Here’s a link to a collection of all the Tweets by Sarah Schmidt during her week running @sdssurvey:
They come in a steady stream: the requests for lost passwords, for aid in correcting a CasJobs query, for insight into the technical details of SDSS photometry, astrometry, and spectroscopy, for help with educational resources and SkyServer, and for general astronomical and database knowledge, all sent to email@example.com. Two or three times a day, they appear in the mailboxes of those on the helpdesk mailing list, representing the hopes and dreams of an astronomer, amateur, student, or professional, to use SDSS data to answer the big questions of the Universe (or at least to get more room on the server).
The task falls to the volunteers, headed up and organized by Ben Weaver, Archive Scientist at New York University (right). Most questions are handled quickly by Ben, Ani Thakar, Archive Scientist at Johns Hopkins University, or Jordan Raddick, one of our Education Directors, also at Johns Hopkins. Promptness is easiest if the questioners have done us the kindness of including relevant information such as the context or URL they are using and the exact query. Occasionally questions about how parameters were derived or why there are changes between Data Releases requires the advice of other SDSS experts. In this case, Ben sends email to the relevant SDSS mailing list. Excellent answers are gratefully accepted from the wider collaboration, who really do know these data. Our thanks to everyone who has stepped up and contributed to making SDSS data scientifically valuable to an astonishing array of people. Above all, we wish to thank the helpdesk regulars. If you have sent an email to the helpdesk or if you know someone who has sent an email to the helpdesk (and you probably do, trust me), send a cheer their way.
Dr. Schmidt studies the lowest mass and most numerous types of stars in our Galaxy – the M and L dwarfs. These types of cool stars have strong magnetic fields on their surfaces which results in special kinds of extra light from the stars, including dramatic flare events, which Dr. Schmidt works to observe and understand.
Within the SDSS collaboration, Dr. Schmidt has worked or is working on observing cool stars using spectroscopy from several different surveys:
1. A study of ultracool dwarfs with data from a BOSS (Baryon Oscillation Spectroscopic Survey) ancillary project
2. A TDSS (Time Domain Spectroscopic Survey) project looking at long timescale magnetic field variations on late-M and early-L dwarfs
3. Studying the colors of late-K and early-M dwarfs with measurements of temperature and metallicity from spectroscopic observations taken for the APOGEE survey.
This can all be summarised as spectroscopy of the lowest mass stars there are, and Sarah is most interested in using these to constrain the stars ages and how this relates to their magnetic activity.
We hope you’ll join the conversation with Sarah and other SDSS scientists on twitter this week so we can all learn more about the magnetic fields of the smallest stars in the Universe.
The spotlight this month is on Jo Bovy, a John Bahcall Fellow and Long-term Member at the Institute for Advanced Study in Princeton. He completed his PhD at New York University. Within APOGEE, he is the Science Working Group Chair for APOGEE-1, and therefore coordinates the scientific analysis of the APOGEE dataset.
Jo uses big datasets from numerous surveys to understand how the Milky Way came to be. To do this, he studies how stars move and what are their chemical compositions. When he first tackled APOGEE’s huge database of stellar spectra in 2011, he realized that APOGEE’s spatial coverage of the Milky Way’s disk allowed the circular velocity of stars in the Milky Way to be measured with greater accuracy than had ever been done before. (The circular velocity is the speed at which a star orbits the center of the Galaxy. This number changes as a function of distance from the center, and precise measurements are required to correctly determine, for instance, the Galaxy’s mass, but also to measure peculiarities in stellar velocity, which help us determine where it might have originated.) The data analysis in his paper is complex, but he was able to draw two important and straight forward conclusions from this work:
- that the circular velocity near the Sun (what we call the “solar neighborhood”), at a distance of 26,000 light-years from the center of the Galaxy, is 218 ± 6 km/s; and
- that the Sun itself is moving 25 km/s faster than other stars at the same distance.
The first result was expected: although the circular velocity in the Sun’s neighborhood was assumed to be about 220 km/s for the last 30 years or so, Jo’s was the first precise measurement to confirm this value. The second result, however, was a surprise: previous measurements had pegged the Sun’s motion relative to nearby stars at something like 12 km/s, not 25 km/s. This result was confirmed using a sub-set of APOGEE data (a mere 19,937 stars, or about 15% of the full APOGEE dataset) known as the APOGEE Red Clump Catalog.
Why does this seemingly small difference matter? From an outsider’s perspective, going from 12 km/s to 25 km/s is still only changing from about 5% to 10% of the circular speed, so either result might seem acceptable! But is is important, and Jo explains why: We orbit the Sun, and the Sun orbits the center of the Galaxy just like every other star. Therefore, every speed that we measure for another star is relative to our own motion. If we can understand how we move in the Galaxy, then we will have a much better understanding of the dynamics of the entire Milky Way.
And understanding the Milky Way is, after all, the whole point!
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.
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.
Brett Andrews is a postdoc in the Physics and Astronomy department at the University of Pittsburgh. He is working on the MaNGA (Mapping Nearby Galaxies at APO) Data Analysis Pipeline and visualisation tools.
Brett’s research focuses on understanding galaxy evolution, particularly the impact of metal production by stars, cosmological gas inflow, and galactic winds. He is interested in using the gas-phase abundances as a way to trace the relatively recent chemical enrichment history of a galaxy as well as the stellar abundances as a tool to provide a fossil record of the abundance of a galaxy over its entire history.
This week is the MaNGA Team meeting, being held at the University of Kentucky, Lexington Kentucky, so it’s a good week for Brett to take over the Twitter account.
So stay tuned for lots of extragalactic science this week from @SDSSurveys
SDSS collaboration members in Europe have enjoyed a partial solar eclipse today (20th March 2015).
At least two groups planned to project the eclipse through SDSS plates (as have been done previously for the October 2014 eclipse). Sadly Portsmouth was clouded out, but in St. Andrews the experiment was a success.
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.
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.
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).
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).
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”.
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.
A paper appearing in Nature today (Xue-Bing Wu et al. 2015, Nature, Feb 25) presents the most massive black hole discovered to date when the Universe, was less than a billion years old – just one-fifteenth of its current age.
A new method to select high-redshift quasars using SDSS observations combined with data from the WISE satellite has resulted in the discovery of new group of quasars at the far reaches of the universe, with redshifts greater than z = 5. One of these quasars, named SDSS J0100+2802, holds a super-massive black hole at a redshift of 6.3 when the Universe was only 900 million years old.
This black hole is estimated to have a mass 12 billion times that of our Sun. The existence of such a massive black hole at such an early stage in the Universe poses a deep mystery whose resolution will improve our understanding of how galaxies form.
For more information, see the following links:
We are beginning a series of spotlights on APOGEE team members, with special emphasis on their interests in APOGEE science. This month, the spotlight is on Duy Nguyen, one of APOGEE’s postdocs. He graduated from the University of Toronto with a PhD in astronomy and astrophysics, and then held postdoc positions at the University of Florida, Stockholm University, and the University of Rochester before joining the APOGEE team.
Duy’s research is on the subject of binary stars. A binary star is actually two stars orbiting each other. The sizes of the binary star orbits are small enough that the two stars cannot usually be distinguished in images. This can confuse the interpretation of starlight; and in a survey like APOGEE where precise velocities of stars are so important, this can be a big hindrance. As a result, a number of different methods have been employed to try to tease out whether a star is a binary or not.
But this post isn’t just about binary stars — it’s about one scientist’s research into better understanding them! And in many ways, Duy sees APOGEE as the best available experiment for binary star studies. APOGEE takes multiple spectra of most stars in its sample over months and even years, and this time sampling enables orbital periods to be measured. APOGEE’s high spectral resolution means that tiny Doppler shifts in a star’s spectral lines can be measured precisely. And most importantly, such a large sample as APOGEE has observed (more than 150,000 stars to date) means that we may be able to get a better handle on the “binary fraction” of stars in the Milky Way — a problem that has been plaguing modern astronomers for decades.
Duy is primarily interested in the dynamic properties of binary stars. These dynamics are primarily observed by means of the Doppler shift. As the stars in the binary pair orbit one another, each star approaches and recedes from the Earth once per orbit. Every time they approach the Earth, their spectral lines move to a slightly smaller (or “bluer” to use astronomical lingo) wavelength. And every time the star recedes, the spectral lines move to a redder wavelength. These small changes can be detected with APOGEE, and the radial velocity variations of the stars can be determined based on how large is the wavelength shift.
Duy and his collaborators are amassing radial velocity information on the stars in the APOGEE sample, looking for candidates with substantial radial velocity shifts. When they find one, they fit the data points with an orbital model to determine what the most likely stellar masses are. Here is an example fit:
On the x-axis is the time in days, and on the y-axis is the velocity of the star relative to the Sun. This plot shows that the best fit to these data suggest that two stars, one that is at least 0.21 times the mass of the Sun and the other that is 1.6 times the mass of the Sun, are orbiting one another every 112.98 days at a distance greater than 0.065 A.U. It’s interesting to note that the less massive star in this binary is eight times smaller than its companion. Large mass discrepancies in binaries are typical, so that one star dominates the other in terms of brightness. This is one reason why binaries are so difficult to detect.
To date, about 12,000 possible stellar binaries from the APOGEE sample have been flagged based on radial velocity shifts, and 4,000 of these are of special interest because they have been visited seven or more times and exhibit significant radial velocity changes. Of these, 1,500 indicate stellar mass companions, such as the one figured above. While the 12,000 possible binaries were found automatically, the 1,500 sources with stellar mass companions have all had to be screened by hand — a process that Duy would like to fully automate.
Analyzing APOGEE’s huge repository of stellar spectra will enable the most comprehensive assessment of binary stars, including details about whether binary star characteristics are different across the Galaxy. And as an added bonus, APOGEE is sensitive enough to spot Jupiter-sized planets using these methods! How many planets are lurking in the APOGEE dataset?
Special thanks to N. Troup, D. Chojnowski, and S. Majewski for assistance preparing this post.
Black holes are intriguing objects. A black hole is a phenomenon whose gravity is so strong that not even light, the fastest traveller in the Universe, can escape from its influence. Once thought mere oddities due to their extreme properties, today, black holes are found to be vital in the formation and lives of galaxies, including our own Milky Way.
But how do we know black holes exist if we can’t see them? Well, even if we can’t see a black hole directly we can observe their influence and indeed the energy and light emitted as gas, dust and stars fall into a black hole; that is, we can see black holes when they are actively “eating” material. When the supermassive black hole, which can be up to a billion times more massive than our Sun, at the center of a galaxy starts to eat new material the resulting process is so bright it can be seen out to ~200 billion lightyears away. Astronomers call the observational result of this process either an active galactic nuclei, or in the most extreme examples a “quasar”. So you might be surprised to find that an object that emits no light can cause the brightest known phenomenon in the Universe!
The light of quasars is not produced by the black hole itself, but instead it comes from the material, mostly gas, that is falling into the black hole. Different types of light are produced by this material at different distances outward from the black hole. Near the surface (or horizon) of the black hole (about the distance of the Earth’s orbit away for supermassive black holes in galaxies) this gas becomes extremely hot and produces X-rays. Stretching out from this to fill a region about the size of our Solar System, a disk of gas shaped like a frisbee is formed. The inside of this disk is closer to the black hole than the outside, so it rotates faster causing friction within the disk. This friction causes the gas to heat up and glow, producing light in the optical to ultraviolet part of the spectrum.
From the edge of the gas disk to a distance of about 3 light years (similar to the distance from the Sun to the next closest star), the temperature becomes low enough that particles of “interstellar dust”, made of carbon and silicon, form. These dust clouds form what is know as the “dusty torus,” a donut shaped ring round the gas disk. Some of the light coming from the gas disk is absorbed by the dust and re-emitted at longer wavelength infrared light. At very large distances from the black hole, some quasars have radio jets coming out along the poles. As the name suggests, this jets produce light at radio wavelengths cased by electrons being accelerated along a strong magnetic field. When these jets are present they can be up to ~300 thousand lightyears (~3 times the diameter of our entire galaxy!) in size.
Not only can a black hole produce light, it can create light at all wavelengths from the radio up to the X-ray, and across an area stretching from the size of the Earth’s orbit out to distances larger than the Milky Way. Therefore, growing black holes, and the regions around them are anything but “black.”
With discoveries from its earliest imaging campaigns, the SDSS extended the study of quasars back to the first billion years after the Big Bang, showing the rapid early growth of black holes and mapping the end stages of the epoch of reionization.
With full quasar samples hundreds of times larger than those that existed before, the SDSS has given us the most accurate descriptions of the growth of black holes over cosmic history. SDSS spectra show that the properties of quasars have changed remarkably little from the early universe to the present day.
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.
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:
- January IYL2015 Post – How SDSS Uses Light to Study the Darkest Objects in the Universe (black holes).
- February IYL2015 Post – How SDSS Uses Light to Measure the Distances to Galaxies (ie. redshifts)
- March IYL2015 Post – How SDSS Uses Light to Understand Stars Inside and Out in the Kepler Field
- April IYL2015 Post – How SDSS Uses Light to Explore the Invisible (Dark Matter)
- May IYL2015 Post – TBC
- June IYL2015 Post – TBC
- July IYL2015 Post – TBC
- August IYL2015 Post – TBC
- September IYL2015 Post – TBC
- October IYL2015 Post – TBC
- November IYL2015 Post – TBC
- December IYL2015 Post – TBC
APOGEE surveyed 156,481 stars in its first three years. And of course APOGEE-2 is going to increase this sample size significantly. But to celebrate the successful end of APOGEE and the Data Releases 11 & 12 (also see here), we’d like to share with you a slice of the kind of data it collected.
Some background: The APOGEE/APOGEE-2 instrument collects near-infrared spectra of distant stars, and the survey is aimed at studying the history of the Milky Way Galaxy. How it does that is explained here. Along the way, it has taken spectra of each known spectral type: from hot O-type stars (with surface temperatures of about 30,000 degrees, or five times the surface of our own Sun) down to M-type stars (about 3,500 degrees, or roughly half the temperature of the Sun). Each of the spectral types (O, B, A, F, G, K, M) is defined based on how many and what kind of atomic or molecular species are seen in their spectrum. For instance, O-type stars have lots of singly-ionized atomic species visible in their spectra, whereas A-type stars have very strong hydrogen lines, and M-type stars have lots of neutral molecules, especially lines of TiO when you look in the visible portion of the spectrum.
These spectral types were defined using the visible portion of the spectrum. So when we look in the near-infrared, do they appear to be different? Here we go:
The O-type star spectrum looks pretty bland — the strongest lines due to ionized Helium in the near-infrared H-band are at 15721 and 16922 Angstroms (the line at 15271 Angstroms is due to interstellar molecules, and is therefore not from the star). The B-type star shows pretty significant absorption lines due to the Brackett series of atomic Hydrogen (those transitions beginning at the n=4 excited state), and those plus a whole bunch of smaller wiggles from other atoms can clearly be seen in the A- and G-type spectra as well. Below that and things look a lot more complicated. If you have experience with data like these, you might be tempted to think that the spectra of the G-, K-, and M-type stars are “noisy”, meaning that they weren’t observed for long enough and therefore weren’t detected well. But that’s not the case: every single spike visible in these spectra is due to an atomic or molecular transition that originates in the photosphere of the star!
All told, these spectra allow us to study sixteen different atomic elements besides hydrogen. Which ones, you ask? Oh all right, I’ll tell you: C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, and Ni. As you can see, this is a truly beautiful, complex dataset. We’ll keep up-to-date science results at this page.
All sorts of SDSS related stuff will be going on at this meeting, from dozens of talks and posters, to demos of SDSS online resources at the SDSS Booth in the Exhibit Hall and not to mention the final data release from SDSS-III. Our “Tweep of the Week” for this exciting week will be SDSS-IV Spokesperson, Jennifer Johnson.
Jennifer Johnson is an Asssociate Professor in the Astronomy Department of The Ohio State University. Her science interests are in stellar abundances, the origin of the elements, nucleocosmochronology and the formation of our own Galaxy and Local Group. She is the Science Team Chair of the APOGEE survey of SDSS-III, and the Spokesperson for SDSS-IV (as well as working on APOGEE-2).
The SDSS Spokesperson has two main roles. She is the main person in charge of making sure the SDSS collaboration is running smoothly and fairly. As part of this, the Spokesperson Chairs the SDSS Collaboration Council (which has a representative from each institutional member of SDSS). This group are the first point of approval for requests for Architect Status (ie. people who have contributed so much to SDSS development they can request to be on any publication) and External Collaborator requests (non-SDSS members working on specific projects), as well as for drafting our publication and other collaboration policies. They also organise the annual SDSS Collaboration Meetings (the next one to be held in Madrid, 20-23rd July 2015).
The SDSS Spokesperson is also responsible for representing SDSS to the press and the public. As such she is responsible for working with the SDSS Communications Director (Jordan Raddick) to draft the text of press releases and maintain the SDSS website, as well as with the SDSS Director of EPO (Karen Masters) on our collective public engagement and outreach efforts.
Added: here’s a storify of Tweets by Jennifer during her week.
Seasons Greetings from all of us in the Sloan Digital Sky Survey Collaboration.
Don’t worry we’ll be keeping our eyes open at the SDSS telescope this year.