When stars are born, they are temporarily surrounded by a protoplanetary disk and have not yet ignited their thermonuclear reactions. These proto-stars are called T-Tauri stars. To study protoplanetary disks, Hubble zoomed in on a triple system with a proto-star, HP Tau.
As usual for years, and despite the James Webb Space Telescope being in service since 2022 now, NASA periodically reveals images taken by the HubbleHubble Space Telescope mainly for their aesthetic beauty, but they can also be discussed for the astrophysics they illustrate.
The latest one concerns a reflection nebula located about 550 light-years away in the Taurus constellationTaurus constellation. It clearly shows a triple system of variable starsvariable stars HPHP Tau, HP Tau G2, and HP Tau G3. These are T-Tauri stars that Hubble has allowed to study to examine their accretion diskaccretion disk. But before contemplating this image, let’s take the opportunity to talk a bit about T-Tauri stars.
It all started regarding them with the discovery of another nebula in the Taurus constellation by British astronomer John Russell Hind in 1852. Called NGCNGC 1555, it is about 470 light-years from the SunSun in the Milky Way. In modern images, it is clear that it is a reflection nebula illuminated by an orange-colored starstar. This star in Hind’s nebula is actually named T-Tauri and serves as a reference to define this type of star. Infrared studies have revealed that it is actually a triple system.
T-Tauri Stars, Proto-Stars
WeWe now know that these are proto-stars, meaning they are young, at most a few million years old, and they are not yet on the main sequence where stars like the Sun are located.
We also now know that stars form in dense, cold, and turbulent molecular clouds that are in conditions conducive to gravitational collapse. These clouds fragment, giving rise to small rotating nebulae. At the core of these nebulae, due to the rotation opposing the gravitational contraction, an accretion disk forms where planets are born around a proto-star heated by compression resulting from the contraction. However, the central temperature is not yet high enough for the thermonuclear reactions to ignite and turn it into a new star on the main sequence.
The material of the contracting nebula falls first onto the protoplanetary disk before reaching the central proto-star, hence the name accretion disk. During this phase, the proto-star is unstable and has large sunspots, causing the star to experience periodic and random fluctuations in brightness. NASA’s statement specifies that these “random variations may be due to the chaotic nature of a developing young star, such as instabilities in the dust and gas accretion disk around the star, material falling onto the star and being consumed, and eruptions on the star’s surface. The periodic changes may be due to the rotation of giant sunspots that appear and disappear.”
Above is the image of the triple star system T-Tauri observed by Hubble, featuring the proto-star HP Tau with a mass comparable to that of the Sun.