In the decades following the launch of NASA’s , astronomers have tallied over 1 trillion galaxies in the universe. But only one galaxy stands out as the most important nearby stellar island to our Milky Way — the Andromeda galaxy. It can be seen with the naked eye on clear autumn nights as a faint oval object roughly the size of the moon.

A century ago, astronomer Edwin Hubble first established that this so-called “spiral nebula” was approximately 2.5 million light years away from our own Milky Way galaxy.

Now, the space telescope named after Hubble has accomplished the most comprehensive survey of this galaxy. The work yields new clues to the evolutionary history of Andromeda — and it looks markedly different from the Milky Way’s history.

91̽�� astronomers presented the findings Jan. 16 in Maryland at a meeting of the , and in an accompanying published the same date in The Astrophysical Journal.

Without Andromeda as an example of a spiral galaxy, astronomers would know much less about the structure and evolution of our own Milky Way. That’s because Earth is embedded inside the Milky Way. This is like trying to understand the layout of New York City by standing in the middle of Central Park.��

“With Hubble we can get into enormous detail about what’s happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,” said principal investigator , a 91̽��research associate professor of astronomy.����

Hubble’s sharp imaging capabilities can resolve more than 200 million stars in the Andromeda galaxy, detecting only stars brighter than our sun. They look like grains of sand across the beach. But the telescope can’t capture everything. Andromeda’s total population is estimated to be 1 trillion stars, with many less massive stars falling below Hubble’s sensitivity limit.��

Photographing Andromeda was a Herculean task because the galaxy is a much bigger target in the sky than the galaxies Hubble routinely observes, which are often billions of light years away. The full mosaic was carried out under two Hubble programs. In total it required over 1,000 Hubble orbits, spanning more than a decade.��

This panorama started about a decade ago with the . Images were obtained at near-ultraviolet, visible and near-infrared wavelengths using instruments aboard Hubble to photograph the northern half of Andromeda.��

This has now been followed by the newly published . This phase added images of approximately 100 million stars in the southern half of Andromeda. This southern region is structurally unique and more sensitive to the galaxy’s merger history than the northern disk mapped earlier.��

Combined, the two programs collectively cover the entire disk of Andromeda, which is seen almost edge on — tilted by 77 degrees relative to the view we see from Earth. The galaxy is so large that the mosaic is assembled from approximately 600 separate fields of view. The mosaic image is made up of at least 2.5 billion pixels.��

“The asymmetry between the two halves — now visually evident in this image — is incredibly intriguing,” said , a 91̽��postdoctoral researcher in astronomy and lead author of the accompanying . “It’s fascinating to see the detailed structures of an external spiral galaxy mapped over such a large, contiguous area.”��

The complementary Hubble survey programs provide information about the age, heavy-element abundance and stellar masses inside Andromeda. This will allow astronomers to distinguish between competing scenarios where Andromeda merged with one or more galaxies. Hubble’s detailed measurements constrain models of Andromeda’s merger history and disk evolution.��

“This ambitious photography of the Andromeda galaxy sets a new benchmark for precision studies of large spiral galaxies,” Chen said.��

Though the Milky Way and Andromeda galaxies formed presumably around the same time many billions of years ago, observational evidence shows that they have very different evolutionary histories, despite growing up in the same cosmological neighborhood. Andromeda seems to be more highly populated with younger stars and unusual features like coherent streams of stars, researchers say. This implies it has a more active recent star formation and interaction history than the Milky Way.��

“This detailed look at the resolved stars will help us to piece together the galaxy’s past merger and interaction history,” Williams said.��

This research was funded by NASA and the Simons Foundation. A full list of co-authors is listed with the .

For more information, contact Williams at benw1@uw.edu or Chen at zczhuo@uw.edu.��

This article was adapted from a by the Space Telescope Science Institute. See related posts from and the .

is a professor in the 91̽�� Department of Astronomy. He is one of working on the , or LSST, which will begin scanning the sky in 2022 from its location atop Cerro Pachón, a mountain in northern Chile.

He has called it “one of the most exciting experiments in astrophysics today,” adding, “it could completely transform our knowledge of the universe, from understanding how dark energy drives the expansion of the universe, to identifying asteroids that may one day impact the Earth.”

Over the years, Connolly has worked on a number of areas in the design and construction of the LSST, from running the 91̽��data management group that develops software to study information that will come from the telescope, to leading a team developing simulations of what this powerful new telescope might see. On his web page he says, “My science focuses on analyzing large astronomical data sets to study the formation and evolution of galaxies and cosmology.”

Throughout his career he has been involved with big data projects. As a postdoctoral researcher he was involved in the , or SDSS, a collaboration of about 200 astronomers at more than 40 institutions on four continents that has been scanning the sky and collecting data since 2000. During a sabbatical in 2006 at Google, Connolly was the project leader for , which incorporated images from the and the SDSS into Google Earth.

Connolly answered a few questions about his work and the promise of big data and tools such as the LSST to astronomy.

Q: Where are you spending the year, and what are you working on?

A.C.: I am in Cambridge (the UK version) for a year. I’m working on a few different areas ranging from the detection of objects whose light has been bent (or gravitationally lensed) by distant galaxies, to studying how we can survey the sky to maximize how quickly we can get science from the LSST.

These may seem like very different questions and problems but they are in fact related. They both involve searching for subtle signals from large complex data sets. Signals that are hard to extract but if we can, we might be able to understand how the universe evolves (driven by dark energy and dark matter).

We have a lot of different ways to look at the sky (different telescopes and instruments) and many tools that can be used when working with data, but it is only when you start applying these techniques to real observations that you can understand how well they will perform in practice. I’m trying to use some of the techniques that we will use on the LSST but on today’s data sets.

So you could say that I am getting my hands dirty with data, which has been a lot of fun, especially with the LSST a few years away.

Q: In your TED talk you say that a single image from the LSST will be equivalent to 3,000 images from the Hubble Space Telescope. How is this achieved?

A.C.: The LSST isn’t the biggest telescope in the world (unlike the new generation of telescopes that will have mirrors 30 meters across), nor does it have the highest-quality images (such as those from space base telescopes like the Hubble).

What it does have is a very large field of view (one image covers an area seven times the width of the full moon) and the largest digital camera in the world (with 3.2 billion pixels). This means it can survey half of the sky every three nights to discover if anything has changed or moved (something Hubble would take about 120 years to do just once).

One of the great aspects of all of the telescopes and instruments we are building today is that they have different and complementary capabilities (e.g. the Hubble can look at great detail at very faint sources but can’t cover large areas of the sky). Combined, we get to reveal both the big picture and the details of how the universe has evolved up to the present day.

Q: What are the challenges that you face in order to answer these “big questions”?

A.C.: Within the next decade new telescopes (on Earth and in space), and new cameras and spectrographs will realize a 1,000-fold increase in the amount of data accessible to astronomers. The size of the data will enable us to answer some of the most fundamental questions in astrophysics today — questions we have been asking since we started looking up at the stars and wondering how they came into being.

Discoveries that might come from the data include:

• Measurements of the shapes of distant galaxies could reveal the properties of dark energy with an accuracy 10 times better than today. This could change our understanding of general relativity if it shows that gravity works differently on large scales.
• Surveys of the faint radio sky may detect the epoch at which stars and galaxies first began to form within the universe.
• Tracking the orbits of asteroids and comets could reveal if the environment in which the Sun formed was responsible for the distribution of the planets in our solar system or identify asteroids that might one day impact the Earth (at distances where we can do something about it).

Some of the most exciting discoveries will be answers to questions that today we don’t even know how to ask.

But this data-rich era comes with a big challenge: Scientific discovery is beginning to be limited not by how we collect or store data, but how we extract the knowledge it contains.

We are reaching a stage where our data are much richer than many of the analyses we apply to them, and where software and algorithms have the potential to become the next instrument for exploring the universe.

Fixing this gap between the science and the amount of data is something that we need to address. The increasing complexity and size of data coming from these instruments means astrophysics is becoming ever more dependent on developments in computing. It also means that there is a great opportunity for discovery if we can prepare the next generation of students and postdocs with the skills that are needed for an era rich in data.

Q: You also mention that “the smart use of data” and new tools will transform astronomy in coming years, “opening up a window in the universe — the window of time.” What new understanding of the cosmos might this bring?

A.C.: There are so many things we know about the universe but don’t understand. We know it is expanding and this expansion is getting faster, but we don’t understand what causes the acceleration.

We know that the dynamics of the universe suggest that most of the matter is not visible, but we don’t understand what particles might make up that matter. We can see the diversity of stars and galaxies that have formed in the universe, but we don’t understand, in detail, the physical processes that drive the formation and evolution of galaxies or the formation of the first stars.

It is a great time to be an astronomer because a new generation of telescopes and surveys might help us unlock these answers by providing a view of the universe that has unprecedented detail. Data will answer these questions (hopefully) and this revolution in data will occur over the next decade.

• Visit for more information about the 91̽��and the LSST.
• Watch a video of Connolly’s 2014 TED talk (and learn more ):

What looked at first like a sort of upside-down planet has instead revealed a new method for studying binary star systems, discovered by a 91̽�� student astronomer.

Working with 91̽��astronomer , doctoral student has confirmed the first “self-lensing” binary star system — one in which the mass of the closer star can be measured by how powerfully it magnifies light from its more distant companion star. Though our sun stands alone, about 40 percent of similar stars are in binary (two-star) or multi-star systems, orbiting their companions in a gravitational dance.

Kruse’s discovery confirms an astronomer’s prediction in 1973, based on stellar evolution models of the time, that such a system should be possible. A by Kruse and Agol was published in the April 18 edition of Science.

Like so many interesting discoveries, this one happened largely by accident.

Astronomers detect planets too far away for direct observation by the dimming in light when a world passes in front of, or transits, its host star. Kruse was looking for transits others might have missed in data from the planet-hunting when he saw something in the binary star system KOI-3278 that didn’t make sense.

“I found what essentially looked like an upside-down planet,” Kruse said. “What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite — it looks like an anti-transit.”

The two stars of KOI-3278, about 2,600 light-years (a light-year is 5.88 trillion miles) away in the Lyra constellation, take turns being nearer to Earth as they orbit each other every 88.18 days. They are about 43 million miles apart, roughly the distance the planet Mercury is from the sun. The white dwarf, a cooling star thought to be in the final stage of life, is about Earth’s size but 200,000 times more massive.

That increase in light, rather than the dip Kruse thought he’d see, was the white dwarf bending and magnifying light from its more distant neighbor through gravitational lensing, like a magnifying glass.

“The basic idea is fairly simple,” Agol said. “Gravity warps space and time and as light travels toward us it actually gets bent, changes direction. So, any gravitational object — anything with mass — acts as a magnifying glass,” though a weak one. “You really need large distances for it to be effective.”

“The cool thing, in this case, is that the lensing effect is so strong, we are able to use that to measure the mass of the closer, white dwarf star. And instead of getting a dip now you get a brightening through the gravitational magnification.”

This finding improves on in 2013 by the California Institute of Technology, which detected a similar self-lensing effect minus the brightening of the light because the two stars being studied were much closer together.

“The effect in this system is much stronger,” said Agol. “The larger the distance, the more the effect.”

Gravitational lensing is a common tool in astronomy. It has been used to detect planets around distant stars within the Milky Way galaxy, and was among the first methods used to confirm Albert Einstein’s general theory of relativity. Lensing within the Milky Way galaxy, such as this, is called microlensing.

But until now, the process had only been used in the fleeting instances of a nearby and distant star, not otherwise associated in any way, aligning just right, before going their separate ways again.

“The chance is really improbable,” said Agol. “As those two stars go through the galaxy they’ll never come back again, so you see that microlensing effect once and it never repeats. In this case, though, because the stars are orbiting each other, it repeats every 88 days.”

White dwarfs are important to astronomy, and are used as indicators of age in the galaxy, the astronomers said. Basically embers of burned-out stars, white dwarfs cool off at a specific rate over time. With this lensing, astronomers can learn with much greater precision what its mass and temperature are, and follow-up observations may yield its size.

By expanding their understanding of white dwarfs, astronomers take a step closer to learning about the age of the galaxy.

“This is a very significant achievement for a graduate student,” Agol said.

The two have sought time to use the Hubble Space Telescope to study KOI-3278 in more detail, and to see if there are other such star systems waiting to be discovered in the Kepler data.

“If everyone’s missed this one, then there could be many more that everyone’s missed as well,” said Kruse.

###

The research was funded by grants from the National Science Foundation (#AST 0645416) and NASA (#12-OSS12-0011). For more information, contact Agol at 206-543-7106 or agol@astro.washington.edu; or Kruse at 845-499-1384 or eakruse@uw.edu.