A type of very young star in an early phase of evolution where contraction is still taking place. The prototype, T Tauri, is an irregular variable within a dark dust cloud in the constellation Taurus. All T Tauri stars vary irregularly. Their absorption line spectra show that their surface temperatures are in the range 3,500-7,000 K. They are found in dense interstellar clouds, usually alongside young, main-sequence O and B stars, but the T Tauri stars are far more luminous than main-sequence stars of the same temperature. Strong emission lines also feature in the spectrum; these come from a low-density envelope of gas around the stars. Large numbers of T Tauri stars have been discovered, notably in the Rho Ophiuchi r Oph dust cloud, through the strong infrared radiation they emit. Loose groupings of T Tauri stars are known as T associations. Strong bipolar outflows twin-lobed jets stream out from T Tauri stars at speeds of several hundred kilometres per second. Where the outflow compresses and heats the interstellar gas, the resulting luminous nebulae are observed as Herbig-Haro objects.