The first observation of a dying star devouring a planet! Only high sensitivity and a large field of view can see it!

From our perspective, the stars in the sky seem to be eternal. However, astronomers have long discovered that all stars, including the sun, have a life cycle. When a star dies, if the planets orbiting it are close enough, the planets will be ruthlessly swallowed up!

Recently, research institutions such as MIT, Harvard University and California Institute of Technology reported in the famous scientific journal "Nature" an event in which a dying star devoured its surrounding planets. Although such a scene has long been predicted, this is the first time it has been actually captured.

So why should we care about something that sounds like it has nothing to do with us? In fact, in the future, the Earth will face the same fate.

From birth to death, stars go through different stages of evolution in their lives. In the entire life cycle, the longest stage a star is in is the "main sequence" stage, and our sun is currently in this stage. After leaving the "main sequence" stage, the star will move towards death, which means that the star's fuel is about to run out. In this dying process, the star will become larger and larger. For example, a star with a mass of 0.7 to 2 times the mass of the sun can reach a maximum radius of about 300 times the radius of the "main sequence" stage during its death process. Such a behemoth will inevitably swallow up some of the planets with smaller orbital radii around it.

In the future, such scenes will also appear in the solar system. The sun is currently about 4.6 billion years old. As it ages, its size will also increase. According to the stellar evolution model, in about 6.4 billion years, the sun will expand rapidly; in about 300 million years, the sun will expand to the size of the orbits of Mercury and Venus and swallow them; after about 200 million years, the sun will expand to the size of the orbits of the earth and Mars, and our earth and Mars will also be swallowed by the sun.

Many exoplanets currently observed by astronomers may eventually be swallowed by their stars, but the probability of directly capturing such an event is very low. It is estimated that there are only a few such events per year in the Milky Way galaxy where we live. To capture such phenomena, the observation requirements are very high: it requires the ability to observe very large areas of the sky at the same time, the need for continuous observation of these areas, and the ability to observe very faint sources. In the words of astronomers, this requires highly sensitive, large-field-of-view time-domain surveys.

In this latest discovery, the researchers used data from the Caltech's ZTF optical time-domain survey, which began in 2017 and can observe 47 square degrees (equivalent to more than 200 times the size of the moon) in one observation, using a telescope with an aperture of about 1.2 meters.

In 2020, researchers accidentally discovered this celestial event while searching for a type of exploding binary star system using ZTF sky survey data. The binary star system the team was searching for would increase in brightness by more than 1,000 times during an explosion, but in this event, the brightness of the celestial body only increased by about 100 times. Subsequently, the research team collected other data and combined it with computer simulations. After ruling out other possibilities, they finally determined that this was most likely an event of "a dying star devouring its surrounding planets."

At present, my country is also intensively developing and building such equipment. For example, the multi-channel photometric survey telescope Mephisto of Yunnan University has been built and can observe in three bands at the same time; the "Sitian Project" under construction can realize large-scale (10,000 square degrees, equivalent to 50,000 times the size of the moon) high-frequency (observation every 30 minutes) observations in the future. The 2.5-meter "Mozi" survey telescope jointly built by the University of Science and Technology of China and Purple Mountain Observatory is expected to start observations this summer. Compared with the ZTF survey, although the field of view is smaller (6.55 square degrees), due to the good site and large aperture of the telescope, the sensitivity is 10 times higher.

With the commissioning of these observation equipment in our country, we will be able to capture events in the future where such stars devour their surrounding planets.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Fang Min

Reviewer: Han Wenbiao, Researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

Source: Starry Sky Project