Shelf Life 05: How to Time Travel to a Star

March 2015 6:37 min

ASHLEY PAGNOTTA (Davis Postdoctoral Fellow, Department of Astrophysics): In astronomy, things either happen almost instantaneously, or over tens of thousands, millions of years. But one of the things that we’re starting to learn is that the things that we previously thought were constant over a hundred years, we’re seeing changes over decades—essentially the course of a human lifetime.

My name is Ashley Pagnotta. I am a Kathryn Davis postdoctoral fellow at the Museum of Natural History. I research stars that explode and I teach future teachers.

[SHELF LIFE TITLE SEQUENCE]

PAGNOTTA: One of the differences between astro collections and natural history collections is that because ours live on hard drives, instead of in cabinets, they take up a whole lot less room. Theoretical and computational astrophysicists run these enormous simulations. So their collections are the outputs of those simulations.

For me, because I’m an observational astronomer, my collections are images.

Now, when we take pictures of the sky, we use basically digital cameras. Before we had that technology, we used glass plates that were coated with a photo emulsion. They would be put onto the end of the telescope, exposed like film, and then developed. And the advantage of this was that they, for the first time, had an objective record of what the sky looked like.

The Harvard Plate Collection is the largest in the world—about half a million glass plates. They date all the way back to 1890. Harvard had telescopes all over the world, and they would do what we call sky patrol—basically, take pictures every night in an organized fashion so they could cover the whole sky. And then they would regularly repeat that, so they could look for things that were changing.

MIKE SHARA (Curator, Department of Astrophysics): Once we had a permanent record on photographic plates, we knew that we could go back a week later, a year later, a century later, and in some sense, recover what galaxies, stars, and so on were like during those observations.

My name is Mike Shara. I’m a curator of astrophysics here at the American Museum of Natural History. What I do is exploding stars.

I can’t send a time machine into the past. We simply have one record of the universe streaming by us, and because the astronomers a century and more ago were snapping pictures, we have a continuous record over an enormously long period of time.

It’s a completely unique history and digitizing their collections is very, very high on the wish list of most of the world’s astronomers.

PAGNOTTA: Harvard has started a scanning program and here at the Museum, we’re also working to help bring the plates into the modern era and to make them more available to us, for the science that we want to do, but also to the broader astronomical community.

So, one type of star that I’m interested in is called a Cepheid variable, and these were discovered by Henrietta Leavitt who worked at the Harvard Observatory in the early 1900s.

She realized that you can use these Cepheid variables to measure distances. Which is really useful because we can’t just stretch a yardstick halfway across the galaxy.

Leavitt in the early 1900s compiled a catalog of all these variable stars in the Large and the Small Magellanic Clouds—mini-galaxies that are actually gravitationally bound to our galaxy, the Milky Way. Later in the 1950s and ‘60s, Cecilia Payne-Gaposchkin updated Henrietta Leavitt’s catalog. So, we have these two fantastic catalogs that are crucial to measure distances.

But we went to look for them in our modern digital catalogs of stars and they weren’t there. So, what we decided to do with a couple of groups of high school students who work here through what we call the SRMP program—the Science Research Mentorship Program—was to update those catalogs.

JULIA KRUK (Science Research Mentoring Program Student): My name is Julia Kruk.

ZACHARY MURRAY (Science Research Mentoring Program Student): I am Zachary Murray. We participated in the SRMP program.

Essentially, they were taking pictures of the sky and printing them on photographic plates, and then Henrietta Leavitt used an X-Y coordinate system to chart the locations of the stars.

What we start with is a sheet with columns of numbers on it. But these positions were only valid for the era in which she did her observations.

KRUK: We had to make sure that it was accurate so we could find these stars today.

MURRAY: So, in order to correct that, we converted those X-Y coordinate positions to modern spherical coordinates.

PAGNOTTA: But they weren’t quite as modern as we use today. Because the Earth kind of wobbles on its axis, that means where we see the stars changes very slightly over time. So, the next step was they had to what we call precess them all the way forward. So, they actually wrote a computer program to get modern coordinates.

KRUK: We’re publishing this data and making it available to the astronomical community— a century’s worth of data in one location.

SHARA: It’s one thing to collect and to generate data. It’s entirely another thing to make sense of it. And that’s what makes it more and more scientifically valuable. By involving a team of students who are willing to get their hands dirty and look at the data, we begin to train the next generation of scientists and at the same time, selfishly, we get something out of it.

PAGNOTTA: Once this catalog is complete—and it’s almost finished—we will have a digital, fully accessible catalog that anyone in the world can use. And then from there, you can start to do science—see how do these stars change over time. We think that they probably do change over 100 years, but we don’t really know what they do. Nobody’s ever looked before.

It’s one thing to collect and to generate data. It’s entirely another thing to make sense of it.

-Michael Shara, Curator, Department of Astrophysics

Stargazing Across Time

Humans have spent eons looking up at the night sky with wonder and mapping it by creating constellations, gathering information, and using it to impose a sense of order on the vastness of space.

From the earliest days of astronomy, though, the answers we've found have served to create further questions, and with them the need for finer instruments that provide more complete data. The naked eye isn’t nearly sufficient for viewing stars closely or tracking their movements in detail, much less plumbing the depths of space. From early devices that used celestial bodies to tell travelers the time of day to the space-based telescopes now capturing images of the cosmos in unprecedented detail, ever-more intricate instruments have helped us get a better look at—and understanding of—the universe and our place in it.

Sundials

Prior to the development of reliable clocks, sundials, which tell the time of day by the position of the Sun, were a method of choice for determining lunch hour, coffee breaks, and quitting time for millennia. By the 17th century, portable dials became popular with people like soldiers and merchants, who were always on the go. Some, like the ivory-encased affair pictured above, could be adjusted to tell the correct time at a variety of different latitudes.

Ivory foldable sundial with an elaborate dial design with a flower at the center.

This portable sundial could tell time accurately from numerous European cities.
© AMNH

Dials

The Sun wasn’t the only heavenly body used for telling time. This rosewood dial crafted in 17th-century Italy was designed to align with the pointer stars of the Big Dipper, letting users determine the time of day even in the dead of night—provided they had a clear sky.

Intricately designed sundial with golden pieces against dark background.

Time and other observations could be made with this dial using starlight using the positions of the stars.
© AMNH

Astrolabes

Astrolabes, which could locate and predict the movement of heavenly bodies like the Moon, planets, and stars, were first developed in the 11th century BC and were refined throughout the ages. Examples like this bronze astrolabe, crafted in Persia during the 18th century, were variously used for surveying, telling time, and predicting and charting the movement of stars and planets.

Circular bronze astronomical model sized to be handheld with intricate designs and a woven cloth handle.

This bronze astrolabe, made in Persia circa 1730–1775, has several inset plates for use by travelers at different latitudes.
D. Finnin /© AMNH

The reasons for gazing up weren’t always about terrestrial concerns like telling time or navigation. People all over the world made maps of the stars in an effort to better understand them. One of the earliest star atlases, Johann Bayer’s 1603 collection of engravings the Uranometria, detailed the known stars and constellations in the night sky. The work also enshrined Bayer’s method for naming stars by assigning them letters in the Greek alphabet in the order of their apparent brightness. While astronomers have developed new naming conventions—the number of stars in a constellation often far exceeds the 24 letters of the Greek alphabet—names like Alpha Centauri are still familiar today.

Illustration of Orion, hunter in Greek myth, with beard, draped robes, a club in one hand and a lion skin in the other against starry background.

The early star atlas Uranometria depicted constellations such as Orion, above.
D. Finnin/© AMNH

Star Maps

Beginning in the 17th century, telescopes came into use, allowing astronomers to get closer looks at the sky, and the objects that populate it, than ever before. New and improving instruments spurred more detailed star mapping.

In the 1800s, Connecticut schoolteacher Elijah Burritt brought astronomy to the masses when he published a popular volume of celestial maps, Burritt’s Atlas, to “illustrate Burritt’s geography of the Heavens.” The Museum’s holdings include an 1856 edition.

Highly illustrated celestial map featuring robed people, a dragon, a giraffe, a small and big bear, a swan and small stars.

Burritt’s Atlas, designed to illustrate the geography of the heavens, featured celestial maps for the masses.
D. Finnin/© AMNH

Burritt had designed his atlas as a cheaper alternative to celestial globes, which illustrated the place of stars and constellations in the sky. While beautiful, globes were out of reach for most stargazers, including school teachers.

Detailed celestial globe in a wooden stand.

Telescopes

in the late 19th century, photographic plates became the gold standard for capturing images that telescopes allowed astronomers to view. The images captured on these plates remain incredibly valuable to science, representing a century of data on stars and galaxies that can never be replaced. The Harvard Plate collection currently being digitized at the Museum consists of images of the sky taken beginning in the 1890s and continuing, uninterrupted, to the 1950s.

“[Harvard astronomers] would basically take a telescope that covered a large part of the sky and take pictures every night of the entire sky in an organized fashion,” says Ashley Pagnotta, a Kathryn W. Davis postdoctoral fellow at the Museum. The result was that about every week or two, the Harvard team would have a full picture of the sky, providing an excellent time lapse of how celestial objects changed over time.

Spiral galaxy surrounded by many more astronomical bodies.

From simple metal tubes with a series of lenses, telescopes have evolved into masterworks of engineering that dominate mountaintops and explore the depths of space with titanic mirrors and radio waves alike.

Today, high-powered digital telescopes, many of them traveling through the cosmos like the Hubble Space Telescope, give us a better look at the universe than ever before, providing answers to astronomical mysteries and introducing new questions.

Beginning in the 1970s, the move to digital imaging marked a revolution in photographing the cosmos. That’s because photographic plates have a quantum efficiency of just 1 percent, meaning that for every photon they record, 99 are going undetected. Modern digital detectors, says Astrophysics Curator Michael Shara, can achieve quantum efficiencies of more than 90 percent.

More sensitive digital detectors are key to the strides made in modern astronomy, but where those detectors are housed is just as important. Moving from Earth-based to space-based telescopes has further improved our ability to visualize the universe around us.

“With the launch of the Hubble Space Telescope, the Chandra X-Ray Observatory, and other infrared telescopes, we can do astronomy in space and see things 100 times fainter than we could before,” says Dr. Shara.

Wide shot of outer space, with multiple colors and many bright stars of various sizes scattered across expanse.

Coronograph

Among those new and improved devices is the Lyot Project Coronagraph, developed at the Museum. Curator Rebecca Oppenheimer has already used the coronagraph, which filters out light from distant stars and makes it easier to see dim objects nearby, to discover the first brown dwarf star ever observed. Future coronographs will aim to see even dimmer and dimmer objects. That's likely to be accomplished courtesy of coronagraphs mounted on space-based telescopes.

"The next goal is to use a coronagraph to see objects that are much fainter, that may just be reflecting light, not radiating it," says Dr. Oppenheimer. "To do that, we'll need better suppression of starlight. That's harder to do from the ground because of Earth's atmosphere, so we'll need to get into space."

On left, very bright, pale star against light background. On right, darker, more subdued star against dark background.

Viewed without a coronagraph, the Sun (left) is so bright that nothing can be seen around it. With a coronagraph in place, planets like Saturn and Jupiter (right) are visible.
R. Makidon, A. Sivaramakrishnan, B.R. Oppenheimer/© AMNH

Next-Generation Technology

As impressive as our tools for observing space are now, there is still plenty of room for improvement. Space-based and terrestrial telescope systems already in development will be able to give us images of the universe in unparalleled detail. One planned device, the Large Synoptic Survey Telescope, will use three huge mirrors and a 3.2-gigapixel digital camera to capture images of a huge swath of the sky every 20 seconds.

“In just a few years, we’re going to be photographing or imaging essentially the entire sky every two or three nights with the Large Synoptic Survey Telescope,” says Shara. “We will be imaging literally all hundred billion galaxies every night, and essentially seeing every supernova as it goes off…at least out to a distance of 10 million light years.”