What if the Sun was half as massive?

The Sun, a fiery ball of hydrogen and helium gas, is our nearest star and the engine for all forms of life on Earth. With a surface temperature close to 6,000 degrees Celsius, diameter roughly 1.4 million kilometres and mass approximately 2E+30 (two nonillion) kilogramme, the Sun is an ordinary star by galactic standards. There are hundreds of billions of stars as much as a few hundred times larger in diameter and a million times more luminous.

At a distance of nearly 150 million kilometres from the Sun, the Earth is in its habitable zone, sometimes referred to as the ‘Goldilocks Zone’, where the temperature is neither too hot, nor too cold for liquid water to exist on a planet. If a planet is located far beyond the habitable zone, odds are, its water will be frozen. Conversely, if it is closer inward, it may be too hot for water to remain in liquid form. For the Sun, the habitable zone ranges between 108 million kilometres from its surface to 340 million kilometres outward. Clearly, if our planet were a little closer to the Sun, a runaway greenhouse effect would have rendered it unliveable.

What if at birth 4.55 billion years ago the Sun was half its mass? What our solar system would be like with the less massive Sun? In order to understand what impact the smaller Sun would have on our solar system and the Earth in particular, we need to have a basic understanding of some of the properties of low-mass stars. For the sake of brevity, let us call the half-solar mass Sun Minisun.

A star’s temperature and colour are determined by its mass. Bigger stars are hotter and bluer than our whitish-yellow Sun, while smaller stars are cooler and redder. As Minisun’s mass is half-solar mass, it will be a red dwarf– half the size of the Sun. Red dwarfs are by far the most common type of star in the neighbourhood of the Sun. We don’t see them because of their low luminosity.

The Minisun’s smaller size means that it burns at a lower effective temperature, reaching a maximum value of 4,000 degrees Celsius. The low temperature means it would be much dimmer than the Sun, emitting less than five percent as much light as the Sun. Thus, the Moon that lights up the night sky would also be dimmer.

Unlike the Sun, which will become a white dwarf – a dead star – after well-nigh five billion years, the Minisun will become a white dwarf after trillions of years. Compared to the age of the Universe, which is 13.7 billion years, the Minisun would be in the infant stage of its life now.

Given its smaller mass and cooler surface temperature, the habitable zone of the Minisun would be small in extent and much closer in. Hence, if our planet orbited the Minisun at the same distance, we wouldn’t stand a chance of surviving. In fact, it is doubtful whether with very little heat and light and frozen water, life could even evolve on Earth.

Mercury would be sitting pretty in the Minisun’s habitable zone. However, being so close to the Minisun, its strong gravitational pull would eventually make the planet’s rotational period equal to its orbital period, leaving the same face always pointing toward the Minisun while the other side will be in perpetual darkness. On a planet with one face scorchingly hot and the other face in deep freeze, the prospect of life arising and evolving seems slim. Nevertheless, extremophiles – hardy organisms capable of thriving in extremely hostile environments – could be a possibility.

Planets move on elliptic (squashed circle) orbits because of the attractive gravitational force exerted on them by the Sun. Greater the mass of the Sun, greater is the strength of the force. If the mass of the Sun is halved, the strength of the attractive force would also be halved. As a result, Earth’s orbit around the Minisun would increase. Moreover, it is highly likely that the orbits of the outer, giant gaseous planets may change so much that they would probably drift out of the solar system. The solar system would be left with only four planets.

The orbital speed of a planet is independent of its mass, but it depends on the mass of the Sun. Closer a planet is to the Sun, greater is its orbital speed. Since the Earth travels about 945 million kilometres in 365.25 days, its orbital speed works out to be close to 108,000 kilometres per hour. If the Minisun replaces the actual Sun, Earth’s orbital speed would decrease by 30 percent, thereby increasing the solar year to 516.2 days.

Finally, paraphrasing the Serbian-American inventor and physicist Nikola Tesla, with the Minisun at the centre of the solar system, the Earth would originate as a frozen mass. It would remain a frozen mass for trillions of years when the dying Minisun will draw it to its doom. n

The writer is a Professor of Physics

at Fordham University, New York.

Photo: Google Image

What if the Sun was half as massive?

The Sun, a fiery ball of hydrogen and helium gas, is our nearest star and the engine for all forms of life on Earth. With a surface temperature close to 6,000 degrees Celsius, diameter roughly 1.4 million kilometres and mass approximately 2E+30 (two nonillion) kilogramme, the Sun is an ordinary star by galactic standards. There are hundreds of billions of stars as much as a few hundred times larger in diameter and a million times more luminous. At a distance of nearly 150 million kilometres from the Sun, the Earth is in its habitable zone, sometimes referred to as the Goldilocks Zone, where the temperature is neither too hot, nor too cold for liquid water to exist on a planet. If a planet is located far beyond the habitable zone, odds are, its water will be frozen. Conversely, if it is closer inward, it may be too hot for water to remain in liquid form. For the Sun, the habitable zone ranges between 108 million kilometres from its surface to 340 million kilometres outward. Clearly, if our planet were a little closer to the Sun, a runaway greenhouse effect would have rendered it unliveable. What if at birth 4.55 billion years ago the Sun was half its mass? What our solar system would be like with the less massive Sun? In order to understand what impact the smaller Sun would have on our solar system and the Earth in particular, we need to have a basic understanding of some of the properties of low-mass stars. For the sake of brevity, let us call the half-solar mass Sun Minisun. A stars temperature and colour are determined by its mass. Bigger stars are hotter and bluer than our whitish-yellow Sun, while smaller stars are cooler and redder. As Minisuns mass is half-solar mass, it will be a red dwarf half the size of the Sun. Red dwarfs are by far the most common type of star in the neighbourhood of the Sun. We dont see them because of their low luminosity. The Minisuns smaller size means that it burns at a lower effective temperature, reaching a maximum value of 4,000 degrees Celsius. The low temperature means it would be much dimmer than the Sun, emitting less than five percent as much light as the Sun. Thus, the Moon that lights up the night sky would also be dimmer. Unlike the Sun, which will become a white dwarf a dead star after well-nigh five billion years, the Minisun will become a white dwarf after trillions of years. Compared to the age of the Universe, which is 13.7 billion years, the Minisun would be in the infant stage of its life now. Given its smaller mass and cooler surface temperature, the habitable zone of the Minisun would be small in extent and much closer in. Hence, if our planet orbited the Minisun at the same distance, we wouldnt stand a chance of surviving. In fact, it is doubtful whether with very little heat and light and frozen water, life could even evolve on Earth. Mercury would be sitting pretty in the Minisuns habitable zone. However, being so close to the Minisun, its strong gravitational pull would eventually make the planets rotational period equal to its orbital period, leaving the same face always pointing toward the Minisun while the other side will be in perpetual darkness. On a planet with one face scorchingly hot and the other face in deep freeze, the prospect of life arising and evolving seems slim. Nevertheless, extremophiles hardy organisms capable of thriving in extremely hostile environments could be a possibility. Planets move on elliptic (squashed circle) orbits because of the attractive gravitational force exerted on them by the Sun. Greater the mass of the Sun, greater is the strength of the force. If the mass of the Sun is halved, the strength of the attractive force would also be halved. As a result, Earths orbit around the Minisun would increase. Moreover, it is highly likely that the orbits of the outer, giant gaseous planets may change so much that they would probably drift out of the solar system. The solar system would be left with only four planets. The orbital speed of a planet is independent of its mass, but it depends on the mass of the Sun. Closer a planet is to the Sun, greater is its orbital speed. Since the Earth travels about 945 million kilometres in 365.25 days, its orbital speed works out to be close to 108,000 kilometres per hour. If the Minisun replaces the actual Sun, Earths orbital speed would decrease by 30 percent, thereby increasing the solar year to 516.2 days. Finally, paraphrasing the Serbian-American inventor and physicist Nikola Tesla, with the Minisun at the centre of the solar system, the Earth would originate as a frozen mass. It would remain a frozen mass for trillions of years when the dying Minisun will draw it to its doom. n The writer is a Professor of Physics at Fordham University, New York. Photo: Google Image