses Light to Study the Darkest Objects in the Universe. This blog post, by Coleman Krawcyzk and Karen Masters (both from the University of Portsmouth in the UK) with help from Nic Ross (Royal Observatory, Edinburgh) was about finding black holes by looking at the light from distant galaxies. Finding objects which are famous for not emitting any light, using light seems contradictory, but this article explains how the light created by the hot material falling onto a black hole can make these objects outshine the entire galaxy they live in.

Quasar

An artist’s rendition of a quasar created by Coleman Krawczyk (ICG Portsmouth). The image is drawn on a radial log scale with the central black hole 1 AU in size.

In February we wrote about How SDSS Uses Light to Measure the Distances to Galaxies. This of course was about the technique of measuring galaxy redshifts (ie. the shift of their light to longer wavelengths caused by the expansion of the Universe) by looking at absorption and emission lines in galaxy spectra and comparing their wavelength to the laboratory measurement. Edwin Hubble, and others, realised over 80 years ago, that this can be used to give distances to galaxies, as the amount of redshift increases with the galaxy’s distance. The original motivation for SDSS (back in the 1990s) was to used this technique to measure distances to a million galaxies, and in SDSS-IV we are continuing to use this in the eBOSS part of the survey, to map distances to ever more distant galaxies.

A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS

A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS

In March, we came back to the most local Universe, with a post by SDSS-IV Spokesperson, Jennifer Johnson (Ohio State University) on How SDSS Uses Light to Understand Stars Inside and Out in the Kepler Field. This was about part of the APOGEE survey, which is measuring spectra from stars which have light curves measured by the Kepler Satellite. This is a valuable experiment, as the combination of spectra and light curves allows us to measure the masses, ages and compositions of these stars.

The Kepler Field. Credit: NASA

The Kepler Field. Credit: NASA

In April, we moved back outside our own Galaxy, to measuring the invisible mass in other galaxies, with a post on How SDSS Uses Light to Explore the Invisible, by the MaNGA Lead Observer, and SDSS-IV Data Release Co-ordinator, Anne-Marie Weijmans from St Andrew University. This post talked about how MaNGA is measuring spectra across the face of nearby galaxies in order to get measurements of the internal motions (again using the redshift/blueshift of the spectra). These measurements give a way to measure the total mass of galaxies, which we find in all cases is much much more than the mass in stars.

MaNGAlogo5small

For May we went back in the history of SDSS, and talked about How SDSS Used Light to Make the Largest Ever Digital Image of the Night Sky. This post was about the the SDSS camera and the SDSS imaging survey, which ran from 2000-2008, and created a image of over 30% of the sky, containing over a trillion pixels (an image which dwarfs others that have also been claimed as the largest).

The SDSS Camera, now in storage in the Smithsonian Museum. Credit: SDSS, Xavier Poultney

The SDSS Camera, now in storage in the Smithsonian Museum. Credit: SDSS, Xavier Poultney

June also saw a post about SDSS imaging, and about an unexpected use for them, finding asteroids, in How SDSS Uses Light to Find Rocks in Space. This has been beautiful visualized in the below video, by Alex Parker.

If our posts in February, March and April confused you because you didn’t understand what astronomers mean by measuring spectra, then the July post was for you: “How SDSS Splits Light into a Rainbow for Science”.  This post explained all about what spectra are, how to create them with gratings, and contained a with bonus activity to make your own spectroscope created by the SDSS Education and Public Outreach group.

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).Credit: Wikimedia

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2).Credit: Wikimedia

Our August post, by the APOGEE survey Public Engagement officer, David Whelan (from Austin College, Texas)  was about the basic physics of the most abundant element in the Universe (hydrogen): “How SDSS Uses Light to Study the Most Abundant Element in the Universe.”

For September, we visited an IYL2015 Exhibit in Dresden with Zach Pace, Graduate Student at the University of Wisconsin, Madison. Zach reported on SDSS plates on display in the exhibit, linking back to an earlier post in which we explain why we need all these big aluminium plates to do our spectroscopic survey. IYL2015 – SDSS Plates (in Retirement).

Technische_Sammlungen3

SDSS Plate on Exhibit in Dresden.

We went back to the APOGEE survey in October, with a post by Gail Zasowski (from John’s Hopkins University) on How SDSS uses mysterious “missing” light to map the interstellar medium. In this post we learned about how SDSS has helped shed light on the the mystery of missing light caused by absorption in the material which is found between stars in our own Galaxy.

Finally last month, we talked about How SDSS Uses Light to Measure the Mass of Stars in Galaxies. Looking back to the post in February, we claimed that the total mass of galaxies is always much much more than the mass we can count in their stars. But how do we know how much mass is in the stars in a galaxy? This post explains how that can be done using measurements of the light from galaxies.

So that wraps up a year of the celebration of light in the SDSS. We certainly haven’t covered all the ways in which SDSS astronomers are using light to learn about the Universe around us, from asteroids in the solar system, to stars in our own Galaxy and galaxies are the furthest edges of the Universe. But we hope it gives you a flavour for the kinds of things the light collected by SDSS (both images and spectra) can be used for.

If you’re looking for a guided entry into SDSS science (especially suitable for educational use), please visit our Voyages.sdss.org site to discover guided journeys through the Universe. As always all SDSS data (through our 12th public data release, DR12) is available free to download, and look out for DR13 (including the first data from SDSS-IV) coming up in mid 2016.

 

How SDSS Uses Light to Measure the Mass of Stars in Galaxies

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Galaxy NGC 3338 imaged by SDSS (the red stars to the right is in our own galaxy). Credit: SDSS

It might sound relatively simple – astronomers look at a galaxy, count the stars in it, and work out how much mass they contain, but in reality interpreting the total light from a galaxy as a mass of stars is fairly complex.

If all stars were the same mass and brightness, it would be easy, but stars come in all different brightnesses, colours and masses, with the lowest mass stars over 600 times smaller than the most massive.

Hertzsprung-Russel Diagram identifying many well known stars in the Milky Way galaxy. Credit: ESO

Hertzsprung-Russell (HR) Diagram, which shows the mass, colour, brightness and lifetimes of different types of stars. This version identifies many well known stars in the Milky Way galaxy. Credit: ESO

And it turns out that most of the light from a galaxy will come from just a small fraction of these stars (those in the upper left of the HR diagram). The most massive stars are so much brighter ounce for ounce than dimmer stars this makes estimating the total mass much more of a guessing game than astronomers would like (while they are 600 times more massive, they are over a million times brighter). So astronomers have to make assumptions about how many stars of low mass are hiding behind the light of their brighter siblings to make the total count.

One of the first astronomers to suggest trying to decode the light from galaxies in this way was Beatrice Tinsley. British born, raised in New Zealand, and working at Yale University in the USA, Dr. Tinsley had a much larger impact on extragalactic astronomy than her sadly shortened career would suggest (she died of cancer in 1981 aged just 40).

Stars of different masses have distinctive spectra (and colours), as first famously classified by Astronomer Annie Jump Cannon in the late 1890s into the OBAFGKM stellar sequence. O stars (at the top left of the HR diagram) are massive, hot, blue and with very strong emission lines, while M stars (at the lower right) are low mass, red and show absorption features from metallic lines in their atmospheres. With a best guess as to the relative abundance of different stars (something we call the “initial mass function“) a stellar population model can be constructed from individual stellar spectra or colours and fit to the total light from the galaxy. Example optical spectra of different types of stars are shown below (or see the APOGEE View of the IR Stellar Sequence)

Example optical spectra of different stellar types. Credit: NOAO/AURA/NSF

Example optical spectra of different stellar types. Credit: NOAO/AURA/NSF.

Using data from SDSS (and other surveys) astronomers use this methods to decode the galaxy light – in fact we can use either the total light observed through different filters in the SDSS imaging, to match the colours of the stars, or if we measure the spectrum of the galaxy we can fit a population of stars to this instead. While in principle the spectrum should give more information, in SDSS (at least before the MaNGA survey) we take spectra through a small fibre aperture (just 2-3″ across), so for nearby galaxies this misses most of the light (e.g. see below), and most galaxies have colour gradients (being redder in the middle than the outskirts), so the extrapolation can add quite a lot of error to the inferred mass.

NGC 3338 with the approximate SDSS fibre size overlaid (note this is an example of a very large galaxy imaged by SDSS). Credit: SDSS, KLM

NGC 3338 with the approximate SDSS fibre size (ie. the part of the galaxy for which we measured spectra) overlaid (note this is an example of a very large galaxy imaged by SDSS, and not representative of most galaxies). Credit: KLM, SDSS

 

Many astronomers prefer to use models based on the total light through different filters (at least for nearby galaxies). The five filters of the SDSS imaging are an excellent start for this, but extending into the UV with the GALEX survey, and IR with a survey like 2MASS or WISE adds even more information to make sure no stars are being missed. However, these fits are still a “best guess” and will still have error –  there is often more than one way to fit the galaxy light (e.g. model galaxies with certain combinations of ages and metallicities can have the same integrated colours), so there’s still typically up to 50% error in the inferred mass.

The SDSS camera filter throughput curves (from left to right ugriz). Credit: SDSS

The SDSS camera filter throughput curves (from left to right ugriz). Credit: SDSS

 

But with galaxies spanning more than 3 orders of magnitude in total mass (ie. the biggest galaxies have more than a 1000 times the stellar mass of the smallest) this is still good enough for many purposes. It gives us an idea of the total mass in stars in a galaxy, which (as you know from earlier post for IYL2015) is almost always way less than the total mass we estimate from looking at the dynamics (ie. the “gravitating mass”). And the properties of galaxies correlate extremely well with their stellar masses, so it’s a really useful thing to have even an estimate of.

This post by Karen Masters is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month in support of the celebration of light. 

IYL2015 Post: SDSS Plates (in Retirement!)

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As part of Dresdner Lichtjahr 2015 [Dresden Year of Light 2015], you can now see a previously-used SDSS plate on display at Technische Sammlungen der Stadt Dresden, a museum located in a former Dresden factory. The exhibit will run through June of 2016, and has some really awesome demonstrations of how light propagates, and how much today’s technology depends on light.  Technische_SammlungenThe SDSS plate (below, designated plate 4385) is suspended above a table illustrating principles of how light propagates, what we can do with light of different wavelengths, and a demonstration of fiber optics. If you’re curious why our telescope might need need a metal plate, read this previous post.

Technische_Sammlungen2Used SDSS plates are available for educational purposes by schools, museums, astronomy clubs, and other educational & community organizations. Just contact someone at your nearest SDSS member institution to get started!

Technische_Sammlungen3Elsewhere in the exhibit and the museum, you can find a working infrared camera (selfie-compatible!), a very challenging puzzle involving prisms and laser light, and other neat activities suitable for children of all ages.

While you’re in Dresden, make sure to also stop by the Mathematische-Physikalische Salon [Royal Cabinet of Mathematical and Physical Instruments], at the Zwinger Palace in the center of Dresden, to have a look at old telescopes, clocks, and surveying tools. Of special interest to telescope enthusiasts are two very early reflector telescopes (i.e., telescopes that use a mirror to focus the incoming light, rather than lenses). You can also see them online in a panoramic view (upstairs in “Instruments of Enlightenment”).

 

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month in support of the celebration of light. 

How SDSS Uses Light to Study the Most Abundant Element in the Universe

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When we spread out the light from a source into a rainbow, we can reveal information about its chemical makeup. This is how we understand the spectral signatures that reveal that stars have different temperatures. But to learn about the objects that we study in space, whether they be stars, interstellar gas, or galaxies, we first have to know something about the chemical properties of the elements that make up these objects. And one of these elements is, by far, the most important to study: Hydrogen.

Why is hydrogen the most important element to study in astronomy? Primarily because it is the most abundant. If you count the number of hydrogen atoms in all of space (stars, gas, and galaxies), it can be shown that nine out of ten atoms in space are hydrogen. The second-most abundant is helium, which makes up almost one out of ten atoms in space, and every other element is present in only trace amounts — which is not to say that they are unimportant! But in this post, we are going to focus on the big one.

 

The Hydrogen atom, consisting of a single proton as the nucleus orbited by a single electron.

The Hydrogen atom, consisting of a single proton as the nucleus orbited by a single electron.

Hydrogen is also the simplest element on the Periodic Table, as the above diagram shows — one lone proton being orbited by one electron. And like all elements, hydrogen is able to absorb and emit light of certain wavelengths. If the electron is hanging out in the ground state (the n=1 state), it can absorb photons that will shimmy it to the n=2, n=3, n=4, etc. state (and there are an infinite number of these states). Likewise, if the electron begins in the n=2 state, then the atom can absorb photons of light to push it into the n=3, n=4, n=5, etc. state.

When a hydrogen atom is in one of these “excited” states (i.e., n ≠ 1), it also has the opportunity to emit a photon and travel back down to a lower energy level. The photons absorbed have the same wavelengths as the photons emitted, so that they always appear in the same place in a spectrum. In the following illustration, the first four energy levels of the hydrogen atom are shown. Three commonly-studied transitions between different energy levels are named, along with their absorption/emission wavelengths in units of Ångströms (= 10-10 m). The colors of the line are the approximate colors that they might appear to your eye — with the exception of the Lyman-α transition, which emits in the ultraviolet and is therefore invisible to the human eye.

An illustration of the four lowest energy levels of the Hydrogen atom, and commonly studied transitions between these energy levels.

An illustration of the four lowest energy levels of the Hydrogen atom, and commonly studied transitions between these energy levels.

 

When studying spectra from space, it is common to study either absorption spectra (spectra with lines that show that atoms are absorbing photons) or emission spectra (spectra with lines that show that atoms are emitting photons). The absorption process is the most common when studying stellar spectra. And for many stars, it is the hydrogen lines that gives us a first indication about the physical properties of the stars. Here, for instance, is the spectrum of an A-type star, i.e., one with strong Hydrogen absorption features:

Spectrum of an A0 star exhibits strong Hydrogen absorption lines where indicated. Other, smaller absorption lines are due to calcium, magnesium, and sodium absorption. This spectrum is taken from the Science Archive Server -- check it out!

Spectrum of an A0 star exhibits strong Hydrogen absorption lines where indicated. Other, smaller absorption lines are due to calcium, magnesium, and sodium absorption. This spectrum is taken from the Science Archive Server — check it out!

Galaxies, which are large conglomerations of stars, can also show hydrogen absorption features. But many galaxies, like spiral galaxies or else irregular galaxies with ongoing star formation, actually produce strong emission features. This is because the hydrogen gas that exists between the stars in these galaxies is heated by the stars, so that individual atoms are excited to higher energy levels. A great example is the spectrum of the irregular galaxy NGC 6052, shown below:

Spectrum of the irregular galaxy NGC 6052, with hydrogen emission lines labeled.

Spectrum of the irregular galaxy NGC 6052, with hydrogen emission lines labeled.

 

You might have noticed in this galaxy spectrum that these hydrogen emission lines appear to sit on top of what look like hydrogen absorption features. The absorption features, as mentioned above, come from the stars in the galaxy, whereas the emission features come from the gas between the stars.

There is other cool stuff that hydrogen can teach us. One of the coolest is called the Lyman-α Forest, which can be used to tell us how much hydrogen gas exists on large scales between galaxies.

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe. 

How SDSS Uses Light to Find Rocks in Space

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It’s an exciting time in solar system exploration, with the Philae lander and Rosetta orbiter exploring Comet 67P Churyumov-Gerasimenko, sending back our most details view ever of a comet. On top of this the New Horizons Mission is approaching the dwarf planet Pluto, and will make its closest pass on 15th July 2015, sending back the highest resolution images of Pluto we have ever seen.

You might think that the Sloan Digital Sky Survey has nothing to say about rocks in space, but you’d be wrong. One of the possibly unexpected discoveries from our  imaging survey has been that of many hundreds of thousands of asteroids.

Because of the way the SDSS Camera Worked, asteroids show up in the SDSS imaging at different times (and therefore different places as they are moving across the sky) in the different filters. This makes them pop out as little strings of almost traffic light coloured dots. These have been popular finds by citizen scientists at Galaxy Zoo as well as identified by computer algorithms.

Asteroids (the three coloured dots) found near galaxies by citizen scientists at Galaxy Zoo.

Asteroids (the three coloured dots) found near galaxies by citizen scientists at Galaxy Zoo.

The below animation by Alex Parker shows the orbital motions of over 100,000 of the asteroids observed by the Sloan Digital Sky Survey (SDSS), with colors illustrating the compositional diversity measured by the SDSS five-color camera. The relative sizes of each asteroid are also illustrated.

 

All main-belt asteroids and Trojan asteroids with orbits known to high precision are shown. The animation has a timestep of 3 days. The fact that the composition of asteroids in the asteroid belt varys systematically is clearly visible, with green Vesta-family members in the inner belt fading through the blue C-class asteroids in the outer belt, and the deep red Trojan swarms beyond that.

Occasional diagonal slashes that appear in the animation are the SDSS survey beams.

The average orbital distances of Mercury, Venus, Earth, Mars, and Jupiter are illustrated with rings.

Colors represented with the same scheme as Parker et al. (2008). Concept and rendering by Alex H. Parker. Music: Tamxr by LJ Kruzer.

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe. We’ve reached the halfway point here in June 2015! 

How SDSS Uses Light to Understand Stars Inside and Out in the Kepler Field

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Stars are not only fascinating objects in their own right — they also help us understand the history of our Milky Way galaxy. Our galaxy was created as dark matter’s pull brought gas together, and the gas formed stars and planets. As part of the APOGEE survey, we wish to map the Milky Way’s star formation throughout cosmic time. As stars died, many of the elements they fused in their interiors during their lives or death throes are mixed back into the remaining gas, changing its composition and the composition of subsequent generations of stars and providing the raw materials for planets (and humans!) and we are exploring this chemical history as well.

A small part of the spectra of a few of the stars observed by APOGEE. The dark lines are caused by absorption of atoms in the star's atmosphere (or sometimes the Earth's). A few of them are highlighted. The bright lines are caused by emission in the Earth's atmosphere ("night sky lines") These particular stars have also been observed by the Kepler satellite.

A small part of the spectra of a few of the stars observed by APOGEE. The dark lines are caused by absorption of atoms in the star’s atmosphere (or sometimes the Earth’s). A few of them are highlighted. The bright lines are caused by emission in the Earth’s atmosphere (“night sky lines”) These particular stars have also been observed by the Kepler satellite.

APOGEE studies stars by passing their infrared light through gratings that spread the light out in wavelength (think infrared rainbows). We do this for > 250 stars at once (one of the reasons why the APOGEE instrument is fantastic). We can tell a lot about stars from studying these spectra. For example, in an earlier blog post, we discussed how we can tell the surface temperature of stars from such data. Another very important property is the composition of the star, for example, how many atoms of iron, calcium, or oxygen it has relative to hydrogen. The image to the left shows a small part of the spectra we gathered for stars that were also observed by the Kepler satellite. The stars do not give off the same amount of light at each wavelength (=color) of light. Instead, there are many dark lines, which are created when atoms in a star’s atmosphere absorb light at very particular wavelengths. Each element has a different pattern of these absorption lines, and by measuring the depth of these lines (+ additional information and math), we can determine the composition of the gas out of which the star formed.

But this doesn’t tell us everything about the star! In particular, we can’t see inside the star where the original composition of the gas is being transformed from hydrogen into helium as the star ages. We have a good idea of how long it takes for a star with a certain mass and original chemical composition to run out of fuse-able hydrogen in its center (about 10 billion years in the case of a star with the mass and composition of the Sun). When that happens, the star undergoes a dramatic change, turning into a red giant or supergiant. So if we can determine the mass to go with the spectral  composition information for red giants that we observe, we can determine the age of those particular stars.

Measuring the mass of a star is hard work, but one possible technique is to use asteroseismology, which is the study of the waves that move through stars. In the outer parts of stars, these waves are actually sound waves that can evocatively be described as ringing the star like a bell (For more information see The Song of the Stars). The motions of these waves cause a star’s brightness to change by small amounts, and thus the frequency of these waves can be measured by studying the lightcurves of red giant stars. The Kepler satellite, in addition to studying many Sun-like stars looking for transiting planets, also measured the brightnesses over many years of thousands of red giants. The favorite frequencies of waves in different stars have been measured by members of the Kepler Asteroseismic Science Consortium. While much can be learned about the insides of stars from these data, we are particularly intrigued by the fact that how long and at what speed waves can move through the star depends on the star’s density and therefore (with some more math) its mass!

Combining together spectra from APOGEE and lightcurves from Kepler therefore gives us a way to figure out the ages of red giant stars in our Galaxy by measuring the masses and composition of stars that have just exhausted their hydrogen. In conclusion, songs and rainbows are good things.

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe. 

How SDSS Uses Light to Measure the Distances to Galaxies

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Here at the Sloan Digital Sky Surveys our mission is to explore and map the Universe, from planets to the edges of the observable Universe. The way we do this is to collect light from specially selected objects we see in the night sky – but we can’t visit them in order to measure how far away they are. So how do we actually know how far away they are in order to make a map of the Universe?

Measuring the distance to objects in the Universe has always been one of the biggest challenges for astronomers. Until we know the distance to something we cannot really understand its physical properties, and the history of astronomy is full of examples where new techniques for measuring distances opened up entirely new areas of study. For example when the “spiral nebulae” were first discovered there was a long debate over if they were small clouds of gas in our own Galaxy, or external galaxies in their own right each made up of millions or billions of stars. Only by measuring their distances was this finally settled, and our understanding of the size of the Universe suddenly jumped many orders of magnitude.

A collection of "spiral nebulae". But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

A collection of “spiral nebulae”. But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

There’s some really useful bits of physics we can use to help measure distances to the galaxies from their light. To do this we need to understand spectroscopy. Once SDSS had finished imaging more than a quarter of the sky with its camera, it became entirely focused on “spectroscopic” surveys. Our telescope in New Mexico collects the light from stars and galaxies and uses instruments called spectroscopes to split it up into its different colours (we actually have two different spectroscopes working right now – the APOGEE spectroscope and the BOSS spectroscope). These measurements split the light into a rainbow (or a spectrum), and we look for the precise colours of series of emission and/or absorption lines to tell us all sorts of things about the light source we’re looking at.

Spectral_lines_en

A hot bright light source (like a star) will have a “continuous spectrum” (with the peak colour depending on its temperature – hot things glow red, even hotter things glow white or blue hot). If the light from that passes through a cool cloud of gas before we measure it, that will create “absorption lines” where very specific colours (or “wavelengths” in proper scientific terms) are absorbed by atoms in the gas cloud. The exact pattern of colours/wavelengths which are absorbed tell you which atoms are in the gas cloud. If the gas cloud gets heated up enough we might instead see emission lines – at the same specific colours, where the atoms are now re-emitting these very specific colours/wavelengths. Each atom has a very distinctive pattern of lines it emits – for example hydrogen (the most abundant element in the Universe) has a very distinctive and bright emission/absorption line in the red part of the spectrum (at a wavelength of 656.3nm).

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Emission spectrum of hydrogen in visible light (wikimedia commons)

Astronomers have been using this technique to work out the materials which make up the Sun and other stars for decades. It’s not always easy (it has been compared to trying to reconstruct a piano from the noise it makes falling down the stairs), but it works. When astronomers first used the technique to look at galaxies however they were very surprised by what they found. The patterns of lines seemed to be in completely the wrong places – for example the famous hydrogen lines weren’t even visible in some cases – they had moved right into the infra-red part of the spectrum.

In order to understand why this could happen we need to learn about another part of physics – the Doppler effect. First proposed in 1842, by a Physicist named Christian Doppler this is the observation that when a source emitting a wave is moving, the waves are shortened if the source is moving towards the observer, and lengthened if it is moving away. Most people are familiar with this effect when they have listened to ambulance sirens passing them on the street; the siren is higher in pitch when the ambulance is moving towards you and lower when it’s moving away (when sound waves are lengthened the pitch drops, and when they are shortened the pitch rises).

Wikimedia commons illustration of the Doppler effect.

Since light is a wave, the same effect happens when light is emitted from a moving source. When the waves of light are shortened the light becomes bluer, and when they are lengthened the light becomes redder.

An astronomer named Vesto Slipher, was the first person to try this out on galaxies, and he found that almost all galaxies he looked at showed enormous “redshifts”, implying that almost all the galaxies were moving away from the Earth at very high speeds.

Edwin Hubble is given the credit for explaining this observation by realising that we live in a Universe which is constantly expanding. In such a Universe any observer will observe almost all other galaxies moving away from them. Hubble published the first description of a relationship between how fast galaxies appear to be moving away from us (their “redshifts”) and their distances – this relationship is now called Hubble’s Law.

It is this relationship that we use to measure the distances to the galaxies from detailed observations of the light they emit, and astronomers are now used to describing the distances to galaxies as simply their “redshift”.

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A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe.

SDSS Celebrates the International Year of Light 2015

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As astronomers, at the Sloan Digital Sky Survey everything we do is based on collecting light from cosmic objects. SDSS is therefore pleased that in 2015 we are celebrating the International Year of Light, and we especially would like to point out the Cosmic Light Theme, supported by the IAU.cosmiclight_color_whitebg

As a small contribution to this celebration, every month in 2015 SDSS will have a special post on here talking about the different ways we use light. Our first post, which will appear before the end of January will be about how we use light to study black holes, something which seems like a contradiction, but has taught us a lot!

This post will be updated to collect all the links as the year progresses: