In this article, Dr Phil Sutton, Lecturer in Astrophysics, University of Lincoln, UK, discusses how the Gaia astrometry mission creates our first full "map of the neighbourhood". This data has immediate supporting value for all astronomical investigation but also has both cosmological and astronautical significance as we begin to contemplate missions to the near stars.
Over 1 billion stars have now been surveyed in extraordinary detail by the Gaia spacecraft since it was launched in late 2013. For the last 3.5 years Gaia has been in a large Lissajous orbit about the L2 Lagrange point of the Sun-Earth system. Spacecraft are often placed into these types of orbits as they require minimal propulsion to stay reasonably stationary relative to Earth for long periods of time.
Gaia’s main aim is to measure the precise position of stars, astrometry, using triangulation between measurements of the stars apparent position as seen from different places in its orbit around the sun. This will give a more detailed 3-dimensional map of the Milky Way that is complemented by spectroscopic measurements of the same stars. Here, along with the precise position of stars, the Doppler Effect is used to find relative velocities of stars by a shift in wavelength of their observed light. The result is a detailed kinematic map of stars in our local neighbourhood, which is important; as we still do not fully understand why stars move the way they do in galaxies.
Observations dating back to the 1930’s suggested that the rotation curves of galaxies do not fit with standard Keplerian orbits. For planets and asteroids in the solar system their orbits are generally well understood and follow Keplerian laws. However, at a galactic scale it was found that galaxies rotated far too fast for the matter we could see. This led to the concept of dark matter, which would give the increase of orbital velocities of stars as that observed. Along with the increased orbital velocities, stars also exhibit some randomness to their motion around the centre of galaxies. All of this points to additional gravitational perturbations which may be significant enough to influence interstellar journeys.
One way we can show the distribution of this elusive dark matter is to study how objects behave in the presence of its gravity, since dark matter is thought to only interact gravitationally. Posti & Helmi (2018) looked at the dynamics of 75 globular clusters from Gaia to map out the dark matter in the Milkyway. The dynamics of these globular clusters, which generally orbit outside of the main galaxy, help constrain the total mass and the distribution of mass. Ultimately, they found that over 2/3 of the total mass inside a radius was dark matter.
When travelling further afield the warping of spacetime due to this dark matter will need to be considered when planning the trajectories of spacecraft.
Creating detailed maps of dark matter in our local neighbour then becomes very useful.
The main science goal of another space telescope, Kepler, was to find new planets around other stars. It succeeded with now thousands of planets discovered and confirmed. However, secondary to the main mission it also discovered many new types of variable stars, like the heartbeat star*.
Due to the nature of the Gaia mission and its measurements, other secondary science is also possible. For example, it could be something simple such as a survey of large asteroids in the Solar System. Or more esoterically the detection of certain frequencies of gravitational waves would help constrain the cosmological constant (the rate at which the universe is known to be expanding).
Gravitational waves are of particular interest to astrophysicists as they exist over a very broad range of wavelengths.
At some of the smaller scales, compact binary systems comprised of black holes or neutron stars can emit gravitational wavelengths on the order of km, while waves from the early universe in the form of a polarisation of the Cosmic Microwave Background can be on the order of Mly. Detecting different wavelengths allows us to probe different physics and astronomical objects and get a better understanding of the universe we reside in. It has been proposed that the signatures of gravitational waves are hidden in the astrometric data of stars measured by Gaia (Klioner 2018). Gravitational waves are disturbances in the curvature of spacetime and will cause the position of distant stars to oscillate slightly over time as they pass through.
A much greater understanding of our local environment will aid our far future endeavours in interstellar travel, with Gaia already delivering unprecedented detail of the nearby stellar population.
If we know the types of stars and their movement within the galaxy more precisely we can better plan our interstellar journeys.
If stars in a galaxy are found to move in a non- Keplerian way due to the gravitational perturbations from dark matter, should we make considerations in spacecraft trajectories? It is also worth noting that there is an element of randomness in the motion of stars in a galaxy that is caused when they pass close to one another. It is almost Brownian in nature which actual increases with the age of the galaxy and is in addition to any effect dark matter might have on the movement of stars. The dark matter increases the global orbital velocities, assuming it is evenly distributed, while the stellar encounters adds in a smaller element of randomness to their motion. Nonetheless, detailed maps of star types in our local environment will also guide our future expeditions.
What type of stars systems do we want to visit? Stars like our Sun with potential habitable planets, young protostars to get a glimpse how stars and planets form up close, or older stars that have moved off the main sequence to give insight into the fate of own solar system? As well as producing a spatial map of stars in our neighbourhood, Gaia data has also created a map of stellar types in the form of the H-R diagram† which might aid in planning our future interstellar missions.
* Heartbeat stars are binary stars with relatively eccentric orbits. "Heartbeat Star" Wikipedia (in German)
† "Hertzsprung–Russell diagram." Wikipedia
Klioner, S.A., 2018. Gaia-like astrometry and gravitational waves. Classical and Quantum Gravity, 35(4), p.045005.
Posti, L. and Helmi, A., 2018. Mass and shape of the Milky Way's dark matter halo with globular clusters from Gaia and Hubble. arXiv preprint arXiv:1805.01408.
About the Author
Dr Phil Sutton, Lecturer in Astrophysics, School of Mathematics and Physics, University of Lincoln, UK.
Phil graduated in Physics with Astrophysics from Nottingham Trent University in 2006. He took his PhD in Astrophysics at Loughborough University in 2015. He worked as a technician and technical tutor at Loughborough University for ten years, involved in teaching observational techniques in astronomy, physics laboratories and astrophysics.