An obsolete form of refracting telescope in which the objective lens was cut into two parts which could be moved relative to each other. Heliometers were formerly used to measure small angular separations.

heliosphere.

The study of the interior of the Sun by the analysis of its natural modes of oscillation, which are observed spectroscopically as Doppler shifts in the absorption line spectrum. See also: Global Oscillation Network Group.

The spherical volume of space extending to between 50 and 100 AU from the Sun, bounded by the zone where the solar wind merges with the interstellar medium. This boundary region is called the heliopause.

A movable flat mirror used to reflect sunlight into a fixed solar telescope. Solar telescopes are large structures with long focal lengths but they need to point only at a small part of the sky. The heliostat is controlled so as to move synchronously with the motion of the Sun across the sky. It is a simple device but produces an image that slowly rotates during the course of a day, so the more complex coelostat is sometimes preferred.

An explosive event in the interior of a low-mass star less than two solar masses marking the start of helium burning when all the hydrogen available for nuclear fusion in the core has been exhausted. See also: stellar evolution.

The problem of explaining why the observed abundance of helium in the universe is about 25 per cent by mass. This is part of the much larger problem of accounting for the observed distribution of all the elements heavier than hydrogen. The amount of helium is much too large for it all to have been synthesized in stars, although that is where all of the heavier elements were probably made. The problem was solved by Gamow in 1946, who proposed a hot Big Bang model of the universe in which helium nuclei are manufactured at the start of the radiation era.

A B star in the spectrum of which the helium lines are abnormally strong.

A popular name for the spiral galaxy NGC 2685 in the constellation Leo.

A large, ring-shaped planetary nebula in the constellation Aquarius. Its apparent diameter is a quarter of a degree half the size of the full Moon and, at a distance of about 500 light years, it is the nearest planetary nebula.

An almost circular impact basin on the surface of Mars. Hellas Planitia is 1,800 kilometres 1,100 miles in diameter and is a feature that has long been recognized and mapped, being conspicuous by its colour, which is lighter than surrounding areas. It was formerly known simply as Hellas.

The fraction of incident light scattered by a surface as a function of the angle of incidence. See also: albedo.

A catalogue of stellar spectra, compiled at Harvard College Observatory. The work was made possible through funds donated by the widow of the pioneering astrophysicist Henry Draper 1837-82, and the catalogue was named as a memorial to him. Under the direction of Edward C. Pickering 1846-1919, Annie Jump Cannon 1863-1941 classified most of the 225,300 stars in the nine-volume catalogue between 1911 and 1915, though the first volume was not ready for publication until 1918 and the ninth did not appear until 1924. The Harvard spectral classification system adopted the sequence of classes still in use today - O, B, A, F, G, K and M. This apparently random order resulted from earlier work in which classes were originally named in alphabetical order. See also: spectral type.

One of a number of peculiar nebulous objects associated with newly forming stars. The first three were discovered on images of the nebula NGC 1999 in Orion in 1946/7 by the American astronomer George Herbig and the Mexican Guillamero Haro. Many more similar objects have since been identified. Herbig-Haro objects are thought to result from the interaction between a strong bipolar outflow from a protostar and the interstellar gas, which is heated and compressed. They are typically between 500 and 4,000 AU in size and have masses in the range 0.5-30 Earth masses, making them among the least massive objects to have been detected outside the solar system. Their velocities are high and, in many cases, their motion can be traced back to T Tauri stars or infrared-emitting objects, suggesting that they have been ejected by young stars. Hubble’s Variable Nebula is an example of an HH object.

A large constellation of the northern sky, included by Ptolemy in his list of 48 c. AD 140. It is named after the hero of classical mythology. There are no first magnitude stars. The brightest globular cluster in the northern hemisphere, M13, lies in Hercules.