A symphony of giant red stars across the sky

Symphony of red giant stars

NASA released the above video on August 4, 2021. It plays the rhythms of three giant red stars towards the constellation Draco the Dragon. These stars are just a few of the more than 158,000 pulsating red giants found via the luminosity measurements of 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 and deeper pulses than smaller ones. And they said their new study provides insight into:

… An unprecedented collection of red giant stars pulsating across the sky. These star rhythms are born from internal sound waves …

TESS stands for Satellite for the study of exoplanets in transit. It is a space telescope that mainly hunts exoplanets. But its sensitive measurements of stellar luminosity make TESS ideal for studying stellar oscillations, according to a statement from NASA. This area of ​​research is not new. It is a decades-old technique for studying certain stars. Called asteroseismology, it stems from a study of the oscillations of our own sun in the 1960s. Astronomers began to comment that their work showed that certain stars “sounded like a bell.”

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

Our initial result, using stellar measurements over the first two years of TESS, shows that we can determine the masses and sizes of these oscillating giants with an accuracy that will only improve as TESS continues. What is truly unprecedented here is that TESS’s wide coverage allows us to perform these measurements evenly across almost any sky.

Hon presented the research at the 2nd TESS Scientific Conference, which was held virtually August 2-6. Astrophysics Journal accepted a document describing the results.

Some stars ring like bells

The scientists’ statement explained:

Sound waves passing through any object – a guitar string, an organ pipe, or the interior of the Earth and the sun – can reflect and interact. Some waves get stronger, others cancel each other out. This can result in an orderly movement called standing waves, which create the sounds of musical instruments.

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

The France-led convection, rotation and planetary transit (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 of 2009 to 2018, found tens of thousands of giant oscillating stars. Now TESS extends this number an additional 10 times.

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

With a sample of this size, giants that might occur only 1% of the time become quite common. Now we can start to think about finding even rarer examples.

Computer training

As mentioned above, TESS is designed for planet hunting. It is not designed to find giant red ringing stars. For their study, astronomers had to sift through what NASA called “a huge 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 models without explicitly programming them, they said.

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

Then, the team found the distances for each giant using data from the ESA (European Space Agency) Gaia mission and plotted the masses of these stars in the sky. Stars more massive than the sun evolve faster, becoming giants at a younger age. 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 bright stripe of the Milky Way in the sky. nocturnal.

A giant red concert

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

Our map shows for the first time empirically that this is indeed the case over almost the entire sky. With Gaia’s help, TESS has now given us tickets to a Red Giant in the Sky concert.

The video below is a visualization showing the new sample of swaying red giant stars (colored dots). The colors correspond to each 24 by 96 degree sky strip observed during the first two years of the TESS mission. The view then changes to show the positions of these stars within our galaxy, based on the 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: Planet researcher TESS and space observatory Gaia have given us tickets to a symphony of giant red stars, which is now being played in our Milky Way galaxy.

By NASA