What Did The Sun Look Like When It Was A Baby Star 4.5 Billion Years Ago? This Study Finds Some Clues

A new study on the footprints of formation found in a young star provides clues about how the Sun looked as a baby star 4.5 billion years ago.

Representational image of The Sun | Photo: NASA

New Delhi: The Sun is possibly the most massive object in our solar system, providing energy to support life on Earth. But how was it like when it formed 4.5 billion years ago? Scientists have been trying to find an answer to this, and a new paper has provided some clues. 

The research paper was recently published in the journal, Nature. Dr Catherine Espaillat, the lead author, is an Associate Professor of Astronomy at Boston University, and studies how young stars and planets form out of protoplanetary disks (disk of gas having 99% gas and 1% dust, orbiting a newly formed star). The team of researchers also included graduate student John Wendeborn and postdoctoral researcher Thanawuth Thanathibodee. NASA and the National Science Foundation funded this study.

First Observation

Espaillat and her team observed a young star about 450 million light-years away from Earth. Their observations confirmed, for the first time, that astronomers' accretion models, calculating the structure of magnetic fields and predicting the creation of 'hotspots', are accurate. Their observation corroborates the theory. 

The young star they closely studied is named GM Aur, and is situated in the Taurus-Auriga molecular cloud of the Milky Way. 

Protoplanetary disks are known by astronomers to be the source of formation of new stars. They theorised that a magnetic field connects protoplanetary disks and stars. At the focal points of the accretion process (agglomeration of solids to form larger and larger objects, and eventually planets), the hotspots are formed. The hotspots are extremely hot and dense. This happens when matter from the disks follow stellar magnetic field lines, and collide into the surface of the growing star. The theory is based on algorithms and calculations with the help of computer models. So far, there was no physical evidence.

Not Photography But Wavelengths

In a statement issued by Boston University, Espaillat says that photography of a faraway star's surface is currently impossible. However, if different parts of a star's surface emit light in different wavelengths, other types of images are possible.

For the study, the researchers took snapshots of light wavelengths emitted from GM Aur's surface every day for a month. They compiled datasets of X-Ray, ultraviolet (UV), infrared and visual light. They observed GM Aur using NASA's Hubble Space Telescope, Transiting Exoplanet Survey Satellite (TESS), Swift Observatory, and the Las Cumbres Observatory global telescope network. 

Observation That Stunned The Team

GM Aur takes a week's time to complete an entire rotation, a time during which the brighter hotspots turn away from Earth, and then turn back around to face our planet. During this period, the brightness levels are expected to soar, or become dim and weak. However, the scientists were stunned by what they observed. 

About a day before all wavelengths other than Ultra-Violet (UV) light reached their peak, UV light was at its brightest, the scientists observed. They thought at first there might have been some mistake in their observations, and hence, they cross-checked their data, only to discover an interesting fact — the hotspot is not completely uniform, and harbours an area within it which is even hotter than the rest of its regions.

The lead author said the hotspot is shaped more like a bow rather than a perfect circle, with one part of the bow being hotter and denser than the remaining parts.

She explained that the offset or misalignment in the light wavelength data of the young star can be attributed to the unique shape of the star. The periodicity in the emission of ultraviolet and optical light curves was because these wavelengths moved in and out during rotation of the star, the study explains. There is a gap of one day between these peaks, indicating that ultraviolet and optical brightness are unevenly distributed over the stellar surface. Such a phenomenon in a hotspot had never been detected previously. 

The scientists explain in the paper that the hotspots have density gradients, and that the difference in emissions could be observed when the high density region of the hot spot, which is smaller, is no longer visible, while the larger low-density region faces the Earth.

Baby Sun Had Hotspots 

Espaillat said that the study teaches everyone that the hotspots on the stellar surface created by the magnetic field are footprints on that surface. There was a time when the Baby Sun had hotspots too, which are different from sunspots. Areas of the Sun cooler than the rest of its surface are known as sunspots.

She explained that the hotspots of the Baby Sun were concentrated in areas where the Baby Sun was consuming particles from a surrounding protoplanetary disk. The study said the regions of the hotspots with different densities have different temperatures, and therefore, emit different radiation at different wavelengths. 

Espaillat added that as protoplanetary disks fade away, they leave behind stars, planets and cosmic objects, making up a stellar system. She said evidence of protoplanetary disks leading to the creation of our solar system could be found in the existence of our asteroid belt and all the planets. In order to understand the birth of Earth, it is essential to study young stars with similar properties, she said.