The Colorful Mass Loss of the Sun

Avi Loeb
4 min readMay 15, 2024


An Aurora display of colors, triggered by a coronal mass ejection from the Sun. Green and red colors reflect excitation of spectral lines of atomic oxygen, whereas blue results from molecular nitrogen in the Earth’s atmosphere. (Image credit: Noppawat Tom Charoensinphon via Getty Images)

Over the past week, the sky over Boston lit up in beautiful aurora colors as a result of the excitation of atomic oxygen (red and green) and molecular nitrogen (blue) in the Earth’s atmosphere by coronal mass ejections from the Sun. An aurora is a manifestation of a geomagnetic storm, triggered by energetic particles streaming out of the sun and interacting with the Earth’s magnetic field. The persistent solar wind also creates the same effects, so there is a weak aurora somewhere even without a major geomagnetic storm.

Coronal mass ejections are triggered by solar flares in which strong magnetic field lines on the surface of the Sun cross each other, tear apart and reconnect like giant ropes glued with hot gas. In 1859, a solar storm known as the Carrington event disabled parts of the telegraph network of the U.S. and triggered fires. The event released on the surface of the Sun a total energy equivalent to a trillion Hiroshima atomic bombs. Today, the damage from a similar energy release would cost trillions of dollars in communication satellites, power grid and other technological infrastructure. More energetic events are rarer but their economic impact was small before the advent of modern science and technology over the past century. Seven years ago, I wrote a paper with my former postdoc, Manasvi Lingam, where we proposed a magnetic shield to deflect charged particles from the Sun and mitigate the huge financial cost of Carrington-like events in our future.

A solar flare with a coronal mass ejection. (Image credit: NASA)

The plume expelled during a coronal mass ejection typically carries about a billion tons of hot gas at speeds between a few hundreds to a few thousands of kilometers per second that cross the Sun-Earth separation over a timescale between a week and 15 hours, respectively. Cumulatively, coronal mass ejections account for about a tenth of the mass loss from the Sun by the solar wind.

The solar wind carries on average 2 million tons of matter per second (or equivalently 0.00003 of the solar mass per billion years). Interestingly, this equals about half the amount of mass lost from the Sun by the radiation it emits, using Einstein’s formula that mass equals energy divided by the speed of light squared. The similarity in magnitude between these two channels for mass loss is a mere coincidence which does not occur for other stars, because the outflow speed of the solar wind is a thousand times smaller than the speed of light. As a result, the kinetic energy of the solar wind is a millionth of its rest-mass energy, and it is powered by a tiny fraction of the solar radiation energy. Altogether, through its radiation and wind, the Sun loses about 6 million tons of mass every second.

But is the Sun only losing mass? As it turns out, Solar system asteroids are raining on the Sun at a rate that could peak around ten million tons per second, potentially compensating for its mass loss. However, the asteroid mass reservoir is limited and interstellar objects are a thousand times less likely to hit the Sun. As a result, the Sun mostly loses mass.

As a result of the mass loss from the Sun, Earth and other planets slowly recede in their orbits from the center of mass of the Solar system. Because of the slow rate of mass loss, the angular momentum of the planetary motion is conserved and their orbital radii increase in inverse proportion to the mass of the Sun.

During the Sun’s lifespan of 12 billion years on the Main sequence, the Sun loses 0.0004 of its mass. But at the end of its life, in about 7.6 billion years from now, the Sun will become a red giant and its radius will engulf the orbital radius of the Earth. This final act would remove two fifths of the mass of the Sun and leave a dense core that will cool to become an Earth-size metallic ball, known as a faint white dwarf.

Take this as a lesson. Whether one does it naturally or by taking Ozempic, losing mass is not always beneficial for promoting a bright future.


Image credit: Chris Michel (October 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".