Spotlight on APOGEE: Jonathan Bird and the Formation of the Milky Way

The spotlight this month is on Jonathan Bird, the Vanderbilt Initiative for Data-Intensive Astrophysics Postdoc (VIDA) at Vanderbilt University. He is also the APOGEE-2 Science co-chair, for which he is responsible of “making sure that APOGEE-2 takes full advantage of the truly ground-breaking dataset the survey has produced.”

Jonathan is fascinated by the structure of the Milky Way Galaxy: Why is it shaped this way? What was it like in the past? And what will it be like in the future?

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Like many people, Jonathan developed a love for astronomy from an early age. His family home in the Santa Monica mountains offered beautiful views of the night sky.

But growing up, his real passion was basketball. He travelled extensively across the west and southwest for tournaments in high school, and was lucky enough to play college ‘ball when he arrived at Caltech — a team in need of little introduction.

Jonathan’s scientific interests have been widespread. Studying radio waves to investigate what determines the large-scale morphology of galaxies; using Cepheid variable stars to measure distances to galaxies; studying Asymptotic Giant Branch stars in order to understand their contribution to stellar synthesis models, a major component of galaxy models; and studying how a disk galaxy is assembled from smaller galaxies. Do you see a theme? In fact, Jonathan’s major interests can best be described by his PhD thesis title: “The Formation and Evolution of Disk Galaxies.” Jonathan’s goal is nothing less than understanding how the Milky Way came to be, how it evolved, and where it is going from here.

Perhaps that is why Jonathan fits in so well with the APOGEE team.

Let’s show one of Jonathan’s models from his 2013 paper on disk galaxy assembly. In the top left panel is shown the distribution of really old stars (11-12 billion years old) in a typical spiral galaxy. From left to right, and then continuing on the bottom, each panel shows the distribution of stars in a different age group (numbers in the bottom right of each panel show the age in billions of years). Notice that the “spiral” shape that we associate with spiral galaxies is found only among stars that are less than about 2 billion years old. As odd as this may seem, this is exactly what astronomers observe: the older stars are found across a large volume across the bulge and halo; whereas younger stars are predominantly found in the disk, where star formation is ongoing.

512_tf_density_agebins

How does this model hold up against the huge APOGEE-1 dataset? Pretty well, actually. For Jonathan, this is heartening — we have begun to piece together the massively complex stellar history of the Milky Way Galaxy, and we can do it with state-of-the-art telescopes and computer codes. You can follow more about what Jonathan is doing, along with the rest of the APOGEE-2 team, by following us on social media.

APOGEE-South: Plate-Pluggers and Tripods – APOGEE-Sur: Conexión de Placas y Trípodes

Recently, a small group of astronomers from Chile has been visiting Apache Point Observatory. Their job will be to assist with operations at APOGEE-South, which is being built for the Irénée du Pont telescope at Las Campanas Observatory. Introducing: Christian Nitschelm, a faculty member at Universidad de Antofagasta; Andrés Almeida, a Masters student from Universidad Andrés Bello; and Jaime Vargas, Masters student at Universidad de La Serena.

Recientemente, un pequeño grupo de astrónomos de Chile ha estado visitando el Observatorio Apache Point (APO por sus siglas en Inglés). Su trabajo consistirá en ayudar con las operaciones en APOGEE-Sur, que se está construyendo para el telescopio Irénée du Pont en el Observatorio Las Campanas. Presentamos a: Christian Nitschelm, profesor en la Universidad de Antofagasta; Andrés Almeida, un estudiante de Maestría de la Universidad Andrés Bello; y Jaime Vargas, estudiante de Maestría de la Universidad de La Serena.

Jamie (left) Christian (center), and Andres (right), unplugging an APOGEE plate after observations. Jamie (a la izquierda), Christian (al centro), y Andrés (a la derecha), desconectando las fibras ópticas de una placa de APOGEE después de las observaciones.

Jamie (a la izquierda), Christian (al centro), y Andrés (a la derecha), desconectando una placa de APOGEE después de las observaciones.
Jamie (left) Christian (center), and Andres (right), unplugging an APOGEE plate after observations.

While at APO, Jamie, Christian, and Andres are learning a number of important skills that they will take back to Las Campanas Observatory. This includes plugging and unplugging plates:

Mientras tanto en el APO, Jamie, Christian y Andrés están aprendiendo una serie de técnicas importantes que llevarán al Observatorio Las Campanas. Esto incluye conectar y desconectar las placas:

Christian and Jamie seen here plugging fibers into a plug plate. Christian y Jaime se ven aquí conectando las fibras en una placa de conexión.

Christian y Jaime se ven aquí conectando las fibras ópticas en una placa de conexión.
Christian and Jamie seen here plugging fibers into a plug plate.

They are also learning to use the new Mock Up and Training Facility tripod, cartridge, and dolly (seen below). This setup will be sent down to Universidad de La Serena so that this crew can train future support staff.

También están aprendiendo a usar la maqueta y trípode de capacitación, el cartucho y carro (observados a continuación). Esta configuración se enviará a la Universidad de La Serena para que este equipo de trabajo pueda entrenar el personal de apoyo futuro.

Christian and Jamie swapping out a plug plate cartridge with the Mock Up and Training Facility tripod (the big steel frame), cartridge (the blue object suspended from the tripod) and dolly, which will be used to transport plug plates to and from the telescope. Christian y Jaime intercambiando el cartucho de la placa conexión con la maqueta y el trípode de capacitación (la estructura de acero grande), el cartucho (el objeto azul suspendido del trípode) y el carro, que será utilizado para transportar las placas de conexión hacia y desde el telescopio.

Christian y Jaime intercambiando el cartucho de la placa conexión con la maqueta y el trípode de capacitación (la estructura de acero grande), el cartucho (el objeto azul suspendido del trípode) y el carro, que será utilizado para transportar las placas de conexión hacia y desde el telescopio.
Christian and Jamie swapping out a plug plate cartridge with the Mock Up and Training Facility tripod (the big steel frame), cartridge (the blue object suspended from the tripod) and dolly, which will be used to transport plug plates to and from the telescope.

“Torquing” the plug plate slightly is a necessary skill so that it aligns with the field of curvature of the telescope. Using a ring around the plate (shown being attached below), the plate can be bent ever so slightly:

“Torcer” ligeramente la placa de conexión es una habilidad necesaria para alinear la placa con el campo de curvatura del telescopio. Usando un anillo alrededor de la placa (mas abajo se ve como se engancha), ésta se puede doblar ligeramente:

Christian and Andres attaching the bending ring around the plate. Christian y Andrés enganchan el anillo de flexión alrededor de la placa.

Christian y Andrés enganchan el anillo de flexión alrededor de la placa.
Christian and Andres attaching the bending ring around the plate.

And, of course, it is important to check your work. In this case, a computer is used to map the locations of fibers on the plate, ensuring that they will be on target when the plug plate is used on the telescope:

Y, por supuesto, es importante revisar su trabajo. En este caso, se utiliza un ordenador para mapear las ubicaciones de fibras en la placa, asegurando que van apuntar al objeto cuando la placa de conexión se use en el telescopio:

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Christian utiliza una computadora para medir el perfil de la placa de conexión después de que ha sido mapeada. Esto asegurará que la placa ha sido “torcida” correctamente.
Christian using a computer to measure the profile of the plug plate after it has been mapped. This will ensure that they have “torqued” the plate properly.

 

Jamie is enjoying his new skills set! Here, he is drawing an overlay on a plug plate to prepare it for plugging. ¡Jaime disfruta de sus nuevas habilidades! Aquí está dibujando una superposición en una placa de conexión para prepararla para la conexión.

¡Jaime disfruta de sus nuevas habilidades! Aquí está dibujando una superposición en una placa de conexión para prepararla para la conexión.
Jamie is enjoying his new skills set! Here, he is drawing an overlay on a plug plate to prepare it for plugging.

Special thanks to Veronica Motta, Professor of Astronomy at Universidad de Valparaíso, for translating the English into Spanish.

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

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. 

APOGEE’s Infrared View of the Stellar Temperature Sequence

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:

apogee_tempsequence_new2

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.