Astronomy Dictionary

Browse through Sky at Night Magazine’s comprehensive astronomical dictionary, with over 300 entries.

H-alpha light

Atoms emit radiation at certain wavelengths based on how the electrons move between certain fixed energies. If an electron jumps to a specific energy level within a hydrogen atom it will emit radiation of a specific wavelength


A class of subatomic particle that includes mesons and baryons. A hadron is defined as a particle that interacts via the strong force.

Heliacal rising

The date when a star first becomes visible in the dawn sky, rising just before the Sun.

Heliocentric model

This literally means ‘Sun-centred’ and is the accepted model for the Solar System that puts the Sun at the centre, with the planets orbiting around it.


Point where the pressure of local interstellar gas and dust overwhelms the solar wind. This stops it expanding into space.


The point where the pressure of the local interstellar medium overwhelms that of the solar wind, stopping it from expanding any further out into space.


The study of the internal structure of the Sun, performed by examining how pressure waves propagate within it.


The study of the internal structure of the Sun, which involves examining how pressure waves propagate within it.


A ‘bubble’ that surrounds our Solar System created by the influence of the Sun’s solar wind and magnetic field interacting with interstellar space.

Herbig-Haro object

These are hot bright patches of gas in space where the impact of fast moving gas (probably from young stellar jets) on the surrounding medium creates a glowing bow shock.

HII star forming region

A large nebulous region of hydrogen gas that is being excited and ionised by the strong ultraviolet radiation from hot newly formed stars.

Histogram display

This is the graph that appears on a camera’s LCD screen plotting pixel brightness along the horizontal axis, and the number of pixels at each brightness value along the vertical axis.


A diagram formulated by astronomer Edwin Hubble that is used to classify galaxies. It is the shape of a tuning fork, beginning at one end with elliptical galaxies and then splitting into spirals and barred spirals.

Hydrogen-alpha (Sun)

A spectral line which has a wavelength of 656.3nm. It is this wavelength of light, from the Sun, which is observed with certain specialist solar telescopes.

Hydrogen-alpha filter

A type of optical filter that’s used for observing the Sun. It blocks all wavelengths of light except for a very narrow portion at 656.3nm.

Hydrogen-beta filter

This filters out all light except that of the hydrogen-beta line that has a wavelength of 486.5nm. It is used to observe very faint objects, such as the Horse Head Nebula and California Nebula, which wouldn’t normally be seen without one.


Unstable massive stars who are constantly loosing mass and who have tremendous luminosities of between 100,000 and a million times that of our Sun.

Hypervelocity stars

These stars are thought to be the result of a violent close encounter of a binary star with a supermassive black hole. As one of the stars is consumed by the black hole, the other is violently thrown away. It’s sent travelling through the galaxy at phenomenal speed.

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