Gaia: science essays

34. Perspective acceleration

For a fixed space velocity, a star's proper motion varies inversely with its distance. However, the tangential velocity changes due to the varying angle between the line-of-sight and its space velocity. The two effects result in a changing proper motion and, for a few nearby stars, an apparent acceleration of the star's motion on the sky.

23 August 2021

33. Nearby stars

Surveys of our solar neighbourhood provide the foundations for defining our Galaxy's stellar luminosity and velocity distributions, the occurrence of binary stars, and the nature of many other types of objects, including brown dwarfs, white dwarfs, and exoplanets. Gaia is revolutionising this knowledge [gif courtesy: galaxymap.org].

16 August 2021

32. Aberration and Galactic rotation

As the Earth orbits the Sun, star positions change due to the combination of the Earth's velocity, and the speed of light arriving from the star. The effect is already at levels imperceptible to the human eye. But a million times more sensitive still, Gaia has recently detected a similar effect due to the Sun's 250 million year orbit around the Galaxy.

9 August 2021

31. The motion of dwarfs spheroidals

Dwarf spheroidal galaxies are small, low-luminosity galaxies comprising old stellar populations. By the late 1990s, their rarity seemed to be in conflict with the 'Lambda cold dark matter' cosmological model, which predicted that massive galaxies like the Milky Way should be surrounded by many dark matter dominated halos.

2 August 2021

30. The motion of globular clusters

The space motions of globular clusters and dwarf spheroidal galaxies depend on our Galaxy's gravitational potential, and therefore its mass distribution. Together with knowledge of their chemistry and ages, these provide strong constraints on theories of formation of our Galaxy, including when and how the halo and disk actually formed.

26 July 2021

29. White dwarf surveys

White dwarfs are of great importance for theories of star formation and evolution, of degenerate matter at extremely high density, for distance scale determination, and for understanding planet survival beyond a star's main-sequence lifetime. Pre-Gaia, known numbers were around 10,000. Gaia is in the process of characterising some 100,000.

19 July 2021

28. Solar activity - and dark matter?

The rotation of the Sun is central to the two main hypotheses which try to explain the 11-year solar activity cycle: attributable either to a turbulent dynamo operating in or below the Sun's convection envelope, or to a large-scale oscillations of a fossil magnetic field in its radiative core. Some particularly speculative ideas could be tested with Gaia.

12 July 2021

27. The celestial reference frame

An absolute reference system is as central to Gaia as it was for Hipparcos, and the accuracy for the link correspondingly more demanding. But Gaia's limiting magnitude, at 20–21 mag, allows very large numbers of quasars to be observed by the instrument itself. And this means that the problem can be tackled much more directly.

5 July 2021

26. Polar motion

The Earth's rotation axis moves, on timescales ranging from years to millennia, as a result of various external and internal forces. The wobbling motion of the Earth has been characterised, back to about 1900, by re-examining star measurements using the Hipparcos star positions. Further insight could be possible with Gaia.

28 June 2021

25. The origin of Oumuamua

The astronomy world was captivated by the announcement of an interstellar traveller in October 2017. Named Oumuamua, from the Hawaaian for 'scout', it was discovered crossing the solar system some 40 days after its closest approach to the Sun. Gaia is in the process of pin-pointing its progenitor star system.

21 June 2021