An all-sky red giant star symphony

Red giant star symphony

NASA released the video above on August 4, 2021. It plays the rhythms of three red giant stars in the direction of the constellation Draco the Dragon. These stars are just a few of the more than 158,000 pulsating red giants found via brightness measurements from NASA’s TESS planet-hunter. ESA’s Gaia space observatory also provided vital data. To produce audible tones in the video, astronomers multiplied the oscillation frequencies of the stars by 3 million times. They said they could see that larger stars produce longer, deeper pulsations than smaller ones. And they said their new study represents a glimpse of:

… an unprecedented collection of pulsating red giant stars all across the sky. These stars rhythms arise from internal sound waves …

TESS stands for Transiting Exoplanet Survey Satellite. It’s a space telescope and primarily hunts exoplanets. But its sensitive measurements of stellar brightness make TESS ideal for studying stellar oscillations, a NASA statement said. This area of research isn’t new. It’s a decades-old technique for studying certain stars. Called asteroseismology, it stems from a study of oscillations in our own sun made in the 1960s. Astronomers began commenting that their work showed some stars “ring like a bell.”

But the scale of this new study is impressive. The lead author of the study, astronomer Marc Hon at the University of Hawaii in Honolulu, said:

Our initial result, using stellar measurements across TESS’s first two years, shows that we can determine the masses and sizes of these oscillating giants with precision that will only improve as TESS goes on. What’s really unparalleled here is that TESS’s broad coverage allows us to make these measurements uniformly across almost the entire sky.

Hon presented the research during the 2nd TESS Science Conference, held virtually from August 2 to 6. The peer-reviewed Astrophysical Journal has accepted a paper describing the findings.

Red giant star symphony: 3 red spheres with mottled surfaces, smallest in the upper left, largest in the lower right.
Artist’s concept of the evolved stars known as red giants, at different distances in this illustration. Astronomers have identified more than 158,000 pulsating red giants across nearly the entire sky. These stars’ rhythms arise from internal sound waves. They provide the opening chords of what NASA called “a symphonic exploration of our galactic neighborhood.” Image via NASA’s Goddard Space Flight Center/ Chris Smith (KBRwyle).

Some stars ring like bells

The scientists’ statement explained:

Sound waves traveling through any object – a guitar string, an organ pipe, or the interiors of Earth and the sun – can reflect and interact. Some waves reinforce, and other cancel out. This can result in orderly motion called standing waves, which create the tones in musical instruments.

Just below the surfaces of stars like the sun, hot gas rises, cools, and then sinks, where it heats up again, much like a pan of boiling water on a hot stove. This motion produces waves of changing pressure – sound waves – that interact, ultimately driving stable oscillations with periods of a few minutes that produce subtle brightness changes. For the sun, these variations amount to a few parts per million. Giant stars with masses similar to the sun’s pulsate much more slowly, and the corresponding brightness changes can be hundreds of times greater.

The French-led Convection, Rotation and planetary Transits (CoRoT) space telescope detected sun-like oscillations in thousands of stars between 2006 and 2013. NASA’s Kepler and K2 missions, which surveyed the sky from 2009 to 2018, found tens of thousands of oscillating giants. Now TESS extends this number by another 10 times.

Co-author Jamie Tayar, also at University of Hawaii, commented:

With a sample this large, giants that might occur only 1% of the time become pretty common. Now we can start thinking about finding even rarer examples.

Smiling, young, dark-haired man.
The lead author of this study, astronomer Marc Teng Yen Hon, is a Hubble Fellow at the University of Hawaii. Image via STScI.

Training a computer

As mentioned above, TESS is designed for planet-hunting. It’s not designed to find ringing red giant stars. For their study, the astronomers had to sift through what NASA called “an immense accumulation of measurements” made by TESS. To do this, Hon and his colleagues taught a computer to recognize pulsating giants. The team used machine learning, a form of artificial intelligence that trains computers to make decisions based on general patterns without explicitly programming them, they said.

To train the system, the team used Kepler light curves (measurements of a star’s light over time) for more than 150,000 stars, of which some 20,000 were oscillating red giants. When the neural network finished processing all of the TESS data, it had identified a chorus of 158,505 pulsating giants.

Next, the team found distances for each giant using data from ESA’s (the European Space Agency’s) Gaia mission, and plotted the masses of these stars across the sky. Stars more massive than the sun evolve faster, becoming giants at younger ages. A fundamental prediction in galactic astronomy is that younger, higher-mass stars should lie closer to the plane of the galaxy, which is marked by the high density of stars that create the glowing band of the Milky Way in the night sky.

A red giant concert

A third co-author, Daniel Huber at the University of Hawaii, said:

Our map demonstrates for the first time empirically that this is indeed the case across nearly the whole sky. With the help of Gaia, TESS has now given us tickets to a red giant concert in the sky.

The video below is a visualization showing the new sample of oscillating red giant stars (colored dots). The colors map to each 24-by-96-degree swath of the sky observed during the TESS mission’s first two years. The view then changes to show the positions of these stars within our galaxy, based on distances determined by the European Space Agency’s Gaia mission. The scale shows distances in kiloparsecs, each equal to 3,260 light-years, and extends nearly 20,000 light-years from our sun.

Bottom line: The TESS planet-finder and Gaia space observatory have given us tickets to a red giant star symphony, now playing in our Milky Way galaxy.


August 8, 2021

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