Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have identified an unprecedented collection of pulsating red giant stars all across the sky. These stars, whose rhythms arise from internal sound waves, provide the opening chords of a symphonic exploration of our galactic neighborhood.
TESS primarily hunts for worlds beyond our solar
system, also known as exoplanets. But its sensitive measurements of stellar
brightness make TESS ideal for studying stellar oscillations, an area of
research called asteroseismology.
“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,” said Marc Hon, a NASA Hubble
Fellow at the University of Hawaii in
Honolulu. “What’s really unparalleled here is that TESS’s broad coverage allows
us to make these measurements uniformly across almost the entire sky.”
This visualization shows
the new sample of oscillating red giant stars (colored dots) discovered by
NASA’s Transiting Exoplanet Survey Satellite. The colors map to each
24-by-96-degree swath of the sky observed during the mission's first two years.
The view then changes to show the positions of these stars within our galaxy,
based on distances determined by ESA’s (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 the Sun.
Credits: Credit: Kristin
Riebe, Leibniz Institute for Astrophysics Potsdam
Download high-resolution video and images from NASA’s
Scientific Visualization Studio
Hon presented the research during the second TESS Science Conference, an event supported by
the Massachusetts Institute of
Technology in Cambridge – held virtually from
Aug. 2 to 6 – where scientists discuss all aspects of the mission. The
Astrophysical Journal has accepted a paper describing the findings, led by Hon.
Sound waves traveling through any object – a guitar string, an organ pipe,
or the interiors of Earth and the Sun – can reflect and interact, reinforcing
some waves and canceling out others. 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.
Oscillations in the Sun were first observed in the 1960s.
Solar-like oscillations were detected in thousands of stars by the
French-led Convection, Rotation
and planetary Transits (CoRoT) space telescope, which
operated from 2006 to 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.
“With a sample this large, giants that might occur only 1% of the time
become pretty common,” said co-author Jamie Tayar, a Hubble Fellow at the University
of Hawaii. “Now we can start thinking about finding even rarer examples.”
The physical differences between a cello and a violin produce their
distinctive voices. Similarly, the stellar oscillations astronomers observe
depend on each star’s interior structure, mass, and size. This means
asteroseismology can help determine fundamental properties for large numbers of
stars with accuracies not achievable in any other way.
Listen to the rhythms of
three red giants in the constellation Draco, as determined by brightness
measurements from NASA’s Transiting Exoplanet Survey Satellite. To produce
audible tones, astronomers multiplied the oscillation frequencies of the stars
by 3 million times. It’s clear that larger stars produce longer, deeper
pulsations than smaller ones.
Credit: NASA/MIT/TESS
and Ethan Kruse (USRA), M. Hon et al., 2021
When stars similar in mass to the Sun evolve into red giants, the
penultimate phase of their stellar lives, their outer layers expand by 10 or
more times. These vast gaseous envelopes pulsate with longer periods and larger
amplitudes, which means their oscillations can be observed in fainter and more
numerous stars.
TESS monitors large swaths of the sky for about a month at a time using its
four cameras. During its two-year primary
mission, TESS covered about 75% of the sky, each camera
capturing a full image measuring 24-by-24 degrees every 30 minutes. In
mid-2020, the cameras began collecting these images at an even faster pace,
every 10 minutes.
NASA’s Transiting
Exoplanet Survey Satellite (TESS) imaged about 75% of the sky during its
two-year-long primary mission. This plot dissolves between the TESS sky map and
a “mass map” constructed by combining TESS measurements of 158,000 oscillating
red giant stars with their distances, established by ESA’s (the European Space
Agency’s) Gaia mission. The prominent band in both images is the Milky Way,
which marks the central plane of our galaxy. In the mass map, green, yellow,
orange, and red show where giant stars average more than 1.4 times the Sun’s
mass. Such stars evolve faster than the Sun, becoming giants at younger ages.
The close correspondence of higher-mass giants with the plane of the Milky Way,
which contains our galaxy's spiral arms, demonstrates that it contains many
young stars.
Credit: NASA/MIT/TESS
and Ethan Kruse (USRA), M. Hon et al., 2021
The images were used to develop light curves – graphs of changing
brightness – for nearly 24 million stars over 27 days, the length of time TESS
stares at each swath of the sky. To sift through this immense accumulation of
measurements, 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.
To train the system, the team used Kepler
light curves 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.
“Our map demonstrates for the first time empirically
that this is indeed the case across nearly the whole sky,” said co-author
Daniel Huber, an assistant professor for astronomy at the University of Hawaii.
“With the help of Gaia, TESS has now given us tickets to a red giant concert in
the sky.”
TESS is a NASA Astrophysics Explorer mission led and
operated by MIT in Cambridge, Massachusetts, and managed by NASA's Goddard
Space Flight Center. Additional partners include Northrop Grumman, based in
Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon
Valley; the Center for Astrophysics | Harvard & Smithsonian in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science
Institute in Baltimore. More than a dozen universities, research institutes,
and observatories worldwide are participants in the mission.
Banner: Red giant stars near and far sweep across the sky in this
illustration. Measurements from NASA’s Transiting Exoplanet Survey Satellite
have identified more than 158,000 pulsating red giants across nearly the entire
sky. Such discoveries hold great potential for exploring the detailed structure
of our home galaxy. Credit: NASA’s Goddard Space Flight Center/Chris Smith
(KBRwyle)
By Francis Reddy
NASA’s Goddard Space
Flight Center, Greenbelt, Md.
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