Tag Archives: observing

The Double Red Giant With Odd Oscillations

My latest paper was accepted for publication on December 31, announced on the astro-ph preprint arXiv on January 5, and will appear soon in The Astrophysical Journal [UPDATE Feb 11! Rawls et al. 2016, ApJ, 818, 108]. It was written online using Authorea. I also wrote several programs in python to analyze data for this paper.

Picture two nearly-identical stars orbiting each other. Something like this:

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An approximation of the double red giant binary KIC 9246715, created using an eclipsing binary simulator. In reality, the two stars are separated by more than 200 times our Sun’s radius, but this simulator maxes out at 60. One orbit takes 170 days rather than the 26 illustrated here. The brightness, or flux, dips when one star passes in front of the other. The eclipses are not evenly spaced in time because the orbit is eccentric.

Even though they usually appear as a single dot of light, binary stars are one of the best tools astronomers have to measure stellar properties. Thanks to the math behind gravity, we can weigh pairs of stars using the relationship between how long an orbit takes and how far apart the things are doing the orbiting. Weighing stars accurately is important because a star’s mass seals its fate. So if every star in the night sky had a secret companion star (or exoplanet!), we could wrap this up pretty neatly and move on to deeper questions about stars’ lives.

Unfortunately, only about half of stars have orbiting companions, and many of those aren’t observable because they don’t happen to be edge-on like the case shown above. This is where starquakes come in.

Weighing stars from the inside out

Some stars, including our own Sun, ring like bells. Pressure waves are excited by convection inside stars, and the waves bounce around at resonant frequencies just enough to make them pulse, or oscillate. Because an oscillating star is changing brightness ever-so-slightly, we can use regular observations of brightness versus time (from Kepler, in my case) to pull out the frequencies of oscillation. Heavy stars oscillate differently from lighter ones, and big stars oscillate differently from smaller ones. Voila—a new technique for weighing stars that doesn’t require anything in orbit!

Of course, the story doesn’t end there. While the study of starquakes (more formally known as asteroseismology) is a powerful way to characterize many stars quickly, it remains relatively untested. We don’t know how accurate of a scale we’re using when we whip out asteroseismology to weigh stars. To address this, my colleagues and I identified about twenty binary systems containing red giants. That’s the kind of star our Sun will become when it runs out of fuel in billions of years. Red giants are convenient targets for asteroseismology because they are bright and oscillate slowly. Both properties make them easier to observe than Sun-like stars. And since our red giant stars all live in binaries, we should be able to weigh them in two independent ways and compare the results.

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Solar-like oscillations make a comb-shaped pattern at different resonant frequencies. The central location of the spikes and their spacing in frequency tell us about a star’s average surface gravity and density, respectively. Red is oscillations from the double red giant binary in my paper. Gray is oscillations of a single red giant star with similar properties, plotted upside-down for reference. Figure 7 from Rawls et al.

The case of the missing oscillations

In my paper, I present a case study of two red giant stars in an eclipsing binary. From binary modeling, I show that the stars are both a little more than two times as massive as the Sun, and over eight times as large. However, I am surprised to only find a single signature of starquakes in the observations. Two similar but not-quite-identical stars should, in principle, both oscillate. The oscillation modes, pictured above, are broader and weaker than expected, too. The same physical process could be fully stopping oscillations in one star and only partially suppressing them in the other.

By harnessing many observations (both images and spectra) and modeling techniques, I thoroughly characterize both stars and investigate why only one of them appears to oscillate. I measure each star’s mass, size, temperature, chemical composition, level of magnetic activity, and tidal force strength, among other things. Then I bring in asteroseismology to see if I can tell which star is oscillating and if its story checks out.

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By simultaneously modeling different observations of this double red giant binary, I can map the geometry of its orbit in space and measure each star’s mass and size. I do this by fitting a model (black) to observations (red and yellow). The top panel shows radial velocities, or how fast each star is moving toward/away from us, and the other panels show a light curve, or brightness versus time, with two eclipses (the bottom is a zoomed view of each eclipse). The x-axis is in units of how long one orbit takes, which is about 170 days for this binary. Figure 6 from Rawls et al.
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Stars emit different amounts of light (flux) at different colors (wavelengths), but the light from a binary contains overlapping information about both stars. As the stars orbit, characteristic dips in flux shift to higher or lower wavelengths depending on how fast they are moving. To create one representative spectrum for each star as shown here (red and yellow), I used a physical model of the stars’ orbit to remove the velocity offset from each observation and then combined them. This process is called disentangling. An example of a single observation containing light from both stars (before disentangling) is plotted in black. Figure 5 from Rawls et al.

As it turns out, the two stars in this binary are similar enough that it’s impossible to say for sure which one the oscillations belong to. Recent work has shown that magnetic fields may suppress oscillations in stars, however, so I strongly suspect the oscillating star is the less magnetically active of the pair. There may be a weak second set of oscillations, but the signal is very noisy and doesn’t appear quite where it should. Either way, the single mass and radius derived from asteroseismology is consistent with that of both stars from binary modeling.

Fraternal twins: born together, but not identical

Because oscillations bounce around inside stars, they carry information about how stellar interiors are structured. Stars of different ages have very different things happening inside: younger red giants are still fueled by hydrogen, while older ones are fueled by helium. The oscillating star in this binary appears to be in an advanced helium-burning stage of its life called the horizontal branch or secondary red clump. I verify this with stellar evolution modeling, and confirm that the two stars most likely were born, grew up, and evolved together. They are about 940 million years old.

The next step is to do a similar analysis for the other red giant binaries my team identified. We are working on two fronts: comparing masses and radii from binary modeling and asteroseismology, and using those results to investigate why about a third of red giants don’t show any oscillation behavior. Our work has important implications for understanding the composition of our Milky Way galaxy, because bright red giants are often surveyed to better understand our galaxy’s history and structure. It’s important to get their stories right.

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When Stars Explode

There’s a new supernova in the skies! Last week, students at the University of London Observatory discovered a strange bright spot in nearby galaxy Messier 82 (M82) during a routine observing training session. As undergraduate student Tom Wright put it, “One minute we’re eating pizza then five minutes later we’ve helped to discover a supernova. I couldn’t believe it.”

Words. Image credit UCL/University of London Observatory.
Two images of galaxy M82. The bottom one shows the location of the new supernova, dubbed SN 2014J. The overall galaxy appears dimmer in the bottom picture because the exposure time was shorter, so less light had time to reach the camera. Image from UCL/University of London Observatory.

What’s the big deal about a supernova? Well, to start with, all the elements in the Universe were formed deep inside stars, and spewed out into space through supernova explosions like this one. Take a moment to let that sink in.

This supernova is a special variety called “Type Ia” (type one-A). This means it is caused by a very dense white dwarf star collecting more mass than it can support and eventually going BOOM! We know this because we see signatures of telltale elements like Silicon in the spectrum of the explosion.

Type Ia supernovae are particularly useful because they are all physically very similar—white dwarf stars can only handle so much mass before they explode—so they are all roughly the same brightness. Astronomers love things that are all the same brightness, because they let us determine distances. How? Let’s pretend you’re staring into a huge, dark, empty room containing nothing but a handful of 100-Watt light bulbs. (Not a bad analogy for an astronomer’s life, really…) You’d like to know how far away the light bulbs are, but you don’t have a measuring tape, plus the room is really big. However, you know how much light each bulb is putting out (100 Watts), so you can figure out the ones that look dimmer are actually farther away. We call the 100-Watt light bulbs of the Universe, such as Type Ia supernovae, “standard candles” because they let us determine distance like this.

If you live in the Northern hemisphere and have access to good binoculars or a telescope, you can try seeing SN 2014J for yourself! It is close to peak brightness, and should be visible for another couple of weeks—the blink of an eye from an astronomical perspective.

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Look for galaxy M82 with binoculars or a telescope near the dipper portion of the Big Dipper in Northern hemisphere skies. Image from Universe Today.

Even if you can’t spot the supernova in M82, the galaxy itself and neighboring galaxy M81 are a lovely sight. They’re also a great example of how light can be deceiving. The image below shows two images of these galaxies: one taken with visible light (inverted so the galaxies appear dark on a light background), and one taken with radio light. There is all kinds of gas and material connecting the galaxies together that you can’t see with your eye!

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Two views of galaxies M81 (the larger one) and M82 (the smaller one above it). These pictures show the same region of space in two different flavors of light. The galaxies appear as two isolated collections of stars in visible light (left), while the multicolor radio image (right) shows gas connecting the galaxies. Observations like these are used to figure out how galaxies have interacted gravitationally in the past. Image from the SEDS Messier Catalog.

What I find particularly mind-boggling is how a galaxy some 12 million light years distant is “nearby” on a cosmic scale. Because light doesn’t travel instantaneously, we are seeing this supernova as it happened 12 million years ago. In contrast, every star in the night sky is located in our own Milky Way galaxy, which is about 100,000 light years across, so the light from these stars (and the planets orbiting them!) is “only” delayed by hundreds or thousands of years, not millions. If the planets in our Solar System are our next-door neighbors, and stars in our galaxy with their own planets are other cities, then M82 is an entirely different country.

I can’t help but wonder… is some alien civilization in our galaxy witnessing this distant explosion just as we are, at this very moment? Are intelligent creatures on a planet we have recently discovered also turning their telescopes to the heavens to study this supernova and learn more about the Universe we share?

Moondance

The moon is one of the coolest things in the sky. In fact, you don’t need a telescope to get a good look at it, and you don’t need to be in a particularly dark location, either. Really! Go outside right now and check out that moon!

What’s that? Maybe you can’t find the moon in the sky at this very moment? Well, the moon can change its appearance. Sometimes you won’t see it for days at a time, or you can only see a small sliver of it.

I’m a little embarrassed to admit that I didn’t really understand why this happened until I went to graduate school. Sure, I knew the earth goes around the sun, and the moon goes around the earth, and the moon reflects light from the sun, and everything rises in the east and sets in the west, and we only see part of the moon sometimes… because, you know, geometry and angles and stuff.

You probably know this too: some evenings you look up and see a crescent moon suspended in the sky. One morning, you might see a not-quite-full moon in the daytime sky. Another night, you could witness a glorious full moon rising. And sometimes the moon is nowhere to be seen.

But, for one reason or another, the beauty of how this cosmic dance works didn’t make it into the deepest parts of my brain for years. The best way to discover and understand the phases of the moon is to observe them – truly, intentionally observe the moon, every day or two for at least two weeks, by taking notes and drawing pictures. The best time to start is two or three days after the “new moon” phase.

Let’s speed these observations up a bit…

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What an interesting pattern. We’ve observed for just shy of two weeks, which is how long it takes for the moon to go from New (completely invisible) to Full (completely bright). But it’s not just the phase, or part of the moon that is illuminated, that gradually changes. The timing of when it rises and sets also does.

If we kept observing, we would see the process happen in reverse. Instead of appearing more illuminated each night, or “waxing,” the moon’s illumination would start getting smaller and dimmer, which is called “waning.” After another two weeks or so, we wouldn’t be able to see it at all.

The pattern of the moon’s waxing and waning is actually pretty easy to predict. If I ask you, “When could I see the First Quarter moon rise?” The answer is always the same – around noon – because the moon always spends half a day (12 hours) in the sky. I remember this because the Full Moon is directly opposite from the Sun in the sky, which means it rises at sunset and sets and sunrise. From there, you can work backwards or forward to any other phase.

If you’re still confused about the phases of the moon, don’t despair. What really made it “click” for me, even after lots of observations and even teaching it to others, was this online tool. Play around with it. See if you can predict what will happen. Challenge yourself to predict which moon phase will always appear highest overhead at, say, 3am.

Even without knowing how the moon’s phases work, it is one of the easiest astronomical objects to share and enjoy. You can see the bright Full Moon at nighttime every 27 days from pretty much anywhere on the planet, so long as it’s not too cloudy.

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A “Full Moon Night” at White Sands National Monument, October, 2013. The dunes are lit by the light of the full moon (with some help from headlights!). You can clearly see the planet Venus in the western sky and the constellation Sagittarius to the left, which looks a bit like a sideways teapot.

Slideshow image credits: Day 1, Day 3, Day 5, Day 7, Day 9, Day 11.