Symphony of stars: The science of stellar sound waves in space

Sound waves in stars.


Astronomers suggest that the stars in space play continuous sounds like a concert. The biggest stars make the lowest, deepest sounds, like tubas and double basses. Small stars have high-pitched voices, like celestial flutes.

According to astronomers, understanding these sounds could help them to make a huge revolution in astronomy. Using telescopes, they can listen to different sound waves in space and can figure out what stars are made of, how old they are, how big they are, and how they contribute to the evolution of our Milky Way galaxy as a whole.

The most commonly used technique for this is referred to as asteroseismology. It can help scientists determine the functions of stars by analyzing vibrations or “starquakes”.

Sound waves of planets in space travel through a star’s inside directly from temperature changes. They start in the star’s convection zone, which is the upper 30 percent of a star’s volume if it is like the Sun.

Hot gas moves upward to the star’s surface, where it chills and falls — however, significantly more violently and turbulently than in your kitchen. Convection, this development of heat rising and falling, makes waves bounce around in the star in various ways.

Convection-driven waves make the entire star extend and contract, ringing the star like a bell. Such huge waves spread on the double that the general stellar surface bumps around like Jell-O, yet so quietly that the movement would not be visible to the eye.

You can’t hear them, but sound waves propagate through stars all the time in thousands of different ways. This artist’s concept shows how a few individual waves travel through a hypothetical star. Some waves propagate only around top layer of a star, while others travel right through the center. The waves cause the star to vibrate and brighten in ways that are too subtle to see with the eye, but which can be detected with telescopes such as NASA’s Kepler Space Telescope. Scientists can determine a star’s interior structure, which gives information about size, composition and age, by detecting these vibrations. Credit: NASA/JPL-Caltech

For example, in a close-up view of the sun, the effects of waves are localized areas of brightening and dimming. These are distinct from the dark spots we know as sunspots on our Sun. Sunspots form in areas where the Sun’s magnetic field lines weaken the amount of energy brought to the surface and represent temporarily cooler regions on the star’s surface.

On the other hand, some waves ripple around the entire circumference of the star, while others dart right through the star’s core. The bigger the star, the longer it takes sound waves to travel in its interior.

In 1977, scientists tracked the Sun’s waves and realized that the star’s convection zone ran much deeper than predicted. From that point forward, helioseismology has picked up a vastly improved comprehension of the Sun’s pivot and inside structure.

Jennifer van Saders, an astronomer at the University of Hawaii, said, “The actual changes in a star’s overall brightness caused by sound waves are unimaginably small: about four parts in a million. Those subtle variations are the equivalent of turning your cell phone flashlight on and off in a room full of very bright spotlights, such as the ones that form the famous light beam at the Luxor Hotel and Casino in Las Vegas.”

One method, Doppler effect, can be used to pick out stellar vibrations. However scientists choose to use Kepler and Tess altogether. Both Kepler and TESS are powerful and sensitive tools for detecting stellar vibrations.

For this revolution, NASA’s Transiting Exoplanet Survey Satellite (TESS), which launched in April 2018, may observe sound waves in up to one million red giants — the massive evolved stars that represent what our Sun will look like in about 5 billion years.

William Chaplin, professor of astrophysics at the University of Birmingham, United Kingdom, said, “We are using seismology to provide an exquisite characterization of the host stars — and hence the planets — we’ve discovered.”

Subtle changes in brightness are hard to discern with ground-based telescopes because Earth’s atmosphere and weather activity get in the way and because daylight interrupts observations. For continuous listening to the stellar orchestra, astronomers needed space telescopes. The Convection, Rotation, and Planetary Transits (CoRoT) satellite, launched in 2006 and led by the French space agency (CNES), and Kepler, launched in 2009, were the pioneers in exploring helioseismology in an expanse of stars in greater detail than ever before.

Sanders said, “Kepler stares at these points of light, and it watches them twinkle — not twinkle as they do here on Earth because that’s the atmosphere causing them to twinkle — but twinkle because they’re actually changing brightness.”

“Kepler gave us — the ability to really test what’s going on in the interiors for stars that aren’t just the Sun.”

While determining stellar sound waves by using Asteroseismology, scientists were surprised to know that in one type of red giant, the core rotates rapidly while the surface rotates slowly.

Tom Barclay, a research scientist with the TESS mission at NASA’s Goddard Space Flight Center, said, “Kepler really started to tell that story — TESS will do the census.”

Dan Huber, an astronomer at the University of Hawaii, Honolulu, said, “Scientists are still exploring how the Sun’s vibrations compare to stars with different masses and ages, and how that “ringing” will change as the Sun ages into a red giant. This is one of the things we are trying to figure out, and that Kepler and TESS and other missions can help us understand.”

However, there are many other important aspects to know about the planet, including size, age, and whether or not it could support life. According to scientists, this information can be gathered only when having information about its host star.

When a planet passes in front of its star, Kepler detects this “transit” by measuring a sharp dip in the star’s brightness as the planet blocks some of its light (very different from the ripple-like quake signatures).

The amount of dimming of the star’s light during transit is related to the planet’s size relative to the star’s size. So, to calculate the planet’s diameter, scientists need the star’s diameter — something that they can determine using asteroseismology.

Similarly, while radial velocity allows scientists to calculate a planet’s mass relative to its star’s mass, scientists must calculate the star’s mass to “weigh” the planet. Asteroseismology is a way to determine the mass.

Chaplin said, “Getting precise estimates of ages is incredibly difficult, but this is something that asteroseismology is really well suited to. The ticking heartbeat of the star allows us to get precise measurements.”

Steve Howell, head of the space and astrobiology division at NASA’s Ames Research Center in California’s Silicon Valley, said, “Much like digging through the archaeological site of an old city, you can look at what happened in each of those ‘rooms’ in our galaxy.”

“By calculating the ages of stars and determining how long they have been in the red giant phase, and knowing their distances, scientists will get a fuller picture of how the stars of the galaxy came together and how it is evolving.”


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