What do earthquakes have to do with astronomy?
They both employ seismology; the scientific study of elastic waves through planetary bodies. Vibrations generated by tectonic activity resonate through the Earth, help scientists to understand its inner structure and composition. The same concept can be applied to stars by studying the waves reverberating from the interior of stellar dynamos. Precisely measuring these stellar waves presented a challenge.
Weighing less than the average person, the Microvariability & Oscillations of STars (MOST) was the first mission to deploy asteroseismology. Launched in 2003, it was Canada's first space telescope and was configured with a high-precision telescope no wider than a pie plate – just 15 cm across. Weighing 60 kg, astronomers dubbed it the "Humble Space Telescope" in reference to the much larger Hubble Space Telescope.
However, MOST was ten times more sensitive than Hubble, and studied tiny variations in a star’s brightness to levels of one part in a million, one ten-thousandth of a percent. Such high sensitivity enabled MOST to also observe the light reflected from nearby planets orbiting around the star, and analyse their atmospheres.
The great majority of stars are so far away that telescopes can only view them as a single, “unchanging” point of light. Subtle changes in brightness are hard to discern with ground-based telescopes because Earth’s atmosphere, weather, and daylight obscure observations. Accordingly, MOST was placed in a polar Sun-synchronous orbit of altitude 820 km and period 101 min, which enabled MOST to collect rapid, high-precision photometry of stars with nearly uninterrupted coverage for up to two months.
Credit: NASA
Changes in brightness are linked to slight temperature changes. Hot gases produced in the core of a star rise upward, where they cool off and fall back down. This convection process creates waves that bounce around in the star, causing the whole star to expand and contract – in effect ringing the star like a bell. The biggest stars make the lowest sounds, like a tuba, while small stars emit high-pitched sounds like a flute.
Size also affects the time it takes sound waves to travel through a given star as well as how long those waves keep bouncing through it. In stars close in size to our sun, a typical wave completes one cycle in five minutes and can last a few days. Red giants, which are dozens of times as big as the Sun, have lower-frequency waves that can propagate for weeks to months.
MOST was the first satellite designed to specifically conduct these asteroseismic observations of stars. Astronomers used that data to calculate the density of the star and the pressure of its surface layers along with the star's temperature and mass, to determine that star's age. This information helps astronomers map the locations of young and old stars, thereby increasing their understanding of how the Milky Way galaxy formed. This is the science of galactic archaeology.
The MOST satellite. Credit: Canadian Space Agency/www.space.gc.ca
The Instrument
MOST consisted of a Rumak-Maksutov telescope, which fed two Teledyne Space Imaging CCD47-20s: one for data collection with a custom optical broadband 350-750 nm filter and integrations typically every 10-100 milliseconds, and one for attitude control with readout every 0.1 seconds. The twin cameras were identical, fully programmable, and exchangeable.
The 1024x1024, frame-transfer CCD47-20s supported the high sensitivity required for MOST to achieve its scientific objectives. MOST’s sensitivity was equivalent to a person standing one kilometre away from a streetlamp being able to notice an increase in luminosity after moving only half a millimetre closer to the light source.
The optical system on the instrument used a dichroic lens to filter certain wavelengths of light while allowing others to pass through. This separated the light into different channels for more effective analysis of the stellar light curves, and optimised diffusion over a greater number of detectors.
MOST had a continuous viewing zone (CVZ) of 54◦ wide to monitor target fields for up to 60 days without interruption. Targets brighter than V∼6 mag were observed in Fabry imaging mode, while fainter targets were observed in direct imaging mode, similar to standard CCD photometry with ground-based instruments.
How Procyon Upended 20 Years of Stellar Observations
In January and February of 2004, the MOST space telescope monitored Procyon – a star bigger than our Sun – over 32 consecutive days. According to stellar models, astronomers expected that the bright star would display oscillations. However, the MOST observations confirmed that Procyon does not oscillate.
This was a major astronomical discovery which contradicted previous observations made from Earth-based telescopes and suggested that long-held theories on the formation and ageing of the Sun and other stars needed to be reconsidered. In fact, a French team that was in the process of planning a space mission to study Procyon's oscillations in 2006 modified their project in light of MOST's discovery.
Achievements
- Observed a giant planet orbiting so close to its host star that the star was forced to synchronise its rotation with that of the planet. Normally, it is the other way around: a planet synchronises its orbit with that of its host star.
- Discovered a new type of variable star — a slowly pulsating B-type supergiant – after continuous observation of the HD 163899 star over a period of 37 days.
- Confirmed the first super-Earth exoplanet around a main-sequence star: 55 Cancri e. These observations allowed a better understanding of this exoplanet's composition.
- Discovered a planet whose atmosphere is either so clear or so hazy that it reflects only 4% of the light it receives from its parent sun. This was an interesting finding as the planet is darker than coal with a sun 400 times brighter than our Sun.
- MOST was a pathfinder for the subsequent
COROT and Kepler missions, pioneering the use of asteroseismology and revealing insights into exoplanet detection. Proposed to last one year and tasked with observing 10 stars, MOST operated for over a decade and observed more than 5,000 stars.
Works Cited
A-N Chené, and Moffat, A. F. J. (2011). Pulsations in Wolf-Rayet stars: observations with MOST. Proceedings IAU Symposium No. 272, 2010, International Astronomical Union.
doi:10.1017/S1743921311011082.
ASTROLab du parc national du Mont-Mégantic (n.d.). The MOST Space Telescope. Canada Under the Stars.
http://astro-canada.ca/le_telescope_spatial_most-the_most_space_telescope-eng
Canadian Space Agency. (2003, July 22). MOST - A New Rising Star.
https://apc.u-paris.fr/~satorchi/media/csa/eng/07_most.asp.html
Government of Canada.( 2019, August 15). MOST - A tiny satellite probes the mysteries of the universe. Canada Space Agency.
https://www.asc-csa.gc.ca/eng/satellites/most/
NASA (2018, July 30). Symphony of stars: The science of stellar sound waves.
https://science.nasa.gov/universe/exoplanets/symphony-of-stars-the-science-of-stellar-sound-waves/
OpenAI. (2025, July 22). GPT-4o mini. [Does the MOST instrument on the MOST satellite have a semi-transparent mirror or dichroic mirror?].
https://duck.ai
Saint Mary’s University. (2010, January). MOST (Microvariability and Oscillations of STars). Department of Astronomy and Physics.
https://www.ap.smu.ca/~guenther/MOST/MOST.html
Science World. (2016, December 15). MOST—Canada’s Space Telescope.
https://www.scienceworld.ca/stories/most-canadas-space-telescope/
SpaceRef. (2008, July 3). MOST—Canada’s Space Telescope. Space News.
https://spacenews.com/canadas-most-space-telescope-celebrates-its-5th-anniversary-in-orbit/
The Institute for Space and Material Science. (1996, July 31). MOST (Microvariability and Oscillations of STars) - 1.13.1 The Instrument.
https://www.astro.utoronto.ca/~rucinski/MOST_proposal_1997.pdf
