Precisely dating a star can have important consequences for understanding stellar evolution and any circling exoplanets. But it’s one of the toughest plights in astronomy with only a few existing techniques.
One method is to find a star with radioactive elements like uranium and thorium, whose half-lives are known and can be used to date the star with certainty. But only about 5 percent of stars are thought to have such a chemical signature.
Another method is to look for a relationship between a star’s age and its ‘metals,’ the astronomer’s slang term for all elements heavier than helium. Throughout cosmic history, the cycle of star birth and death has steadily produced and dispersed more heavy elements leading to new generations of stars that are more heavily seeded with metals than the generation before. But the uncertainties here are huge.
The latest research is providing a new technique, showing that protostars can easily be dated by measuring the acoustic vibrations — sound waves — they emit.
Stars are born deep inside giant molecular clouds of gas. Turbulence within these clouds gives rise to pockets of gas and dust with enough mass to collapse under their own gravitational contraction. As each cloud — protostar — continues to collapse, the core gets hotter, until the temperature is sufficient enough to begin nuclear fusion, and a full-blown star is born.
Our Sun likely required about 50 million years to mature from the beginning of collapse.
Theoretical physicists have long posited that protostars vibrate differently than stars. Now, Konstanze Zwintz from KU Leuven’s Institute for Astronomy, and colleagues have tested this prediction.
The team studied the vibrations of 34 protostars in NGC 2264, all of which are less than 10 million years old. They used the Canadian MOST satellite, the European CoRoT satellite, and ground-based facilities such as the European Southern Observatory in Chile.
“Our data show that the youngest stars vibrate slower while the stars nearer to adulthood vibrate faster,” said Zwintz in a press release. “A star’s mass has a major impact on its development: stars with a smaller mass evolve slower. Heavy stars grow faster and age more quickly.”
Each stars’ vibrations are indirectly seen by their subtle changes in brightness. Bubbles of hot, bright gas rise to the star’s surface and then cool, dim, and sink in a convective loop. This overturn causes small changes in the star’s brightness, revealing hidden information about the sound waves deep within.
You can actually hear this process when the stellar light curves are converted into sound waves. Below is a video of such singing stars, produced by Nature last year.
“We now have a model that more precisely measures the age of young stars,” said Zwintz. “And we are now also able to subdivide young stars according to their various life phases.”
A simple, yet elegant method of measuring the surface gravity of a star has just been discovered. These computations are important because they reveal stellar physical properties and evolutionary state – and that’s not all. The technique works equally well for estimating the size of hundreds of exoplanets. Developed by a team of astronomers and headed by Vanderbilt Professor of Physics and Astronomy, Keivan Stassun, this new technique measures a star’s “flicker”. Continue reading “Flicker… A Bright New Method of Measuring Stellar Surface Gravity”
Scientists have discovered a new planet orbiting a Sun-like star, and the exoplanet is the smallest yet found in data from the Kepler mission. The planet, Kepler-37b, is smaller than Mercury, but slightly larger than Earth’s Moon. The planet’s discovery came from a collaboration between Kepler scientists and a consortium of international researchers who employ asteroseismology — measuring oscillations in the star’s brightness caused by continuous star-quakes, and turning those tiny variations in the star’s light into sounds.
“That’s basically listening to the star by measuring sound waves,” said Steve Kawaler, from Iowa State University in the US, and a member of the research team. “The bigger the star, the lower the frequency, or ‘pitch’ of its song.”
The measurements made by the astroseismologists allowed the Kepler research team to more accurately measure the tiny Kepler-37b, as well as revealing two other planets in the same planetary system: one slightly smaller than Earth and one twice as large.
While Kepler 37b is likely a rocky planet, this would not be a great place for humans to live. It’s likely very hot — with a smoldering surface and no atmosphere.
“Owing to its extremely small size, similar to that of the Earth’s moon, and highly irradiated surface, Kepler-37b is very likely a rocky planet with no atmosphere or water, similar to Mercury,” the team wrote in their paper, which was published this week in Nature. “The detection of such a small planet shows for the first time that stellar systems host planets much smaller as well as much larger than anything we see in our own Solar System.”
The host star, Kepler-37, is about 210 light-years from Earth in the constellation Lyra. All three planets orbit the star at less than the distance Mercury is to the Sun, suggesting they are very hot, inhospitable worlds. Kepler-37b orbits every 13 days at less than one-third Mercury’s distance from the Sun. The estimated surface temperature of this smoldering planet, at more than 800 degrees Fahrenheit (700 Kelvin), would be hot enough to melt the zinc in a penny. Kepler-37c and Kepler-37d, orbit every 21 days and 40 days, respectively.
The size of the star must be known in order to measure the planet’s size accurately. To learn more about the properties of the star Kepler-37, scientists examined sound waves generated by the boiling motion beneath the surface of the star.
“The technique for stellar seismology is analogous to how geologists use seismic waves generated by earthquakes to probe the interior structure of Earth,” said Travis Metcalfe, who is part of the Kepler Asteroseismic Science Consortium.
The sound waves travel into the star and bring information back up to the surface. The waves cause oscillations that Kepler observes as a rapid flickering of the star’s brightness. The barely discernible, high-frequency oscillations in the brightness of small stars are the most difficult to measure. This is why most objects previously subjected to asteroseismic analysis are larger than the Sun.
“Studying these oscillations been done for a long time with our own Sun,” Metcalfe told Universe Today, “but the Kepler mission expanded that to hundreds of Sun-like stars. Kepler-37 is the coolest star, as well as the smallest star that has been measured with asterosiesmology.”
Kepler-37 has a radius just three-quarters of the Sun. Metcalfe said the radius of the star is known to 3 percent accuracy, which translates to exceptional accuracy in the planet’s size.
Metcalfe launched a non-profit organization to help raise research funds for the Kepler Asteroseismic Science Consortium. The Pale Blue Dot Project allows people to adopt a star to support asteroseismology, since there is no NASA funding for asteroseismology.
“Much of the expertise for this exists in Europe and not in the US, so as a cost saving measure NASA outsourced this particular research for the Kepler mission,” said Metcalfe, “and NASA can’t fund researchers in other countries.”
The Kepler spacecraft carries a photometer, or light meter, to measure changes in the brightness of the stars it is focusing on in the Cygnus region in the sky.
Metcalfe said this discovery took a long time to verify, as the signature of this very small exoplanet was hard to confirm, to make sure the signature wasn’t coming from other sources such as an eclipsing binary star.
Kawaler said Kepler is sending astronomers photometry data that’s “probably the best we’ll see in our lifetimes,” he said, adding that this latest discovery shows “we have a proven technology for finding small planets around other stars.”
“We uncovered a planet smaller than any in our solar system orbiting one of the few stars that is both bright and quiet, where signal detection was possible,” said Thomas Barclay, lead author of Nature paper. “This discovery shows close-in planets can be smaller, as well as much larger, than planets orbiting our sun.”
And are there more small planets like this out there, just waiting to be found?
As the team wrote in their paper, “While a sample of only one planet is too small to use for determination of occurrence rates it does lend weight to the belief that planet occurrence increases exponentially with decreasing planet size.”
Asteroseismology is a relatively new field in astronomy. This branch uses sound waves in stars to explore their nature in the same way seismologists on Earth have used waves induced by tectonic activity to probe the interior of our planet. These waves aren’t heard directly, but as they strike the surface they can cause it to undulate, shifting the spectral lines this way and that, or compress the outer layers causing them to brighten and fade which can be detected with photometry. By studying these variations, astronomers have begun peering into stars. This much is generally known, but some of the specific tricks aren’t often brought up when discussing the topic. So here’s five things you can do with asteroseismology you may not have known about!
1. Determine the Age of a Star
From high school science you should know sound will travel through a medium at a characteristic speed for a given temperature and pressure. This information tells you something about the chemical composition of the star. This is a fantastic thing since astronomers can then check that against predictions made by stellar models. But astronomers can also take that one step further. Since the core of a star slowly converts hydrogen to helium over its lifetime, that composition will change. How much it has changed from its original composition towards the point where there’s no longer enough hydrogen to support fusion, tells you how far through the main sequence lifetime a star is. Since we know the age of the solar system very well from meteorites, astronomers have calibrated this technique and begun using it on other stars like α Centauri. Spectroscopically, this star is expected to be nearly identical to the Sun; it has very similar spectral type and chemical composition. Yet a 2005 study using this technique pinned α Cen as 6.7 ± 0.5 billion years which is about one and a half billion years older than the Sun. Obviously, this still has a rather large uncertainty to it (nearly 10%), but the technique is still new and will certainly be refined in the future.
And if that wasn’t cool enough by itself, astronomers are now beginning to use this technique on stars with known planets to get a better understanding of the planets! This can be important in many cases since planets will initially glow more brightly in younger systems since they still retain heat from their formation and this amount of extra light could confuse astronomers on just how might light is being reflected leading to inaccurate estimates of other properties like size or reflectivity.
2. Determine Internal Rotation
We already know that stars rotation is a bit funny. They rotate faster at their equator than at their poles, a phenomenon known as differential rotation. But stars are also expected to have differences in rotation as you get deeper. For stars like the Sun, this effect is related to a difference in energy transport mechanisms: radiative, where energy is conducted by a flow of photons in the deep interior, to convective, where energy is carried by bulk flow of matter, creating the boiling motion we see on the surface. At this boundary, the physical parameters of the system change and the material will flow differentially. This boundary is known as the tachocline. Within the Sun, we’ve known it’s there, but using asteroseismology (which, when used on the Sun is known as helioseismology), astronomers actually pinned it down. It’s 72% the way out from the core.
3. Find Planets
Until very recently, the most reliable way to find planets has been to look for the spectroscopic wiggle as the planets tug the star around. This technique sounds very straightforward, and it can be, unless the star has a lot of wiggle of its own due to the effects that make asteroseismology possible. Those effects can easily be much larger than those created by planets. So if you want to find planets lost in the forest of noise, you’d best understand the effects caused by the pulsating stellar surface. After astronomers cancelled out those effects on V391 Pegasi, they discovered a planet. And what a weird one it was. This planet is orbiting a sub-dwarf star, which is the helium core of a post-main sequence star which has ejected its hydrogen envelope. Of course, this occurs during the red giant phase when the star should have swollen up to engulf the gas giant planet in orbit. But apparently the planet survived, or somehow came along later.
4. Find Buried Sunspots
Turning to recent news, helioseismology recently found some sunspots. This wouldn’t be a big deal. Anyone with a properly filtered telescope can find them. Except these ones were buried some 60,000 km beneath the Sun’s surface. By using the seismic data, astronomers found an overdense region beneath the surface. This region was caused, just as sunspots are, by a tangle in the magnetic field keeping the material in place. As it rose to the surface, it became a sunspot. Here’s the vid:
5. Make “Music”
Because many of the events that create the soundwaves in stars are periodic, they are rhythmic in nature. This has prompted many explorations into using these naturally created beats to make music. A direct example is this one which simply assigns tones to the modes of pulsation. The site also notes that the beat created by one of the stars, has been used as a base for club music in Belgium. This has also been done for longer “symphonies” by Zoltan Kollath.